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Overview of the VERDON-ISTP Program and main insights from the VERDON-2 air ingress test
Annals of Nuclear Energy 101 (2017) 109–117
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Overview of the VERDON-ISTP Program and main insights from the VERDON-2 air ingress test A. Gallais-During a,⇑, S. Bernard a, B. Gleizes a, Y. Pontillon a, J. Bonnin b, P.-P. Malgouyres a, S. Morin c, E. Hanus a, G. Ducros a a b c
CEA, DEN, DEC, SA3C, F-13108 Saint-Paul-lez-Durance, France CEA, DEN, DTN, STCP, F-13108 Saint-Paul-lez-Durance, France IRSN, BP3, F-13115 Saint-Paul-lez-Durance, France
a r t i c l e
i n f o
Article history: Received 16 June 2016 Received in revised form 22 September 2016 Accepted 27 September 2016 Available online 14 November 2016
a b s t r a c t In order to reduce the uncertainties concerning source term assessment under LWR severe accident conditions, the International Source Term Program has been launched. Four main R&D research axes have been addressed in this program: iodine behaviour in the RCS and containment, study of the boron carbide effect on fuel degradation and FP release, study of the air effect on fuel and FP behaviour, FP release from high burn-up UO2 and MOX fuels. As far as the source term quantification is concerned, four VERDON tests have been defined and were performed in the VERDON laboratory at the CEA Cadarache Centre. We present an overview of these four tests, devoted to the study of FP release from high burn-up UO2 and MOX fuels. We focus on the VERDON-2 air ingress test performed on MOX fuel with a specific loop dedicated to FP release and transport. It addressed two issues: Ru release under mixed steam-air conditions following lower head failure and transport/re-volatilization of Ru and volatile FPs in the primary circuit. The latter was performed thanks to the implementation of thermal gradient tubes exposed at different phases of the sequence. In this configuration, the VERDON facility combines the ability of the analytical approach to identify and understand the physical phenomena involved, with a semi-integral approach, which allows the results to be applied to source term assessments for reactor application. This VERDON-2 test has shown a rather unexpected low release of ruthenium from the MOX sample and no re-volatilization from the thermal gradient tubes. On the contrary, a significant re-volatilization of iodine has been observed when switching to mixed steam-air conditions, as well as a low, but measurable amount of iodine transported under gaseous form downstream of the thermal gradient tubes. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In order to assess the Source Term in the case of a hypothetical LWR severe accident and the ensuing risks for the population and the environment, a large number of research programmes have been undertaken in various countries (Schwarz et al., 1999; Lorenz and Osborne, 1995; Lui et al., 1983; Hidaka, 2011; Lewis et al., 2008). Among them, the HEVA (Leveque et al., 1994) – VERCORS (Pontillon et al., 2010; Pontillon and Ducros, 2010a,b) programmes, financed jointly by IRSN and EDF and conducted by the CEA led to reduce the uncertainties concerning FP behaviour under LWR severe accident conditions. However, some major uncertainties still remain in some fields so it was decided to build an international co-operative research ⇑ Corresponding author. E-mail address: [email protected] (A. Gallais-During). http://dx.doi.org/10.1016/j.anucene.2016.09.045 0306-4549/Ó 2016 Elsevier Ltd. All rights reserved.
programme, called the ‘‘International Source Term Program (ISTP)” (Clément and Zeyen, 2005). ISTP was launched by CEA, EdF and IRSN, with contributions from the European Commission (EC, Europe), Tractebel Engineering (GDF SUEZ, Belgium), Paul Scherrer Institute (PSI, Switzerland), Atomic Energy of Canada Limited (AECL, Canada) and United States Nuclear Regulatory Commission (USNRC, United States). Four main R&D research axes, based on separate-effect experiments, have been addressed in this program. They concern iodine behaviour in the RCS and the containment, study of the boron carbide effect on fuel degradation and FP release, study of the air effect on fuel and FP behaviour and study of the FP release from the fuel. The results of these separate-effect experiments would allow improving models used for Source Term evaluation studies. They contribute to the resolution of a high priority safety issue defined by SARNET (Klein-Heßling et al., 2014; Albiol et al., 2010), which was endorsed by NUGENIA (SNETP, 2013).
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As far as the Source Term quantification is concerned, four VERDON tests have been identified within the framework of the ISTP and were performed in the VERDON laboratory at the CEA Cadarache Centre. In the first part of this paper, we present an overview of these four tests, devoted to the study of FP release from high burn-up UO2 and MOX fuels. In the second part, we focus on the VERDON-2 air ingress test performed with a specific loop dedicated to FP release and transport. 2. Overview of the 4 VERDON-ISTP tests 2.1. The VERDON experiments The four VERDON tests of the ISTP were conducted between 2011 and 2014 in an entirely new laboratory at the CEA Cadarache Centre in the LECA-STAR facility (Ferroud-Plattet et al., 2009). The VERDON laboratory is constituted of two high activity cells (C4 and C5) and a glove-box. The C4 cell is dedicated to pre/post-test operations, such as sample reception and preparation. This cell also contains a gamma spectrometry bench dedicated to gamma scanning of the fuel before and after the test and of the different elements of the loop after dismantling. The C5 cell, represented in Fig. 1, is dedicated to the accidental sequence carrying out and to on-line measurements (both thermal-hydraulics and gamma spectrometry). It contains the experimental circuit itself (i.e. VERDON loop) as well as three complementary on-line gamma spectrometry stations for the furnace, filter and the May-Pack in order to follow the FP
release kinetics from the fuel sample located inside the furnace during the experiment (Ducros et al., 2009). The glove-box main functions are to analyze and store the fission and carrier gases. Two complementary loops are available: one dedicated to release studies (see Fig. 1a), the other to transport and revolatilization studies (see Fig. 1b). It was necessary to develop a specific high frequency (HF) furnace (Gallais-During et al., 2012), key element of both loops, since it is where the fuel sample is heated up in conditions representative of a severe accident in terms of temperature (up to 2600 °C) and oxidizing-reducing conditions (fluid flow constituted of a mixture of helium, water steam, hydrogen and air). Downstream of the furnace, the FPs emitted by the fuel are carried by the gas flow in the VERDON loop. The Release Loop is devoted to the precise characterization of FP release by trapping FPs under aerosol forms in a total aerosol filter located just above the furnace; the precise quantification of the released fraction is allowed on-line by gamma spectrometry and/ or after the test by chemical analysis. Downstream of the aerosol filter, only gases (carrier gases, fission gases Xe and Kr and potential gaseous forms of iodine) are transported along the circuit. The May-Pack filter is located downstream of the aerosol filter and is designed to trap gaseous iodine on successive stages, according to chemical forms of iodine (molecular or organic). The main difference of the Transport Loop compared with the Release Loop is the system of Multiple Thermal Gradient Tubes (TGTM) located between the HF furnace and the aerosol filter. This device enables the study of FP transport in the primary circuit of a nuclear power plant thanks to thermal gradient tubes which
Fig. 1. VERDON C5 cell: a. Release Circuit, b. Transport Circuit.
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temperature is representative of the hot leg and cold leg temperatures under severe accident conditions: the temperature along these tubes linearly decreases from 700 °C at the entrance to 150 °C at the exit, and is monitored by a series of thermocouples located every 100 mm as shown in Fig. 2. There, the FP vapours deposit according to their condensation temperature, giving an indication of their chemical forms. In addition, FP aerosols can deposit (by thermophoresis) along the tubes. A specificity of this TGTM device is that it is constituted of 4 tubes assembled on a rotating system. During the accidental sequence, 2 tubes are swept along in parallel by the FPs and they can be rotated of a quarter turn at specific moment of the sequence, when changing the oxidizing-reducing conditions, for instance. By this way, one of the tubes is not affected by the new conditions, the second one is affected by both the previous and new conditions and a third one is only affected by the new conditions. The study of FP revaporisation is then possible, by comparing FP deposits along the 3 tubes linked to specific oxidizing-reducing conditions.
2.2. The VERDON samples For the 4 VERDON-ISTP tests, the samples were taken from a UO2 or from a MOX fuel rod, previously irradiated in a PWR operated by the French operator EDF. The sample consists of two irradiated pellets in their original cladding and two half-pellets of depleted (and un-irradiated) uranium oxide placed at each end of the sample and held there by crimping the cladding so that the cladding is not fully sealed. Before the experimental sequence, the sample was re-irradiated at low linear power (15–20 W/cm) in the OSIRIS material testing reactor for about a week, in order to recreate the short half-life FPs without any in-pile release. As a consequence, these FPs (i.e. 99 Mo, 132Te, 133I, 131I, 140Ba. . .), important for their radiobiological effects, are measurable by using on-line c spectrometry during the experiment. The fuel sample used for VERDON-1 was a high burn up UO2 fuel very similar to VERCORS-RT6 (Pontillon et al., 2004) (same fuel assembly, same power history and very close in burn-up). For VERDON-2 to -4, the samples were made up of a fuel section taken from the same span of the MOX fuel rod so that they are identical.
2.3. VERDON-ISTP tests main results The main parameters explored throughout the 4 tests of the programme were mainly the maximal temperature, the oxidizing-reducing conditions of the vector fluid, the high burnup and the nature of the fuel sample (UO2 or MOX). For the 4 tests, an intermediate temperature plateau was maintained at 1500 °C (in a steam and hydrogen atmosphere for VERDON-1 and -3, pure steam for VERDON-2 and -4) in order to fully oxidise the cladding before the temperature ramps up to the final phase of the test. The duration of this intermediate plateau was about 10–15 min, except for VERDON-1 for which it was 50 min long. The full test grid is described in Table 1 for each test. Most of the former experimental data bases concern low burnup fuel in both oxidizing and reducing conditions and high burn-up fuel under oxidizing conditions only (Pontillon et al., 2010). Therefore, the main issue addressed by the VERDON-1 test concerns high burn up UO2 fuel behaviour – and corresponding FPs releases – under reducing conditions at very high temperature (up to 2600 °C). Moreover, the first part of the test (up to the end of the oxidation plateau at 1500 °C) was performed under the same atmosphere conditions as the VERCORS RT6 test (Gallais-During et al., 2014), which was conducted with a very similar high burnup UO2 fuel, in order to check the continuity between VERCORS and VERDON data bases. It was performed with the Release Circuit. During this first VERDON-1 test, the good overall performance of the VERDON loop in terms of tightness, thermal-hydraulics, furnace ceramics behaviour, etc.. . . and of the gamma scanning and sighting have been clearly demonstrated. As a consequence it can be asserted that the performance of the VERDON facility has been confirmed. A comparison with VERCORS RT6 has been possible and yielded the following conclusion: there were similar FP released fractions at 1500 °C, there was confirmation of high burn-up effect on release kinetics and demonstration of the atmosphere effect on Mo, Ba releases. The VERDON loop was thus qualified in ‘‘release configuration”, and as giving results that are consistent with the VERCORS experiments. From a general point of view, the VERDON-1 test does not result in a global and/or strong relocation of the fuel sample at the end of the test. This behaviour, at temperatures as high as 2610 °C, is one of the major lessons of the VERDON-1 test. This lack of global relocation must be connected
1000 Flow inlet: Experimental He = 6 mg/s, Susceptor He = 5 mg/s, flow temperature Without H2O (phase 1), wall temperature
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Table 1 Main characteristics of the 4 VERDON-ISTP tests. Test
VERDON-1
VERDON-2
VERDON-3
VERDON-4
Date of test Fuel PWR irradiation Sample burn-up (GWd/t) Re-irradiation
09-2011 UO2 Gravelines 72 Osiris
06-2012 MOX Chinon 60 Osiris
04-2013 MOX Chinon 60 Osiris
10-2014 MOX Chinon 60 Osiris
Mixed H2O steam + air
H2O steam
H2
2.6 0 4.2
2.5 0 0
0 0.4 0
2100 90 min at Tmax = 2100 °C Ru release 6 60%
2300 10 min at Tmax = 2300 °C Ba release 6 70% No global fuel relocation
2530 Beginning of fuel relocation Ba release 6 80% Tmax 6 2600 °C (10 min)
Test conditions (after oxidation plateau) Atmosphere Mixed H2O steam + H2 (He at the end) H2O steam flow rate (mg/s) 0.3 H2 flow rate (mg/s) 0.33 Air flow rate (mg/s) 0 End of the test Max fuel temperature (°C) Ending criterion Other criteria to be respected
2610 Tmax = 2600 °C reached None
to the atmospheres experienced during the last part of the test (i.e. reducing then neutral conditions) (Barrachin et al., 2008), and post-test micro-analyses are currently being performed in order to better understand this phenomenon. The released fractions measured by the on-line gamma stations and the data obtained via pre and post-test gamma scannings have been carefully assessed. This has yielded a FP general classification (Pontillon et al., 2010; Pontillon and Ducros, 2010a,b), in relation to their released fractions and specific behaviours, was again obtained similar to previous work with: (1) volatile FPs (fission gases, iodine, caesium) with an almost total release; (2) semi-volatile FPs (molybdenum and barium), with high sensitivity to oxidizingreducing conditions and significant released fractions; (3) low volatile FPs (ruthenium, europium) with generally (very) low released fractions (except for ruthenium under highly oxidizing conditions which can be associate to semi-volatile FPs in these cases) and (4) non-volatile FPs composed of zirconium. VERDON-3 and -4 are two complementary experiments. Their main objective is the quantification of FP release from a high burn-up MOX fuel; under steam oxidizing conditions for VERDON-3 and H2 reducing conditions for VERDON-4. The two tests were conducted at high temperature but without reaching global fuel relocation and with a particular interest in semivolatile fission products during post-test micro-analysis. They were performed with the Release Circuit. The main observations in terms of fuel sample behaviour and fission product release after VERDON-3 are the following: the comparison of the non-volatile FP distributions along the fuel sample before and after the test confirms the integrity of the fuel sample after the test; some FP traces are however detected under the crucible, meaning that fuel melting was probably imminent; the behaviour of ruthenium is specific with a very low release (a few percent); the comparison with Ru release for VERCORS HT2 (65%) and VERCORS RT6 (28%) obtained on UO2 fuel in oxidizing condition (Pontillon and Ducros, 2010b) leads us to consider a positive effect of the MOX nature of the fuel in reducing the Ru release. as expected, the release of semi-volatile FPs depends on atmosphere: with steam oxidizing conditions as in VERDON-3, the release of Mo is larger than for Ba (total release for Mo and about 55–60% for Ba) and associated with fast kinetics; the release of volatile FP is almost total and is associated with fast kinetics; regarding iodine, about 1% of gaseous species were found in the May-Pack filter and for the first time their release
kinetics could be measured thanks to the May-Pack gamma station (Pontillon et al., 2016). Regarding VERDON-4, the results are still under investigation but some qualitative issues can already be discussed. The test was conducted under H2 atmosphere until about 20% loss of ZrLa activity was detected on the on-line furnace gamma station, representative of a beginning of melting of the fuel sample, at a temperature of about 2530 °C, comparable to that of VERDON-1 test performed under mildly reducing conditions. As expected, in the reducing conditions of the VERDON-4 test, the release of Ba is larger than that of Mo since semi-volatile FP release depends on atmosphere. However, what was unexpected is a quite low release and slow kinetics of Ba release: the released fraction of Ba is about 55% at the end of the test. The main insights of the VERDON-2 air ingress test are presented in the following part.
3. VERDON-2 air ingress test TheVERDON-2 test concerns high burn-up MOX fuel behaviour – and corresponding FP release and transport – under air ingress conditions: air may contact fuel in a number of accident situations including reactor core meltdown accidents (after failure of the pressure vessel lower head or after drying out of a spent fuel pool). If, in the early phase of the accident, the fuel cladding has been nearly completely oxidised by steam, air ingress will also result in a significant increase of fuel superstoichiometric deviation and oxygen potential. This will accelerate the release of certain FPs, notably due to oxidation of metallic precipitates in the fuel which will trigger a large release of ruthenium, a highly radiotoxic radio-nuclide. The VERDON-2 test objective was to specifically monitored the ruthenium released from the fuel. The only previous experiment with air ingress is VERCORS RT8 for which the amount of air injected was too low to obtain a significant release of ruthenium and for which the fuel was a high burn up UO2 fuel (Pontillon and Ducros, 2010b). VERDON-2 was the first test to be performed with the Transport Circuit. The second objective of this test was to study the transport/re-volatilization of ruthenium and previously deposited volatile FPs in the primary circuit of a LWR, thanks to the TGTMs of the Transport Circuit. This test then complements, in a semi-integral approach using real irradiated fuel, other separate effect tests performed in different R&D laboratories and relevant to the impact of air ingress on ruthenium
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release (Lui et al., 1983) and on its transport in the reactor coolant system (Pontillon et al., 2016). 3.1. Thermal-hydraulic sequence The thermal-hydraulic sequence is represented in Fig. 3. The criterion to end the sequence was either to detect 60% of ruthenium release (1st stop criterion) or to stay one hour at 2100 °C to perform the last TGTM rotation with an additional 30 min in the same atmosphere (2nd stop criterion). Phase 1 and 2 were similar to VERDON-1 test without H2 injection, that is, steam alone (Gallais-During et al., 2014). Phase 3, under steam conditions, was dedicated to the measurement of release, transport and deposit of FPs without air injection, simulating LWR severe accident conditions before the lower head failure. The first TGTM rotation occurred (so that the exposed tubes 1–2 were switched to tubes 2–3) at the end of this phase. Phase 4, under air and steam conditions (molar ratio of 1 between steam and air) has been maintained for 70 min. Since the ruthenium release had not reached the 1st stop criterion of 60% released, another heat-up phase (phase 5) was performed up to 2100 °C. The 2100 °C plateau lasted for 90 min. During this plateau, the TGTM mechanism was turned after 60 min (from exposing tubes 2–3 to exposing tube 3–4). After the rotation, the tubes 3 and 4 stayed opened for 30 min under the same atmosphere to study the FP deposits as specified by the 2nd stop criterion. VERDON-2 was the first test to be performed with the Transport Circuit and the TGTM operated as specified without problem. In the following parts, we focus on three issues addressed by this test: 1) FP release and more specifically 2) ruthenium release and transport under mixed steam-air conditions following lower head failure and 3) transport/re-volatilization of volatile FPs in the primary circuit. 3.2. FP release under mixed steam-air conditions In order to illustrate the general behaviour of the FPs belonging to the four volatility categories (Pontillon et al., 2010), Fig. 4 com-
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pares the release kinetics of Zr (non or low-volatile), Ba, Mo, Ru (semi-volatile), Cs (volatile). The FP release kinetics presented here were measured by the furnace on-line gamma spectrometry station. They are measurements of the FP remaining in the fuel as a function of temperature, which are relatively imprecise when the release is low, in particular below 10%. Indeed, the changes in the object geometry measured during heating (swelling, fracturing, then fuel collapse, etc.), just like the axial migration of the FP, is significantly complicated to reduce the measurement accuracy and can explain some negative values of the released fractions. We found good agreement with previous VERCORS test results, in particular regarding semi-volatile FPs: The release of Mo starts at the beginning of the 1500 °C plateau as soon as the cladding is fully oxidized. It increases with rapid kinetics to reach approximately 30% of the initial inventory at the end of the plateau and 100% of the initial inventory after around 15 min at 2000 °C. The release of Ba starts after the oxidation plateau with the increase of temperature. A high Ba kinetics release is observed to reach up to 30% of the initial inventory at the beginning of the 2000 °C plateau and around 80% of the initial inventory at the end of the test. The release rate decreases after the air injection confirming that Ba volatility lies in the range of oxygen potentials corresponding to steam and air atmosphere. It seems to increase later with the increase of temperature. However, a major difference comparing to all the other VERCORS tests on standard UO2 fuel is observed here: it concerns the very fast release kinetics for volatile and semi-volatile FPs during the oxidizing phase. These fast kinetics were also measured during VERDON-3, on the same fuel in oxidizing conditions and seem to be linked to the MOX nature of the fuel. Moreover, even though the final atmosphere of the test was strongly oxidizing, the barium release continues (although it is slightly slower after air is added to steam injection) and is relatively high (80%) at the end of the test.
Fig. 3. Thermal-hydraulic sequence of VERDON-2 test.
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Fig. 4. Release kinetics of Zr (non-volatile), Ba, Mo, Ru (semi-volatile), Cs (volatile) for VERDON-2 test.
Table 2 Atmosphere conditions of TGTM tubes.
Blue shades means the tube is exposed (or open) to the corresponding atmoshere. White means the tube is un-exposed (or closed).
3.3. Ruthenium release and transport/re-volatilization under mixed steam-air conditions The specific ruthenium release is mainly due to the atmosphere conditions of the test (Auvinen et al., 2008; Brillant et al., 2010; Beuzet et al., 2012). The ruthenium release starts with the air ingress during the 2000 °C plateau and reaches about 70% of the initial inventory at the end of the sequence. The release rate slightly increases between the 2000 °C and 2100 °C plateaus. From a general point of view, the ruthenium kinetics seem to be relatively low compared to what could be expected due to the final temperature and atmosphere of the test (2.5 h under mixed stem-air conditions, 1 h at 2000 °C and 1.5 h at 2100 °C). This low value can be the consequence of the MOX nature of the fuel, as was confirmed in VERDON-3 (Section 2.3). As explained in Section 2.1, the Transport Circuit main feature is to use four thermal gradient tubes (TGTM) that can be operated sequentially with two in parallel in order to study deposits as well as re-vaporisation. Fig. 2 displays the temperatures of the tubes measured during the commissioning tests and Table 2 summarizes the different atmosphere conditions during which each tube was exposed and then when FP deposits occur. After the test sequence, all the TGTM tubes were dismounted and measured on the c spectrometry bench (C4 cell). The comparison of the deposits on each tube gives information on the possible re-vaporisation of the volatile FP deposits as well as deposits of lower volatile FPs, such as ruthenium, linked to air injection. Regarding the specific behaviour of ruthenium with air ingress, we observe in Fig. 5 that Ru-103 did not deposit in tube 1, exposed only during steam injection. The profiles of tubes 2 and 3 are similar, which is consistent with a Ru release that started with the air injection. On these tubes we observe a large accumulation
Fig. 5. Distribution of Ru-103 along the TGTM tubes for VERDON-2 test.
of Ru in the mid-plane area of the tubes, at around 400–350 °C. Another peak is observed at the end of the tubes, at around 200 °C. We suspect that this deposit is due to thermophoresis, since all the other FPs deposited at this location (see Fig. 7 for Cs and I). The quantity of deposits on tube 3 are greater than those on tube 2, which is normal considering its longer period of use under air injection. We observe on both tubes a large accumulation in the mid-plane area of the tubes. Tube 4 shows a rather homogeneous profile of low amplitude. The sum of the profiles for tubes 2 and 4 is similar to tube 3, which shows there was no significant revolatilization of ruthenium deposits at the end of the test. In addition to these gamma spectrometry measurements, chemical analyses and Raman spectroscopy measurements are planned on samples of tubes where Ru is present in order to discriminate which Ru compounds deposited. To further study the transport of ruthenium, we analysed the P4VP filter (one of the final safety filters, made of ‘‘poly vinylpyridine” and intended to trap the potential gaseous RuO4 produced during the experiment. This filter was totally free from ruthenium (and from any other traces of FPs), meaning that no gaseous RuO4 was released downstream of the TGTM.
3.4. Transport/re-volatilization of volatile FPs in the primary circuit The transport of volatile FPs (caesium and iodine) along the TGTM is very different: for caesium, the deposit profile is qualitatively similar in each tube (almost all the caesium was released by
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Fig. 6. Distribution of Cs-137 along the TGTM tubes for VERDON-2 test.
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the test. This latter point clearly highlights the difference, in terms of transport, which exists between Cs and I, difference which can impact chemistry occurring in the RCS during a severe accident. The May-Pack filter is located just downstream of the aerosol filter and was designed to trap gaseous iodine. It includes a glass filter stage, followed by three stages containing silver coated zeolite for molecular iodine trapping and three stages containing activated charcoal for the trapping of residuals traces of FPs, in our case mainly inorganic iodine. Only I-131 and Cs-137 could be measured in significant quantities on this filter. The profiles of these two FPs down the filter are given in Fig. 8. Cs-137 shows a local and small aerosol deposit at the filter inlet in the self-sealing connection, the remainder of the signal probably being background noise due to the cell contamination. I-131 shows several typical activity peaks of trapping of its gaseous species, the largest being located on the first zeolite stage (so, mainly molecular iodine) while the others are located on the third zeolite stage and the first stage of activated charcoal (residual inorganic iodine traces: I2 or HI). We are not expecting to find organic iodine RI since these chemical forms of iodine are mainly due to molecular iodine interactions with paints in severe accident conditions and there are no paints in the VERDON Transport Loop. The fraction of gaseous iodine having gone through the aerosol filter at 150 °C is nevertheless low (evaluated around 0.13% of the initial inventory) but significant. This is the first time that gaseous iodine has been detected in the May-Pack of the whole VERCORS/ VERDON programmes. The May-Pack is shown to be well designed to measure gaseous forms of iodine. 4. Concluding remarks
Fig. 7. Distribution of I-131 along the TGTM tubes for VERDON-2 test.
2000 °C), showing no effect of air injection (see Fig. 6), whereas for iodine, we observe in Fig. 7 that the highest I-131 deposits were located in the middle of tube 1 and at the outlet of tube 2. The central deposit observed on tube 1 has been completely re-volatilized during the air injection since it no longer appeared on tube 2. Deposits on tube 3 are very low, which shows that deposits at the outlet of tube 2 have been also re-volatilized at the end of
The four VERDON tests identified within the framework of the ISTP were performed in the VERDON laboratory at the CEA Cadarache Centre. They were devoted to the study of FP release from high burn-up UO2 and MOX fuels, one test on MOX fuel being dedicated to air ingress scenario (VERDON-2). We presented here an overview of these four tests, with a focus on the VERDON-2 air ingress test performed with a specific loop dedicated to FP release and transport. In this configuration, using the TGTMs, which are exposed at different phases of the sequence, the VERDON facility combines the ability of the analytical approach to identify and understand the physical phenomena involved, with a rather semi-integral approach, which allows the
Fig. 8. Distribution of I and C down the May-Pack filter.
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results to be applied to Source Term assessments for reactor application. From a general point of view, the main insights gained from the four VERDON tests are the following: A rapid release kinetics was observed, beginning at unexpectedly low temperatures, for volatile (I, Cs, Te) and even for the semi-volatile Mo, Ba FPs, for high burn-up UO2 and MOX fuels; However, a major difference comparing to all the other VERCORS tests on standard UO2 fuel is observed here: it concerns the very fast release kinetics for volatile and semi-volatile FPs during the oxidizing phase. This fast kinetics is also measured during VERDON-3, on the same fuel in oxidizing conditions. Several key parameters have an influence on release kinetics, particularly the burn-up, the type of atmosphere, the type of fuel (UO2 or MOX). By comparison with the VERCORS tests, it was also possible to identify the influence of one of these parameters on another. From a general point of view, the release rate increases with the burn up, in oxidizing condition and with MOX compared to UO2 fuels (Pontillon et al., 2016); An unexpected behaviour of ruthenium was observed for MOX fuel in VERDON-2 and -3 with a low release and relatively slow kinetics compared to what could be expected in oxidizing conditions. The VERDON-3 test was performed with the same MOX fuel as VERDON-2, but under pure steam conditions up to 2300 °C. Only a few percent of Ru was released from this test, which is very low in comparison with the Ru measured with UO2 fuel, for some VERCORS tests in similar conditions (see Section 2.3). This result confirms the lower Ru release in case of MOX fuel compared to UO2 fuel, probably linked to a difference in the oxidation of the fuel matrix and suggests that oxygen potentials in MOX fuel may be lower and stabilise the ruthenium in metallic precipitates so that it may require longer exposure to air to enable its release as volatile oxides. In VERDON-2 no deposits of Ru was observed to take place in the TGTM before the air ingress phase, which is consistent with the Ru release observations. Afterwards, an accumulation of Ru on tube surfaces was noticed in the course of the test. Ruthenium was not observed downstream of the condenser (coupled with a May-Pack filter in the upstream) at the circuit outlet, thus indicating that no gaseous Ru was transported so far; Re-volatilization of iodine deposited in the thermal gradient tubes has been observed during air injection in the VERDON-2 test, on the contrary to Cs. This latter point clearly highlights the difference, in terms of transport, which exists between Cs and I, difference which can impact chemistry occurring in the RCS during a severe accident. Finally, gaseous iodine has been quantified downstream in the VERDON circuit simulating its release into the containment.
5. Perspectives Concerning the air ingress topic, even though important results have been obtained from the VERDON-2 tests and other experiments performed by AECL on CANDU fuels, for instance, it is clear that this topic is not yet totally covered. To complement the experimental data concerning FP release and transport from high burn-up fuel under air ingress conditions a new test is considered. This ‘‘Air ingress” test, called VERDON-5, will be performed on UO2 fuel (same high burn-up fuel as VERDON-1), using the Transport Circuit and similar atmosphere conditions as for the VERDON-2 test. It will give complementary information regarding the potential impact of the Pu content in high burn-up UO2 fuel on Ru release, by comparison with the MOX fuel itself.
In addition boron will be injected during the initial phase under steam. This would allow the study of boron impact on the iodine speciation. The potentiality of boron to induce a high gaseous iodine fraction has been evidenced on separate effect tests (Bottomley et al., 2013). With the VERDON loop in transport configuration, thanks to the May-Pack filter located downstream of the TGTM, gaseous iodine fraction can be measured at a location simulating the entrance into the containment, in a semi-integral way. This opportunity is of great interest in the context of the Fukushima events, where the iodine gaseous fraction released in the environment seems to have been in good agreement with PHEBUS FPT3 (Haste et al., 2012), which is still not fully explained (80% of gaseous iodine entering the containment). Thus, this additional VERDON-5 test would confirm or not this important issue for Source Term assessments under a more integral and representative test. The VERDON laboratory is unique in the world for studies on reirradiated nuclear fuels up to high temperatures and can be used for further severe accident studies such as air ingress scenario at moderate and low temperatures, coupling between FP release and fuel degradation. It is also possible to investigate new field of research, for instance loss of coolant accident type experiments. These new topics are currently under evaluation. Acknowledgements ISTP was launched by CEA, EdF and IRSN, with contributions from the European Commission (EC, Europe) grant no. 3428-8807-TP-ISP-F, Tractebel Engineering (GDF SUEZ, Belgium), Paul Scherrer Institute (PSI, Switzerland), Atomic Energy of Canada Limited (AECL, Canada) and United States Nuclear Regulatory Commission (USNRC, United States) whose support is gratefully acknowledged, and the research has benefitted from technical discussions in SARNET working groups. References Albiol, T., Van Dorsselaere, J.P., Chaumont, B., Haste, T., Journeau, C., Meyer, L., Sehgal, B.R., Schwinges, B., Beraha, D., Annunziato, A., Zeyen, R., 2010. SARNET: Severe accident research network of excellence. Prog. Nucl. Energy 52–1, 2–10. Auvinen, A., Brillant, G., Davidovich, N., Dickson, R., Ducros, G., Dutheillet, Y., Giordano, P., Kunstar, M., Kärkelä, T., Mladin, M., Pontillon, Y., Séropian, C., Vér, N., 2008. Progress on ruthenium release and transport under air ingress conditions. Nucl. Eng. Des. 238, 3418–3428. Barrachin, M., Chevalier, P.Y., Cheynet, B., Fischer, E., 2008. New modelling of the U– O–Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions. J. Nucl. Mater. 375, 397–409. Beuzet, E., Lamy, J.-S., Perron, H., Simoni, E., Ducros, G., 2012. Ruthenium release modelling in air and steam atmospheres under severe accident conditions using the MAAP4 code. Nucl. Eng. Des. 246, 157–162. Bottomley, P.D.W. et al., Revaporisation of Fission Product deposits in the primary circuit and its impact on accident Source Term. In: 6th European Review meeting on Severe Accident Research (ERMSAR 2013), 2–4 October, 2013, Avignon, France. Brillant, G., Marchetto, C., Plumecocq, W., 2010. Ruthenium release from fuel in accident conditions. Radiochim. Acta 98 (5), 267–275. Clément, B., Zeyen, R., The Phebus Fission Product and Source Term International Programmes. In: Proc. Int. Conf. Nuclear Energy for New Europe, Bled, Slovenia, 2005. Ducros, G., Bernard, S., Ferroud-Plattet, M.P., Ichim, O., Use of gamma spectrometry for measuring fission product releases during a simulated PWR severe accident: Application to the VERDON experimental program. In: Proc. Int. Conf. ANIMMA International Conference, Marseilles, France, 2009. Ferroud-Plattet, M.P., Bonnin, J., Gallais-During, A., Bernard, S., Grandjean, J.P., Ducros, G., CEA VERDON Laboratory at Cadarache: new hot cell facilities devoted to studying irradiated fuel behaviour and fission product releases under simulated accident conditions. In: Proc. HotLab International Conference, Prague, Czech Republic, 2009. Gallais-During, A., Bonnin, J., Malgouyres, P.-P., Bernard, S., Pontillon, Y., Hanus, E., Ducros, G., VERDON Laboratory: performances of the experimental LWR severe accident device and first results of fission products release on high burn-up UO2 fuel. In: Proc. Int. Conf. Nuclear Energy for New Europe, Ljubljana, Slovenia, 2012. Gallais-During, A., Bonnin, J., Malgouyres, P.-P., Morin, S., Bernard, S., Gleizes, B., Pontillon, Y., Hanus, E., Ducros, G., 2014. Performances and first results of fission
A. Gallais-During et al. / Annals of Nuclear Energy 101 (2017) 109–117 products release and transport provided by the VERDON facility. Nucl. Eng. Des. 277, 117–123. Haste, T., Payot, F., Dominguez, C., March, Ph., Simondi-Teisseire, B., Steinbrück, M., 2012. Study of boron behaviour in the primary circuit of water reactors under severe accident conditions: a comparison of Phebus FPT3 results with other recent integral and separate-effects data. Nucl. Eng. Des. 246, 147–156. Hidaka, A., 2011. Outcome of VEGA program on radionuclide release from irradiated fuel under severe accident conditions. J. Nucl. Sci. Technol. 48, 85–102. Klein-Heßling, W., Sonnenkalb, M., Jacquemain, D., Clément, B., Raimond, E., Dimmelmeier, H., Azarian, G., Ducros, G., Journeau, C., Herranz Puebla, L.E., Schumm, A., Miassoedov, A., Kljenak, I., Pascal, G., Bechta, S., Güntay, S., Koch, M. K., Ivanov, I., Auvinen, A., Lindholm, I., 2014. Conclusions on severe accident research priorities. Ann. Nucl. Energy 74, 4–11. Leveque, J.P., Andre, B., Ducros, G., Le Marois, G., Lhiaubet, G., 1994. The HEVA experimental programme. Nucl. Technol. 108, 33–44. Lewis, B.J., Dickson, R., Iglesias, F.C., Ducros, G., Kudo, T., 2008. Overview of experimental programs on core melt progression and fission product release behaviour. J. Nucl. Mater. 380, 126–143. Lorenz, R.A., Osborne, M.F., A summary of ORNL fission product release tests with recommended release rates and diffusion coefficients, Report ORNL/TM-12801, NUREG/CR-6261, 1995. Lui, Z., Cox, D.S., Dickson, R.S., Elder, P., A summary of CRL fission product release measurements from UO2 samples during post-irradiation annealing (19831992). Report COG-92-377, 1994.
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Pontillon, Y., Ducros, G., 2010a. Behaviour of fission products under severe PWR accident conditions: the VERCORS experimental programme—Part 2: Release and transport of fission gases and volatile fission products. Nucl. Eng. Des. 240, 1853–1866. Pontillon, Y., Ducros, G., 2010b. Behaviour of fission products under severe PWR accident conditions: the VERCORS experimental programme—Part 3: Release of low-volatile fission products and actinides. Nucl. Eng. Des. 240, 1867–1881. Pontillon, Y., Ducros, G., Durin, M., Lessons learnt about fission product release from high burn-up UO2 fuels under severe accident conditions: the VERCORS 6 and RT6 tests. In: Proc. CSARP meeting, Washington, United States, 2004. Pontillon, Y., Ducros, G., Malgouyres, P.P., 2010. Behaviour of fission products under severe PWR accident conditions: the VERCORS experimental programme—Part 1: General description of the programme. Nucl. Eng. Des. 240, 1843–1852. Pontillon, Y., Bernard, S., Gallais-During, A., Gleizes, B., Hanus, E., Ducros, G., Main outcomes relative to iodine behaviour from the VERDON/ISTP programme in OECD. In: International Iodine Workshop: Full Proceedings, Marseille, France, 30 March – 1 April 2016, OECD/NEA/CSNI/R(2016) 5, May 2016. Schwarz, M., Hache, G., von der Hardt, P., 1999. PHEBUS FP: a severe accident research programme for current and advanced light water reactors. Nucl. Eng. Des. 187, 47–69. SNETP (Sustainable Nuclear Energy Technology Platform), ‘‘Strategic Research and Innovation Agenda” (SRIA 2013), February 2013.
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Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Overview of the VERDON-ISTP Program and main insights from the VERDON-2 air ingress test A. Gallais-During a,⇑, S. Bernard a, B. Gleizes a, Y. Pontillon a, J. Bonnin b, P.-P. Malgouyres a, S. Morin c, E. Hanus a, G. Ducros a a b c
CEA, DEN, DEC, SA3C, F-13108 Saint-Paul-lez-Durance, France CEA, DEN, DTN, STCP, F-13108 Saint-Paul-lez-Durance, France IRSN, BP3, F-13115 Saint-Paul-lez-Durance, France
a r t i c l e
i n f o
Article history: Received 16 June 2016 Received in revised form 22 September 2016 Accepted 27 September 2016 Available online 14 November 2016
a b s t r a c t In order to reduce the uncertainties concerning source term assessment under LWR severe accident conditions, the International Source Term Program has been launched. Four main R&D research axes have been addressed in this program: iodine behaviour in the RCS and containment, study of the boron carbide effect on fuel degradation and FP release, study of the air effect on fuel and FP behaviour, FP release from high burn-up UO2 and MOX fuels. As far as the source term quantification is concerned, four VERDON tests have been defined and were performed in the VERDON laboratory at the CEA Cadarache Centre. We present an overview of these four tests, devoted to the study of FP release from high burn-up UO2 and MOX fuels. We focus on the VERDON-2 air ingress test performed on MOX fuel with a specific loop dedicated to FP release and transport. It addressed two issues: Ru release under mixed steam-air conditions following lower head failure and transport/re-volatilization of Ru and volatile FPs in the primary circuit. The latter was performed thanks to the implementation of thermal gradient tubes exposed at different phases of the sequence. In this configuration, the VERDON facility combines the ability of the analytical approach to identify and understand the physical phenomena involved, with a semi-integral approach, which allows the results to be applied to source term assessments for reactor application. This VERDON-2 test has shown a rather unexpected low release of ruthenium from the MOX sample and no re-volatilization from the thermal gradient tubes. On the contrary, a significant re-volatilization of iodine has been observed when switching to mixed steam-air conditions, as well as a low, but measurable amount of iodine transported under gaseous form downstream of the thermal gradient tubes. Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction In order to assess the Source Term in the case of a hypothetical LWR severe accident and the ensuing risks for the population and the environment, a large number of research programmes have been undertaken in various countries (Schwarz et al., 1999; Lorenz and Osborne, 1995; Lui et al., 1983; Hidaka, 2011; Lewis et al., 2008). Among them, the HEVA (Leveque et al., 1994) – VERCORS (Pontillon et al., 2010; Pontillon and Ducros, 2010a,b) programmes, financed jointly by IRSN and EDF and conducted by the CEA led to reduce the uncertainties concerning FP behaviour under LWR severe accident conditions. However, some major uncertainties still remain in some fields so it was decided to build an international co-operative research ⇑ Corresponding author. E-mail address: [email protected] (A. Gallais-During). http://dx.doi.org/10.1016/j.anucene.2016.09.045 0306-4549/Ó 2016 Elsevier Ltd. All rights reserved.
programme, called the ‘‘International Source Term Program (ISTP)” (Clément and Zeyen, 2005). ISTP was launched by CEA, EdF and IRSN, with contributions from the European Commission (EC, Europe), Tractebel Engineering (GDF SUEZ, Belgium), Paul Scherrer Institute (PSI, Switzerland), Atomic Energy of Canada Limited (AECL, Canada) and United States Nuclear Regulatory Commission (USNRC, United States). Four main R&D research axes, based on separate-effect experiments, have been addressed in this program. They concern iodine behaviour in the RCS and the containment, study of the boron carbide effect on fuel degradation and FP release, study of the air effect on fuel and FP behaviour and study of the FP release from the fuel. The results of these separate-effect experiments would allow improving models used for Source Term evaluation studies. They contribute to the resolution of a high priority safety issue defined by SARNET (Klein-Heßling et al., 2014; Albiol et al., 2010), which was endorsed by NUGENIA (SNETP, 2013).
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As far as the Source Term quantification is concerned, four VERDON tests have been identified within the framework of the ISTP and were performed in the VERDON laboratory at the CEA Cadarache Centre. In the first part of this paper, we present an overview of these four tests, devoted to the study of FP release from high burn-up UO2 and MOX fuels. In the second part, we focus on the VERDON-2 air ingress test performed with a specific loop dedicated to FP release and transport. 2. Overview of the 4 VERDON-ISTP tests 2.1. The VERDON experiments The four VERDON tests of the ISTP were conducted between 2011 and 2014 in an entirely new laboratory at the CEA Cadarache Centre in the LECA-STAR facility (Ferroud-Plattet et al., 2009). The VERDON laboratory is constituted of two high activity cells (C4 and C5) and a glove-box. The C4 cell is dedicated to pre/post-test operations, such as sample reception and preparation. This cell also contains a gamma spectrometry bench dedicated to gamma scanning of the fuel before and after the test and of the different elements of the loop after dismantling. The C5 cell, represented in Fig. 1, is dedicated to the accidental sequence carrying out and to on-line measurements (both thermal-hydraulics and gamma spectrometry). It contains the experimental circuit itself (i.e. VERDON loop) as well as three complementary on-line gamma spectrometry stations for the furnace, filter and the May-Pack in order to follow the FP
release kinetics from the fuel sample located inside the furnace during the experiment (Ducros et al., 2009). The glove-box main functions are to analyze and store the fission and carrier gases. Two complementary loops are available: one dedicated to release studies (see Fig. 1a), the other to transport and revolatilization studies (see Fig. 1b). It was necessary to develop a specific high frequency (HF) furnace (Gallais-During et al., 2012), key element of both loops, since it is where the fuel sample is heated up in conditions representative of a severe accident in terms of temperature (up to 2600 °C) and oxidizing-reducing conditions (fluid flow constituted of a mixture of helium, water steam, hydrogen and air). Downstream of the furnace, the FPs emitted by the fuel are carried by the gas flow in the VERDON loop. The Release Loop is devoted to the precise characterization of FP release by trapping FPs under aerosol forms in a total aerosol filter located just above the furnace; the precise quantification of the released fraction is allowed on-line by gamma spectrometry and/ or after the test by chemical analysis. Downstream of the aerosol filter, only gases (carrier gases, fission gases Xe and Kr and potential gaseous forms of iodine) are transported along the circuit. The May-Pack filter is located downstream of the aerosol filter and is designed to trap gaseous iodine on successive stages, according to chemical forms of iodine (molecular or organic). The main difference of the Transport Loop compared with the Release Loop is the system of Multiple Thermal Gradient Tubes (TGTM) located between the HF furnace and the aerosol filter. This device enables the study of FP transport in the primary circuit of a nuclear power plant thanks to thermal gradient tubes which
Fig. 1. VERDON C5 cell: a. Release Circuit, b. Transport Circuit.
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temperature is representative of the hot leg and cold leg temperatures under severe accident conditions: the temperature along these tubes linearly decreases from 700 °C at the entrance to 150 °C at the exit, and is monitored by a series of thermocouples located every 100 mm as shown in Fig. 2. There, the FP vapours deposit according to their condensation temperature, giving an indication of their chemical forms. In addition, FP aerosols can deposit (by thermophoresis) along the tubes. A specificity of this TGTM device is that it is constituted of 4 tubes assembled on a rotating system. During the accidental sequence, 2 tubes are swept along in parallel by the FPs and they can be rotated of a quarter turn at specific moment of the sequence, when changing the oxidizing-reducing conditions, for instance. By this way, one of the tubes is not affected by the new conditions, the second one is affected by both the previous and new conditions and a third one is only affected by the new conditions. The study of FP revaporisation is then possible, by comparing FP deposits along the 3 tubes linked to specific oxidizing-reducing conditions.
2.2. The VERDON samples For the 4 VERDON-ISTP tests, the samples were taken from a UO2 or from a MOX fuel rod, previously irradiated in a PWR operated by the French operator EDF. The sample consists of two irradiated pellets in their original cladding and two half-pellets of depleted (and un-irradiated) uranium oxide placed at each end of the sample and held there by crimping the cladding so that the cladding is not fully sealed. Before the experimental sequence, the sample was re-irradiated at low linear power (15–20 W/cm) in the OSIRIS material testing reactor for about a week, in order to recreate the short half-life FPs without any in-pile release. As a consequence, these FPs (i.e. 99 Mo, 132Te, 133I, 131I, 140Ba. . .), important for their radiobiological effects, are measurable by using on-line c spectrometry during the experiment. The fuel sample used for VERDON-1 was a high burn up UO2 fuel very similar to VERCORS-RT6 (Pontillon et al., 2004) (same fuel assembly, same power history and very close in burn-up). For VERDON-2 to -4, the samples were made up of a fuel section taken from the same span of the MOX fuel rod so that they are identical.
2.3. VERDON-ISTP tests main results The main parameters explored throughout the 4 tests of the programme were mainly the maximal temperature, the oxidizing-reducing conditions of the vector fluid, the high burnup and the nature of the fuel sample (UO2 or MOX). For the 4 tests, an intermediate temperature plateau was maintained at 1500 °C (in a steam and hydrogen atmosphere for VERDON-1 and -3, pure steam for VERDON-2 and -4) in order to fully oxidise the cladding before the temperature ramps up to the final phase of the test. The duration of this intermediate plateau was about 10–15 min, except for VERDON-1 for which it was 50 min long. The full test grid is described in Table 1 for each test. Most of the former experimental data bases concern low burnup fuel in both oxidizing and reducing conditions and high burn-up fuel under oxidizing conditions only (Pontillon et al., 2010). Therefore, the main issue addressed by the VERDON-1 test concerns high burn up UO2 fuel behaviour – and corresponding FPs releases – under reducing conditions at very high temperature (up to 2600 °C). Moreover, the first part of the test (up to the end of the oxidation plateau at 1500 °C) was performed under the same atmosphere conditions as the VERCORS RT6 test (Gallais-During et al., 2014), which was conducted with a very similar high burnup UO2 fuel, in order to check the continuity between VERCORS and VERDON data bases. It was performed with the Release Circuit. During this first VERDON-1 test, the good overall performance of the VERDON loop in terms of tightness, thermal-hydraulics, furnace ceramics behaviour, etc.. . . and of the gamma scanning and sighting have been clearly demonstrated. As a consequence it can be asserted that the performance of the VERDON facility has been confirmed. A comparison with VERCORS RT6 has been possible and yielded the following conclusion: there were similar FP released fractions at 1500 °C, there was confirmation of high burn-up effect on release kinetics and demonstration of the atmosphere effect on Mo, Ba releases. The VERDON loop was thus qualified in ‘‘release configuration”, and as giving results that are consistent with the VERCORS experiments. From a general point of view, the VERDON-1 test does not result in a global and/or strong relocation of the fuel sample at the end of the test. This behaviour, at temperatures as high as 2610 °C, is one of the major lessons of the VERDON-1 test. This lack of global relocation must be connected
1000 Flow inlet: Experimental He = 6 mg/s, Susceptor He = 5 mg/s, flow temperature Without H2O (phase 1), wall temperature
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H2O=25mg/s (phase 2), wall temperature
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Fig. 2. Flow and wall temperature in the TGTM.
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Table 1 Main characteristics of the 4 VERDON-ISTP tests. Test
VERDON-1
VERDON-2
VERDON-3
VERDON-4
Date of test Fuel PWR irradiation Sample burn-up (GWd/t) Re-irradiation
09-2011 UO2 Gravelines 72 Osiris
06-2012 MOX Chinon 60 Osiris
04-2013 MOX Chinon 60 Osiris
10-2014 MOX Chinon 60 Osiris
Mixed H2O steam + air
H2O steam
H2
2.6 0 4.2
2.5 0 0
0 0.4 0
2100 90 min at Tmax = 2100 °C Ru release 6 60%
2300 10 min at Tmax = 2300 °C Ba release 6 70% No global fuel relocation
2530 Beginning of fuel relocation Ba release 6 80% Tmax 6 2600 °C (10 min)
Test conditions (after oxidation plateau) Atmosphere Mixed H2O steam + H2 (He at the end) H2O steam flow rate (mg/s) 0.3 H2 flow rate (mg/s) 0.33 Air flow rate (mg/s) 0 End of the test Max fuel temperature (°C) Ending criterion Other criteria to be respected
2610 Tmax = 2600 °C reached None
to the atmospheres experienced during the last part of the test (i.e. reducing then neutral conditions) (Barrachin et al., 2008), and post-test micro-analyses are currently being performed in order to better understand this phenomenon. The released fractions measured by the on-line gamma stations and the data obtained via pre and post-test gamma scannings have been carefully assessed. This has yielded a FP general classification (Pontillon et al., 2010; Pontillon and Ducros, 2010a,b), in relation to their released fractions and specific behaviours, was again obtained similar to previous work with: (1) volatile FPs (fission gases, iodine, caesium) with an almost total release; (2) semi-volatile FPs (molybdenum and barium), with high sensitivity to oxidizingreducing conditions and significant released fractions; (3) low volatile FPs (ruthenium, europium) with generally (very) low released fractions (except for ruthenium under highly oxidizing conditions which can be associate to semi-volatile FPs in these cases) and (4) non-volatile FPs composed of zirconium. VERDON-3 and -4 are two complementary experiments. Their main objective is the quantification of FP release from a high burn-up MOX fuel; under steam oxidizing conditions for VERDON-3 and H2 reducing conditions for VERDON-4. The two tests were conducted at high temperature but without reaching global fuel relocation and with a particular interest in semivolatile fission products during post-test micro-analysis. They were performed with the Release Circuit. The main observations in terms of fuel sample behaviour and fission product release after VERDON-3 are the following: the comparison of the non-volatile FP distributions along the fuel sample before and after the test confirms the integrity of the fuel sample after the test; some FP traces are however detected under the crucible, meaning that fuel melting was probably imminent; the behaviour of ruthenium is specific with a very low release (a few percent); the comparison with Ru release for VERCORS HT2 (65%) and VERCORS RT6 (28%) obtained on UO2 fuel in oxidizing condition (Pontillon and Ducros, 2010b) leads us to consider a positive effect of the MOX nature of the fuel in reducing the Ru release. as expected, the release of semi-volatile FPs depends on atmosphere: with steam oxidizing conditions as in VERDON-3, the release of Mo is larger than for Ba (total release for Mo and about 55–60% for Ba) and associated with fast kinetics; the release of volatile FP is almost total and is associated with fast kinetics; regarding iodine, about 1% of gaseous species were found in the May-Pack filter and for the first time their release
kinetics could be measured thanks to the May-Pack gamma station (Pontillon et al., 2016). Regarding VERDON-4, the results are still under investigation but some qualitative issues can already be discussed. The test was conducted under H2 atmosphere until about 20% loss of ZrLa activity was detected on the on-line furnace gamma station, representative of a beginning of melting of the fuel sample, at a temperature of about 2530 °C, comparable to that of VERDON-1 test performed under mildly reducing conditions. As expected, in the reducing conditions of the VERDON-4 test, the release of Ba is larger than that of Mo since semi-volatile FP release depends on atmosphere. However, what was unexpected is a quite low release and slow kinetics of Ba release: the released fraction of Ba is about 55% at the end of the test. The main insights of the VERDON-2 air ingress test are presented in the following part.
3. VERDON-2 air ingress test TheVERDON-2 test concerns high burn-up MOX fuel behaviour – and corresponding FP release and transport – under air ingress conditions: air may contact fuel in a number of accident situations including reactor core meltdown accidents (after failure of the pressure vessel lower head or after drying out of a spent fuel pool). If, in the early phase of the accident, the fuel cladding has been nearly completely oxidised by steam, air ingress will also result in a significant increase of fuel superstoichiometric deviation and oxygen potential. This will accelerate the release of certain FPs, notably due to oxidation of metallic precipitates in the fuel which will trigger a large release of ruthenium, a highly radiotoxic radio-nuclide. The VERDON-2 test objective was to specifically monitored the ruthenium released from the fuel. The only previous experiment with air ingress is VERCORS RT8 for which the amount of air injected was too low to obtain a significant release of ruthenium and for which the fuel was a high burn up UO2 fuel (Pontillon and Ducros, 2010b). VERDON-2 was the first test to be performed with the Transport Circuit. The second objective of this test was to study the transport/re-volatilization of ruthenium and previously deposited volatile FPs in the primary circuit of a LWR, thanks to the TGTMs of the Transport Circuit. This test then complements, in a semi-integral approach using real irradiated fuel, other separate effect tests performed in different R&D laboratories and relevant to the impact of air ingress on ruthenium
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release (Lui et al., 1983) and on its transport in the reactor coolant system (Pontillon et al., 2016). 3.1. Thermal-hydraulic sequence The thermal-hydraulic sequence is represented in Fig. 3. The criterion to end the sequence was either to detect 60% of ruthenium release (1st stop criterion) or to stay one hour at 2100 °C to perform the last TGTM rotation with an additional 30 min in the same atmosphere (2nd stop criterion). Phase 1 and 2 were similar to VERDON-1 test without H2 injection, that is, steam alone (Gallais-During et al., 2014). Phase 3, under steam conditions, was dedicated to the measurement of release, transport and deposit of FPs without air injection, simulating LWR severe accident conditions before the lower head failure. The first TGTM rotation occurred (so that the exposed tubes 1–2 were switched to tubes 2–3) at the end of this phase. Phase 4, under air and steam conditions (molar ratio of 1 between steam and air) has been maintained for 70 min. Since the ruthenium release had not reached the 1st stop criterion of 60% released, another heat-up phase (phase 5) was performed up to 2100 °C. The 2100 °C plateau lasted for 90 min. During this plateau, the TGTM mechanism was turned after 60 min (from exposing tubes 2–3 to exposing tube 3–4). After the rotation, the tubes 3 and 4 stayed opened for 30 min under the same atmosphere to study the FP deposits as specified by the 2nd stop criterion. VERDON-2 was the first test to be performed with the Transport Circuit and the TGTM operated as specified without problem. In the following parts, we focus on three issues addressed by this test: 1) FP release and more specifically 2) ruthenium release and transport under mixed steam-air conditions following lower head failure and 3) transport/re-volatilization of volatile FPs in the primary circuit. 3.2. FP release under mixed steam-air conditions In order to illustrate the general behaviour of the FPs belonging to the four volatility categories (Pontillon et al., 2010), Fig. 4 com-
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pares the release kinetics of Zr (non or low-volatile), Ba, Mo, Ru (semi-volatile), Cs (volatile). The FP release kinetics presented here were measured by the furnace on-line gamma spectrometry station. They are measurements of the FP remaining in the fuel as a function of temperature, which are relatively imprecise when the release is low, in particular below 10%. Indeed, the changes in the object geometry measured during heating (swelling, fracturing, then fuel collapse, etc.), just like the axial migration of the FP, is significantly complicated to reduce the measurement accuracy and can explain some negative values of the released fractions. We found good agreement with previous VERCORS test results, in particular regarding semi-volatile FPs: The release of Mo starts at the beginning of the 1500 °C plateau as soon as the cladding is fully oxidized. It increases with rapid kinetics to reach approximately 30% of the initial inventory at the end of the plateau and 100% of the initial inventory after around 15 min at 2000 °C. The release of Ba starts after the oxidation plateau with the increase of temperature. A high Ba kinetics release is observed to reach up to 30% of the initial inventory at the beginning of the 2000 °C plateau and around 80% of the initial inventory at the end of the test. The release rate decreases after the air injection confirming that Ba volatility lies in the range of oxygen potentials corresponding to steam and air atmosphere. It seems to increase later with the increase of temperature. However, a major difference comparing to all the other VERCORS tests on standard UO2 fuel is observed here: it concerns the very fast release kinetics for volatile and semi-volatile FPs during the oxidizing phase. These fast kinetics were also measured during VERDON-3, on the same fuel in oxidizing conditions and seem to be linked to the MOX nature of the fuel. Moreover, even though the final atmosphere of the test was strongly oxidizing, the barium release continues (although it is slightly slower after air is added to steam injection) and is relatively high (80%) at the end of the test.
Fig. 3. Thermal-hydraulic sequence of VERDON-2 test.
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Fig. 4. Release kinetics of Zr (non-volatile), Ba, Mo, Ru (semi-volatile), Cs (volatile) for VERDON-2 test.
Table 2 Atmosphere conditions of TGTM tubes.
Blue shades means the tube is exposed (or open) to the corresponding atmoshere. White means the tube is un-exposed (or closed).
3.3. Ruthenium release and transport/re-volatilization under mixed steam-air conditions The specific ruthenium release is mainly due to the atmosphere conditions of the test (Auvinen et al., 2008; Brillant et al., 2010; Beuzet et al., 2012). The ruthenium release starts with the air ingress during the 2000 °C plateau and reaches about 70% of the initial inventory at the end of the sequence. The release rate slightly increases between the 2000 °C and 2100 °C plateaus. From a general point of view, the ruthenium kinetics seem to be relatively low compared to what could be expected due to the final temperature and atmosphere of the test (2.5 h under mixed stem-air conditions, 1 h at 2000 °C and 1.5 h at 2100 °C). This low value can be the consequence of the MOX nature of the fuel, as was confirmed in VERDON-3 (Section 2.3). As explained in Section 2.1, the Transport Circuit main feature is to use four thermal gradient tubes (TGTM) that can be operated sequentially with two in parallel in order to study deposits as well as re-vaporisation. Fig. 2 displays the temperatures of the tubes measured during the commissioning tests and Table 2 summarizes the different atmosphere conditions during which each tube was exposed and then when FP deposits occur. After the test sequence, all the TGTM tubes were dismounted and measured on the c spectrometry bench (C4 cell). The comparison of the deposits on each tube gives information on the possible re-vaporisation of the volatile FP deposits as well as deposits of lower volatile FPs, such as ruthenium, linked to air injection. Regarding the specific behaviour of ruthenium with air ingress, we observe in Fig. 5 that Ru-103 did not deposit in tube 1, exposed only during steam injection. The profiles of tubes 2 and 3 are similar, which is consistent with a Ru release that started with the air injection. On these tubes we observe a large accumulation
Fig. 5. Distribution of Ru-103 along the TGTM tubes for VERDON-2 test.
of Ru in the mid-plane area of the tubes, at around 400–350 °C. Another peak is observed at the end of the tubes, at around 200 °C. We suspect that this deposit is due to thermophoresis, since all the other FPs deposited at this location (see Fig. 7 for Cs and I). The quantity of deposits on tube 3 are greater than those on tube 2, which is normal considering its longer period of use under air injection. We observe on both tubes a large accumulation in the mid-plane area of the tubes. Tube 4 shows a rather homogeneous profile of low amplitude. The sum of the profiles for tubes 2 and 4 is similar to tube 3, which shows there was no significant revolatilization of ruthenium deposits at the end of the test. In addition to these gamma spectrometry measurements, chemical analyses and Raman spectroscopy measurements are planned on samples of tubes where Ru is present in order to discriminate which Ru compounds deposited. To further study the transport of ruthenium, we analysed the P4VP filter (one of the final safety filters, made of ‘‘poly vinylpyridine” and intended to trap the potential gaseous RuO4 produced during the experiment. This filter was totally free from ruthenium (and from any other traces of FPs), meaning that no gaseous RuO4 was released downstream of the TGTM.
3.4. Transport/re-volatilization of volatile FPs in the primary circuit The transport of volatile FPs (caesium and iodine) along the TGTM is very different: for caesium, the deposit profile is qualitatively similar in each tube (almost all the caesium was released by
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Fig. 6. Distribution of Cs-137 along the TGTM tubes for VERDON-2 test.
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the test. This latter point clearly highlights the difference, in terms of transport, which exists between Cs and I, difference which can impact chemistry occurring in the RCS during a severe accident. The May-Pack filter is located just downstream of the aerosol filter and was designed to trap gaseous iodine. It includes a glass filter stage, followed by three stages containing silver coated zeolite for molecular iodine trapping and three stages containing activated charcoal for the trapping of residuals traces of FPs, in our case mainly inorganic iodine. Only I-131 and Cs-137 could be measured in significant quantities on this filter. The profiles of these two FPs down the filter are given in Fig. 8. Cs-137 shows a local and small aerosol deposit at the filter inlet in the self-sealing connection, the remainder of the signal probably being background noise due to the cell contamination. I-131 shows several typical activity peaks of trapping of its gaseous species, the largest being located on the first zeolite stage (so, mainly molecular iodine) while the others are located on the third zeolite stage and the first stage of activated charcoal (residual inorganic iodine traces: I2 or HI). We are not expecting to find organic iodine RI since these chemical forms of iodine are mainly due to molecular iodine interactions with paints in severe accident conditions and there are no paints in the VERDON Transport Loop. The fraction of gaseous iodine having gone through the aerosol filter at 150 °C is nevertheless low (evaluated around 0.13% of the initial inventory) but significant. This is the first time that gaseous iodine has been detected in the May-Pack of the whole VERCORS/ VERDON programmes. The May-Pack is shown to be well designed to measure gaseous forms of iodine. 4. Concluding remarks
Fig. 7. Distribution of I-131 along the TGTM tubes for VERDON-2 test.
2000 °C), showing no effect of air injection (see Fig. 6), whereas for iodine, we observe in Fig. 7 that the highest I-131 deposits were located in the middle of tube 1 and at the outlet of tube 2. The central deposit observed on tube 1 has been completely re-volatilized during the air injection since it no longer appeared on tube 2. Deposits on tube 3 are very low, which shows that deposits at the outlet of tube 2 have been also re-volatilized at the end of
The four VERDON tests identified within the framework of the ISTP were performed in the VERDON laboratory at the CEA Cadarache Centre. They were devoted to the study of FP release from high burn-up UO2 and MOX fuels, one test on MOX fuel being dedicated to air ingress scenario (VERDON-2). We presented here an overview of these four tests, with a focus on the VERDON-2 air ingress test performed with a specific loop dedicated to FP release and transport. In this configuration, using the TGTMs, which are exposed at different phases of the sequence, the VERDON facility combines the ability of the analytical approach to identify and understand the physical phenomena involved, with a rather semi-integral approach, which allows the
Fig. 8. Distribution of I and C down the May-Pack filter.
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results to be applied to Source Term assessments for reactor application. From a general point of view, the main insights gained from the four VERDON tests are the following: A rapid release kinetics was observed, beginning at unexpectedly low temperatures, for volatile (I, Cs, Te) and even for the semi-volatile Mo, Ba FPs, for high burn-up UO2 and MOX fuels; However, a major difference comparing to all the other VERCORS tests on standard UO2 fuel is observed here: it concerns the very fast release kinetics for volatile and semi-volatile FPs during the oxidizing phase. This fast kinetics is also measured during VERDON-3, on the same fuel in oxidizing conditions. Several key parameters have an influence on release kinetics, particularly the burn-up, the type of atmosphere, the type of fuel (UO2 or MOX). By comparison with the VERCORS tests, it was also possible to identify the influence of one of these parameters on another. From a general point of view, the release rate increases with the burn up, in oxidizing condition and with MOX compared to UO2 fuels (Pontillon et al., 2016); An unexpected behaviour of ruthenium was observed for MOX fuel in VERDON-2 and -3 with a low release and relatively slow kinetics compared to what could be expected in oxidizing conditions. The VERDON-3 test was performed with the same MOX fuel as VERDON-2, but under pure steam conditions up to 2300 °C. Only a few percent of Ru was released from this test, which is very low in comparison with the Ru measured with UO2 fuel, for some VERCORS tests in similar conditions (see Section 2.3). This result confirms the lower Ru release in case of MOX fuel compared to UO2 fuel, probably linked to a difference in the oxidation of the fuel matrix and suggests that oxygen potentials in MOX fuel may be lower and stabilise the ruthenium in metallic precipitates so that it may require longer exposure to air to enable its release as volatile oxides. In VERDON-2 no deposits of Ru was observed to take place in the TGTM before the air ingress phase, which is consistent with the Ru release observations. Afterwards, an accumulation of Ru on tube surfaces was noticed in the course of the test. Ruthenium was not observed downstream of the condenser (coupled with a May-Pack filter in the upstream) at the circuit outlet, thus indicating that no gaseous Ru was transported so far; Re-volatilization of iodine deposited in the thermal gradient tubes has been observed during air injection in the VERDON-2 test, on the contrary to Cs. This latter point clearly highlights the difference, in terms of transport, which exists between Cs and I, difference which can impact chemistry occurring in the RCS during a severe accident. Finally, gaseous iodine has been quantified downstream in the VERDON circuit simulating its release into the containment.
5. Perspectives Concerning the air ingress topic, even though important results have been obtained from the VERDON-2 tests and other experiments performed by AECL on CANDU fuels, for instance, it is clear that this topic is not yet totally covered. To complement the experimental data concerning FP release and transport from high burn-up fuel under air ingress conditions a new test is considered. This ‘‘Air ingress” test, called VERDON-5, will be performed on UO2 fuel (same high burn-up fuel as VERDON-1), using the Transport Circuit and similar atmosphere conditions as for the VERDON-2 test. It will give complementary information regarding the potential impact of the Pu content in high burn-up UO2 fuel on Ru release, by comparison with the MOX fuel itself.
In addition boron will be injected during the initial phase under steam. This would allow the study of boron impact on the iodine speciation. The potentiality of boron to induce a high gaseous iodine fraction has been evidenced on separate effect tests (Bottomley et al., 2013). With the VERDON loop in transport configuration, thanks to the May-Pack filter located downstream of the TGTM, gaseous iodine fraction can be measured at a location simulating the entrance into the containment, in a semi-integral way. This opportunity is of great interest in the context of the Fukushima events, where the iodine gaseous fraction released in the environment seems to have been in good agreement with PHEBUS FPT3 (Haste et al., 2012), which is still not fully explained (80% of gaseous iodine entering the containment). Thus, this additional VERDON-5 test would confirm or not this important issue for Source Term assessments under a more integral and representative test. The VERDON laboratory is unique in the world for studies on reirradiated nuclear fuels up to high temperatures and can be used for further severe accident studies such as air ingress scenario at moderate and low temperatures, coupling between FP release and fuel degradation. It is also possible to investigate new field of research, for instance loss of coolant accident type experiments. These new topics are currently under evaluation. Acknowledgements ISTP was launched by CEA, EdF and IRSN, with contributions from the European Commission (EC, Europe) grant no. 3428-8807-TP-ISP-F, Tractebel Engineering (GDF SUEZ, Belgium), Paul Scherrer Institute (PSI, Switzerland), Atomic Energy of Canada Limited (AECL, Canada) and United States Nuclear Regulatory Commission (USNRC, United States) whose support is gratefully acknowledged, and the research has benefitted from technical discussions in SARNET working groups. References Albiol, T., Van Dorsselaere, J.P., Chaumont, B., Haste, T., Journeau, C., Meyer, L., Sehgal, B.R., Schwinges, B., Beraha, D., Annunziato, A., Zeyen, R., 2010. SARNET: Severe accident research network of excellence. Prog. Nucl. Energy 52–1, 2–10. Auvinen, A., Brillant, G., Davidovich, N., Dickson, R., Ducros, G., Dutheillet, Y., Giordano, P., Kunstar, M., Kärkelä, T., Mladin, M., Pontillon, Y., Séropian, C., Vér, N., 2008. Progress on ruthenium release and transport under air ingress conditions. Nucl. Eng. Des. 238, 3418–3428. Barrachin, M., Chevalier, P.Y., Cheynet, B., Fischer, E., 2008. New modelling of the U– O–Zr phase diagram in the hyper-stoichiometric region and consequences for the fuel rod liquefaction in oxidising conditions. J. Nucl. Mater. 375, 397–409. Beuzet, E., Lamy, J.-S., Perron, H., Simoni, E., Ducros, G., 2012. Ruthenium release modelling in air and steam atmospheres under severe accident conditions using the MAAP4 code. Nucl. Eng. Des. 246, 157–162. Bottomley, P.D.W. et al., Revaporisation of Fission Product deposits in the primary circuit and its impact on accident Source Term. In: 6th European Review meeting on Severe Accident Research (ERMSAR 2013), 2–4 October, 2013, Avignon, France. Brillant, G., Marchetto, C., Plumecocq, W., 2010. Ruthenium release from fuel in accident conditions. Radiochim. Acta 98 (5), 267–275. Clément, B., Zeyen, R., The Phebus Fission Product and Source Term International Programmes. In: Proc. Int. Conf. Nuclear Energy for New Europe, Bled, Slovenia, 2005. Ducros, G., Bernard, S., Ferroud-Plattet, M.P., Ichim, O., Use of gamma spectrometry for measuring fission product releases during a simulated PWR severe accident: Application to the VERDON experimental program. In: Proc. Int. Conf. ANIMMA International Conference, Marseilles, France, 2009. Ferroud-Plattet, M.P., Bonnin, J., Gallais-During, A., Bernard, S., Grandjean, J.P., Ducros, G., CEA VERDON Laboratory at Cadarache: new hot cell facilities devoted to studying irradiated fuel behaviour and fission product releases under simulated accident conditions. In: Proc. HotLab International Conference, Prague, Czech Republic, 2009. Gallais-During, A., Bonnin, J., Malgouyres, P.-P., Bernard, S., Pontillon, Y., Hanus, E., Ducros, G., VERDON Laboratory: performances of the experimental LWR severe accident device and first results of fission products release on high burn-up UO2 fuel. In: Proc. Int. Conf. Nuclear Energy for New Europe, Ljubljana, Slovenia, 2012. Gallais-During, A., Bonnin, J., Malgouyres, P.-P., Morin, S., Bernard, S., Gleizes, B., Pontillon, Y., Hanus, E., Ducros, G., 2014. Performances and first results of fission
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