THAI test facility for experimental research on hydrogen and fission product behaviour in light water reactor containments

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THAI test facility for experimental research on hydrogen and fission product behaviour in light water reactor containments S. Gupta a,∗ , E. Schmidt a , B. von Laufenberg a , M. Freitag a , G. Poss a , F. Funke b , G. Weber c a b c

Becker Technologies GmbH, Koelner Strasse 6, 65760 Eschborn, Germany AREVA GmbH, P.O. Box 1109, 91001 Erlangen, Germany Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, Forschungszentrum, Boltzmannstraße 14, 85748 Garching, Germany

h i g h l i g h t s • • • • •

Large scale facility for investigating representative LWR severe accident scenarios. Coupled effect tests in the field of thermal-hydraulics, hydrogen, aerosol and iodine. Measurement techniques improved and adapted for severe accident conditions. Testing of passive mitigation systems (e.g. PAR) under accident conditions. THAI data application for validation and development of CFD and LP codes.

a r t i c l e

i n f o

Article history: Received 14 September 2015 Accepted 22 September 2015

a b s t r a c t The test facility THAI (thermal-hydraulics, hydrogen, aerosol, and iodine) aims at addressing open questions concerning gas distribution, behaviour of hydrogen, iodine and aerosols in the containment of light water reactors during severe accidents. Main component of the facility is a 60 m3 stainless steel vessel, 9.2 m high and 3.2 m in diameter, with exchangeable internals for multi-compartment investigations. The maximal design pressure of the vessel is 14 bar which allows H2 combustion experiments at a severe accident relevant H2 concentration level. The facility is approved for the use of low-level radiotracer I-123 which enables the measurement of time resolved iodine behaviour. The THAI test facility allows investigating various accident scenarios, ranging from turbulent free convection to stagnant stratified containment atmospheres and can be combined with simultaneous use of hydrogen, iodine and aerosol issues. THAI experimental research also covers investigations related to mitigation systems employed in light water reactor containments by performing experiments on, e.g. pressure suppression pool hydrodynamics, performance behaviour of passive autocatalytic recombiners, and spray interaction with hydrogen–steam–air flames in phenomenon orientated and coupled-effects experiments. The THAI experimental data have been widely used for the validation and further development of Lumped Parameter and Computational Fluid Dynamics codes with 3D capabilities, e.g. International Standard Problems ISP-47 (thermal hydraulics, gas distribution) and ISP-49 (hydrogen combustion), EUSARNET/SARNET2 code-benchmark exercises involving THAI data on iodine/surface interactions, iodine mass transfer, passive autocatalytic recombiner performance, iodine oxide behaviour and iodine transport in multi-compartment behaviour. The present paper provides an overview of the THAI experiments related to hydrogen and fission products issues performed in the frame of national and international projects. From the comprehensive THAI experimental database, a selection of typical results is presented to illustrate the multi-functionality of the THAI facility and the broad variety of the experimental investigations. © 2015 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +49 6196 936 115; fax: +49 6196 936 100. E-mail address: [email protected] (S. Gupta). http://dx.doi.org/10.1016/j.nucengdes.2015.09.013 0029-5493/© 2015 Elsevier B.V. All rights reserved.

Please cite this article in press as: Gupta, S., et al., THAI test facility for experimental research on hydrogen and fission product behaviour in light water reactor containments. Nucl. Eng. Des. (2015), http://dx.doi.org/10.1016/j.nucengdes.2015.09.013

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1. Introduction In the event of a nuclear reactor accident, a large amount of hydrogen and fission products may be generated by interactions of the core melt with containment structures and water. The behaviour and the distribution of hydrogen and the fission products can be affected by a variety of chemical and physical interactions taking place inside the containment. These interactions involve the disciplines of thermal hydraulics, hydrogen distribution and deflagrations, fission products chemistry and material interactions, aerosol physics, and effectiveness of mitigation measures among others. In order to develop a high level of confidence that nuclear reactor containment systems and components establish an acceptable level of safety, both experimental as well as analytical efforts are required to understand the aforesaid phenomena and processes that may occur during an accident transient. The need is further affirmed by the severe nuclear accident which occurred at the Fukushima Daiichi nuclear power station. Explosion of the released hydrogen breached the containment integrity and impeded onsite emergency response efforts (SNETP, 2013). After the accident, significant amounts of radioactivity were released in the environment. In the aftermath of Fukushima accident, action plans prepared by regulatory bodies in many countries included directives to consider implementing the safety measures, such as passive autocatalytic recombiners (PAR) and filtered containment venting systems (FCVS) to manage severe accidents and mitigate their consequences (OECD/NEA, 2013). To ensure an optimal performance of these mitigation systems, their effectiveness will need to be assessed under a wide range of postulated accident scenarios covering not only high but also low probability events to consider cliff-edge effects. In this context, large efforts are also underway at international level to more precisely model and predict the phenomena occurring in reactor containments in case of severe accidents, and to assess the safety benefits from particular accident management measures. Conventional LP codes are being continuously improved by implementing more detailed models, and CFD codes with 3D capabilities are being developed for containment applications. The application of both code types requires

further experimental validation to enlarge their modelling capabilities for the reactor case. Furthermore, the performance of these safety analysis codes for plant application is often validated through large scale experiments reasonably representative of dominant accident sequences or conditions. Large scale experiments used for validation purpose need to consider the impact of physical processes which govern the mixing of containment atmosphere such as, forced convection, natural convection, condensation for an accurate prediction of iodine and aerosol transport and distribution behaviour. In the above-mentioned context, an extensive experimental program on nuclear severe accident in light water reactors (LWR) has already been pursuing at the THAI test facility since its construction in the year 2000. The THAI hydrogen related experiments deal with, hydrogen stratification and gas mixing (by imposed natural or forced convections) phenomena in condensing or superheated steam–air–gas atmospheres, hydrogen deflagrations and their interaction with containment sprays, and performance of PARs under severe accident typical conditions. The THAI fission product related experiments aimed at investigating fission product transport, depletion and resuspension processes in combination with the hydrogen issues and thermal-hydraulic phenomena. The size of the facility allows studying the combined effects of physical advection–dispersion in the atmosphere, mass exchanges at walls and water surfaces, chemical reactions with structural surfaces or materials dissolved in water, and gas phase interaction between fission products (e.g. iodine/aerosol) as well as interaction of fission products with mitigation devices, e.g. spray, PARs. Table 1 provides a list of already conducted or planned THAI experiments and the related project frame. The THAI reactor safety research programs conducted so far in the frame of previous national projects and OECD/NEA joint nuclear safety projects provided a sound database for development and validation of safety analysis codes. Major progress in measuring spatial hydrogen distributions, slow hydrogen deflagration behaviour, performance of PARs under accident-typical conditions, fission product distribution and interaction with containment surfaces, aerosols and water pools has been demonstrated with the THAI tests. Improved models based on THAI experimental data have demonstrated reliable

Table 1 THAI experimental research programs with investigated issues. Experimental program

Duration

Investigated issues

National THAI-I

1998–2003

National THAI-II

2003–2007

National THAI-III

2006–2009

National THAI-IV

2009–2013

National THAI-V

2013–ongoing

OECD/NEA THAI

2007–2009

OECD/NEA THAI2

2011–2014

Gas distribution–stratification (including the effects of condensation, turbulence), I2 distribution and interaction with steel surfaces Stratified flows in containment, I2 mass transfer, multi-compartment I2 distribution and behaviour, iodine interaction with painted surfaces, iodine-oxides distribution and behaviour, dry resuspension of aerosol material by hydrogen deflagration, wet aerosol resuspension from a boiling sump, PAR operation influence on iodine volatility Stratified flows in containment, I2 gas/liquid mass transfer, I2 interaction with painted surfaces Initially stable gas stratification and dissolution by natural or forced convective flows, I2 deposition and washdown behaviour from condensing painted surfaces, aerosol (soluble/insoluble mixture) washdown from surfaces, soluble/insoluble aerosol resuspension from a boiling sump, BWR pressure suppression pool hydrodynamics (thermal stratification, bubble dynamics, pool swelling) Stratified flows in containment-constant pressure, water hydrodynamics-incomplete condensation, I2 multi-compartment-paint//aerosol/relative humidity, air-borne aerosol and iodine wash out by spray, iodine/Ag reaction in sump, PAR performance under counter-current flow conditions Gas distribution (stratification, condensation), PAR performance under SA conditions – startup-ignition, oxygen starvation, H2 deflagration, I2 –(soluble and insoluble) aerosol interaction, aerosol (soluble) washdown from surfaces PAR performance – start-up and ignition behaviour in reduced O2 conditions, H2 combustion–spray interaction, release of gaseous iodine from a flashing jet (PWR DBA scenario)

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Table 2 Application of THAI experiments for code benchmark exercises. Project/frame

Issue

THAI experiment

OECD-THAI 2 project

Based on THAI test HR-35

OECD-THAI project

PAR performance in gas atmosphere containing low O2 concentration Stratified flows in containment

OECD-NEA/ISP-47 OECD-NEA/ISP-49

Containment thermal hydraulics Hydrogen combustion

SARNET-1, WP1 SARNET-2, WP8 SARNET-2, WP8 SARNET-2, WP7 German CFD network

Iodine mass transfer Iodine oxide behaviour Multi-compartment iodine behaviour Passive autocatalytic recombiners Gas distribution, thermal-hydraulics

simulation of complex experiments related to hydrogen and fission product issues. A selection of code benchmark exercises carried out based on THAI experimental data is given in Table 2. 2. THAI test facility and instrumentation THAI is a technical-scale test facility built for carrying out the experimental research in the field of nuclear reactor containment safety. The facility allows investigating processes and phenomena which might occur during design basis accident (DBA) and severe accident (SA) scenarios. The size of the test vessel is large enough to establish inhomogeneous atmosphere conditions (e.g. stratification) as well as natural convection flow loops by controlled wall heating or cooling. The vessel dimensions, its modular multi-compartment configuration and the well-controlled boundary conditions facilitate coupling of flow distribution studies with fission products to investigate the influence of independent thermal-hydraulic conditions prevailing in different compartments on deposition/resuspension behaviour of reactive/nonreactive aerosols and chemically reactive iodine in the presence of steel/painted surfaces or water pools. Technical specifications of the THAI test facility are given in Table 3. Sketch of the THAI test facility and its mains dimensions are shown in Fig. 1a and b, respectively. The inner space of the THAI

test vessel can be sub-divided into five main compartments. Starting from the top, the upper part is called “dome” compartment and its lower edge is on the elevation of the top of the 1.4 m outer diameter hollow inner cylinder. The inner cylinder divides the centre part of the vessel into two compartments – “annulus” compartment is space outside of the cylindrical structure and “inner cylinder” compartment which is inside of the structure. The space below the cylindrical structure also consists of two parts – “bottom of vessel” compartment and “sump” compartment. The annulus compartment is further sub-divided into upper and lower parts by the set of condensate trays, which partly blocks the flow through the annulus between upper and lower part of vessel. All the internal structures in THAI are removable. The cylindrical part of the test vessel wall is equipped with three independent heating/cooling jackets over the height for controlled wall temperature conditioning by means of external thermal oil circuits. The heating/cooling power of each jacket is determined from measurements of oil mass flow and inlet/outlet temperature difference. Top and bottom vessel heads including the sump part can be heated electrically. The sump water basin at the bottom of the THAI vessel has a diameter of 1.4 m and is additionally equipped with a 20 kW electrical heating coil. The outer sides of the vessel are thermally insulated by 12 cm mineral wool to minimize the heat losses. Typical heat losses are in the order of 8 kW at 110 ◦ C operation temperature. 2.1. Feed systems

Table 3 Technical specifications of THAI. Total volume of it: Sump compartment Dome compartment Inner cylinder compartment Total THAI steel surface (including internals) Total THAI steel surface (single compartment, without internals) Permissible overpressure Operational temperature Medium

60 m3 1.7 m3 17.7 m3 6.05 m3 163 m2

3 heating/cooling mantles

Total volume Permissible overpressure Operational temperature Medium

0.9 m3 1 bar 180 ◦ C Heat transfer oil

Material

Stainless steel DIN 1.4571 ≈ AISI/SAE 316 Ti

Other features

Use of radioactive tracer I-123 Licensed amount of tracer per test: 5 GBq

Vessel

Erosion of stable atmospheric stratification layer by a buoyant plume: based on THAI test HM-2 using H2 Based on THAI test TH-13 Based on THAI hydrogen deflagration tests HD-2R, HD-12, HD-22, and HD-23 and French test facility ENACCEF (non-uniformity of hydrogen concentration the fast combustion regime) Joint interpretation of THAI Iod-9 and code benchmark Joint interpretation of THAI tests Iod-13 and Iod-14 Benchmark on THAI multi-compartment tests Iod-11 and Iod-12 Benchmark on THAI PAR tests PAR-2 and PAR-4 THAI thermal hydraulics separate- and coupled-effects experiments for blind and open calculations with CFD and LP codes

100 m2

14 bar 180 ◦ C Steam, water, air, hydrogen, helium

Feed systems are available for injection of steam, air, gas (e.g. N2 , O2 , He, H2 , O3 ), iodine and aerosol at variable positions. A 108 kW steam generator provides up to 36 g/s saturated steam for release into the test vessel. Elemental iodine (I2 ) is labelled with I-123 radiotracer by melting the solid I2 together with I-123 placed inside a small glass flask. The injection of gaseous radio-labelled I2 into the gas phase of the THAI vessel is then performed by hot carrier gas (e.g. synthetic air or helium). Injection of radio-labelled I2 into the sump water is carried out by using a cold aqueous I2 solution in a pressure vessel placed inside a glove box, which can be directly connected to the vessel sump. The THAI facility is equipped with various aerosol generation techniques to produce severe accident prototypical poly-disperse aerosols (CsI, Ag, LiNO3 , SnO2 , etc.) at specified thermal-hydraulic conditions and aerosol mass concentration up to 4 g/m3 . 2.2. Instrumentation Conventional thermal hydraulic instrumentation is provided for pressure, fluid and wall temperature, relative humidity, feed mass flow, wall heating/cooling power, water level and condensate mass flow measurements. It is supplemented by a thermal conductivity

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Fig. 1. THAI test facility (a) THAI facility including its auxiliary rooms and accessories provided for the application of radioactive I-123 tracer for iodine distribution measurements. (b) THAI vessel and its main dimensions.

based gas sampling system for continuous light-gas (H2 , He) concentration monitoring and with oxygen concentration analysers based on electrochemical detection principle. Furthermore, a spectral photometer (FASP) is available for in situ measurements of fog droplet size and airborne liquid water content (fog density). For flow measurements THAI can be equipped with vane wheel transducers for point velocity measurements, a 2-D Laser Doppler Anemometer (LDA) for velocity profile measurement, and a Particle Image Velocimetry (PIV) for velocity field measurement. Iodine distribution measurements are mainly based upon radioactive iodine I-123 which is used as a tracer for inactive iodine, and liquid or gas samples are taken at numerous locations for immediate gamma-ray evaluation. To avoid errors by adsorption of gaseous iodine within the sampling lines several small gas scrubbers have been installed at the measuring points inside the test vessel. They are filled with an aqueous iodine absorber

(alkaline sodium thiosulphate) which retains the iodine from the gas sample sucked through the liquid. Filling, draining and purging of the scrubbers, and gas flow control are managed from outside. The iodine concentration in the vessel atmosphere is determined from the amount of I-123 tracer in the absorber liquid and from the gas flow through the scrubber. In addition, external Maypack filters are applied to discriminate molecular, organic and aerosol-borne iodine in the test vessel atmosphere. Samplings from condensates in trays, collecting tanks, and from sump water at different heights allows offline evaluation of I-123. I-123 sensitive scintillation detectors are focused onto deposition coupons mounted inside the vessel and are monitoring online the iodine deposits through glass windows. The material of the surface of the deposition coupons is chosen to be representative for the tests objectives. Coupons can be heated separately to study iodine desorption. Additionally, specific chemical trace analyses of iodine (including

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Fig. 2. Examples of the gas distribution tests conducted in THAI.

iodine speciation measurements) are performed on-site or in AREVA’s Radiochemical Laboratory, Erlangen, Germany. Aerosol concentration is measured by filter samplers which are installed outside the THAI vessel. Additionally, an in-line aerosol concentration measurement is performed by laser light extinction measurement, which provides a continuous and non-intrusive measurement. Low-pressure cascade impactors are used for determining the aerosol particle size distribution. A condensation nuclei counter and a differential mobility analyzer are available for measuring aerosol particles in sub-micron range. The modular data acquisition system is designed to register both short-term data (up to 1 kHz) and long-term data (over several days). A large top flange and two manholes provide access to the interior of the vessel to facilitate modifications of internals and instrumentation for the individual experimental requirement. Flanges on five levels at five circumferential positions allow installation of in situ optics based measuring devices, e.g. Particle Image Velocimetry, Laser Doppler Anemometry, and laser light extinction measurement. 3. Experimental investigations in THAI 3.1. Thermal-hydraulics and hydrogen distribution Investigations related to gas transport and mixing in a containment building are important thermal-hydraulic phenomena to determine hydrogen related risks (OECD/NEA, 1999). The prediction of hydrogen distribution behaviour under postulated severe accident conditions is required not only to develop adequate accident management procedures but also to determine the integral design parameters, such as local loads resulting from a combustion event. It is possible that the global pressure rise is below some certain safety level for a containment, but local loads which are more sensitive to hydrogen distribution are capable to damage seriously specific containment components, internal walls and safety equipment (Bielert et al., 2001). Depending on hydrogen release location and distribution inside the containment building, atmospheric stratification and locally enhanced hydrogen concentration in the LWR containments contribute to the risk of early containment failure. Although good success has been reported in using improved modelling capabilities to calculate the formation and dissolution of stratified flows in the reactor containments, the improved numerical and

physical modelling methods need further investigation (Fischer and Kanzleiter, 2008; Schwarz et al., 2009; Andreani et al., 2015). In particular, the database used for the development and the assessment of the models is sparse and mostly related to integral test conditions closer to the conditions of a reactor accident. The use of such database from integral tests might allow validating the integral prediction capability of codes, but does not allow identifying individual model shortcomings. In this context, THAI experiments have been performed to address individual modelling features by means of separate effect tests and increased to higher complexity by successively adding more interactions to enhance the prediction capabilities of safety analysis codes for reactor-relevant accident conditions. The gas distribution experiments in THAI deal with stratification and natural convection phenomena in dry atmospheres. By adding steam, two-phase phenomena are introduced which influence the flow behaviour by condensation and/or evaporation in the gas atmosphere or on the vessel walls. Fig. 2 shows the schematics of recent gas distribution experiments conducted in THAI and the investigated phenomena. In the OECD/NEA International Standard Problem ISP-47, stratified atmospheric conditions were extensively studied by using the THAI experiment TH13 (Allelein et al., 2007). Analysis of the test results indicated that the light gas cloud erosion by the buoyant plume from the lower steam injection in the experiment was over-predicted by nearly all CFD- and LP-codes resulting in fully mixed atmospheric conditions. Application of such codes with a tendency to predict lower uniform concentration instead of locally enhanced hydrogen concentration to a reactor case could lead to non-conservative underestimation of the risk from hydrogen combustion. Based on the experience gained with ISP-47 exercise for CFD and LP codes, several recommendations were made to improve the analysis results. For LP codes, recommendation was postulated to improve the general guidelines for the nodalisation, whereas CFD codes were required to further improve modelling of condensation and turbulence (Allelein et al., 2007). The follow-up modelling work carried out with e.g. COCOSYS using THAI test TH13 confirmed that a suitable horizontal as well as vertical nodalisation scheme is required for the proper simulation of atmospheric stratifications and their dissolutions (Burkhardt et al., 2009). Sufficient refinement in the light gas cloud was considered necessary in order to calculate proper density gradients in stratified flows. These nodalisation requirements have

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Fig. 3. Progression of density interface by erosion in HM-1, HM-2 and TH13 (ISP-47) tests.

been considered in another THAI gas distribution experiment HM2 and confirmed their validity by the improved simulation results (Schwarz et al., 2009; Duspiva, 2009). Analysis of the THAI tests TH13 and HM2 by LP- and CFD-codes also confirmed that neglecting droplet settlement or rainout can have an unphysical impact on the simulation of the plume buoyancy (Schwarz et al., 2009). Apart from providing data for code validation purpose, the main objective of the HM2 test was to validate the transferability of experimental findings with helium to hydrogen problems. Helium and hydrogen differ in their thermodynamic properties, such as thermal conductivity, heat capacity, diffusivity, density, etc. Therefore, the suitability of helium based gas distribution data for current containment simulation code validation to predict hydrogen distribution and mixing in case of a postulated accident need to be assessed by means of a gas distribution experiment. For this purpose two tests were conducted: one with He (test HM1), the other with H2 (test HM2). Both tests consisted of two phases. In phase 1, an atmospheric stratification was established by injecting helium or hydrogen near the half height of the facility. In phase 2 the stratification was gradually dissolved by a steam injection into the lower part of the facility. Fig. 3 shows the progression of the density interface over time in the upper plenum of the THAI vessel for the HM tests as well as for the TH13 test. Both HM tests and the TH13 test indicate comparable erosion velocities of the stratified light gas layers. It was shown that comparable atmospheric distributions, pressure and temperature levels can be obtained if the volumetric concentrations of hydrogen and helium are comparable. This conclusion is based on volumetric light gas concentrations up to 40 vol% according to the experimental parameter ranges. The thermalhydraulic processes in the containment atmosphere, in particular the stratification and mixing phenomena are primarily governed by the density differences in the atmosphere. Since both hydrogen and helium have much smaller molecular weights as compared to air or steam, the differences in the molecular weights of these light gases give only very small contributions to the atmospheric density differences, provided that similar volumetric concentrations are established. Other differences in physical properties, like diffusivity, thermal conductivity or heat capacity, are of minor influence. From these findings it can be concluded that experiments on containment atmosphere flow dynamics using helium instead of hydrogen can provide meaningful data for identification of flow phenomena and code validation. The experimental configuration for the gas distribution experiment TH18 was designed for the validation of LP- and CFD-codes for mass transport and turbulence. The stationary forced air flow in the THAI vessel was established by means of a fan blower. Turbulent interactions were mainly involved by fan jet entrainment, wall

friction and flow separation. The CFD simulations over-predicted the flow resistance of the vents between the condensate trays by a factor of 3 and largely under-predicted the jet dispersion of the flow away from the vents. These deviations are related to the turbulent momentum exchange, which was simulated by variants of the k-ε model. This experiment indicated some substantial shortcomings of the used variants of the k-ε model, which should be overcome by suitable extensions of the turbulence closure relations (Fischer and Kanzleiter, 2008). The experiment TH18 was followed by a series of thermal hydraulic experiments with successively enhanced interaction of heat and mass transfer mechanisms. In the test TH20, buoyancy interactions were added, resulting from the inhomogeneous distribution of an injected helium cloud. The fan jet was used to dissolve helium-air stratification in the vessel from below. Blind analytical activities performed on the experiment TH20 resulted into erosion velocities which have been far from the measured experimental values. Analysis of the TH20 test with lumped parameter codes which utilize supplementary systems to simulate a momentum driven fluid jet indicated that optimization of the jet model is necessary for the realistic modelling of the physical jet behaviour with respect to the build-up of atmospheric stratifications (Kopper and Koch, 2014). The subsequent THAI test TH21 was designed to study natural convection produced by means of differentially heating and cooling of the vessel walls. Due to the thermal boundary conditions, a specific distribution of flow velocity and temperature in the atmosphere is formed which is characterized by thermal buoyancy and natural convection. In this case the turbulent heat exchange with the walls was superimposed on the effects of momentum exchange and buoyancy forces. In the test TH22 the first test phase consisted of the establishment of a natural convection loop (similar to the test TH21) and thereafter the test proceeded by a fast release of helium into the upper plenum of the test vessel to create a stratified light gas cloud. The build-up and dissolution of the light gas cloud by means of the natural convective flow pattern was made part of the investigation. In the TH24 test which follows the basic test design as used in the TH21 and TH22 tests, saturated steam is injected into the upper third of the vessel forming a stratification layer which is then mixed by a superposed natural convection. Between THAI tests TH22 and TH24, at comparable wall temperature difference it has been found that the erosion of the steam air cloud is much faster than the erosion of a comparable helium air cloud. This faster erosion can be attributed to the lower density difference between air and steam compared to air and helium (Schmidt et al., 2014). In the frame of German CFD network, a blind and open simulation benchmark exercise based on the TH24 test has been conducted to evaluate the prediction capability of the CFD and LP simulations (Freitag et al., 2014). The TH24 benchmark results indicated improvement in capability of codes in predicting the build-up of the stratification in the upper vessel section. However, the strength of the stratification, the duration for its dissolution and the associated mixture of water vapour at lower levels lacks good prediction quality. One of the recommendations to improve the code prediction capabilities was related to the enhancement of the condensation models to deliver the balance between injected and condensed steam to allow an accurate prediction of pressure. 3.2. Passive autocatalytic recombiners (PAR) Use of PARs based mitigation system is one of the established strategies for hydrogen removal by recombining it with atmospheric oxygen, in the presence of a catalyst, generating heat and water vapour. However, during recombination process, H2 reacts with available O2 much more slowly than in a deflagration and only partial recombination of hydrogen available at the PAR inlet takes

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place. The efficiency of catalyst material and its distribution inside a PAR plays a significant role in determining the PAR performance behaviour under accident conditions. The catalyst provides a suitable surface for the absorption of hydrogen and oxygen atoms and for desorption of the recombination product steam to surrounding gas. The presence of certain impurities can have an adverse impact on the catalyst functioning by blocking the active catalyst surface. Additionally, the hydrogen recombination reaction in PAR is controlled by the external mass transfer and their performance may be beyond design limits if exposed to high H2 release rates. Precise information about H2 release rate and the gas composition and distribution inside sub-compartments where PARs may be located is a prerequisite to design an optimal H2 control system based on PARs. Data on the behaviour of PAR types under accident conditions were elaborated earlier by the vendors and also in some research projects carried out by Battelle Frankfurt in the 640 m3 BMC test facility (Kanzleiter, 1997), tests in 90 m3 Surtsey vessel at Sandia (Blanchat and Malliakod, 1999), and by CEA Cadarache in the 16 m3 KALI vessel (Studer et al., 1999), and others. However, availability, consistency and completeness of the PAR performance data under a range of accident typical conditions are limited. In order to fill the PAR related knowledge gap, three different available PAR designs based on plate-type catalysts (provided by AREVA GmbH, Germany, and AECL (now CNL), Canada), and pellet-type catalyst (provided by NIS Ingenieurgesellschaft mbH, Germany) have been investigated in a comparable manner in the THAI test facility. The large vessel volume allows PAR operation with unrestricted natural convection which includes interaction of PAR performance and vessel atmosphere distribution. The investigated PAR units differ in their geometry, size and use of catalyst material. For the PAR performance tests, the plate type PAR units were scaled down to the size of the THAI test facility by reducing the number of catalytic elements. A limited number of tests have also been conducted with the smallest available 1/8th module of the pellet-type NIS PAR without modification. The total catalytic surface of the investigated PAR units was in the range of 1.44–1.89 m2 . The variation in test parameters to investigate PAR performance and ignition behaviour included: initial pressure between 1.0 bar and 3.0 bar, initial gas temperature between ambient and 117 ◦ C; atmosphere steam content of 0–60 vol%; variation in O2 concentration; and PAR overload by high hydrogen concentration. Tests started with an initially air-filled atmosphere. The vessel instrumentation allowed the measurement of pressure, temperatures, gas injection rates and distribution of gas concentrations in the vessel atmosphere; the PAR instrumentation provided the inlet flow velocity, the inlet and outlet gas temperatures, gas concentrations (H2 , O2 ) and the local catalytic surface temperatures. Majority of the PAR performance tests consisted of two test phases with two consecutive H2 injections. In the first test phase, H2 is released at a low rate (∼0.15 g/s) into the test vessel. Immediately after onset of PAR operation, H2 is switched to higher injection rate (∼0.30 g/s) resulting into further increase of H2 concentration and H2 recombination rate. Hydrogen injection is interrupted as soon as a level of approximately 5.5 vol% H2 at the PAR inlet (i.e. below the expected PAR ignition level) has been reached. Measurements of decreasing H2 concentrations and other relevant parameters are used to determine the PAR performance. Prior to starting the second test phase, O2 in vessel atmosphere is replenished if necessary for a test with “ignition” or further reduced by injecting nitrogen for a test with “oxygen starvation”. In the second test phase, H2 injection is again resumed at mass flow rate of about 0.30 g/s. For the investigation of PAR ignition, H2 concentration is increased until the operating PAR becomes so heavily loaded that ignition occurs. Immediately following an ignition, H2 release has been terminated. In principle, the performance behaviour of the three investigated PARs varied within a well specified range. Some differences

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occurred due to specific design features, such as gas velocity and gas residence time between the catalyst elements inside the PAR which in-turn depend on the chimney height of the respective PAR design. 3.2.1. PAR start-up behaviour Minimum hydrogen concentration required for the onset of H2 recombination is an important feature of a PAR. In the THAI PAR tests, onset of recombination has been studied in the first test phase with slowly increasing H2 concentration (typically 0.16 vol% H2 /min). The first indication of hydrogen recombination onset is a moderate increase in catalyst temperature. As soon as catalyst temperature further increases, buoyancy-induced convection inside the PAR housing starts and H2 concentration at the PAR outlet drops forming a second criterion for recombination onset. For the complete PAR performance test series, test results indicate that H2 concentration required for the onset of hydrogen recombination by PARs varies from 0.2 vol% to 4.4 vol% depending on temperature, pressure, and steam content. Dry atmosphere, elevated pressure and temperature promoted early recombination onset. Steam-saturated conditions resulted in delayed onset. Once being heated up the PARs remain operating until a lower concentration threshold of approximately 0.3 vol% H2 has been reached. Knowledge on PAR start-up behaviour during different time points of an accident transient, which might have different thermalhydraulics as well as gas composition, is also necessary. E.g. start-up or continuation of hydrogen recombination should also be confirmed at low or even extremely low oxygen concentrations, as possible in some accident scenarios (e.g. late accident phase with MCCI or air ingress into containment), for which no data are available. In the recently concluded OECD/NEA THAI2 project (Gupta et al., 2014; Gupta, 2015b), additional tests using AREVA and NIS PARs have been conducted to investigate the PAR onset and performance behaviour under O2 lean (almost inert) atmosphere containing superheated or saturated steam (up to 60%) and elevated pressure conditions (up to 3 bar). The tests started with purging of the vessel with N2 to reduce O2 content in the vessel atmosphere near to an inert level. Once the pre-defined thermal-hydraulic test conditions were established with an initial H2 content of 4 vol% in N2 or N2 -steam atmospheres, O2 release was commenced. Test results indicated prompt onset of H2 recombination with measured value of O2 concentration at PAR inlet below 0.5 vol%. 3.2.2. Hydrogen recombination rate and H2 depletion efficiency The demonstration of adequate hydrogen recombination rate (= amount of H2 recombined per unit time) and H2 depletion efficiency are the important criteria to confirm the PAR performance. In THAI tests, the hydrogen recombination rate is calculated by use of data measured at PAR: inlet temperature, inlet flow velocity, inlet/outlet gas concentrations, and the vessel pressure. Typical PAR behaviour under different pressures and PAR inlet H2 concentrations is shown in Fig. 4. After PAR onset, the recombination rate increases with H2 concentration and with pressure. The effect of increasing steam content combined with an increasing temperature in THAI tests was determined to be very small on the measured hydrogen recombination rate. As shown in Fig. 5, at a given H2 concentration of 4 vol% at PAR inlet, increasing the steam concentration from 0 vol% to 60 vol% results in about 30% reduction in recombination rate. The observed effect might be due to temperature rather than steam as increase in temperature from ambient to 97 ◦ C (associated with 60 vol% steam) will decrease the buoyancy by about same order of magnitude for a fixed hydrogen depletion efficiency. The buoyancy force calculated from the difference in gas density at the PAR inlet and outlet, is the driving force for convective flow and, in turn, affects the hydrogen recombination rate.

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Fig. 4. PAR recombination rate as a function of H2 concentrations and pressures. Tests without steam, initial gas temperature 24–32 ◦ C.

Fig. 6. Effect of pressure on hydrogen depletion efficiency. Tests without steam, initial gas temperature 24–32 ◦ C.

mainly depends on the gas residence time in the PAR catalyst zone and on the diffusion length from the vertically flowing gas to the catalyst surface (and probably also on catalyst material). For the three investigated PAR designs, the hydrogen depletion efficiency was determined to be varying between 40 and 60% in an atmosphere containing sufficient oxygen surplus. As depicted in Fig. 6, THAI test results indicate that hydrogen depletion efficiency significantly decreases with increasing pressure. This behaviour may be explained by the increase of gas diffusion resistance with pressure.

Fig. 5. Effect of steam content on H2 recombination rate (pressure = 1.5 bar, saturated steam, oxygen surplus ratio ˚ > 2.2).

Hydrogen recombination in a PAR is incomplete and this can be quantified by hydrogen depletion efficiency  (in %) calculated from the measured H2 concentrations at the PAR inlet and outlet,  = (CH2 in − CH2 out )/CH2 in · 100. Hydrogen depletion efficiency at a given H2 concentration is also PAR design specific as it

3.2.3. Oxygen starvation effect The aforesaid range of 40–60% H2 depletion efficiency remains unaffected as long as sufficient oxygen is available for H2 recombination at the PAR inlet. The test results provide evidence that an oxygen-to-hydrogen ratio higher than stoichiometric is required for PAR to operate at design capacity. A minimum oxygen surplus ratio defined as ˚ = 2 × CO2 /CH2 between 2 and 3 (depending on the PAR design) is necessary to ensure unimpaired PAR performance independently from steam content. In the used definition of O2 surplus ratio, CO2 and CH2 are the volumetric oxygen and hydrogen concentrations measured at the PAR inlet. The minimum value of oxygen surplus ratio is significantly higher than

Fig. 7. Effect of O2 starvation on catalyst temperatures and H2 recombination rate.

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strength of combustion after ignition by the PAR. In an environment with homogeneous H2 distribution, initiation of combustion with more marked pressure development has to be expected (Blanchat and Malliakod, 1999). Due to limited number of experiments conducted with the pellet-type PAR, no clear picture of PAR induced ignition behaviour could be drawn. However, during the tests it was observed that in case of very high load (>5.2 vol% H2 ), the PAR releases glowing particles into the surrounding atmosphere. This visible effect coincides with a marked additional H2 –O2 recombination in the bulk which under the investigated test conditions significantly supports H2 recombination by PAR without relevant pressure effects. The THAI PAR ignition test data did not indicate any flame acceleration under investigated test conditions. Fig. 8. PAR specific temperatures. Initial test conditions: 1.5 bar, 90 ◦ C, 47% steam.

the stoichiometric ratio (˚ = 1). As shown in Fig. 7, as soon as ˚ falls below 2, the hydrogen recombination rate and catalyst temperature decrease drastically. At oxygen surplus ratio ˚ = 1, the PAR capacity falls below 50% of the design capacity. The need for a significantly high surplus of oxygen can be explained by the large differences in molecular diffusivity of hydrogen and the other gases involved (O2 , N2 , and water vapour). The low diffusivity of the bigger molecules must be compensated by a higher specific concentration to guarantee immediate recombination of each hydrogen molecule that approaches the catalyst surface, i.e. to achieve full recombination capacity. 3.2.4. PAR induced ignition and the resulting hydrogen deflagration behaviour Potential hydrogen deflagration in the containment which might be initiated by a PAR exposed to elevated H2 concentrations is a serious safety concern. The PAR tests conducted in the THAI test facility markedly improved the level of knowledge on ignition potential by PAR. The test data also provided inlet conditions at which PAR induces an ignition. The details are important in determining the possible combustion mode that can occur in the compartment outside of the PAR. THAI test data indicated that ignition is directly correlated with the PAR catalyst surface temperature which in turn depends on the H2 concentration present at the PAR inlet. Fig. 8 depicts the dependency between H2 concentration (ascending and descending) at the PAR inlet and the PAR specific temperatures. The prerequisite for the PAR induced ignition varies with the specific PAR design. Nevertheless, from the THAI experiments, a narrow range of parameters at the time of ignition could be identified. Table 4 provides an overview of the conditions required to be fulfilled for PAR induced ignition. The maximum relative pressure rise P/P0 (P0 = absolute vessel pressure prior to ignition, P = pressure rise during deflagration) due to ignition obtained during the THAI tests remained below 1 for all the tests conducted mainly with plate-type PARs. However, it should be noted that the atmospheric stratification developing in the THAI vessel during PAR operation significantly affects the Table 4 Conditions required to be fulfilled for PAR induced ignition. Gas atmosphere

Measured catalyst surface temperature (maximum)

Measured H2 concentration (minimum) at the PAR inlet

Dry condition (without steam or superheated steam) Wet conditions (condensing steam)

890–920 ◦ C

5.5–7.5 vol% H2

960–1005 ◦ C

8–9 vol% H2 with 45 vol% steam

3.2.5. PAR interaction with fission products In addition to the PAR performance tests, two experiments under severe accident typical conditions were conducted to study the effect of fission product on performance behaviour of an operating PAR. The first experiment (HR31) designed to investigate conversion of caesium iodide (CsI) aerosol to gaseous iodine by an operating PAR considered a specific accident phase in which steam and hydrogen release has already occurred and PAR units are operational. Test boundary conditions were established in particular to achieve high catalyst temperatures (>800 ◦ C) to maximize CsI conversion. The corresponding hydrogen concentration was adjusted to 8–9 vol% at the PAR inlet. The gas mixture in the vessel atmosphere was rendered inert by adding steam (>60%) to avoid H2 ignition risks during the experiment. The conversion yield of aerosol-borne CsI to gaseous iodine has been calculated according to Eq. (1), by using the measured concentration of gaseous iodine at PAR outlet and the relationship of gas densities at PAR inlet and PAR outlet as follows: Conversion (%) =

CIgas (PARout ) CCsIaerosol (PARin )

·

PARin PARout

· 100

(1)

The evaluation with Eq. (1) considers that no gaseous iodine enters the PAR. This has also been confirmed by gaseous iodine concentration measurements in the vicinity of the PAR inlet. The measured particle Sauter mean diameter at the PAR inlet was in the range of 0.6–0.8 ␮m. Considering the PAR inlet crosssection dimension and the measured inlet velocity, residence time of about 0.15 s was estimated in between two catalyst plates. The test results indicate the conversion rates of CsI aerosol to gaseous iodine in the range of 1–3%. Another important result of THAI experiment HR-31 was the shifting in aerosol particle distribution to lower Sauter mean diameter at the PAR outlet. The second experiment (HR32) was designed to investigate the poisoning effects of fission product on PAR performance. The PAR poisoning test considered challenging test conditions which are conceivable during a severe accident. Subjected conditions for the test included PAR exposure to fission products at very low inlet H2 concentration (<2 vol%), elevated concentration of realistic aerosol mixture (hygroscopic and inert aerosols), use of reactive molecular iodine and saturated steam conditions prevailing inside the test vessel. Test phases were defined to ensure fission products arrival at PAR inlet before the onset of PAR recombination occurs. During the tests, the PAR was exposed to high concentrations (1.5–2.5 g/m3 ) of insoluble tin oxide (SnO2 ) aerosol, highly hygroscopic lithium nitrate (LiNO3 ) droplets, steam and radio-labelled molecular iodine (injected amount 1.7E−6 g/l). Test results indicate that PAR H2 depletion efficiency remained unaffected and comparable to the results of the other PAR tests with the identical thermal-hydraulic conditions but without aerosols and iodine. The onset of recombination was delayed and occurred at inlet hydrogen

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concentration of about 4.0 vol% and under saturated steam conditions. However, the delay in H2 recombination onset was also observed for other PAR-tests conducted with comparable thermal hydraulic conditions in the presence of saturated steam content but without fission products. 3.3. Hydrogen deflagration Hydrogen combustion behaviour is strongly influenced by the composition of the atmosphere, thermodynamic processes (local turbulence, convective flows, various heat and mass transfer mechanisms), geometry, operation of the safety and mitigation systems (fans, sprays) and by presence of ignition sources (Bujar, 1993). Though PARs and other available hydrogen mitigation systems drastically reduce the amount of hydrogen that can be present in the containment, deflagration of hydrogen may not be completely excluded. Therefore, it is necessary to predict the combustion regime and to calculate the consequence of combustion in terms of temperature and pressure loads. THAI experiments are mainly focused on hydrogen deflagrations with maximum H2 concentration limited to 10–12 vol%. Such experiments are representative of accident scenarios where ignition is induced either by an operating PAR or by a random ignition source as already observed during TMI accident (Alvares, 1986). In TMI accident, the combustion was produced at relatively low H2 concentration of approximately 6–8 vol% and gave rise to a slight overpressure of about 2 bar which was safely contained given the large volume and adequate design pressure of the containment building. However, the multi-compartment H2 deflagration experiments conducted in 640 m3 Battelle Model Containment (BMC) test facility (Kanzleiter, 1993) demonstrated that flame acceleration is also possible at H2 concentration as low as 9–10 vol% depending on initial pressure, steam concentration, and geometry of compartments. The flame acceleration without any symptoms of detonation occurred due to jet ignition effect produced by prevalence of specific distribution of hydrogen and geometry of the vent in the connected compartments. The estimated flame speed was up to 250 m/s resulting into high local over-pressure with its maximum close to the adiabatic-isochoric pressure. Therefore, detailed investigations on slow hydrogen deflagration behaviour and their coupling with potential accidental conditions and other mitigation measures, such as turbulence level, containment sprays, geometry effect, etc., is a necessary prerequisite to ensure efficient hydrogen mitigation measures. In THAI hydrogen deflagration experiments, the main parameters varied were hydrogen and steam concentration, temperature, pressure, burn direction upward and downward, well-mixed and stratified atmosphere. Most of the tests have been conducted with an initial pressure of 1.5 bar and an initial gas temperature between ambient and 140 ◦ C. The initial volumetric hydrogen concentration has been varied between 6 and 12 vol%. The test results indicate a curved flame front in most tests of almost axis-symmetric shape. Erratic flame front occurs only in case of slow burns or stratification. The addition of steam (at saturation state) until mixture composition is close to the ignition limit indicated an extremely unsteady combustion and a significant increase of combustion time. The tests with complete combustion indicated that measured peak pressures and temperatures are lower than the respective theoretical AICC (adiabatic isochoric complete combustion) values. The non-adiabatic thermodynamic process with heat transfer to structures is the main reason for the difference between measured and AICC values. A shorter combustion time reduces heat losses and the transient comes closer to the adiabatic change of state. In case of premixed quiescent hydrogen–air–(steam) atmosphere data indicate that the tests conducted with low hydrogen

Fig. 9. Effect of initial H2 concentration on upward flame front propagation. Tests with H2 –air gas mixture at 20 ◦ C and 1.5 bar.

concentration (<7 vol%), upward burn direction and ambient initial temperature show an unsteady combustion, a relatively long combustion time and an incomplete combustion. The flame front propagation speed is enhanced by increasing the initial H2 content and combustion is complete for the test cases with initial H2 content higher than 7.0 vol%. The effect of initial H2 concentration on upward flame front propagation is shown in Fig. 9. Upward flame propagation is supported by buoyancy, it proceeds at comparatively low hydrogen concentration with higher velocity and shows convex flame surfaces. For the test cases with burn direction “downward”, the limit concentration for flame propagation was determined to be 8.7 vol% H2 in air (at 1.5 bar and 20 ◦ C gas temperature) and 12 vol% H2 in a steam–air mixture containing 47 vol% saturated steam (at 1.5 bar and 90 ◦ C). The test results for the flame propagation in dry gas mixture at initial test conditions of 20 ◦ C gas temperature and 1.5 bar pressure are shown in Fig. 10. The influence of initial temperature on upward flame front propagation is marked with respect to flame speed, which decreases with increasing initial temperature because of changes in buoyancy forces. A higher initial temperature also influences reaction kinetics in a way, that combustion becomes almost complete also for lean mixtures with 6 vol% hydrogen (upward burn direction), which results in higher peak pressures. Higher initial temperatures (up to 140 ◦ C), which are more typical for severe accidents conditions, lead to lower peak pressures (because of the lower density/energy inventory in the gas mixture) and also give rise to more steady flame front propagation and a more complete combustion. A high steam content (48 vol%, at saturation state) in the combustible gas mixture leads to an irregular (“erratic”) combustion

Fig. 10. Effect of initial H2 concentration on downward flame propagation. Tests with H2 –air gas mixture at 20 ◦ C and 1.5 bar.

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Fig. 11. Effect of steam content on (upward) flame front propagation. Tests with 10 vol% H2 , 90 ◦ C, and variation in steam content.

both for upward and downward burn direction with lower flame velocities and lower peak pressures as compared to “dry” mixtures. Fig. 11 shows the effect of steam content on flame front propagation behaviour. Comparison between the test cases with upward and downward burn direction at identical initial test conditions depicts that the downward combustion is slower and produces slightly lower peak temperatures and peak pressures than upward combustion. Combustion is complete in case of downward burn with hydrogen concentration >8.7 vol%. In the tests with initially stratified atmosphere and upward burn direction, after the initiation of the combustion process a large scale convection was generated, which displaces hydrogen-rich mixture into hydrogen-lean, originally non-burnable mixture, which then becomes burnable. In the experiments with downward burn direction, the combustion does not produce much displacement of well-burnable mixture ahead of the flame downwards, and combustion stops once the flame enters into non-burnable mixture, i.e. H2 concentration below 8.7 vol%. 3.4. Aerosol distribution and transport behaviour During the course of a hypothetical core melt accident in a LWR, several physical processes lead to the formation of radioactive and non-radioactive aerosols (CSNI, 2009). Non-radioactive aerosols are mainly formed during interaction between the core debris and the concrete floor. The aerosol produced during core concrete interaction phase amounts to about 3% in PWRs and 2% in BWRs of the total aerosol inventory and may also be accompanied during the release with the remaining fission product inventory (Wichner and Spence, 1985). Most of the aerosol inventory comes from control rod materials (73% in PWRs and 75% in BWRs) and fission products (24% PWRs and 23% BWRs) (Wichner and Spence, 1985) which dominates the distribution of dose rate, decay heat and hence humidity in the containment which in turn has a large influence on the aerosol depletion behaviour. The aerosols entering the containment are mostly assumed to have a near log normal distribution of particles with aerodynamic mass median diameter ranging from 0.1 to 5 ␮m. Particle size may change depending on a particular accident sequence, prevailing thermal-hydraulic conditions or even steam condensation on particles may influence particle size drastically. Aerosol particle size in sub-micron range may also occur from secondary sources present inside the containment, e.g. radiolytic oxidation of iodine, aerosol resuspension or revolatization processes occurring during an accident. It has been established that under most accident conditions the iodine released from fuel into containment would be

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primarily in its reduced state as caesium iodide (CSNI, 2007). However, the Phebus-FP experiments indicated that caesium iodide is not the only likely species, especially for those reactors using silver–indium–cadmium control roads. The airborne concentration of these metal iodides in the containment is mainly governed by aerosol physics and the prevailing thermal-hydraulic conditions (Jacquemain et al., 1999). It is also important to consider that during an accident transient aerosol behaviour can be complex due to their multi-component composition and also because different thermalhydraulic conditions can affect aerosol distribution and transport behaviour, e.g. high relative humidity or water content can accelerate aerosol depletion from the containment atmosphere. There still remains uncertainty regarding presence of multi-component aerosols in the containment atmosphere and whether only consideration of density to represent multi-component aerosols is an adequate parameter for aerosols originating from multiple sources, such as structural material, fission product or control rods (CSNI, 2009). The experiments conducted in the KAEVER test facility (Poss, 2013) investigated soluble (CsI, CsOH), insoluble (Ag, SnO2 ) and aerosol mixtures (Ag/CsI, Ag/CsOH, CsI/CsOH and CsI/CsOH/Ag) under a wide range of thermal-hydraulic conditions from strongly supersaturated to strongly superheated containment atmospheres. Fig. 12 shows a relative comparison of aerosol depletion velocities for different aerosol systems and thermal hydraulics. KAEVER results indicated that in condensing atmospheres soluble particles and aerosol-mixtures with a soluble component deplete significantly faster than insoluble species depending strongly on the airborne liquid water content. The distribution of the aerosol particles inside the containment is governed by agglomeration, deposition on walls and advective transport and dependent upon the thermal hydraulic conditions in many aspects and therefore required to be investigated systematically in order to enhance the predictive capabilities of safety analysis tools. THAI aerosol experiment database has been built by investigating a wide range of test parameters required for stepwise validation and further development of aerosol models implemented in safety analysis tools while considering severe accident representative aerosol material (single component or mixture), particle size distribution, aerosol surface loadings, and air-borne mass-concentrations. In coupled-effects tests, aerosol behaviour has been combined with other fission product such as iodine or with accident mitigation systems. e.g. PARs, sprays, water pools. 3.4.1. Aerosol wash-down and retention behaviour Aerosol particles deposited on floors and walls in the containment are washed off by condensate flow into puddles or intermediate water pools in the containment sub-compartments. Depending on the accident scenario and the prevailing thermal hydraulic conditions, the aerosols entering from primary side to the containment building may consist of particles of various sizes, it can also be mixture of aged and fresh aerosols, and may be composed of more than one species with different chemical nature. Therefore, it is desirable to experimentally investigate in-containment aerosol behaviour comprising of material from different sources, such as fission product, control rod, or structure material. The wash-down leads to relocation of fission products and thus to relocation of decay heat and radiation sources inside containment. Depending on the wash-down efficiency of a particular aerosol or aerosol-mixture, different effects on containment thermalhydraulic (e.g. pressure, relative humidity) and source-term are therefore expected. Present modelling of fission product washdown shows that this process is not yet sufficiently understood even for soluble aerosols (Hoehne and Weber, 2010). Processes such as agglomeration, sedimentation, diffusiophoresis, thermophoresis, and hygroscopicity promote the deposition of aerosols on surfaces. The deposited aerosol may be washed down with the

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Fig. 12. Effect of thermal-hydraulic conditions on multi-component aerosol depletion behaviour, KAEVER experiments 1997. Reproduced from Poss (2013).

wall condensate flow through existing drains and arbitrary paths into the sump. The wash-down behaviour of soluble and insoluble aerosols is different. Intermediate small water pools or puddles on the floors may also have an impact on the aerosol wash-down process, e.g. time scale, wash-down efficiency. A number of THAI tests have been designed to investigate the aerosol wash-down behaviour from containment surfaces and to evaluate the effect of puddles (intermediate water pools of limited maximum height) on aerosol retention capacity. To support necessary model validation and development work, a systematic experimental approach was followed by conducting the tests with single component aerosols of different solubility (CsI, Ag) and complexity was enhanced during an experiment conducted with an aerosol mixture (CsI/SnO2 ). In THAI test AW, wash down of soluble aerosol CsI was investigated. During preconditioning phase, CsI solution was injected into the vessel atmosphere filled with air at 130 ◦ C that facilitated immediate and efficient evaporation of the CsI containing water droplets. The total injected dry mass of aerosol was about 1178 g. The aerosol horizontal surface loading measured with deposition coupons was of the order of 80 g/m2 . Extrapolated to the total horizontal surfaces (the cross-section area of the vessel) of 7.63 m2 , this corresponds to 610 g deposited on the horizontal surfaces. The difference to 1178 g (i.e. 568 g) was deposited on the vertical surfaces. Considering the ratio of vertical to horizontal surface area in THAI vessel (approximately 10), it could be concluded that the specific surface loading of the vertical walls was about 10 times lower than on the horizontal surfaces. The wash-down process was started by cooling down the vertical vessel walls and simultaneously starting the steam injection. The cooling power of the vessel mantles was adjusted to allow complete steam condensation on the vertical vessel walls in a quasi-stationary condition and no volume condensation occurred during the test. The injected steam condensed on the vertical walls (specific condensate rate: 0.30 g/s m2 ) and the condensate was drained over the aerosol loaded horizontal surface (inclined by 2◦ ), which consisted of 16 individual plate sections (separated by small vertical walls) and a puddle section with 30 l capacity. The puddle section surface was one fourth of the total horizontal surface, and the height of the rim was 4 cm, relevant for overflow during increase of water level due to in-coming condensate. The condensate drained out from the plates and the puddle was analyzed separately. The measurements of CsI concentration in the condensate flow from the plate- and

puddle-sections show the concentration peak values at the beginning of the wash-down phase. The concentration in the plate runoff water decreased rapidly and the deposited CsI aerosol was almost completely washed-down after 2.5 h. However, the concentration in the puddle runoff water decreased much slower and shows draining out of CsI aerosol even after 23 h of washing time. The puddle water acts as an intermediate storage of dissolved aerosol material, which leads to a considerable delay in the wash-down transport. In THAI test AW2, wash down behaviour of CsI/SnO2 aerosol mixture from vertical walls and horizontal surfaces (including a puddle) by steam condensate was investigated. The methodology and hardware used during the test was similar to the AW test, except that the horizontal stainless steel surfaces used during AW test were coated with artificially aged paint with an estimated age of 15 years (Funke et al., 2015). Total horizontal surface loading for CsI and SnO2 aerosols was about 30 g/m2 and 60 g/m2 , respectively. The mass median diameter of the injected CsI and SnO2 aerosols was 1 ␮m with a geometric standard deviation of 2. The test results indicated that CsI-aerosol was almost completely washed down from the horizontal and vertical surfaces as well as from the puddle within 20 h. The wash-down behaviour of CsI aerosol was in agreement with the previous AW test. However, wash-down of nonsoluble SnO2 aerosol remained far from being complete. Assuming, complete wash-down of aerosols from the vertical surfaces, the estimated total wash-down efficiency of the aerosol mixture from deposition surfaces was of the order of 50% consisting to a large extent of the soluble CsI aerosol. The unwashed aerosol mass which remained on the surfaces consisted of 95% of the non-soluble SnO2 aerosol. The test results indicate that the aerosol wash-down from deposition surfaces is a stochastic process governed by changing rivulets on the plate sections or aerosol concentration stratification in case of the puddle. Wash-down efficiencies of CsI and SnO2 aerosols were found to be independent from each other. Table 5 provides an overview of the aerosol loadings and composition as measured before and after the wash-down phase. Experimental results and the associated analysis conducted with COCOSYS indicated that in the modelling approach, partial aerosol wash-down due to rivulet formation instead of a closed water film at low wall condensation rates, and strong retention of even soluble aerosols due to concentration stratification in the water puddle should be considered (Hoehne and Weber, 2010).

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Table 5 Wash down behaviour of CsI and SnO2 aerosols.

Prior to wash down Deposited mass horizontal surface Composition horizontal surface Deposited mass vertical surface Composition vertical surface After wash down Mass retained horizontal surface Composition horizontal surface

Plate (16 sectors)

Puddle (4 sectors)

Aerosol loadings/composition

529 g

174 g

703 g 65% SnO2 + 35% CsI 395 g 90% SnO2 + 10% CsI

370 g

124 g

494 g 94.5% SnO2 + 5.1% CsI

Following wash-down of deposited aerosols into the containment sump, the potential of a re-release from the containment sump during a severe accident may depend on, accident sequence, sump convection state, other chemical substances present in the sump, chemical boundary conditions, e.g. sump water redox potential, pH, and the prevailing thermal-hydraulic conditions. In a recently conducted THAI experiment AW3, wash-down behaviour of the non-soluble aerosol Ag has been investigated together with the effect of washed down Ag on the fixation of volatile iodine (I2 ) in the sump water in a second part of the test. In order to maintain a link with the previous THAI tests, the test procedure for the aerosol deposition and wash-down phases of the AW3 test was oriented towards THAI tests AW and AW2. After determining the wash-down efficiency of Ag aerosol from surfaces by condensing steam, experimental complexity was enhanced by successively investigating interaction between iodine and silver aerosols in the sump. Regarding, I2 –Ag reaction in sump, the specific surface area of the silver particles in the sump used in the iodine/silver model has been identified as one of the influential uncertain parameters determined in the uncertainty and sensitivity study on a COCOSYSAIM (Weber and Funke, 2009) calculation on PHEBUS test FPT1 (Weber et al., 2014). These analyses entrained a reduction of the modelled specific Ag surface area in containment sumps by a factor of 35, which was rationalized by the silver particles producing a precipitation on the sump bottom. The second part of THAI test AW3 was designed to quantify the I2 /Ag reaction rate in the case of I2 being dissolved in stagnant water above a layer of settled Ag particles. First results and modelling are provided in Weber et al. (2015). 3.4.2. Aerosol resuspension In the event of a severe accident, aerosols deposited onto containment walls or trapped into water pools may be re-entrained or re-suspended to containment atmosphere depending on an accident transient (Parozzi et al., 1995). Among other effects, resuspension may result into an increase in aerosol concentrations in the containment atmosphere which in turn may have an influence on the radiological source term from containment to the environment. As resuspended aerosols e.g. from water pools are typically in the submicron range and may remain airborne for long time depending on the accident transient, the details on resuspended aerosols (mass concentration, particle size) are important in verifying longterm removal efficiency of a scrubber based filtered containment venting system (CSNI, 2014). In total 7 experiments have been conducted so far in the THAI test facility with objective to investigate “wet aerosol resuspension” behaviour in the containment atmosphere. The parameters varied included aerosol type (soluble or insoluble), and their characteristics (e.g. initial concentration in sump, particle size). Resuspended fraction refers to the fission products in droplets and is defined as a product of entrainment factor (mass ratio of released droplets and released gas) and aerosol enrichment factor (ratio of concentration in droplets and in sump water). Measurements are carried out over a broad range of superficial velocity (defined as the

volume flow of the released gas and steam bubbles per unit area of the pool) covering bubble flow, transition, and churn turbulent flow regimes. In the case of containment sump boiling which can occur either due to core melt, or in case of fast depressurization, e.g. due to containment breach or containment venting, the steam bubbles bursting at the surface of the boiling pool generate a large number of very small droplets. The smaller the ratio of droplet terminal velocity to the gas superficial velocity, the greater become the likelihood of aerosol particles being carried away from the sump into the containment atmosphere. For soluble fission products one can assume the same fission product concentration in the pool water and in the droplet implying that aerosol enrichment factor is equal to 1. However, in the case of an insoluble aerosol, concentrations in water pool and entrained droplets may differ significantly. The REST experiments (Bunz et al., 1992), where soluble and insoluble aerosol resuspension fraction were measured under comparable conditions indicated an aerosol enrichment factor of 20 for insoluble aerosol (mass median diameter of 0.65 ␮m, geometric standard deviation 1.7). In the THAI “wet aerosol resuspension” experiments release of fission products carried by water droplets from the surface of a boiling reactor sump into the containment atmosphere has been investigated for soluble aerosols and non-soluble aerosol. In the case of non-soluble calcium carbonate aerosol, two different primary mass-median diameters of 0.065 ␮m and 0.9 ␮m have been injected into the sump. A general sketch of the THAI test facility configuration as used during the aerosol resuspension tests and comparison of the THAI tests TH14 and TH15 with REST experiments are shown in Fig. 13. The details on test procedure and measurement technique can be consulted in Schmidt et al. (2015). Investigated aerosols and main results from THAI “wet aerosol resuspension” test series are summarized in Table 6. For soluble aerosols, the entrainment factor obtained during THAI test cases confirmed former REST laboratory-scale test data with entrainment factors varying between 1.5E−05 and 1.0E−04 for the investigated range of gas superficial velocity from 0.044 m/s to 0.185 m/s (Bunz et al., 1992). However, as a result of the more sophisticated aerosol instrumentation applied in THAI, the explicitly measured size of the residual nuclei from dried out droplets turned out to be significantly smaller, and, accordingly, the number of droplets to be higher by orders of magnitude compared to the data reported earlier. As a consequence, the respective input data for source term calculations should be reconsidered. For the THAI experiments with non-soluble aerosols, the investigated superficial velocities ranged from 0.024 m/s to 0.13 m/s. A decreasing trend of the resuspension fraction for insoluble particles was found as the superficial velocity increases. The effect of primary particle sizes on the resuspension fraction was found to be rather small compared to the effect of the superficial velocity. The largest values obtained at superficial velocity of 0.024 m/s were approximately 4.1E−04 and reduce to 1.9E−05 for high superficial velocities of about 0.13 m/s. Test results indicate that mainly very small aerosols in the range of 0.06–0.11 ␮m (mass median diameter) are

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Fig. 13. Example of the test-configuration for THAI aerosol resuspension tests.

released from a boiling pool, which in turn might remain airborne for a long time. Such a small particle size represents unfavourable value for the design of the accident management measure filtered containment venting because filters normally show a minimum efficiency in this particle size range (“Filter gap”). In addition to wet aerosol resuspension process, dry aerosol resuspension can also increase the aerosol concentration in the containment atmosphere during late phase of an accident transient. The potential of hydrogen deflagration to resuspend the deposited aerosol has also been part of the THAI experimental investigations. Fig. 14 shows the experimental configuration used in the dry aerosol resuspension experiment. In the first step, a layer of aerosol depositions has been prepared on a floor close to the vessel bottom by injecting CsI aerosol into the test vessel and subsequent settling over a 24 h period. Then the 5.5 m long vertical deflagration tube has been filled with a hydrogen–air mixture. No rupture membrane is needed because its low density keeps the mixture inside the vertical deflagration tube and prevents its downward release through the open bottom pipe. In the step 2, after ignition at the lower end of the deflagration tube the hydrogen–air mixture burns in upward direction towards the closed top end. The burnt gases expand and escape through the bottom pipe and are directed by a slit nozzle in horizontal direction over the aerosol deposits. The escaping gas jet velocities over the deposition plate are in the range of 17–67 m/s. Significant portions of the deposited aerosol become re-suspended, and the resulting aerosol concentration in the gas atmosphere is measured by bulk-filter sampling. Before initiation of the deflagration, majority of the aerosol particles had been deposited over surfaces and only very fine particles contribute to the measured low mass concentration in the gas atmosphere. Immediately after the deflagration, the measured particle size shows a large fraction of coarse particles. The test conditions before and after ignition in

AER-1, AER-3, and AER-4 tests are compiled in Fig. 14. By using THAI experimental data, a source term calculation was performed with COCOSYS (Nowack and Allelein, 2007). For the calculation purpose, a hydrogen deflagration was assumed in a KONVOI type reactor. The performed COCOSYS calculations show that high air velocities produced by energetic deflagration may resuspend already deposited aerosols resulting into an increase of the released aerosol mass up to a factor of 10 higher than aerosol mass available for release without resuspension. 3.5. Iodine volatility, distribution and transport behaviour Volatile iodine will be present in the containment atmosphere entering either from reactor cooling system or from the containment sump water (CSNI, 2007). The kinetics of reactions by iodine species entering from primary side to the containment is complex and mainly determined by the presence of radiation, thermal hydraulic conditions and the type of gas and materials available for interaction, such as gas-mixtures, structural surfaces coated with decontamination paint, water pools and their pH values, etc. Therefore, from source-term perspective, it is important to know the amount, composition and time of release of the potential volatile radioactive species which could be present inside containment and become available for release to the environment during an accident either due to a containment breach or via controlled release pathways (e.g. containment venting). Additionally, in large containment geometries atmospheric conditions are generally inhomogeneous. At the same time, gas atmosphere can be under-saturated in one compartment and supersaturated (with fog formation) in another compartment. Such conditions which require coupling of thermalhydraulic and fission products cannot be generated in small scale experiments.

Table 6 THAI wet aerosol resuspension experiments. Aerosol material

TH-14 KI + CsCl

TH-15 KI + CsCl

TH-16 Cs2 SO4 + Li2 SO4

TH-17 Cs2 SO4 + Li2 SO4

TH-25.1 KI + CsCl

TH-25.2 CaCO3

TH-25.3 CaCO3

Initial aerosol concentration in sump, g/l Vessel pressure, bar Mean droplet size, ␮m Particle size (MMD), ␮m Gas superficial velocity, m/s Entrainment/resuspension fraction

6.5 (KI) + 0.9 (CsCl)

8.2 (KI) + 1.2 (CsCl)

19.1

10.4

1.63–1.96 0.35–0.62 0.051–0.093 0.029–0.069

4.61 (Cs2 SO4 ) + 1.54 (Li2 SO4 ) 1.56–1.99 0.57–0.80 0.073–0.10 0.02–0.075

8.44 (KI) + 1.51 (CsCl)

2.11–2.38 0.5–0.65 0.07–0.09 0.034–0.046

4.65 (Cs2 SO4 ) + 1.55 (Li2 SO4 ) 1.78–1.86 0.48–0.82 0.06–0.10 0.018–0.057

2.18–2.25 0.2–0.4 0.061–0.081 0.024–0.133

2.21–2.24 – 0.063–0.092 0.024–0.128

2.19–2.21 – 0.061–0.11 0.024–0.129

2.03E−05 to 8.27E−05

3.7E−05 to 6.53E−07

3.05E−05 to 3.29E−04

2.38E−05 to 3.26E−04

9.59E−07 to 8.01E−04

4.54E−06 to 9.49E−4

1.02E−05 to 1.63E−3

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Fig. 14. Dry aerosol resuspension experiment in THAI.

The availability of experimental data investigating fission product behaviour in large scale experimental facilities is very sparse and thus, desirable for the further development and validation of fission products models which are incorporated into accident analysis codes to reduce the uncertainties involved in source term quantification. Iodine experiments performed in the THAI test facility provide a basis to understand the source term relevant iodine behaviour inside the containment. THAI experiments fill gaps and also enlarge the existing knowledge on iodine behaviour in gas/liquid phase under severe accident typical thermal hydraulic conditions and interactions with exposed steel/painted surfaces and aerosols. An overview of the iodine and aerosol related tests conducted up to now in the THAI test facility is given in Table 7. Although, most of the containment surfaces are coated with decontamination paint, iodine interaction with steel surfaces is of high interest due to the two different reasons. First, there are some components with stainless steel in the reactor building, such as reflective insulation material used for thermal isolation of pipes or surfaces in condensation chamber of BWRs. Second, and even more important, the performance of iodine tests in the THAI vessel with its walls and internals providing large steel surface areas entrains a significant effect from the iodine–steel interaction, which needs to be understood and quantified in order to study the containmentrelevant iodine issues in THAI and also to analyze results in another test facilities made of stainless steel, e.g. PHEBUS tests. Several THAI experiments were conducted to investigate the influence of temperature and relative humidity on iodine deposition and resuspension behaviour from stainless steel surfaces. THAI test data indicate that the chemisorption at steel is weak at low relative humidity but increases significantly with relative humidity. Increasing surface temperatures may result in a release of a small fraction of the chemisorbed iodine (Weber et al., 2010). In one of the THAI tests performed with dry initial conditions, the influence of an enhanced atmospheric convection and turbulence, generated by a blower, on I2 deposition and resuspension was investigated. The test data indicate less pronounced or even negligible influence of natural and turbulent convection states on I2 /steel deposition and resuspension behaviour under the investigated experimental conditions. Based on the knowledge gained from the THAI experiments, an iodine/steel model was developed and implemented in COCOSYS/AIM which describes the iodine/steel reaction as a sequence of physisorption (I2 deposition and resuspension) and chemisorption (formation of non-volatile iron iodide) for the THAI stainless steel surface as function of temperature and relative humidity (Weber and Funke, 2009). The model was extended by use of the data from two steel samples measured in the OECD/NEA BIP programme (OECD/NEA, 2012).

Due to high capacity of painted surfaces in adsorbing iodine and their large surface areas in a containment, the iodine/paint interaction was studied in THAI experiments, using artificially aged epoxy paint. In order to provide a good interlink between the tests, artificial ageing was always performed in the same way, i.e. by heating the painted surfaces for 21 h at 160 ◦ C or for 24 h at 155 ◦ C, to simulate an estimated 15 years of ageing at normal plant operation conditions (Funke et al., 2015). Thermal hydraulic conditions were established within temperature range of 80–140 ◦ C and relative humidity between 20 and 75%. The data were used to refine the COCOSYS/AIM model (Weber and Funke, 2009), by describing the interaction of I2 with dry painted surfaces based upon physisorption followed by chemisorption analogously to the iodine/steel model. The effect of relative humidity was not yet considered in the model, but the availability of more recent THAI tests suggests a significant dependency of iodine deposition on paint from very dry to nearly saturated conditions. In the event of a severe accident, analysis of the gas borne iodine concentration and hence the iodine source term depends not only on the chemical reactivity but also on the prediction of the gaseous iodine transport as a function of the local thermal hydraulic conditions in multi-compartment volumes (CSNI, 2007). In THAI multi-compartment tests, the iodine distribution by atmospheric flows is measured in the THAI vessel sub-divided into 5-compartments representing dome, upper and lower annulus, sump and a dead end room in a reactor containment. The parameter variation take into account the initial relative humidity, mixing mechanisms (wall heating, helium or steam injection), and surface characteristics (stainless-steel or painted). The transport behaviour of chemically reactive iodine is compared with inert gas helium to quantify the complex multicompartment effects of I2 distribution. The THAI tests Iod-10, Iod-11 and Iod-12 in stainless steel environments showed that the distribution of gaseous iodine remained inhomogeneous over long time periods and therefore, multi-compartment effects need to be considered in best-estimate source term calculations. Fig. 15 shows the typical test configuration used for multi-compartment iodine tests in THAI. To achieve the 5-compartment geometry two intermediate decks made of chromium-nickel-steel-sheets were installed in the THAI vessel. Five inter-compartmental flow openings in these decks allowed gas exchanges. The five compartments were: dome, upper and lower annulus, central compartment (inner cylinder) and bottom compartment including the sump. The test strategy was generally to start with a stratified phase with high temperature in the dome compartment and low temperatures below. The three main phases of test Iod-11 are distinguished by different flow conditions in the

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Table 7 Overview of THAI tests investigating iodine and other fission product related issues. Tests not specifically marked have been conducted in the frame of national THAI projects (2000–2015). Topic

THAI test

Main conditions

I2 deposition onto steel

Iod-6, Iod-7, Iod-9 (first phase), Iod-16, Iod-18 Iod-22 Iod-8

Dry steel walls; temperature and humidity transients Dry steel walls; atmospheric convection forced by a blower Wet steel walls; weak and strong wall condensation

I2 deposition onto paint

Iod-15, Iod-17, Iod-20 Iod-21, Iod-24

Large paint area at dry condition; temperature and humidity transients Large painted areas at wet condition; different wall condensation rates

I2 /ozone reaction

Iod-13, Iod-14

IOx aerosol formation and behaviour at different I2 to O3 ratios; superheated atmosphere

I2 mass transfer gas/sump

Iod-9 (second phase) Iod-23

I2 transfer from atmosphere into sump; sump stratified and mixed I2 transfer from sump to atmosphere; sump alternating stratified and mixed

I2 deposition onto aerosols

Iod-25 (OECD/NEA THAI2) Iod-26 (OECD/NEA THAI2)

I2 deposition onto non-reactive SnO2 aerosol I2 deposition and chemisorption onto reactive Ag aerosol

I2 transport in a multi-compartment geometry

Iod-10 Iod-11 Iod-19 (repetition) Iod-12

I2 distribution in 5-compartment geometry with steel walls at dry atmosphere (Iod-10), humid atmosphere (Iod-11) and with wall condensation (Iod-12)

I2 transport in a multi-compartment geometry with painted areas

Iod-27a Iod-28 Iod-30

I2 distribution in 5-compartment geometry painted areas and steel walls at very dry atmosphere (Iod-27a), humid atmosphere (Iod-28) and with an additional Ag aerosol injection (Iod-30)

Gaseous iodine release by a flashing jet

Iod-29 (OECD/NEA THAI2)

Quantification of gaseous iodine release under thermal hydraulic conditions of a steam generator tube rupture during reactor shutdown. Simulation of 40 bar pressure drop by use of an external pressure vessel

Aerosol wash down

AW (OECD/NEA THAI)

Wash down of soluble CsI aerosol by wall condensate; retention in a puddle Wash down of a mixed CsI/SnO2 aerosol (soluble/insoluble) by wall condensate; retention in a puddle Wash down of insoluble Ag aerosol by wall condensate Reaction of iodine with particulate Ag in sump at mixed and stagnant conditions Airborne CsI aerosol washout by spray

AW2 AW3 (part 1) AW3 (part 2) AW4

vessel atmosphere, which were stratified, transient and mixed. At the beginning of the stratified phase, 8.4E−04 kg of gaseous I2 was injected into the dome within several minutes. The beginning of injection was the start of the test time (t = 0). The iodine spread slowly in the dome atmosphere, which was also slightly stratified, and reached the gas scrubber near the bottom of the dome after about 1.5 h. One part of the I2 was adsorbed on the steel surface. During the stratified phase almost no I2 reached the lower compartments. In the transient phase the mixing of the vessel atmosphere was stimulated by a controlled injection of heat, helium and steam. The vessel atmosphere was heated by the middle and lower vessel jackets and the sump water was heated electrically. 8.8 standard-m3 of helium were injected into the bottom compartment to support the atmospheric mixing and to serve as a tracer. The released helium was rapidly distributed by the convective flows and after 3 h it was completely mixed (Fig. 16). Iodine behaved differently. It was transported from the dome into the lower compartments but its concentration was not homogenized in the vessel as the iodine gas scrubber measurements show (also Fig. 16). This incomplete mixing is due to I2 adsorption and desorption processes onto/from the steel surfaces. This I2 mass transfer between atmosphere and steel surfaces lasted until the end of the test. In the mixed phase the well mixed conditions were maintained by keeping atmospheric and sump temperatures nearly stationary. The I2 distribution in the vessel changed only gradually, but the concentration decreased in all compartments due to chemisorption. At the end of the mixed phase the I2 concentration in the dome was still one order of magnitude higher than in the lower compartments. In the lower compartments the I2 concentration differed up to a factor 2, which is clearly above the measurement error of ±30%.

The THAI multi-compartment tests Iod-11 and Iod-12 have been analyzed and interpreted in the THAI benchmark performed in the frame of SARNET2 (Weber et al., 2013). The benchmark results also highlighted the need of a detailed thermal-hydraulic modelling to precisely model the correct relative humidity as a basis for the correct application of the iodine/steel model and thus to accurately simulate the I2 transport in a multi-compartment geometry. More recently, multi-compartment tests with painted surfaces, high relative humidity and aerosol have been performed in the THAI test facility and data evaluation is on-going. In addition to the containment surfaces, interaction between iodine and aerosol could have an impact on iodine source-term by converting gaseous iodine into aerosol-bound iodine. Two tests were conducted to investigate the interactions between molecular iodine and chemically inactive (tin oxide, SnO2 ) and chemically reactive (silver, Ag) aerosols. Both tests have been conducted in an air filled atmosphere and at a temperature of 70 ◦ C. The observed removal of gas-borne total iodine by Ag aerosols was faster than the removal by SnO2 aerosols by a factor of 25. With Ag aerosol, I2 was quickly converted into aerosol-bound iodine, and settling of iodine aerosols was the main process in this test (Iod-26). With SnO2 aerosol, I2 was only weakly adsorbed onto aerosol, but mainly remained in the I2 form (Iod-25), and I2 resuspension from deposited iodine aerosols was also taking place. Real containment aerosol can be expected to show removal effects lying between these bounding cases, depending on its surface reactivity defined by the chemical phases available for reaction with gaseous I2 . These data were analyzed using the ART code (Ishikawa and Maruyama, 2014), where the adsorption velocity of I2 onto chemically inert tin oxide aerosol was estimated to be in the order to 10–5 m/s to 10–4 m/s, whereas it was two orders of magnitude higher with reactive silver (Ishikawa and Maruyama, 2014).

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Fig. 15. Example of the THAI test configuration for multi-compartment iodine tests.

Air radiolysis products such as ozone and nitrogen oxides would react with I2 to form iodine oxides and nitroxides, leading primarily to non-volatile iodine species (CSNI, 2007). The gas to solid oxide particle conversion has been investigated in the THAI tests Iod-13 and Iod-14 (Funke et al., 2012). As no radiation source is available in THAI, the radiation field was simulated by directly injecting ozone as a representative of air radiolysis products into

the THAI vessel, where it reacted with the gaseous I2 . The two tests were conducted at an atmospheric temperature of 100 ◦ C and a relative humidity of 60–70%. In test Iod-13 ozone was injected into the I2 -loaded atmosphere. In Iod-14 test, ozone was injected first to a high concentration level expected to immediately oxidize all I2 . The test results indicate that iodine behaviour in Iod-13 is mainly governed by the gaseous iodine, as the ozone concentration

Fig. 16. THAI test Iod-11: measured gaseous iodine and helium concentrations.

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was low. However, in Iod-14 test performed with ozone-saturated conditions, I2 is rapidly converted into IOx and the behaviour of the atmospheric iodine is governed by iodine oxides as aerosol particles. The observed iodine oxides form submicron particles with mean volume related diameter of about 0.35 ␮m. Over time, agglomeration of particles is observed. The slow depletion of the gas-borne IOx concentration indicated that diffusive deposition was more efficient than sedimentation. The exchange of gaseous iodine between atmosphere and sumps, and the deposition of gaseous iodine into a falling wall film in condensing conditions was measured in test Iod-9. Analysis of the test Iod-9 (gas to sump mass transfer) within SARNET project (Weber et al., 2009) clearly demonstrated that correlations are required for coupling the iodine mass transfer coefficient with containment thermal-hydraulics. In another THAI test Iod-23, mass transfer of molecular iodine at the water pool–gas interface has been investigated as a function of water motion. I2 was dissolved in a water volume, and the iodine released from the water was carried immediately to a filter by continuously flushing the air space. From the iodine accumulation on the filter, the iodine mass flow and the mass transfer coefficient could be evaluated. Two different states of water motion were repeatedly established: a stagnant pool, and an agitated pool with well-defined flow distribution, in order to cover a wide range of conditions for I2 mass transfer. The measurements show that the mass transfer coefficient is strongly dependent upon the water motion, with much higher values for the agitated pool. The experimental data can be correlated by means of a water surface film renewal model superimposed to the established two-film theory (Fischer et al., 2012). 3.6. Water hydrodynamics The THAI experimental research on water hydrodynamics is focused to investigate the fluid-dynamic features produced by a gas/steam discharge in water pools. A spectrum of the postulated loss of coolant accident (LOCA) scenario in a reactor has been considered in the THAI water hydrodynamic (WH) tests (Gupta et al., 2011; Balewski et al., 2014; Gupta, 2015a). The experimental investigations covered various phenomena, such as complete and incomplete steam condensation, thermal stratification in the water pool, bubble-column induced gas-liquid hydrodynamics, and air blowdown behaviour in the water pool to quantify dynamic pressure loadings. The results from THAI WH experiments provide useful database related to the water pool hydrodynamics for validation and further development of LP and CFD codes. 4. Conclusion An overview has been given on the multi-compartment THAI containment test facility and its experimental programs conducted in the frame of German national and OECD/NEA joint nuclear safety research projects. Selected examples are provided to illustrate the THAI experimental research which includes coupled-effect investigations on thermal-hydraulics, hydrogen, and fission product (aerosols and iodine) behaviour in LWR containments under severe accident conditions. The results obtained from the different series of tests demonstrated the capabilities of the THAI facility in producing valuable data on gas mixing/stratification issues, slow hydrogen deflagration, passive autocatalytic recombiners performance under severe accident conditions, and fission product behaviour. Based on THAI experimental findings, important progress has been achieved in aerosol and iodine modelling and their coupling with containmenttypical thermal hydraulics. The improved models have demonstrated reliable simulation of complex experiments, e.g. hydrogen distribution (ISP-47 and OECD/NEA THAI code benchmark),

hydrogen combustion behaviour (ISP-49), hydrogen mitigation by PARs (OECD/NEA THAI-2 code benchmark), iodine/surface interactions, iodine mass transfer, and iodine transport and multi-compartment behaviour (EU-SARNET and EU-SARNET 2). A follow-up of OECD/NEA THAI-2 project is currently under discussion. Hydrogen related investigations on PAR performance under counter-current flow conditions and hydrogen deflagration tests in two-compartment system are proposed. Additionally, in light of Fukushima Daiichi accident, experimental investigations are foreseen to study the release of fission product (aerosols and gaseous I2 ) from water pool at elevated temperature due to continuous heat-up of pool or depressurization (venting) induced boiling. Another planned experiment is related to “delayed” source-term in order to investigate re-suspension of aerosols as well as iodine deposits from steel/painted surfaces due to hydrogen combustion. Acknowledgments THAI experimental research program is funded by the German Federal Ministry for Economic Affairs and Energy, on the basis of a decision of the German Bundestag (Projects number 1501218, 1501272, 1501325, 1501326, 1501361, 1501420, 1501455). The sponsorship by the countries of the OECD/NEA THAI and OECD/NEA THAI2 projects is gratefully acknowledged. References Allelein, H.-J., Fischer, K., Vendel, J., Malet, J., Studer, E., Schwarz, S., Houkema, M., Paillère, H., Bentaib, A., 2007. International Standard Problem ISP-47 on Containment Thermal Hydraulics, Report NEA/CSNI/R(2007)10. Alvares, N.J., 1986. Assessment of extent and degree of thermal damage to polymeric materials in the Three Mile Island unit 2 reactor building. In: Proceedings of the First International Symposium on fire Safety Science. Andreani, A., Badillo, A., Kapulla, R., 2015. Synthesis of the OECD/NEA-PSI CFD Benchmark Exercise, CFD4NRS-5, OECD-NEA and IAEA Workshop , September 9–11. Balewski, B., Gupta, S., Fischer, K., Poss, G., 2014. Experimental investigation of air bubble flows in a water pool. In: Proceedings of NURETH-14 Conference, Paper 036, Toronto, Canada. Bielert, U., Breitung, W., Kotchourko, A., Royl, P., Scholtyssek, W., Veser, A., Beccantini, A., Dabbene, F., Paillere, H., Studer, E., Huld, T., Wilkening, H., Edlinger, B., Poruba, C., Mohaved, M., 2001. Multi-dimensional simulation of hydrogen distribution and turbulent combustion in severe accidents. Nucl. Eng. Des. 209, 165–172. Blanchat, T.K., Malliakod, A., 1999. Analysis of hydrogen depletion using a scaled passive autocatalytic recombiner. Nucl. Eng. Des. 187, 229–239. Bujar, A., 1993. Hydrogen Problems Related to Reactor Accidents. Technical Report Risø R-706. Risø National Laboratory, Denmark, ISBN 87-550-1925-0. Bunz, H., Koyro, M., Propheter, B., Schöck, W., Wagner-Ambs, W., 1992. Resuspension of Fission Products from Sump Water. Final Report EUR 14635 EN. Burkhardt, J., Schwarz, S., Koch, M.-K., 2009. Analysis of the nodalisation influence on simulating atmospheric stratifications in the experiment TH13 with the containment code system COCOSYS. Nucl. Eng. Technol. 41 (9). CSNI, 2007. State of the Art Report on Iodine Chemistry. NEA/CSNI/R(2007)1. CSNI, 2009. State of the Art Report on Nuclear Aerosols. NEA/CSNI/R(2009)5. CSNI, 2014. Status Report on Filtered Containment Venting. NEA/CSNI/R(2014)7. Duspiva, J., 2009. Post-test calculation of OECD THAI HM-2 experiment with MELCOR 1.8.6 code. In: Proceedings of the 13th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH-13, Kanazawa City, Japan, September 27–October 2. Fischer, K., Kanzleiter, T., 2008. Large scale experiments on gas distribution in the containment. In: Proceedings of the International Topical Meeting on Safety of Nuclear Installations TOPSAFE, Dubrovnik, Croatia, September 30–October 3. Fischer, K., Weber, G., Funke, F., Langrock, G., 2012. Experimental determination and analysis of iodine mass transfer coefficients from THAI test Iod-23. In: 5th European Review Meeting on Severe Accident Research, ERMSAR, Cologne, Germany, March 21–23. Freitag, M., Schmidt, E., Gupta, S., Poss, G., 2014. Simulation Benchmark Based on THAI-Experiment on Dissolution of a Steam Stratification by Natural Convection, CFD4NRS-5, OECD-NEA and IAEA Workshop , September 9–11. Funke, F., Langrock, G., Kanzleiter, T., Poss, G., Fischer, K., Kühnel, A., Weber, G., Allelein, H.-J., 2012. Iodine oxides in large-scale THAI tests. Nucl. Eng. Des. 245, 206–222. Funke, F., Gupta, S., Weber, G., Langrock, G., Poss, G., 2015. Interaction of gaseous I2 with painted surfaces and aerosols in large-scale THAI tests. In: Proceedings of the International OECD-NEA/NUGENIA-SARNET Workshop on the Progress in Iodine Behaviour for NPP Accident Analysis and Management, Paper no. 3–5, Marseille, France, March 30–April 1.

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Please cite this article in press as: Gupta, S., et al., THAI test facility for experimental research on hydrogen and fission product behaviour in light water reactor containments. Nucl. Eng. Des. (2015), http://dx.doi.org/10.1016/j.nucengdes.2015.09.013