Influence of corium oxidation on fission product release from molten pool

Nuclear Engineering and Design 240 (2010) 1229–1241

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Influence of corium oxidation on fission product release from molten pool S.V. Bechta a,∗ , E.V. Krushinov a , S.A. Vitol a , V.B. Khabensky a , S.Yu. Kotova a , A.A. Sulatsky a , V.V. Gusarov b , V.I. Almyashev b , G. Ducros c , C. Journeau c , D. Bottomley d , B. Clément e , L. Herranz f , S. Guentay g , K. Trambauer h , A. Auvinen i , V.V. Bezlepkin j a

Alexandrov Scientific-Research Institute of Technology (NITI), Sosnovy Bor, Russia Grebenschikov Institute of Silicate Chemistry of the Russian Academy of Sciences (ISC RAS), St. Petersburg, Russia CEA, DEN, Cadarache, F-13108 St. Paul lez Durance, France d Joint Research Centre Institut für Transurane (ITU), Karlsruhe, Germany e Institut de Radioprotection et Sûreté Nucléaire (IRSN), St. Paul lez Durance, France f CIEMAT, Madrid, Spain g PSI, Würenlingen, Switzerland h GRS, München, Germany i VTT, Espoo, Finland j SPbAEP, St. Petersburg, Russia b c

a r t i c l e

i n f o

Article history: Received 4 September 2009 Received in revised form 24 December 2009 Accepted 13 January 2010

a b s t r a c t Qualitative and quantitative determination of the release of low-volatile fission products and core materials from molten oxidic corium was investigated in the EVAN project under the auspices of ISTC. The experiments carried out in a cold crucible with induction heating and RASPLAV test facility are described. The results are discussed in terms of reactor application; in particular, pool configuration, melt oxidation kinetics, critical influence of melt surface temperature and oxidation index on the fission product release rate, aerosol particle composition and size distribution. The relevance of measured high release of Sr from the molten pool for the reactor application is highlighted. Comparisons of the experimental data with those from the COLIMA CA-U3 test and the VERCORS tests, as well as with predictions from IVTANTHERMO and GEMINI/NUCLEA codes are made. Recommendations for further investigations are proposed following the major observations and discussions. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Fission products may be released in a late phase of a severe accident from molten corium pools. In three different locations in the plant the molten pool may be accumulated, (a) in the lower plenum of reactor pressure vessel, (b) on the concrete pit and (c) in the core catcher. In case of latter two locations the reactor lower head will have been breached. Current analytical tools can predict the release of fission products (FP) from a molten corium pool during the late phase of a severe accident with a large uncertainty regarding the released amounts. Difficulty stems from characterizing the complexity of the melt pool regarding its oxidation level, and its constituents and as well as describing thermal-hydraulic conditions governing the transport of the released fission products with sufficient confidence.

∗ Corresponding author at: NITI, Sosnovy Bor, Leningrad Oblast 188540, Russia. Tel.: +7 81369 60 675/23672; fax: +7 81369 60 675/23672. E-mail address: [email protected] (S.V. Bechta). 0029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2010.01.008

Nowadays, present codes, either Russian (i.e. RATEG/SVECHA/ GEFEST and SOKRAT) or Western (i.e. ASTEC-ELSA, MELCOR), contain mechanistic models for estimating the FP release from the molten pools, which are mostly based on mechanistic treatment of the release as a function of only melt composition and temperature. An alternative approach to the evaluation of FP release from the molten pool based on more phenomenological treatment is the coupled modeling of the pool thermal hydrodynamics, gas–aerosol flow above the pool and thermodynamic equilibrium between the melt surface and confined volume of gas adjacent to it. In both cases of model treatment high quality experimental data are indispensable for further development and verification of models. In addition, the data are required to assess severe accident system codes and reduction of the related uncertainty in the late phase FP release as well as gaining insight into the processes and phenomena affecting the radionuclide release from oxidized molten pools. A brief account of experimental investigations that have provided relevant data on the FP release from fuel and corium under different conditions is given below. Several separate effects experiments were previously performed (Lewis et al., 2008) to quantify the FP release from irradiated

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fuels. The recent out-of-pile VERCORS (CEA, France) and VEGA (JAEA, Japan) programs provided data on the FP release covering all phases of the release from intact to molten stage of the fuel. The tests used small samples of irradiated UO2 or MOX fuel at different burn-ups and heated-up in different mixtures of atmospheres composed of steam and hydrogen up to fuel melting. Before the tests, the samples were re-irradiated at low power in a material testing reactor (MTR) for a few days prior to the tests, to re-create the main short half-life FPs, and to extend the database. The objective of these experiments concerned more particularly the FP release from the solid fuel up to the melting state. The tests were generally stopped before or just after the melting and showed a decrease in the FP release rate when the fuel sample collapsed. FPRMP Project was conducted to quantify FP and core material release from molten corium (Benson et al., 1999). Experiments were performed in AEAT, England and in LSK, Russia to determine the release kinetics of the key FP and actinides from both metallic and oxidic melts. These tests involved FP simulants (FPS) and addressed the effect of temperature, oxygen potential, vaporization versus physical release and slag formation on the nature and magnitude of the release. A further set of experiments was conducted to examine the volatilization of UO2 as a function of fuel stoichiometry and temperature. The COLIMA CA-U3 (Journeau et al., 2007) test provided some information on the aerosol release over an in-vessel type corium pool in a reducing (Ar–2%H2 ) atmosphere. COLIMA CA-U3 corium load was made up of about 2 kg depleted uranium dioxide pellets and powders of other oxides (to represent oxidized cladding, steel and FPs). A corium melt temperature of 3033 K was reached and kept for more than 50 min. The aerosols collected in a thermal gradient tube, filters and impactors, were analyzed as well as the remaining corium pool. The observed aerosol release was compared with thermodynamic calculations made using GEMINI and NUCLEA (Bakardjieva et al., 2010), and ELSA codes (Godin-Jacqmin et al., 2006). The Phébus FPT-4 in-pile experiment (Bottomley et al., 2005) investigated the release of FPs and actinides from a 3.8 kg debris bed of irradiated fuel fragments and oxidized cladding shards including a molten pool phase. The materials were released in a steam/hydrogen flow (80/20% molar). The mentioned separate-effect experiments together with the integral Phebus-FP program demonstrated a strong influence of the atmosphere composition on the FP release from overheated fuel and highlighted a need for additional data in the conditions of air ingress into the system (Gauntt, 2007). The separate effect test program as presented in this paper was conducted for a better understanding of local phenomena influencing the volatilization of FPS and melt components and for the extension of a verified database on the high-temperature FP release from corium molten pool. Experimental objectives included a qualification and quantification of the influence of the melt temperature and oxidation index1 on the FPS release rate. Melt composition and temperature were chosen to represent a certain transitory condition of the top oxidic layer of an in-vessel melt before the vessel failure and corium relocation into the catcher, which is specific to certain reactor designs. As the melt oxidation index can vary substantially depending on the accident scenarios (e.g. large LOCA and station blackout), a broad range of melt oxidation index was adopted in the experimental program. The experimental program was designed with two step-tests. The first test had the methodological objectives: to determine the

1 Here the oxidation index indicates the extent of metallic Zr oxidation in the M −M initial corium: Cn = Zr,˙M Zr,met 100, % where MZr, and MZr, met are masses of Zr in Zr,˙

ZrO2 + Zr and of metallic Zr, respectively.

superheating of the melt of different oxidation degrees that could be achieved; to test the operation of the gas–aerosol system and performance of its components; to determine the optimal accidentfree flow-rate of the oxidizer and the possibilities for the on-line control of the melt oxidation rate. In this experiment the molten corium did not contain FPS. After the operability analysis of the whole system and a review of the experimental data on the release of melt components, an adjustment of the induction furnace and gas–aerosol system was made and the procedure of the main experiment was adjusted. EVAN-1FP test is the real test carried out with the addition of FPS to the molten pool to achieve a chemically prototypic corium. 2. Experimental setup and methodology Test EVAN-1FP determined the release of low-volatile FPs, uranium and zirconium from molten corium and was performed in the medium-scale experimental facility Rasplav-3 with a corium charge of ∼2 kg. It used the technique of induction melting in a cold crucible (IMCC, Petrov, 1983) in argon atmosphere during steady state regimes with corium oxidation indexes 70 ◦ C, 85, 100 and in argon–oxygen mixture during the transitions of melt oxidation regimes from 70 to 85 ◦ C and from 85 to 100 ◦ C. The sweeping gas flow method was employed to transport the released vapour from the evaporating melt. In this method the carrier gas passed above the melt surface with a constant flow-rate became saturated with the vapour of the studied substance. Along the transport path the volatilized substances were deposited as aerosol particles on the inner surfaces of components of the furnace, sampling system and analytical filters. The aerosols in each location were collected and analyzed with different methods leading to the determination of the amount of the fission products released from the molten pool. The schematic diagram of the furnace is presented in Fig. 1a. The carrier gas was fed into the lower part of the furnace, flows through the coaxial gap between the cold crucible (5) and the quartz vessel (4), then through the gaps between the cold crucible tubes above the melt, and passed out through an outlet in the lid. In order to reduce the transport losses of aerosols (i.e. their deposition on the furnace components) the molten pool (6) was produced in the highest position of the cold crucible (5); the quartz partition (3) separated the above-melt area from the rest of the furnace. For the same purpose the water-cooled pyrometer shaft designed for blowing off aerosol particles from the pyrometer sighting spot during temperature measurements of the melt surface was only briefly inserted into the furnace for quick temperature measurements and, if necessary, melt sampling through the shaft. For the rest of time the shaft was raised so that its tip was behind the upper conical lid surface (1). Structure of the gas flow in the furnace (Fig. 1b) was optimized in order to reduce aerosol depositions by the furnace design, which was based on direct numerical simulation of the gas flow pattern. Fig. 1b also presents the distribution of gas flow velocity and temperature in the furnace. Fig. 2 shows the gas supply diagram and aerosol collection system. An electromechanical vibrator (11) was mounted on the wall of the main line (12) to reduce aerosol deposition in it. For aerosol collection during experiments the filters F1 (7) and F2 (6) were used in parallel and consecutively replaced. To collect gaseous ruthenium oxides two spargers (8) were installed after the filters. They contained potassium persulfurate solution (0.1 M solution K2 S2 O8 ) and potassium hydroxide (1 M solution KOH). High-purity substances were used as FPS: powders of metallic Mo and Ru, La2 O3 and CeO2 , Ba and Sr metazirconates prepared from BaO and Sr(OH)2 ·8H2 O by sintering them with zirconium oxide. Experimental conditions are given in Table 1.

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Fig. 1. Induction furnace diagram (a), the gas velocity field and the temperature distribution in the furnace (b). 1: furnace lid; 2: pyrometer shaft; 3: quartz partition; 4: quartz vessel; 5: cold crucible; 6: corium melt; 7: inductor coil; 8: corium crust; and 9: bottom calorimeter.

Fig. 2. Schematic diagram of the gas–aerosol system. 1: gas canisters; 2: gas flow regulators; 3: gas mixing unit; 4: electrochemical oxygen sensors; 5: induction furnace; 6: F2-large exposition aerosol filter (replaceable); 7: F1-small exposition analytical filter (replaceable); 8: bubblers; 9: gas flow meter; 10: fan; 11: vibrator; 12: main line (electrochemically polished copper tube); and 13: three-way valve.

Table 1 EVAN-1FP experimental conditions. Parameter

Characteristics

Melt composition, mass%

UO2 : 72.64 ZrO2 : 19.38 Zr: 6.15 1.2 70–100 2808–2969 Ar or Ar + 20 vol.% ofO2 SrO: 0.14 BaO: 0.23 CeO2 : 0.45 La2 O3 : 0.20 Ru: 0.35 Mo: 0.46

Atomic ratio U/Zr Melt oxidation index, Cn (%) Temperature range of the melt pool surface, K Atmosphere FPS content, mass%

The experimental procedure included formation and homogenization of the molten pool produced from oxidic powders (UO2 , ZrO2 ), compact metallic Zr and FPS. This was followed by the pool being held at steady-state regimes with fixed temperature of the surface. During this regimes aerosol samples were taken. The regimes were changed either by temperature adjustment using the inductor voltage regulator or by changing melt oxidation index, that is, by blowing an argon–oxygen mixture over the melt surface. In this way the volatilization of melt components was established for three melt oxidation indexes at different temperatures (Table 2). In order to monitor the variations in the composition of the melt pool direct samples from the pool were taken and quenched. To monitor the gaseous RuO4 in the carrier gas liquid samples were taken from bubblers on a regular basis. The experiment was terminated at first by disconnecting the induction heating (i.e. inductor voltage), thereby allowing the melt to crystallize and later cooling the corium ingot in argon atmosphere.

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Table 2 Characteristics of temperature regimes and mass release rates. Regime

I II III IV V VI VII VIII a

Melt oxidation degree (Cn ), %

Above-melt atmosphere (carrier-gas)

Carrier-gas flow-rate (G), l/min

Ar

70

Ar/O2 (80:20) Ar

70–85

10

85 Ar/O2 (80:20) Ar

85–100 100

Temperature of the melt surface (T), K

Mass release rate (V)a , mg/cm2 h

2868 2808 – 2912 2969 2928 – 2925

249 137 – 154 533 373 – 334

Average rate for F1 filters taking transport losses into account.

Post-test studies of the ingot included the visual examination, photography, weighing of all solidified molten components, preparation of samples and physicochemical analyses. The samples included the molten pool samples taken during the test and aerosols collected from the furnace components and the F1 and F2 filters. The content of main components (U, Zr) in the corium ingot, melt samples and aerosols was determined by using the X-ray fluorescence (XRF) (Losev, 1966), and the content of FPs (Sr, Ba, Ce, La, Mo, Ru)—by mass-spectrometry with induction-coupled plasma (ICP MS) (Isaev, 1998). The degree of Zr oxidation in the molten pool samples taken during the experiment was determined by direct volumetry of free Zr (Asmolov et al., 2000). The U4+ and U6+ contents of the frozen corium ingot and aerosol samples were determined by the photometry with arsenazo-III dye (Goriushina et al., 1961; Ryabchikov and Senyavin, 1962). Errors in the release rates of FP and melt components were largely determined by the errors in the analytical methods applied for the sample composition. The relative error in determining the content of main components is less than 5%, and for all FPs 10–50%. The error in determining the zirconium oxidation degree is 10% relative. The integral error of the FPs, uranium and zirconium release rates determined from the filter deposits in the most unfavourable conditions (i.e. at short filter exposures) is not more than 11% (Bechta et al., 2007). The instrument error in the surface temperature measurement of melt, which is used to determine the release rates, is 30 K.

Fig. 3. Aerosol deposition masses collected in different locations.

3. Experimental results Table 2 summarizes the measurements of release rate. Note that in spite of all technical measures taken to reduce the mass of aerosols deposited on the furnace components and in the lines before the filters, aerosol mass transport losses were quite substantial–approximately 60% relative (Fig. 3). In view of this the transport losses were included into the calculation of release rate for certain regimes. The integral transport losses during the whole test were distributed between regimes on the assumption that the aerosol deposition rate is proportional to the aerosol mass concentration in the flow. The methodology of calculations is described in detail in Bechta et al. (2007). It also should be noted that Table 2 does not give release rates in the transient regimes, when the melt was under oxidation, because they are difficult to interpret, as the

Table 3 Aerosol masses and compositions determined by the XR/ICP-MS and corresponding FPS release rates in different test regimes. Element

Composition of melt and aerosols collected during regimes I–VII, mass% Melt

U Zr Sr Ba La Ce Ru Mo

63.48 19.22 0.12 0.20 0.15 0.37 0.35 0.45

I

II 74.05 2.93 5.57 2.66 0.18 0.2 0.13 0.96

73.26 2.67 6.62 3.42 0.2 0.07 0.1 0.58

Exposition time and collected aerosol masses – Exp. time, ca – Mass, mgb

360 987.2

630 946.9

Element specific release rate, mg/(cm2 h) U – Zr – Sr – Ba – La – Ce – Ru – Mo –

185 7.3 13.9 6.63 0.46 0.49 0.32 2.38

100 3.65 9.05 4.67 0.27 0.09 0.13 0.79

a b

Total operation time of all F1 filters during the regime. Calculated mass of aerosols deposited on F1 filters, transport losses taken into account.

IV

V 83.81 1.09 0.45 0.75 0.09 0.18 0.12 0.87

727 1231.6 129 1.68 0.69 1.16 0.14 0.27 0.18 1.34

VI 84.53 1.01 0.32 0.55 0.11 0.1 0.19 0.89

320 1877 451 5.39 1.71 2.92 0.6 0.54 0.99 4.76

VIII 85 1 0.26 0.53 0.07 0.18 0.22 0.57

450 1847.6 317 3.73 0.97 1.96 0.25 0.65 0.83 2.11

79.32 0.77 0.1 0.14 0.04 0.12 0.09 7.45 420 1541.4 265 2.57 0.33 0.47 0.12 0.39 0.29 24.8

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Fig. 4. Location (left) and view (right) of metallic inclusion in corium ingot.

melt surface temperature was not measured during those regimes. It was likely to grow at the constant inductor voltage due to the heat of oxidation reactions deposited in the melt surface layer and changes of power in the melt resulting from evolution of composition and, consequently, of electric conductivity. It is difficult to quantify these phenomena. During oxidation regimes of the main experiment and pretests the melt oxidation rate was found to be unchanged with the decrease of free zirconium content in the melt; it was sensitive only to the gas flow-rate and gas dynamics. The oxidation kinetics of a melt having the crust-free surface is likely to be controlled by the supply of oxygen from gas mixture to the surface, but not by the transport of unoxidized melt components to the surface. Table 3 shows the data provided by the XRF and ICP-MS analyses of aerosol samples and element specific release rates calculated with consideration of transport losses for the stationary regimes. It can be seen from the table that in comparison with the melt, aerosols are enriched with uranium and depleted in zirconium. Strontium and barium concentration in aerosols reduces substantially as the melt oxidation index grows; ruthenium concentration is practically stable (taking into account its determination error of 10–50 relative % mentioned in Section 2); and molybdenum concentration grows substantially. It should be noted that the analysis of aliquots from bubblers has not shown a noticeable release of gaseous ruthenium tetroxide or any other (semi-)volatile form of ruthenium. The measured Ru concentrations in sparger solutions corresponded to the threshold of detectability (10 ppb). The macrostructure of corium ingot is important for understanding Ru and Mo behaviour during the experiment. A metallic inclusion of 5.11 g was found in the ingot bottom (approximately 5 mm from the lower edge) during the ingot post-test crushing. According to the XRF analysis it contained 88.8 mass% of Ru; 11.0% of Mo and 0.2% of Zr. Fig. 4 shows its view and location. Ru mass in metallic inclusion was 76% of the initial mass of ruthenium.

be higher than the oxide density: metallic inclusion had the density of 12.01 g/cm3 , and an average oxide ingot sample 8.65 g/cm3 . The small quantity of metallic melt indicates that the integral composition at the final test regime was close to the miscibility gap boundary. Thermodynamic modeling of final composition using GEMINI2/NUCLEA.v.07 1 does not show the second liquid (Fig. 5). The Ru-enriched phase (HCP A3(1), Fig. 5) forms only at 2550 K, i.e. after melt crystallization started. But if this was the case, the metallic phase would be distributed along the crystallization front, i.e. a single compact inclusion would not be found and metal would be located near to the shrink pore of the ingot (Fig. 4), where the last remaining liquid is crystallized. 4.2. Influence of corium temperature and oxidation degree on the release rates It is evident that temperature has a decisive influence on the release rate. Temperature plots were constructed for release rates of melt components and FPs (Fig. 6) using Arrhenius coordinates. Noticeable are close activation energies of Ba, Sr and La release,

4. Discussion of results 4.1. Morphology of the molten pool The metallic inclusion and its location indicate that the melt stratified into oxidic and metallic liquids; the two liquids existed at least at the final test regime when corium had the final oxidation index 100 ◦ C, after which it froze upon the test’s completion. The density of the metallic inclusion at room temperature was found to

Fig. 5. Thermodynamic modeling of final corium phase composition versus temperature.

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Fig. 6. Arrhenius plots of element specific release rates of U (a), Zr (b), Sr (c), Ba (d), Ce (e), La (f), Ru (g) and Mo (h).

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which in some cases can indicate similar latent heats of volatilization. Oxidation changed melt composition and thus it considerably increased corium liquidus temperatures, which are approximately 2723 and 2823 K for 70 and 100 ◦ C compositions, respectively. Due to this difference and a limited melt superheating in the IMCC conditions, same temperatures could not be reached at the different oxidation indexes of the melt.

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For determination of the influence of corium oxidation index on the release rates of fission products the experimental data (Fig. 6) were extrapolated to the temperature, at which the overall extrapolation would be minimal, i.e. to 2925 K. This temperature was then used in the thermodynamic modeling by GEMINI2 computer codes with NUCLEA.v.07 1 and IVTANTHERMO databases on the assumption that a local thermodynamic equilibrium was reached between the melt and a limited volume of gas passing above it.

Fig. 7. Experimental and calculated element specific release rates of U (a), Zr (b), Sr (c), Ba (d), La (e), Ce (f), Ru (g) and Mo (h) versus melt oxidation index at 2925 K.

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This volume of gas was evaluated using the data measured in the regimes with melt oxidation on the assumption that oxygen is fully absorbed by the melt only from a limited volume of the Ar/O2 mixture passing over it and which was in equilibrium with the melt. In the experiment the volume fraction of oxygen absorbed by the melt was about 40% of the total volume of oxygen fed into the furnace. For this reason the calculations modeled the equilibrium of melt with argon using 40% of its total volume flowing above the melt for each regime. Melt composition was specified on the assumption that the melt mass and its components concentration varied linearly from the initial charge to the final composition. The latter was determined by the average oxide ingot and inclusion samples. When applying the GEMINI2 code Ce and Mo were not included in the calculations due to their unavailability in the database. Experimental and calculated data on element specific release rates versus oxidation index at 2652 ◦ C are given in Fig. 7. It is evident from Fig. 7 that in the experiment, the release rates of most elements decrease, as the melt oxidation degree grows. For Sr and Ba this decrease is quite substantial, while the Ru release rate changes insignificantly. The Mo release rate is approximately constant at oxidation index from 70 to 85 ◦ C but then grows at further oxidation to 100 ◦ C. We see that Zr, Ce and La are involatile and tend to decrease with increasing oxidation index as the models predict. By contrast U release rate remains constant with increasing oxidation index which is against the models’ predictions. Minor change of Ru release versus corium oxidation index in the test conditions could be associated with incomplete system oxidation, lack of oxygen in the carrier gas and separation of metallic liquid phase, containing around 80% of the initial Ru inventory. This metallic liquid at the bottom of the corium pool (Fig. 4) could trap Ru since the melt oxidation was terminated after reaching C 100 index. The comparison of experimental and calculated data is useful since it gives a general idea on the performance of current predictive tools. Generally, the release rates of the majority of elements calculated by both codes and their measured release rates differ by several orders. This non-systematic discrepancy cannot be explained by the reported experimental errors (see the last paragraph of Section 2). There is no clear trend that one code is more conservative than the other. Consequently, extension of databases and validation of release models are urgent objectives for the future work. The following particular observations can be made synthesizing the comparison of experimental and calculated data. The GEMINI thermodynamic code coincides well with experimental data for Ba and U in the oxidation range from 70 to 85 ◦ C, which is especially important for uranium in view of the fact that it has the highest volatilization rates relative to the other elements. However for GEMINI modeling of the C 100 melt a large volume of UO3 is volatilized from the melt, but the experiment did not confirm such a high intensity of its release. Possibly there is inaccuracy in the UO3 data or there is a significant buffering effect of the other elements on the U oxide volatilization. This latter effect, in view of the large amounts of uranium, in comparison to the FPS seems unlikely. IVTANTHERMO results quantitatively agree with measured data only for zirconium for the whole range of oxidation indexes (Fig. 7b). IVTANTHERMO predicts U release approximated to the experimental one and to the GEMINI forecast for suboxidized corium but it significantly underestimates release for oxidized corium (Fig. 7a). This can be explained by the use of a simplified ‘ideal mixture’ model. The necessity of using a more complex model of the melt is confirmed by the experimentally observed liquid immiscibility, which is possible only with higher mixing enthalpies, in contrast to the ideal mixture, in which the mixing enthalpy is assumed to be zero. Note that in accordance with IVTANTHERMO at 100 ◦ C, uranium is mostly released as UO2 , which has a qual-

Fig. 8. Integral release of FPS in EVAN-1FP test.

itative agreement with experimental data, but diverges from the GEMINI2 results. IVTANTHERMO gives the Sr release higher by one order and 2 orders than experiment for C 70 and C 85 corium respectively, but very close to experimental results for C 100. For GEMINI2 it is the reverse situation in which it tends to slightly overestimate the experimental values in C 70 and C 85 corium, but is two orders lower than experimental for C 100. Therefore the experimental data have an intermediate position between Sr release values calculated by the two codes. The reasons of the listed differences for Sr and Ru release, which are important for calculation of radiological consequences, are likely to be associated with complex chemistry of these elements and problems of their modeling. Fig. 8 shows the integral releases in all regimes as percentage of the initial content of the elements in the melt. In the beginning of the experiment with 70 ◦ C corium, a considerable part of strontium was evaporated, which is actually unexpected. Strong Sr evaporation influenced its further release. Let us qualitatively compare the integral releases in all regimes of the EVAN-1FP test with FP’s release tests from solid fuels obtained from the VERCORS and VEGA programs. These programs have made possible to identify similar behaviour between FPs and to classify them into four categories of volatility: volatile, semivolatile, low volatile and non-volatile (Ducros et al., 2007). The degree of volatility depends on various physical parameters, such as temperature, oxygen potential, fuel burn-up and fuel nature (UO2 versus MOX fuels, as well as “rod geometry” versus “debris bed geometry”). The VERCORS HT1 test, for instance, can be considered as a good candidate for this comparison, since it was performed on an UO2 fuel sample at 50 GWd/t, including an 1 h plateau at 1773 K under steam atmosphere in order to totally oxidise the cladding, then an heat-up phase under mixed He/H2 flow up to the fuel liquefaction, reached at around 2900 K. From this comparison, we can observe the following tendencies: • The semi-volatile behaviour of Ba and Mo is still evidenced: around 15% of release in EVAN-1FP, 50% in VERCORS HT1. In addition, the effect of the corium oxidation degree on Ba and Mo release rate (increased for Ba with a sub-oxidised corium, the opposite for Mo) is consistent with the findings of the whole VERCORS program (higher Ba release in reducing conditions, higher release for Mo under oxidising conditions). • The low volatile behaviour of La, Ce and Ru is confirmed: around 2% of release in EVAN-1FP, 5–10% in VERCORS HT1. The higher release rate of Ce and La for sub-oxidised corium is consistent with what was observed on the VERCORS program. In addition, the formation of large precipitates, mainly composed of Ru and, in a lesser extent, of Mo was also identified in the VERCORS corium.

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Fig. 9. GEMINI2 post-test calculations of COLIMA CA-U3.

• The non-volatile behaviour of Zr is confirmed in the EVAN-1FP test as well as in the whole VERCORS program, with a very low released fraction, generally non-detectable and always below 1%. More contrasting is the behaviour of Sr, which exhibits an unexpectedly high release around 55% in EVAN-1FP, when this element is classically considered as low volatile from solid fuel (even between low and non-volatile category). This result, which could have a significant impact on the radiological effect in the long term, highlights the need to improve the physico-chemical properties of this element in the thermodynamic databases. High Sr release was also observed in COLIMA CA-U3 test and estimated during TMI2. A thermodynamic modeling of the release in COLIMA CA-U3 test (Journeau et al., 2007) has been made with GEMINI2 and the NUCLEA database (version 2005). Post-test calculations are reported in Fig. 9 for two cases considering that the pool was in contact with 5% of the gas flow (fraction estimated by CFD calculations before the test) or with all the gas flow, for equilibrium calculations at 3033 K. Most of the strontium has been released from the melt (contrary to the expected value of 10% (Baichi, 2001). Except for strontium, all the tendencies for the elements in the database (Ag represents Te, La represents all the lanthanides) are satisfactorily fitted. The effect of pool temperature uncertainties has been verified that it does not modify the calculation results qualitatively. For steel components (Fe, Cr), the high release rates seem to indicate that, assuming correct modeling, most of the gas flow was in thermodynamic equilibrium with the corium pool. Both EVAN and COLIMA tests indicate the same trend concerning strontium, so the thermodynamic modeling of this element will have to be carefully verified. The releases measured in the Phébus FPT-4 are low for ruthenium (1.8%), uranium (0.41%) and plutonium (0.31%) that is, consistent with EVAN-1FP results. The release of strontium is also low (1.4%). This is not inconsistent with EVAN-1FP results, which shows a decrease of strontium release with increase of melt oxidation. Indeed the FPT-4 molten pool was fully oxidised and the experiment was conducted in a mixture of steam with approximately 20 mol% of hydrogen. One should however be cautious in the comparison since in FPT-4 a part of the released vapours/particles was trapped in an upper vault of solid debris located above the molten pool.

(Cu K␣-radiation), equipped with the automatic system DifWin, Etalon-PTC. The diffractograms with the marked peaks representing different phases in % are shown in Fig. 10. F1 filters can be grouped in the following way: F1 filters # 9, 24, 35, 40, 62 with aerosols deposited during release in the inert atmosphere; one UO2 -based phase is identified; F1 filters # 21, 50, 56 with aerosols deposited during the melt oxidation, main identified phases are UO3 , U3 O8 , relatively small UO2 reflections are found too; F1 filter # 18, which also collected aerosols generated during the release in the melt oxidation regime; the identified phase–SrUO4 . The filter had a different colour of deposits (Fig. 10). Relative 0.2 shifts of diffractograms in relation to each other and to the reference diffractogram, are probably explained by the height of sections. A broad peak of constant intensity in all measurements at 36◦ , which is typical for organic compounds, is explained by the film material covering the filter surface. 4.4. Morphology and size distribution of aerosol particles Aerosol size distribution was measured both for aerosols collected by F1 filters and for those deposited on the cold surfaces and in the gas line. Aerosol depositions are characterized by a rather high density and good adhesion to the substrate materials—copper, quartz, and stainless steel. Let us consider aerosol morphology. Fig. 11 presents microphotographs of aerosols collected in similar conditions, during

4.3. Phase composition of aerosols The analysis of aerosols deposited on F1 filters (Fig. 10) was made by XRF analysis using the X-ray diffractometer DRON-3 M

Fig. 10. X-ray diffractograms of filters with deposited aerosols.

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Fig. 11. Examples of SEM and optical views of corium aerosol particles.

experiment (Bechta et al., 1999) in filters and different steps of impactor of the Rasplav-2 facility. Particles are visible that range from 100 to ∼5 ␮m size. Nevertheless they are heavily loaded and so the original size range of the arriving aerosol particles will lie below the lower limit of this range. SEM-images of cross section of aerosol layer deposited on the cold crucible sections in Pr1-EV1 test are presented in Fig. 12. Both Figs. 11 and 12 confirm that the shape of particles is close to the regular and rounded (i.e. aerosols are not rod or plate-like), this form enables the optical methods for determining particle sizes to be applied. The aerosol size distribution was measured by the method of small-angle light scattering using the laser particle analyzer Microsizer-201A manufactured by VA Instruments. Samples for analysis had a mixture of particles: small aerosol particles likely to result from agglomerates of initial clusters on which vapours condense; particles produced by the volumetric coagulation in gas flow, and particle-agglomerates as lumps and flakes. The ultrasonic dispersion of aerosol particles in the wetting carrier-liquid (Na4 P2 O7 solution) enabled most of the agglomerates to be broken up. In addition the size of secondary particles produced in the process of volumetric coagulation – rather stable particles – could be determined. It should be noted that effectiveness of ultrasonic dispersion depends on the nature of agglomerates, dispersion time and liquid. The last 2 factors were optimized during post-test study of corium aerosols (Benson et al., 1999). However some agglomerates (e.g. formed from sintered particles) could be very stable and their dispersion is not possible by this method. Apparently, sintered agglomerates quantity is low relative to the overall amount

of agglomerates as the lifetime of such particles at very high temperatures is small. It has been found that the character of the aerosol size distribution depends on the sampling location, melt temperature (Fig. 13), its composition (Fig. 14) and above-melt atmosphere (Fig. 15). As the facility did not have a long thermal gradient tube, the influence of distance between the sampling place and the melt could not be taken into account in determining aerosol particle size distribution. Generally the measured size distributions are bimodal or trimodal and there are a submicron range and a micronic one. A minor number of particles close to ∼1 ␮m is evident; this can be a consequence of agglomerates splitting since there are very low odds of forming micron-sized particles. It can be seen from Fig. 13 that melt temperature change does not produce any noticeable influence on the characteristic sizes of aerosol particles as determined by ultrasonic dispersion (although it is of limited sensitivity). Changes in the melt temperature influence only the ratio between the small particles and larger agglomerates with a shoulder of the larger (agglomerate) distribution appearing at the lower temperature. Thus a higher temperature seems to result in a certain increase of small particles and decrease of agglomerates. Fig. 14 shows that at melt oxidation from 70 to 100 ◦ C and temperature growth from 2808 to 2925 K the fraction of large particles (11–12.6 ␮m in diameter) has grown noticeably. Otherwise the higher oxidation and temperature particle distribution appears to be similar to that just observed at lower oxidation index and temperature.

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Fig. 12. Microphotographs of aerosols collected from crucible sections in Pr1-EV1.

Presence of oxygen in the above-melt atmosphere significantly changes the measured distribution pattern of released particles (Fig. 15). Aerosols contain a much larger number of small particles (sizes below 1 ␮m), and the content of other particles (with modes at ∼1 and ∼3 ␮m, as a remnant of the previous large agglomerate peak) is very small. A final small mode is seen at ∼12 ␮m as seen from the C-100 distribution in Fig. 14. In the COLIMA CA-U3 test, an 8-stage impactor had been installed at the outlet of a pressure gradient tube and collected the aerosols at the plateau temperature of 2760 ◦ C, for an oxidic melt pool in an (Ar–2%H2 ) reducing atmosphere. Fig. 16 presents the repartition of the collected aerosols. Even though the num-

ber of stages is lower, a bimodal repartition is also observed, with peaks around 2–3 and 4.7–9 ␮m. The larger diameters correspond to the bigger particle peaks observed in the EVAN test. Further investigations are necessary to explain the discrepancies at lower diameters. Uranium was found to be more present in larger particles, while barium is more centered around 3 ␮m, iron around 2 ␮m and tellurium is more abundant in the smallest particles. In the VERCORS HT1 experiment, the aerosols emitted during the last minutes of the test, just after melting of the fuel, were trapped in an impactor composed of 6 stages and 2 bead bed filters. Mo, Te, I, Cs and Ba were mainly measured in the last stage and

Fig. 13. Aerosol particle size distribution at the same melt oxidation degree (C 70) and different temperatures.

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Fig. 14. Aerosol particle size distribution at different melt oxidation degrees.

Fig. 15. Aerosol particle size distribution at different above-melt atmospheres.

the two bead beds, indicating fine particles with an aerodynamic diameter of around 1 ␮m and below. Qualitative character of aerosol distributions measured by optical and impaction method and their comparison should be stressed, as the both applied methods have some uncertainties. 4.5. Melt oxidation rate An important result of completed experiments is nearly constant oxygen absorption rate by the melt versus the concentration of free zirconium in the corium. The magnitude of melt oxidation rate was 15.0 (±0.8) mg/s.

The constant rate of melt oxidation in the stable hydrodynamic conditions means that the rate-determining process is the transport and oxygen supply to the melt surface, not the Zr-free supply to the surface and not the kinetics of chemical reaction itself. It was also determined that melt oxidation does not stop after the complete oxidation of all available Zr. As the melt oxidation rate practically is the same after the corium reaches C 100. This can be explained by the further oxidation of uranium dioxide. This gives further evidence that oxygen absorption by the melt is limited by the availability of oxidant on the pool surface. The test results have not confirmed that temperature changes of the pool surface (in the limited range studied) influence the oxygen absorption rate. 5. Conclusions

Fig. 16. Relative masses deposited in the 8 stages of the impactor in CA-U3.

The produced quantitative data confirm the critical influence of melt temperature and oxidation state on the release rates of uranium and zirconium oxides and such fission products as Sr, Ba, La, Ce oxides, Ru and Mo. High release of Sr should be stressed in comparison with some previous tests and calculations. It was much higher than anticipated with evaporation of more than one half of the initial inventory. This effect confirmed earlier observations in the FPRMP and COLIMA CAU3 tests, as well as in the TMI accident, and could be important for long-term radiological consequences and should be studied more systematically.

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The growth of melt oxidation index from 70 to 100 ◦ C results in a considerable reduction of strontium and barium release rates, approximately 20 and 10 times, respectively, whereas the release rate of molybdenum grows approximately 3 times. The total aerosol release rate drops approximately 5 times. Ruthenium release rate is insensitive to the corium oxidation degree in the studied range of oxidation index C. In the reported experimental conditions the formation of a volatile ruthenium tetroxide has not been detected because melt oxidation was insufficient for formation of this or any other volatile ruthenium oxide in noticeable quantity. Practically all Ru remained in a second metallic immiscible liquid found in the bottom of molten pool. From analysis of the total inventory data one concludes that melt oxidation to a stoichiometric composition is a basically positive phenomenon, which makes retention of the main FPs in the melt (Sr, Ba, La, and Ce) more efficient. The experimental data confirmed the presence of a liquid miscibility gap for C 100 corium melt having prototypic content of ruthenium. Practically the whole volume of ruthenium and a substantial part of molybdenum concentrate in the metallic liquid phase, which was denser and thus relocated to the pool bottom. This phenomenon can also reduce the release rate of these components if the density of metallic melt is higher than that of oxidic melt. Such a configuration of a molten pool can be formed during corium in-vessel retention, if the corium oxidation index and steel/corium mass ratio is low. It is also necessary to note that the thermodynamic modeling, which indicates the absence of a 2nd liquid, confirms the experimentally observed change of molybdenum release rate and insignificant ruthenium release to the gas phase under the test conditions. Qualitative analysis of the aerosol particle size distribution has shown that it is generally bimodal, with a small aerosol particle population having a characteristic size of <1 ␮m, i.e. they are non-settling particles. A noticeable difference was found in the measured size distribution of aerosol particles formed in an inert (Ar) atmosphere and during an oxygen supply to the melt surface. It should also be noted that the aerosol particle has a capability to form dense deposits with a good adhesion to cold surfaces and with substantial thermal and diffusion resistance. These can, in principle, influence the efficiency of containment emergency systems, e.g. heat exchangers of passive heat removal systems, catalytic hydrogen recombiners, etc. The presented experimental data can be used for extension of the databases for FP release from molten corium and for the further development and validation of corresponding numerical models.

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Acknowledgements This work has been supported by EU and coordinated by International Science and Technology Centre (Project # 3345 EVAN). References Asmolov, V.G., Strizhov, V.F., Degaltsev, Yu, G., 2000. RASPLAV Final Report, Attachment A, Post-test Examinations Methodology and Results. Baichi, M., 2001. Contribution à l’étude du corium d’un réacteur nucléaire accidenté: Aspects puissance résiduelle et thermodynamique des systèmes U–UO2 et UO2 –ZrO2 . PhD Thesis, INPG, Grenoble. Bakardjieva, S., Barrachin, M., Bechta, S., Bottomley, D., Brissonneau, L., Cheynet, B., Fischer, E., Journeau, C., Kiselova, M., Mezentseva, L., Piluso, P., Wiss, T., 2010. Improvement of the European thermodynamic database NUCLEA. Progr. Nucl. Energ. 52, 84–96. Bechta, S.V., Khabensky, V.B., Krushinov, E.V., Vitol, S.A., 1999. Investigation of Fission Product Release from Molten Corium on the late phase of a severe accident at NPP with VVER. Project report, Essentials of the unique Russian experimental facilities, NITI, p. 75 (in Russian). Bechta, S.V., Khabensky, V.B., Vitol, S.A., et al., 2007. Ex-vessel Source Term Analysis. Experiment EVAN. Progress Report. ISTC Project, p. 44. Benson, C.G., Bechta, S., Bowsher, B.R., et al., 1999. Fission Product Release from Molten Pools: Final Report. AEA Technology, AEAT-5893. Bottomley, P.D.W., Carbol, P., Glatz, J.P., Knoche, D., Papaioannou, D., Solatie, D., Van Winckel, S., Grégoire, A.C., Grégoire, G., Jacquemain, D., 2005. Fission products and actinide release from the Debris bed Phébus FPT4: synthesis of the posttest analyses and the revaporisation testing of the plenum samples performed at ITU. In: International Congress on Advanced Power Plants (ICAPP-05), Seoul, Korea, May 15–19. Ducros, G., Pontillon, Y., Malgouyres, P.P., 2007. An overview of the VERCORS experimental programme. In: International VERCORS seminar, Gréoux les bains, France, October 15–16. Gauntt, R.O., 2007. In: Synthesis of VERCORS Data in Severe Accident Codes and Application to USNRC Regulatory Practices, Presentation at International VERCORS seminar, Gréoux les bains, France, October 15–16, p. 29. Godin-Jacqmin, L., Journeau, C., Piluso, P., 2006. Analysis of the COLIMA CA-U3 Test with the ELSA Module of ASTEC. Nuclear Energy New Europe, Portoroˇz, Slovenia. Goriushina, V.G., Romanova, E.V., Archakova, T.A., 1961. Colorimetric methods for determining zirconium in melts. Ind. Lab. 27 (7), 795–797 (in Russian). Isaev, L.K., 1998. Control of Chemical and Biological Parameters of the Environment. Ecologo-analytical information center “Soyuz”, SPb (in Russian). Journeau, C., Piluso, P., Correggio, P., Godin-Jacqmin, L., 2007. The PLINIUS/COLIMA CA-U3 Test on Fission Product Release over a VVER-type Corium Pool. CEA Report, CEA-R-6160. 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. Losev, N.F., 1966. Quantitative X-ray Fluorescence Analysis. Nauka, M., p. 366 (in Russian). Petrov, Yu.B., 1983. Induction Melting of Oxides. Energoatomizdat, Leningrad (in Russian). Ryabchikov, D.I., Senyavin, M.M., 1962. Analytical Chemistry of Uranium. The USSR ASc, M. (in Russian).