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Nuclear Engineering and Design 240 (2010) 1229–1241

Contents lists available at ScienceDirect

Nuclear Engineering and Design

journa l homepage: www.e lsev ier .com/ locate /nucengdes

nfluence of corium oxidation on fission product release from molten pool

.V. Bechtaa,∗, E.V. Krushinova, S.A. Vitola, V.B. Khabenskya, S.Yu. Kotovaa, A.A. Sulatskya,.V. Gusarovb, V.I. Almyashevb, G. Ducrosc, C. Journeauc, D. Bottomleyd, B. Clémente,. Herranzf, S. Guentayg, K. Trambauerh, A. Auvineni, V.V. Bezlepkin j

Alexandrov Scientific-Research Institute of Technology (NITI), Sosnovy Bor, RussiaGrebenschikov Institute of Silicate Chemistry of the Russian Academy of Sciences (ISC RAS), St. Petersburg, RussiaCEA, DEN, Cadarache, F-13108 St. Paul lez Durance, FranceJoint Research Centre Institut für Transurane (ITU), Karlsruhe, GermanyInstitut de Radioprotection et Sûreté Nucléaire (IRSN), St. Paul lez Durance, FranceCIEMAT, Madrid, SpainPSI, Würenlingen, SwitzerlandGRS, München, GermanyVTT, Espoo, FinlandSPbAEP, St. Petersburg, Russia

r t i c l e i n f o

rticle history:eceived 4 September 2009eceived in revised form4 December 2009

a b s t r a c t

Qualitative and quantitative determination of the release of low-volatile fission products and core mate-rials from molten oxidic corium was investigated in the EVAN project under the auspices of ISTC. Theexperiments 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 oxida-

ccepted 13 January 2010tion kinetics, critical influence of melt surface temperature and oxidation index on the fission productrelease rate, aerosol particle composition and size distribution. The relevance of measured high releaseof Sr from the molten pool for the reactor application is highlighted. Comparisons of the experimentaldata with those from the COLIMA CA-U3 test and the VERCORS tests, as well as with predictions fromIVTANTHERMO and GEMINI/NUCLEA codes are made. Recommendations for further investigations are

ajor

proposed following the m

. Introduction

Fission products may be released in a late phase of a severe acci-ent from molten corium pools. In three different locations in thelant the molten pool may be accumulated, (a) in the lower plenumf reactor pressure vessel, (b) on the concrete pit and (c) in theore catcher. In case of latter two locations the reactor lower headill have been breached. Current analytical tools can predict the

elease of fission products (FP) from a molten corium pool dur-ng the late phase of a severe accident with a large uncertaintyegarding the released amounts. Difficulty stems from character-zing the complexity of the melt pool regarding its oxidation level,

nd its constituents and as well as describing thermal-hydrauliconditions governing the transport of the released fission productsith sufficient confidence.

∗ Corresponding author at: NITI, Sosnovy Bor, Leningrad Oblast 188540, Russia.el.: +7 81369 60 675/23672; fax: +7 81369 60 675/23672.

E-mail address: bechta@sbor.spb.su (S.V. Bechta).

029-5493/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2010.01.008

observations and discussions.© 2010 Elsevier B.V. All rights reserved.

Nowadays, present codes, either Russian (i.e. RATEG/SVECHA/GEFEST and SOKRAT) or Western (i.e. ASTEC-ELSA, MELCOR), con-tain mechanistic models for estimating the FP release from themolten pools, which are mostly based on mechanistic treatmentof the release as a function of only melt composition and temper-ature. An alternative approach to the evaluation of FP release fromthe molten pool based on more phenomenological treatment is thecoupled modeling of the pool thermal hydrodynamics, gas–aerosolflow above the pool and thermodynamic equilibrium between themelt surface and confined volume of gas adjacent to it.

In both cases of model treatment high quality experimentaldata are indispensable for further development and verification ofmodels. In addition, the data are required to assess severe acci-dent system codes and reduction of the related uncertainty in thelate phase FP release as well as gaining insight into the processesand phenomena affecting the radionuclide release from oxidized

molten pools. A brief account of experimental investigations thathave provided relevant data on the FP release from fuel and coriumunder different conditions is given below.

Several separate effects experiments were previously per-formed (Lewis et al., 2008) to quantify the FP release from irradiated

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uels. The recent out-of-pile VERCORS (CEA, France) and VEGAJAEA, Japan) programs provided data on the FP release coveringll phases of the release from intact to molten stage of the fuel. Theests used small samples of irradiated UO2 or MOX fuel at differenturn-ups and heated-up in different mixtures of atmospheres com-osed of steam and hydrogen up to fuel melting. Before the tests,he samples were re-irradiated at low power in a material test-ng reactor (MTR) for a few days prior to the tests, to re-create the

ain short half-life FPs, and to extend the database. The objectivef these experiments concerned more particularly the FP releaserom the solid fuel up to the melting state. The tests were generallytopped before or just after the melting and showed a decrease inhe FP release rate when the fuel sample collapsed.

FPRMP Project was conducted to quantify FP and core mate-ial release from molten corium (Benson et al., 1999). Experimentsere performed in AEAT, England and in LSK, Russia to determine

he release kinetics of the key FP and actinides from both metal-ic and oxidic melts. These tests involved FP simulants (FPS) andddressed the effect of temperature, oxygen potential, vaporiza-ion versus physical release and slag formation on the nature and

agnitude of the release. A further set of experiments was con-ucted to examine the volatilization of UO2 as a function of fueltoichiometry and temperature.

The COLIMA CA-U3 (Journeau et al., 2007) test provided somenformation on the aerosol release over an in-vessel type coriumool in a reducing (Ar–2%H2) atmosphere. COLIMA CA-U3 corium

oad was made up of about 2 kg depleted uranium dioxide pel-ets and powders of other oxides (to represent oxidized cladding,teel and FPs). A corium melt temperature of 3033 K was reachednd kept for more than 50 min. The aerosols collected in a ther-al gradient tube, filters and impactors, were analyzed as well as

he remaining corium pool. The observed aerosol release was com-ared with thermodynamic calculations made using GEMINI andUCLEA (Bakardjieva et al., 2010), and ELSA codes (Godin-Jacqmint al., 2006).

The Phébus FPT-4 in-pile experiment (Bottomley et al., 2005)nvestigated the release of FPs and actinides from a 3.8 kg debrised of irradiated fuel fragments and oxidized cladding shards

ncluding a molten pool phase. The materials were released in ateam/hydrogen flow (80/20% molar).

The mentioned separate-effect experiments together with thentegral Phebus-FP program demonstrated a strong influence of thetmosphere composition on the FP release from overheated fuelnd highlighted a need for additional data in the conditions of airngress into the system (Gauntt, 2007).

The separate effect test program as presented in this paper wasonducted for a better understanding of local phenomena influ-ncing the volatilization of FPS and melt components and for thextension of a verified database on the high-temperature FP releaserom corium molten pool. Experimental objectives included a qual-fication and quantification of the influence of the melt temperaturend oxidation index1 on the FPS release rate. Melt composition andemperature were chosen to represent a certain transitory condi-ion of the top oxidic layer of an in-vessel melt before the vesselailure and corium relocation into the catcher, which is specifico certain reactor designs. As the melt oxidation index can varyubstantially depending on the accident scenarios (e.g. large LOCA

nd station blackout), a broad range of melt oxidation index wasdopted in the experimental program.

The experimental program was designed with two step-tests.he first test had the methodological objectives: to determine the

1 Here the oxidation index indicates the extent of metallic Zr oxidation in thenitial corium: Cn = MZr,˙−MZr,met

MZr,˙100, % where MZr,� and MZr, met are masses of Zr in

rO2 + Zr and of metallic Zr, respectively.

nd Design 240 (2010) 1229–1241

superheating of the melt of different oxidation degrees that couldbe achieved; to test the operation of the gas–aerosol system andperformance of its components; to determine the optimal accident-free flow-rate of the oxidizer and the possibilities for the on-linecontrol of the melt oxidation rate. In this experiment the moltencorium did not contain FPS. After the operability analysis of thewhole system and a review of the experimental data on the releaseof melt components, an adjustment of the induction furnace andgas–aerosol system was made and the procedure of the main exper-iment was adjusted. EVAN-1FP test is the real test carried out withthe addition of FPS to the molten pool to achieve a chemically pro-totypic corium.

2. Experimental setup and methodology

Test EVAN-1FP determined the release of low-volatile FPs, ura-nium and zirconium from molten corium and was performed inthe medium-scale experimental facility Rasplav-3 with a coriumcharge of ∼2 kg. It used the technique of induction melting in a coldcrucible (IMCC, Petrov, 1983) in argon atmosphere during steadystate regimes with corium oxidation indexes 70 ◦C, 85, 100 andin argon–oxygen mixture during the transitions of melt oxidationregimes from 70 to 85 ◦C and from 85 to 100 ◦C.

The sweeping gas flow method was employed to transport thereleased vapour from the evaporating melt. In this method the car-rier gas passed above the melt surface with a constant flow-ratebecame saturated with the vapour of the studied substance. Alongthe transport path the volatilized substances were deposited asaerosol particles on the inner surfaces of components of the furnace,sampling system and analytical filters. The aerosols in each locationwere collected and analyzed with different methods leading to thedetermination of the amount of the fission products released fromthe 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, flowsthrough the coaxial gap between the cold crucible (5) and thequartz vessel (4), then through the gaps between the cold cru-cible tubes above the melt, and passed out through an outlet inthe 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); thequartz partition (3) separated the above-melt area from the restof the furnace. For the same purpose the water-cooled pyrometershaft designed for blowing off aerosol particles from the pyrom-eter sighting spot during temperature measurements of the meltsurface was only briefly inserted into the furnace for quick tem-perature measurements and, if necessary, melt sampling throughthe shaft. For the rest of time the shaft was raised so that its tipwas behind the upper conical lid surface (1). Structure of the gasflow in the furnace (Fig. 1b) was optimized in order to reduceaerosol depositions by the furnace design, which was based ondirect numerical simulation of the gas flow pattern. Fig. 1b alsopresents the distribution of gas flow velocity and temperature in thefurnace.

Fig. 2 shows the gas supply diagram and aerosol collection sys-tem. An electromechanical vibrator (11) was mounted on the wallof the main line (12) to reduce aerosol deposition in it. For aerosolcollection during experiments the filters F1 (7) and F2 (6) were usedin parallel and consecutively replaced. To collect gaseous ruthe-nium oxides two spargers (8) were installed after the filters. Theycontained potassium persulfurate solution (0.1 M solution K2S2O8)

and potassium hydroxide (1 M solution KOH).

High-purity substances were used as FPS: powders of metallicMo and Ru, La2O3 and CeO2, Ba and Sr metazirconates preparedfrom BaO and Sr(OH)2·8H2O by sintering them with zirconiumoxide. Experimental conditions are given in Table 1.

S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241 1231

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 regula6: F2-large exposition aerosol filter (replaceable); 7: F1-small exposition analytical filter(electrochemically polished copper tube); and 13: three-way valve.

Table 1EVAN-1FP experimental conditions.

Parameter Characteristics

Melt composition, mass% UO2: 72.64ZrO2: 19.38Zr: 6.15

Atomic ratio U/Zr 1.2Melt oxidation index, Cn (%) 70–100Temperature range of the melt pool surface, K 2808–2969Atmosphere Ar or Ar + 20 vol.% ofO2

FPS content, mass% SrO: 0.14BaO: 0.23CeO2: 0.45La2O3: 0.20Ru: 0.35Mo: 0.46

tors; 3: gas mixing unit; 4: electrochemical oxygen sensors; 5: induction furnace;(replaceable); 8: bubblers; 9: gas flow meter; 10: fan; 11: vibrator; 12: main line

The experimental procedure included formation and homoge-nization of the molten pool produced from oxidic powders (UO2,ZrO2), compact metallic Zr and FPS. This was followed by thepool being held at steady-state regimes with fixed temperature ofthe surface. During this regimes aerosol samples were taken. Theregimes were changed either by temperature adjustment using theinductor voltage regulator or by changing melt oxidation index, thatis, by blowing an argon–oxygen mixture over the melt surface. Inthis way the volatilization of melt components was established forthree melt oxidation indexes at different temperatures (Table 2). Inorder to monitor the variations in the composition of the melt pooldirect samples from the pool were taken and quenched. To moni-

tor the gaseous RuO4 in the carrier gas liquid samples were takenfrom bubblers on a regular basis. The experiment was terminatedat first by disconnecting the induction heating (i.e. inductor volt-age), thereby allowing the melt to crystallize and later cooling thecorium ingot in argon atmosphere.

1232 S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241

Table 2Characteristics of temperature regimes and mass release rates.

Regime Melt oxidationdegree (Cn), %

Above-melt atmosphere(carrier-gas)

Carrier-gasflow-rate (G),l/min

Temperature of themelt surface (T), K

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

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a Average rate for F1 filters taking transport losses into account.

Post-test studies of the ingot included the visual examina-ion, photography, weighing of all solidified molten components,reparation of samples and physicochemical analyses. The sam-les included the molten pool samples taken during the test anderosols collected from the furnace components and the F1 and F2lters.

The content of main components (U, Zr) in the corium ingot,elt samples and aerosols was determined by using the X-ray flu-

rescence (XRF) (Losev, 1966), and the content of FPs (Sr, Ba, Ce,a, Mo, Ru)—by mass-spectrometry with induction-coupled plasmaICP MS) (Isaev, 1998). The degree of Zr oxidation in the molten poolamples taken during the experiment was determined by directolumetry of free Zr (Asmolov et al., 2000). The U4+ and U6+ contentsf the frozen corium ingot and aerosol samples were determinedy the photometry with arsenazo-III dye (Goriushina et al., 1961;yabchikov and Senyavin, 1962).

Errors in the release rates of FP and melt components wereargely determined by the errors in the analytical methods appliedor the sample composition. The relative error in determining theontent of main components is less than 5%, and for all FPs 10–50%.he error in determining the zirconium oxidation degree is 10% rel-tive. The integral error of the FPs, uranium and zirconium release

ates determined from the filter deposits in the most unfavourableonditions (i.e. at short filter exposures) is not more than 11%Bechta et al., 2007). The instrument error in the surface tempera-ure measurement of melt, which is used to determine the releaseates, is 30 K.

able 3erosol masses and compositions determined by the XR/ICP-MS and corresponding FPS r

Element Composition of melt and aerosols collected during regime

Melt I II

U 63.48 74.05 73.26Zr 19.22 2.93 2.67Sr 0.12 5.57 6.62Ba 0.20 2.66 3.42La 0.15 0.18 0.2Ce 0.37 0.2 0.07Ru 0.35 0.13 0.1Mo 0.45 0.96 0.58

Exposition time and collected aerosol massesExp. time, ca – 360 630Mass, mgb – 987.2 946.9

Element specific release rate, mg/(cm2 h)U – 185 100Zr – 7.3 3.65Sr – 13.9 9.05Ba – 6.63 4.67La – 0.46 0.27Ce – 0.49 0.09Ru – 0.32 0.13Mo – 2.38 0.79

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

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

3. Experimental results

Table 2 summarizes the measurements of release rate. Notethat in spite of all technical measures taken to reduce the massof aerosols deposited on the furnace components and in thelines before the filters, aerosol mass transport losses were quitesubstantial–approximately 60% relative (Fig. 3). In view of this thetransport losses were included into the calculation of release ratefor certain regimes. The integral transport losses during the wholetest were distributed between regimes on the assumption that the

aerosol deposition rate is proportional to the aerosol mass concen-tration in the flow. The methodology of calculations is described indetail in Bechta et al. (2007). It also should be noted that Table 2does not give release rates in the transient regimes, when the meltwas under oxidation, because they are difficult to interpret, as the

elease rates in different test regimes.

s I–VII, mass%

IV V VI VIII

83.81 84.53 85 79.321.09 1.01 1 0.770.45 0.32 0.26 0.10.75 0.55 0.53 0.140.09 0.11 0.07 0.040.18 0.1 0.18 0.120.12 0.19 0.22 0.090.87 0.89 0.57 7.45

727 320 450 4201231.6 1877 1847.6 1541.4

129 451 317 2651.68 5.39 3.73 2.570.69 1.71 0.97 0.331.16 2.92 1.96 0.470.14 0.6 0.25 0.120.27 0.54 0.65 0.390.18 0.99 0.83 0.291.34 4.76 2.11 24.8

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S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241 1233

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It is evident that temperature has a decisive influence on therelease rate. Temperature plots were constructed for release ratesof melt components and FPs (Fig. 6) using Arrhenius coordinates.Noticeable are close activation energies of Ba, Sr and La release,

Fig. 4. Location (left) and view (rig

elt surface temperature was not measured during those regimes.t was likely to grow at the constant inductor voltage due to theeat of oxidation reactions deposited in the melt surface layer andhanges of power in the melt resulting from evolution of compo-ition and, consequently, of electric conductivity. It is difficult touantify these phenomena.

During oxidation regimes of the main experiment and pre-ests the melt oxidation rate was found to be unchanged with theecrease of free zirconium content in the melt; it was sensitive onlyo the gas flow-rate and gas dynamics. The oxidation kinetics of a

elt having the crust-free surface is likely to be controlled by theupply of oxygen from gas mixture to the surface, but not by theransport of unoxidized melt components to the surface.

Table 3 shows the data provided by the XRF and ICP-MS analy-es of aerosol samples and element specific release rates calculatedith consideration of transport losses for the stationary regimes.

t can be seen from the table that in comparison with the melt,erosols are enriched with uranium and depleted in zirconium.trontium and barium concentration in aerosols reduces substan-ially as the melt oxidation index grows; ruthenium concentrations practically stable (taking into account its determination error of0–50 relative % mentioned in Section 2); and molybdenum con-entration grows substantially. It should be noted that the analysisf aliquots from bubblers has not shown a noticeable release ofaseous ruthenium tetroxide or any other (semi-)volatile form ofuthenium. The measured Ru concentrations in sparger solutionsorresponded to the threshold of detectability (10 ppb).

The macrostructure of corium ingot is important for under-tanding Ru and Mo behaviour during the experiment. A metallicnclusion of 5.11 g was found in the ingot bottom (approximatelymm from the lower edge) during the ingot post-test crushing.ccording to the XRF analysis it contained 88.8 mass% of Ru; 11.0%f Mo and 0.2% of Zr. Fig. 4 shows its view and location. Ru mass inetallic inclusion was 76% of the initial mass of ruthenium.

. Discussion of results

.1. Morphology of the molten pool

The metallic inclusion and its location indicate that the melttratified into oxidic and metallic liquids; the two liquids existed ateast at the final test regime when corium had the final oxidationndex 100 ◦C, after which it froze upon the test’s completion. Theensity of the metallic inclusion at room temperature was found to

metallic inclusion in corium ingot.

be higher than the oxide density: metallic inclusion had the den-sity 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 com-position at the final test regime was close to the miscibility gapboundary.

Thermodynamic modeling of final composition using GEM-INI2/NUCLEA.v.07 1 does not show the second liquid (Fig. 5). TheRu-enriched phase (HCP A3(1), Fig. 5) forms only at 2550 K, i.e. aftermelt crystallization started. But if this was the case, the metallicphase would be distributed along the crystallization front, i.e. asingle compact inclusion would not be found and metal would belocated near to the shrink pore of the ingot (Fig. 4), where the lastremaining liquid is crystallized.

4.2. Influence of corium temperature and oxidation degree on therelease rates

Fig. 5. Thermodynamic modeling of final corium phase composition versus tem-perature.

1234 S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241

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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S.V. Bechta et al. / Nuclear Enginee

hich in some cases can indicate similar latent heats of volatiliza-ion.

Oxidation changed melt composition and thus it considerablyncreased corium liquidus temperatures, which are approximately

723 and 2823 K for 70 and 100 ◦C compositions, respectively. Dueo this difference and a limited melt superheating in the IMCC con-itions, same temperatures could not be reached at the differentxidation indexes of the melt.

Fig. 7. Experimental and calculated element specific release rates of U (a), Zr (b), Sr (c

nd Design 240 (2010) 1229–1241 1235

For determination of the influence of corium oxidation index onthe release rates of fission products the experimental data (Fig. 6)were extrapolated to the temperature, at which the overall extrap-olation would be minimal, i.e. to 2925 K. This temperature was then

used in the thermodynamic modeling by GEMINI2 computer codeswith NUCLEA.v.07 1 and IVTANTHERMO databases on the assump-tion that a local thermodynamic equilibrium was reached betweenthe melt and a limited volume of gas passing above it.

), 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 inhe regimes with melt oxidation on the assumption that oxygen isully absorbed by the melt only from a limited volume of the Ar/O2

ixture passing over it and which was in equilibrium with the melt.In the experiment the volume fraction of oxygen absorbed by

he melt was about 40% of the total volume of oxygen fed intohe furnace. For this reason the calculations modeled the equi-ibrium of melt with argon using 40% of its total volume flowingbove the melt for each regime. Melt composition was specified onhe assumption that the melt mass and its components concentra-ion varied linearly from the initial charge to the final composition.he latter was determined by the average oxide ingot and inclu-ion samples. When applying the GEMINI2 code Ce and Mo wereot included in the calculations due to their unavailability in theatabase. Experimental and calculated data on element specificelease rates versus oxidation index at 2652 ◦C are given in Fig. 7.

It is evident from Fig. 7 that in the experiment, the release ratesf most elements decrease, as the melt oxidation degree grows. Forr and Ba this decrease is quite substantial, while the Ru releaseate changes insignificantly. The Mo release rate is approximatelyonstant at oxidation index from 70 to 85 ◦C but then grows at fur-her oxidation to 100 ◦C. We see that Zr, Ce and La are involatilend tend to decrease with increasing oxidation index as the modelsredict. By contrast U release rate remains constant with increas-

ng oxidation index which is against the models’ predictions. Minorhange of Ru release versus corium oxidation index in the test con-itions could be associated with incomplete system oxidation, lackf oxygen in the carrier gas and separation of metallic liquid phase,ontaining around 80% of the initial Ru inventory. This metallic liq-id at the bottom of the corium pool (Fig. 4) could trap Ru since theelt oxidation was terminated after reaching C 100 index.The comparison of experimental and calculated data is useful

ince it gives a general idea on the performance of current predic-ive tools. Generally, the release rates of the majority of elementsalculated by both codes and their measured release rates dif-er by several orders. This non-systematic discrepancy cannot bexplained by the reported experimental errors (see the last para-raph of Section 2). There is no clear trend that one code is moreonservative than the other. Consequently, extension of databasesnd validation of release models are urgent objectives for the futureork.

The following particular observations can be made synthesizinghe comparison of experimental and calculated data. The GEMINIhermodynamic code coincides well with experimental data fora and U in the oxidation range from 70 to 85 ◦C, which is espe-ially important for uranium in view of the fact that it has theighest volatilization rates relative to the other elements. How-ver for GEMINI modeling of the C 100 melt a large volume of UO3s volatilized from the melt, but the experiment did not confirmuch a high intensity of its release. Possibly there is inaccuracy inhe UO3 data or there is a significant buffering effect of the otherlements on the U oxide volatilization. This latter effect, in viewf the large amounts of uranium, in comparison to the FPS seemsnlikely.

IVTANTHERMO results quantitatively agree with measured datanly for zirconium for the whole range of oxidation indexesFig. 7b). IVTANTHERMO predicts U release approximated to thexperimental one and to the GEMINI forecast for suboxidizedorium but it significantly underestimates release for oxidizedorium (Fig. 7a). This can be explained by the use of a simplifiedideal mixture’ model. The necessity of using a more complex model

f the melt is confirmed by the experimentally observed liquidmmiscibility, which is possible only with higher mixing enthalpies,n contrast to the ideal mixture, in which the mixing enthalpy isssumed to be zero. Note that in accordance with IVTANTHERMOt 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 theGEMINI2 results.

IVTANTHERMO gives the Sr release higher by one order and2 orders than experiment for C 70 and C 85 corium respectively,but very close to experimental results for C 100. For GEMINI2 itis the reverse situation in which it tends to slightly overestimatethe experimental values in C 70 and C 85 corium, but is two orderslower than experimental for C 100. Therefore the experimental datahave an intermediate position between Sr release values calculatedby the two codes.

The reasons of the listed differences for Sr and Ru release, whichare important for calculation of radiological consequences, arelikely to be associated with complex chemistry of these elementsand problems of their modeling.

Fig. 8 shows the integral releases in all regimes as percentageof the initial content of the elements in the melt. In the begin-ning of the experiment with 70 ◦C corium, a considerable part ofstrontium was evaporated, which is actually unexpected. Strong Srevaporation influenced its further release.

Let us qualitatively compare the integral releases in all regimesof the EVAN-1FP test with FP’s release tests from solid fuelsobtained from the VERCORS and VEGA programs. These programshave made possible to identify similar behaviour between FPs andto classify them into four categories of volatility: volatile, semi-volatile, low volatile and non-volatile (Ducros et al., 2007). Thedegree of volatility depends on various physical parameters, such astemperature, oxygen potential, fuel burn-up and fuel nature (UO2versus MOX fuels, as well as “rod geometry” versus “debris bedgeometry”). The VERCORS HT1 test, for instance, can be consideredas a good candidate for this comparison, since it was performed onan UO2 fuel sample at 50 GWd/t, including an 1 h plateau at 1773 Kunder steam atmosphere in order to totally oxidise the cladding,then an heat-up phase under mixed He/H2 flow up to the fuelliquefaction, 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. Inaddition, the effect of the corium oxidation degree on Ba andMo release rate (increased for Ba with a sub-oxidised corium, theopposite for Mo) is consistent with the findings of the whole VER-CORS program (higher Ba release in reducing conditions, higherrelease 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 higherrelease rate of Ce and La for sub-oxidised corium is consistentwith what was observed on the VERCORS program. In addition,the formation of large precipitates, mainly composed of Ru and, ina lesser extent, of Mo was also identified in the VERCORS corium.

S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241 1237

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4

m

in the gas line. Aerosol depositions are characterized by a ratherhigh density and good adhesion to the substrate materials—copper,quartz, and stainless steel.

Let us consider aerosol morphology. Fig. 11 presents micropho-tographs of aerosols collected in similar conditions, during

Fig. 9. GEMINI2 post-test

The non-volatile behaviour of Zr is confirmed in the EVAN-1FPtest as well as in the whole VERCORS program, with a very lowreleased fraction, generally non-detectable and always below 1%.

More contrasting is the behaviour of Sr, which exhibits annexpectedly high release around 55% in EVAN-1FP, when this ele-ent is classically considered as low volatile from solid fuel (even

etween low and non-volatile category). This result, which couldave a significant impact on the radiological effect in the long term,ighlights the need to improve the physico-chemical properties ofhis element in the thermodynamic databases. High Sr release waslso observed in COLIMA CA-U3 test and estimated during TMI2.

A thermodynamic modeling of the release in COLIMA CA-U3est (Journeau et al., 2007) has been made with GEMINI2 andhe NUCLEA database (version 2005). Post-test calculations areeported in Fig. 9 for two cases considering that the pool was in con-act with 5% of the gas flow (fraction estimated by CFD calculationsefore the test) or with all the gas flow, for equilibrium calculationst 3033 K. Most of the strontium has been released from the meltcontrary to the expected value of 10% (Baichi, 2001). Except fortrontium, all the tendencies for the elements in the database (Agepresents Te, La represents all the lanthanides) are satisfactorilytted. The effect of pool temperature uncertainties has been ver-

fied that it does not modify the calculation results qualitatively.or steel components (Fe, Cr), the high release rates seem to indi-ate that, assuming correct modeling, most of the gas flow was inhermodynamic equilibrium with the corium pool.

Both EVAN and COLIMA tests indicate the same trend concern-ng strontium, so the thermodynamic modeling of this element willave to be carefully verified.

The releases measured in the Phébus FPT-4 are low for ruthe-ium (1.8%), uranium (0.41%) and plutonium (0.31%) that is,onsistent with EVAN-1FP results. The release of strontium is alsoow (1.4%). This is not inconsistent with EVAN-1FP results, whichhows a decrease of strontium release with increase of melt oxi-ation. Indeed the FPT-4 molten pool was fully oxidised and thexperiment was conducted in a mixture of steam with approxi-ately 20 mol% of hydrogen. One should however be cautious in the

omparison since in FPT-4 a part of the released vapours/particlesas trapped in an upper vault of solid debris located above theolten pool.

.3. Phase composition of aerosols

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

lations of COLIMA CA-U3.

(Cu K�-radiation), equipped with the automatic system DifWin,Etalon-PTC. The diffractograms with the marked peaks represent-ing different phases in % are shown in Fig. 10. F1 filters can begrouped in the following way:

F1 filters # 9, 24, 35, 40, 62 with aerosols deposited during releasein the inert atmosphere; one UO2-based phase is identified;F1 filters # 21, 50, 56 with aerosols deposited during the meltoxidation, main identified phases are UO3, U3O8, relatively smallUO2 reflections are found too;F1 filter # 18, which also collected aerosols generated during therelease 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 andto the reference diffractogram, are probably explained by the heightof sections. A broad peak of constant intensity in all measurementsat 36◦, which is typical for organic compounds, is explained by thefilm material covering the filter surface.

4.4. Morphology and size distribution of aerosol particles

Aerosol size distribution was measured both for aerosols col-lected by F1 filters and for those deposited on the cold surfaces and

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

1238 S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241

tical v

eifsbotsad

oMatcfls(ucd

ducatt

Fig. 11. Examples of SEM and op

xperiment (Bechta et al., 1999) in filters and different steps ofmpactor of the Rasplav-2 facility. Particles are visible that rangerom 100 to ∼5 �m size. Nevertheless they are heavily loaded ando the original size range of the arriving aerosol particles will lieelow the lower limit of this range. SEM-images of cross sectionf aerosol layer deposited on the cold crucible sections in Pr1-EV1est are presented in Fig. 12. Both Figs. 11 and 12 confirm that thehape of particles is close to the regular and rounded (i.e. aerosolsre not rod or plate-like), this form enables the optical methods foretermining particle sizes to be applied.

The aerosol size distribution was measured by the methodf small-angle light scattering using the laser particle analyzericrosizer-201A manufactured by VA Instruments. Samples for

nalysis had a mixture of particles: small aerosol particles likelyo result from agglomerates of initial clusters on which vapoursondense; particles produced by the volumetric coagulation in gasow, and particle-agglomerates as lumps and flakes. The ultra-onic dispersion of aerosol particles in the wetting carrier-liquidNa4P2O7 solution) enabled most of the agglomerates to be brokenp. In addition the size of secondary particles produced in the pro-ess of volumetric coagulation – rather stable particles – could beetermined.

It should be noted that effectiveness of ultrasonic dispersionepends on the nature of agglomerates, dispersion time and liq-

id. The last 2 factors were optimized during post-test study oforium aerosols (Benson et al., 1999). However some agglomer-tes (e.g. formed from sintered particles) could be very stable andheir dispersion is not possible by this method. Apparently, sin-ered agglomerates quantity is low relative to the overall amount

iews of corium aerosol particles.

of agglomerates as the lifetime of such particles at very high tem-peratures is small.

It has been found that the character of the aerosol size distribu-tion depends on the sampling location, melt temperature (Fig. 13),its composition (Fig. 14) and above-melt atmosphere (Fig. 15). Asthe facility did not have a long thermal gradient tube, the influ-ence of distance between the sampling place and the melt couldnot be taken into account in determining aerosol particle sizedistribution.

Generally the measured size distributions are bimodal or tri-modal and there are a submicron range and a micronic one. A minornumber of particles close to ∼1 �m is evident; this can be a conse-quence of agglomerates splitting since there are very low odds offorming micron-sized particles.

It can be seen from Fig. 13 that melt temperature changedoes not produce any noticeable influence on the characteristicsizes of aerosol particles as determined by ultrasonic dispersion(although it is of limited sensitivity). Changes in the melt tempera-ture influence only the ratio between the small particles and largeragglomerates with a shoulder of the larger (agglomerate) distribu-tion appearing at the lower temperature. Thus a higher temperatureseems to result in a certain increase of small particles and decreaseof 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 par-ticles (11–12.6 �m in diameter) has grown noticeably. Otherwisethe higher oxidation and temperature particle distribution appearsto be similar to that just observed at lower oxidation index andtemperature.

S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241 1239

collec

c((apf

itmt

Fig. 12. Microphotographs of aerosols

Presence of oxygen in the above-melt atmosphere significantlyhanges the measured distribution pattern of released particlesFig. 15). Aerosols contain a much larger number of small particlessizes below 1 �m), and the content of other particles (with modest ∼1 and ∼3 �m, as a remnant of the previous large agglomerateeak) is very small. A final small mode is seen at ∼12 �m as seenrom the C-100 distribution in Fig. 14.

In the COLIMA CA-U3 test, an 8-stage impactor had beennstalled at the outlet of a pressure gradient tube and collectedhe aerosols at the plateau temperature of 2760 ◦C, for an oxidic

elt pool in an (Ar–2%H2) reducing atmosphere. Fig. 16 presentshe repartition of the collected aerosols. Even though the num-

Fig. 13. Aerosol particle size distribution at the same melt

ted from crucible sections in Pr1-EV1.

ber of stages is lower, a bimodal repartition is also observed,with peaks around 2–3 and 4.7–9 �m. The larger diameters cor-respond to the bigger particle peaks observed in the EVAN test.Further investigations are necessary to explain the discrepanciesat lower diameters. Uranium was found to be more present inlarger particles, while barium is more centered around 3 �m, ironaround 2 �m and tellurium is more abundant in the smallest

particles.

In the VERCORS HT1 experiment, the aerosols emitted duringthe last minutes of the test, just after melting of the fuel, weretrapped in an impactor composed of 6 stages and 2 bead bed fil-ters. Mo, Te, I, Cs and Ba were mainly measured in the last stage and

oxidation degree (C 70) and different temperatures.

1240 S.V. Bechta et al. / Nuclear Engineering and Design 240 (2010) 1229–1241

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

tion a

td

ca

4

sor

Fig. 15. Aerosol particle size distribu

he two bead beds, indicating fine particles with an aerodynamiciameter of around 1 �m and below.

Qualitative character of aerosol distributions measured by opti-al and impaction method and their comparison should be stressed,s the both applied methods have some uncertainties.

.5. Melt oxidation rate

An important result of completed experiments is nearly con-tant oxygen absorption rate by the melt versus the concentrationf free zirconium in the corium. The magnitude of melt oxidationate was 15.0 (±0.8) mg/s.

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

t different above-melt atmospheres.

The constant rate of melt oxidation in the stable hydrodynamicconditions means that the rate-determining process is the trans-port and oxygen supply to the melt surface, not the Zr-free supplyto the surface and not the kinetics of chemical reaction itself.

It was also determined that melt oxidation does not stop afterthe complete oxidation of all available Zr. As the melt oxidation ratepractically is the same after the corium reaches C 100. This can beexplained by the further oxidation of uranium dioxide. This givesfurther evidence that oxygen absorption by the melt is limited bythe availability of oxidant on the pool surface. The test results havenot confirmed that temperature changes of the pool surface (in thelimited range studied) influence the oxygen absorption rate.

5. Conclusions

The produced quantitative data confirm the critical influence ofmelt temperature and oxidation state on the release rates of ura-nium 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 someprevious tests and calculations. It was much higher than anticipated

with evaporation of more than one half of the initial inventory. Thiseffect confirmed earlier observations in the FPRMP and COLIMA CA-U3 tests, as well as in the TMI accident, and could be important forlong-term radiological consequences and should be studied moresystematically.

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S.V. Bechta et al. / Nuclear Enginee

The growth of melt oxidation index from 70 to 100 ◦C resultsn a considerable reduction of strontium and barium release rates,pproximately 20 and 10 times, respectively, whereas the releaseate of molybdenum grows approximately 3 times. The total aerosolelease rate drops approximately 5 times. Ruthenium release rates insensitive to the corium oxidation degree in the studied rangef oxidation index C. In the reported experimental conditions theormation of a volatile ruthenium tetroxide has not been detectedecause melt oxidation was insufficient for formation of this or anyther volatile ruthenium oxide in noticeable quantity. Practicallyll Ru remained in a second metallic immiscible liquid found in theottom of molten pool. From analysis of the total inventory datane concludes that melt oxidation to a stoichiometric compositions a basically positive phenomenon, which makes retention of the

ain FPs in the melt (Sr, Ba, La, and Ce) more efficient.The experimental data confirmed the presence of a liquid mis-

ibility gap for C 100 corium melt having prototypic content ofuthenium. Practically the whole volume of ruthenium and a sub-tantial part of molybdenum concentrate in the metallic liquidhase, which was denser and thus relocated to the pool bottom.his phenomenon can also reduce the release rate of these com-onents if the density of metallic melt is higher than that of oxidicelt. Such a configuration of a molten pool can be formed dur-

ng corium in-vessel retention, if the corium oxidation index andteel/corium mass ratio is low. It is also necessary to note that thehermodynamic modeling, which indicates the absence of a 2nd liq-id, confirms the experimentally observed change of molybdenumelease rate and insignificant ruthenium release to the gas phasender the test conditions.

Qualitative analysis of the aerosol particle size distribution hashown that it is generally bimodal, with a small aerosol parti-le population having a characteristic size of <1 �m, i.e. they areon-settling particles. A noticeable difference was found in theeasured size distribution of aerosol particles formed in an inert

Ar) atmosphere and during an oxygen supply to the melt surface.t should also be noted that the aerosol particle has a capability toorm dense deposits with a good adhesion to cold surfaces and withubstantial thermal and diffusion resistance. These can, in princi-le, influence the efficiency of containment emergency systems,

.g. heat exchangers of passive heat removal systems, catalyticydrogen recombiners, etc.

The presented experimental data can be used for extension ofhe databases for FP release from molten corium and for the furtherevelopment and validation of corresponding numerical models.

nd Design 240 (2010) 1229–1241 1241

Acknowledgements

This work has been supported by EU and coordinated by Inter-national Science and Technology Centre (Project # 3345 EVAN).

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