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Nuclear Engineering and Design 240 (2010) 1853–1866

Contents lists available at ScienceDirect

Nuclear Engineering and Design

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

ehaviour of fission products under severe PWR accident conditionshe VERCORS experimental programme—Part 2: Release and transportf fission gases and volatile fission products

ves Pontillon ∗, Gérard Ducrosommissariat à l’Energie Atomique, Cadarache centre, DEN/DEC/SA3C, BP 1, 13108 Saint Paul lez Durance, France

r t i c l e i n f o

rticle history:eceived 20 January 2009eceived in revised form 10 June 2009ccepted 25 June 2009

a b s t r a c t

The VERCORS programme is an analytical experimental programme focusing on the release of fissionproducts (FP) and actinides from an irradiated fuel rod during a severe accident in a PWR. Following onfrom the HEVA programme, VERCORS was conducted in a similar manner in a high activity cell at the CEAGrenoble Centre over a 14-year period between 1989 and 2002. A total of 17 tests were carried out, in 3experimental phases, on a facility that continued to evolve over time. The instrumentation progressivelybecome more sophisticated, thus making it possible to extend the knowledge database to include thetransport of FP (HT loop), or on the contrary focus on the release objective by associating the study ofactinides (RT loop). The general description of the programme has already been presented in a previously

published article, which is the first in a series of three articles on the same subject. The specific behaviourof fission gases (Xe and Kr) and volatile FP (I, Cs, Te, Sb, Ag, Rb and Cd) in terms of global release, releaserate and transport is the subject of this second article. The third and the final part of this series of articlesfocusing on the VERSCORS programme will study the release of actinides and less volatile FP, respectingthe classification previously identified, i.e. (1) semi-volatile FP composed of Mo, Ba, Rh, Pd, Tc, (2) verylow-volatile FP composed of Sr, Y, Nb, Ru, La, Ce, Eu, and finally (3) non-volatile FP composed of Zr, Nd and Pr.

. Introduction

One of the most important areas of research concerning a hypo-hetical severe accident in a pressurised water reactor (PWR), onoth a French and an international level, is determining the sourceerm, i.e. quantifying the nature, release rate and global releasedraction of fission products (FP) and other radioactive materials.his is in great part due to the consequences of the Three Milesland (1979) and Chernobyl (1986) accidents. In the course of thisype of event, the chain of events can effectively result in primaryoolant boiling and draining, meaning that it is no longer coolinghe core. One direct result is the core melting and the release ofP and structural and/or activated control rod elements (e.g. acti-ation products, AP) into the containment building. If there is aailure in the various protective barriers, the FP/AP can leak out of

he containment building and be released into the environment.

A large number of research programmes have thus been under-aken on this subject in various countries. In line with this approach,RSN (France) has been the driving force and has conducted pro-

∗ Corresponding author.E-mail address: yves.pontillon@cea.fr (Y. Pontillon).

029-5493/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.nucengdes.2009.06.024

© 2009 Elsevier B.V. All rights reserved.

grammes specifically focusing on determining the source term,with particular efforts devoted to understanding the mechanismsthat lead to the release of FP. This is because only a very com-plete knowledge of the phenomena governing the behaviour ofFP/AP under such constraints will make it possible to define theactions that need to be planned to minimise emissions and optimisethe protection of both the people and the environment. The HEVA(Leveque et al., 1994)/VERCORS (Ducros et al., 2001) programmeswere thus initiated by the CEA.

These analytic experiments simulating severe PWR accidentswere financed jointly by IRSN and EDF (French Electricity Com-pany). Their aim was to quantify the released fraction and releaserates of FP from irradiated nuclear ceramics (UO2 or MOX, typicallythree PWR pellets in their original sheath), quantify the nature ofthe vapours and aerosols emitted (particle size analysis and chem-ical composition), and understand how the fuel degrades. Theseexperimental sequences were carried out in a very high activity celland were commonly considered to be complementary to the PHE-

BUS PF (Schwarz et al., 1999; von der Hardt and Tattegrain, 1992)integral tests and comparable with certain tests carried out abroad:HI/VI(Lorenz and Osborne, 1995) in the United States, VEGA (Kudoet al., 2005) in Japan or the programme conducted at the CRL inCanada (Lui et al., 1994).

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of this station come from: (i) its ability for direct measurement atthe source (all FP were measured, unlike the other stations where

854 Y. Pontillon, G. Ducros / Nuclear Engi

In addition to the HEVA experimental grid (first part of therogramme carried out between 1983 and 1989) which made itossible to obtain FP releases up to fuel temperatures of 2370 K, theERCORS tests (VERCORS 1–6) have made it possible to attain fuel

emperatures of around 2600 K, and to specify the nature of theerosols emitted (up until 1994). The last part of the programmeas conducted from 1996 to 2002: VERCORS HT (high temperature)

nd VERCORS RT (release of transuranium elements). Thanks to aew experimental set-up, the range of temperatures was extendedo include fuel melting. It was thus possible to (1) study the releasef non-volatile FP and transuranium elements, (2) measure theinetics of FP deposits and finally, (3) analyse the interaction ofP with structural materials (Ag-In-Cd, B4C, etc.) (Pontillon et al.,003a,b,c) during the final phase of an accident.

The general description of the programme has already been pre-ented in a previously published article, which is first in a series ofhree articles on the same subject (Pontillon et al., submitted forublication). This also made it possible to classify FP into differentategories of behaviour, in relation to their released fraction andpecific behaviour: (1) volatile FP including fission gases, iodine,aesium, antimony, tellurium, cadmium, rubidium and silver, with aery high released fraction (almost total) at temperatures of around350 ◦C; (2) semi-volatile FP, a category composed of molybdenum,hodium, barium, palladium and technetium, with high sensitivityo oxidising–reducing conditions and released fractions of around0–100% of those of the volatile FP but with their redepositsarkedly close to the emission point; (3) FP that are low volatile,

uthenium, cerium, strontium, yttrium, europium, niobium andanthanum, with low but significant released fractions of around–10% on average, capable of reaching 20–40% under certain con-itions; (4) non-volatile FP composed of zirconium, neodymium andraseodymium. They do not present any level of release that can beeasured by gamma spectrometry under the envelope conditions

f the VERCORS test grids. Actinides include uranium, plutonium,eptunium, americium and curium. They, too, each have their ownpecific behaviour. It was nevertheless possible to divide them intowo categories, the first including U and Np with released fractionsf up to 10% and behaviour similar to those of low-volatile FP. Theecond category (Pu, Am and Cm) has very low released fractions,ypically considerably less than 1%.

The specific behaviour of fission gases (Xe and Kr) and volatile FPI, Cs, Te, Sb, Ag, Rd and Cd) in terms of released fractions and trans-ort is the subject of this second article. The FP grouped together

n this category will be dealt with in three parts in relation to theirolatility and own specific characteristics. First, the behaviour ofhe fission gases will be detailed, then that of caesium and iodine,efore describing the behaviour of tellurium, antimony and silver.owever, although an integral part of the first article, the main char-cteristics of the experimental sequences and VERCORS test loopill be briefly summarised (preamble part) to make this article

asier to read and as self-contained as possible.The third and final part of this series of articles focusing on the

ERSCORS programme will study the release of actinides and lessolatile FP, respecting the classification previously identified, i.e. (1)emi-volatile FP composed of Mo, Ba, Rh, Pd, Tc, (2) low-volatile FPomposed of Sr, Y, Nb, Ru, La, Ce, Eu, and finally (3) non-volatile FPomposed of Zr, Nd and Pr.

. Preamble

With the exception of 2 tests,1 the fuel sample was composedf a PWR rod section containing 3 pellets in their original cladding.

1 RT3 and RT4 were carried out with an initial fuel configuration in the form ofebris beds to simulate the advanced phase of a severe accident.

g and Design 240 (2010) 1853–1866

This sample was taken from a father rod irradiated for several yearsin one of EDF’s power reactors. At each end, there was a fresh half-pellet of depleted uranium, blocked against the fissile column andheld in place by crimping, closing the rod with no particular leak-tightness.

In most of the tests, this sample was re-irradiated for severaldays in a pool-type material test reactor (MTR) at the CEA (SILOE orOSIRIS) so as to reconstitute the FP inventory with short half-lives2

at a level that can be detected by gamma spectrometry similar tothat of FP with long half-lives (obtained from standard irradiationin a PWR).

The experimental sequence was then carried out in the LAMAhigh activity cell in as short a delay as possible following re-irradiation in the reactor pool, typically two or three days later.

In the course of the 14-year programme, the test loop, installedin a high activity cell at the LAMA (CEA Grenoble Centre) benefitedfrom a certain number of adaptations and upgrades to satisfy exper-imental requirements. Two types of test loops were thus developed:the RT and the HT configurations.

The RT configuration involves a compact loop designed to collectall the elements released, as near as possible to the source of emis-sion. This was required to improve the measurement precision forthe released fractions and to extend the release results databaseto (a) FP which do not emit gamma rays that are important forsafety, and (b) the heavy nuclei U, Np, Pu, Am and Cm. In addi-tion to the FP release objective, the HT loop is used to study the FPtransport and their potential interaction with silver, indium, cad-mium and boron resulting from the degradation of PWR neutronabsorbers.

Fig. 1 shows a schematic diagram of the most complete version(HT loop) in the VERCORS experimental facility. The furnace designwas very similar to that of the RT loop. The crucible, its supportsand the column delimiting the internal canal (sample canal) andthe external canal (“susceptor” canal) were made of dense tho-ria. The column was composed of a series of two thicknesses oftubular sleeves in a staggered arrangement to guarantee dynamicleaktightness between the two canals. The susceptor was made ofsolid tungsten and formed an extractible sheath supporting boththe column and the crucible. The thermal insulators, interspacedbetween the susceptor and the quartz tube, were made of porousthoria (internal insulator) and porous zirconia (external insulator).A thermal gradient tube (TGT) designed to isolate FP in the form ofvapours by their condensation temperature was placed above thefurnace. Its temperature was regulated with a profile that decreasedin a linear manner from an inlet temperature of 830 ◦C to an outlettemperature of 130 ◦C. Beyond the TGT, an elbow made possible thelink to a horizontal stainless steel tube with the same diameter asthe TGT. This tube, heated to 130 ◦C, ended in a fork and two parallelconnections onto the large capacity aerosol filter and the impactor.The circuit was completed with a gas counting capacity, then thecondensers and driers.

The FP release kinetics for the entire programme were measuredby means of several complementary gamma spectrometry stations:

• One station with a direct view of the fuel sample. This made it pos-sible to measure the FP remaining in the fuel. The two advantages

there may have been deposits upstream) and (ii) its ability to indi-cate the moment at which the fuel relocated (for the HT/RT series)

2 These FP with short half-lives, which are important because of their radiologi-cal consequences, include numerous isotopes of volatile FP (iodine and tellurium),fission gases (xenon) and very low-volatile FP (zirconium, niobium, molybdenum,ruthenium, barium, lanthanum, cerium, etc.).

Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866 1855

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by detecting the disappearance (or considerable decrease) in thesignal from the non-volatile FP.One station with a direct view of the impactor, present in theVERCORS 1–6 series and HT 1–3. This station gave very precisemeasurements of the FP deposited at this point.One station with a direct view of a portion of the TGT for the HTseries. It provided qualitative information regarding the depositand potential revaporisation phenomena of certain volatile FP.One station with a direct view of the large capacity aerosol filterfor the HT and RT series. This station gave very precise mea-surements of the FP deposited at this point where most of thevolatile FP were found (station similar to the impactor stationin the VERCORS 1–6 series). It was highly complementary to the“fuel” station.One station with a direct view of the capacity for measur-ing fission gases. It provided important information about thebehaviour of fission gases and their release mechanisms, partic-ularly by differentiating between the gases with long half-lives(85Kr) produced by standard PWR irradiation3 and those withshort half-lives (135Xe, 133Xe, etc.) produced by re-irradiation in anMTR and present, for the most part, in the form of atoms dissolvedin the matrix.

In addition to the on-line gamma spectrometry measurements,he overall release results were obtained by quantitative gammapectrometry of all the components in the loop. The measurements

ere made on a gamma scanning bench in the high activity cell.

he initial FP concentrations were first established by gamma scan-ing the fuel sample before the experimental sequence4 and it wasompleted by calculation for the non-gamma-emitting elements.

3 Present in a variety of atomic forms or bubbles, in the matrix or at the grainoundary, in proportions depending on the irradiation conditions.4 These initial inventories were often carried out in two phases: before re-

rradiation in the experimental reactor for precise measurement of the FP with longalf-lives, then just after this re-irradiation to measure the FP with short half-lives.

HT loop.

The main parameters explored throughout the three phases ofthe programme were: (1) the temperature and duration of the finalplateau, (2) the burn-up of the sample, and (3) the nature and initialstate of the fuel (UO2 or MOX, initial “rod” or “debris bed” configura-tion). For most of the tests,5 an intermediary temperature plateau(1300 ◦C or 1500 ◦C depending on the test) was carried out in asteam and hydrogen atmosphere so as to fully oxidise the claddingbefore ramping the temperature up to the final plateau. For the HT2,HT3 and all the RT tests, this intermediary plateau was followed by asuccession of 10-min plateaus every 100 ◦C from 2000 ◦C or 2100 ◦C,up until the sample “relocated”. The full test grids are shown inTable 1, Table 2 and Table 3 respectively for the VERCORS 1–6, RT1–8 and HT 1–3 series.

3. Release, kinetics and transport of volatile FP and fissiongases

As explained in the introduction, this part will successively dis-cuss (1) fission gases, (2) the caesium/iodine couple, and (3) Te, Sband Ag. For each of these three cases, their behaviour in terms ofreleased fractions will first be detailed, followed by their releasekinetics and finally the “transport” characteristics of these variousFP.

3.1. Fission gases

The fission gases in question (Kr and Xe) are composed of iso-topes whose half-lives have a very different radiological impact overtime under PWR severe accident conditions:

• Long half-life for krypton (10.71 years for 85Kr); the radiologicaleffects range over the mid and long term. The other tracer iso-topes of the element have sufficiently short half-lives for there

5 With the exception of VERCORS 1, 3 and 6, where cladding oxidation occurredduring the temperature ramp up to the final plateau.

1856 Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866

Table 1VERCORS 1–6 test conditions and summary of the fractions released at the end of the test.

Test VERCORS 1 VERCORS 2a VERCORS 3 VERCORS 4b VERCORS 5c VERCORS 6

Date of test 11-1989 06-1990 04-1992 06-1993 11-1993 06-1994

Fuel UO2 UO2 UO2 UO2 UO2 UO2

PWR irradiation Fessenheim Bugey Bugey Bugey Bugey GravelinesFuel burn-up (GWd/tU) 42.9 38.3 38.3 38.3 38.3 60Re-irradiation Siloe Siloe Siloe Siloe Siloe Siloe

Test conditionsMax fuel temperature (K) 2130 2150 2570 2570 2570 2620Atmosphere (end of test) Mixed H2O + H2 Mixed H2O + H2 Mixed H2O + H2 Hydrogen Steam Mixed H2O + H2

Last plateau duration (min) 17 13 15 30 30 30Steam flow rate (g/min) 0.15 1.5 1.5 1,5-0 1.5 1.5Hydrogen flowrate (g/min) 0.003 0.027 0.03 0.012 0 0.03

FP released fraction (%)Sr (via 91Sr) <6 <6 <6Y (via 93Y) 17Zr(via95Zrand 97Zr) <3 <4 <4Nb (via95Nb) 0.2Mo (via 99Mo) 15 42 47 92 79Ru (via 106Ru and 103Ru) 0.36 6 6 0.6Rh (via105Rh) 0.52 45 20 3.6Sb(via125Sband 127Sb) 2 7 69 97 98 98–100Te (via132Te) 4 18 76 100 >98 98–100I (via 131I and 133I) 30 23 70 87 93 98–100Xe (via 133Xe) 33 23 77 86 87 ∼100Cs (via 134Cs and 137Cs) 42 30 70 93 93 97Ba (via 140Ba) 4 4 13 80 55 28La (via 140La) <4 <3 <3 <3Ce (via 141Ce, 143Ce and 144Ce) 3 <3 0.2Eu (via 154Eu) <6 <5 <3 <4Np (via 238Np and 239Np) 0.006 0.016 0.4 6 <4 0.3Ud ∼2 ∼2Pud ∼0,2 ∼0,2

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cannot to be any significant impact in the hours following reactorshutdown, with the exception of 85mKr (half-life of 4.48 h) whoseeffects are felt a little longer.Short half-lives for the main isotopes of xenon (2.19 days, 5.24days and 9 h respectively for 133mXe, 133Xe and 135Xe); the radio-logical effects are felt in the short term.

Release of these fission gases was not complete during the VER-ORS 1–5 tests, suggesting partial trapping of the atoms and/orubbles still contained in the UO2 grains. The release thus dependedn the final temperature level of the test, e.g. 23% and 33% in VER-ORS 2 and VERCORS 1 for Xe at a temperature of 1860–1880 ◦C,ersus 77% and 86–87% in VERCORS 3 and VERCORS 4–5 at 2300 ◦C.urthermore, it should be noted that the released fraction alsoepended on the duration of the high temperature plateau see-

ng that the highest released fractions corresponded to the longesturations for same temperature: 13 min for VERCORS 2 versus7 min for VERCORS 1 and 15 min for VERCORS 3 versus 30 minor VERCORS 4 and 5. However, for the entire VERCORS HT/RTrid where the fuel was brought up to melting point, there wasotal release of the gases. These observations are coherent with theelease mechanisms predicted for these species (see below).

The instantaneous release kinetics corresponding to the RT/HTrid were characterised by a sequence of burst release. An exam-le of these kinetics is given in Fig. 2(a) and (b) respectively for

ERCORS RT7 and RT6. The following observations were made:

The first burst release represented the start of gas releases ataround 500–600 ◦C (maximum amplitude around 700–800 ◦C, atwhich the amplitude was greater for 85Kr than for 133Xe),

7 and 30 min.at 1300 ◦C for 60 min.0 ◦C for 30, 30 and 70 min.es, correlated with 137Cs measurement.

• There was a second burst release around 1100–1200 ◦C with sig-nificantly greater amplitude for the krypton-85 in relation to thexenon with a short half-life,

• The following burst release was almost systematically provokedby the reactivation of the temperature ramps. The last two puffsrespectively indicated the start of relocation and the completedrainage of the residual intragranular inventory of fission gasesfollowing sample melting.

Generally speaking, although each burst release was syn-chronous between the two types of gas, the correspondingamplitudes were different until the end of the cladding oxidationplateau, after which they coincided perfectly. These phenomenashow the location differences of 85Kr and 133Xe. 133Xe is gener-ally located in the intragranular region, whereas 85Kr is generallyfound in the intergranular and intragranular regions with an inter-granular versus intragranular ratio which varied in relation to theirradiation history and burn-up (Noirot et al., 2004; Noirot, 2005).This behaviour is perfectly in line with current knowledge of fissiongas releases under transient temperature conditions (Pontillon etal., 2003b; Pontillon et al., 2004a). For temperatures below 1200 ◦Cand for fuel with a high burn-up, it is now generally accepted thatreleases are greatly due to the release of the gas located at the grainboundary and accumulated at this level during standard irradia-tion, with this release occurring by means of two mechanisms: (1)

decohesion of the grain boundary (T < 1000 ◦C) and (2) interconnec-tion of the intergranular bubbles (1000 ◦C < T < 1200 ◦C). For highertemperatures, “classic” diffusion of the intragranular gases occurs(as the inter-gases have already been released), which explains thesuperimposition of the kinetics of the two gas types.

Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866 1857

Table 2VERCORS RT test conditions and summary of the fractions released at the end of the test.

Test VERCORS RT1 VERCORS RT2 VERCORS RT4 VERCORS RT3a VERCORS RT7a,b VERCORS RT6 VERCORS RT8

Date of test 03-1998 04-1998 06-1999 11-1999 04-2000 09-2002 11-2002

Fuel UO2 MOX (AUC) Debris UO2/ZrO2 Debris UO2 MOX (AUC) UO2 UO2

PWR irradiation Gravelines Gravelines Gravelines BR3 Saint Laurent B1 Gravelines GravelinesFuel burn-up (GWd/tU) 47.3 45.6 37.6 39 43.0 71.8 70Re-irradiation No No No Yes (partly) Yes Yes Yes

Test conditionsMax fuel temperature (◦C) 2300 2170 2250 2700 2620 2200 2380Atmosphere (end of test) Mixed H2O + H2 Mixed H2O + H2 Mixed “oxidising” Mixed “reducing” Hydrogen Mixed H2O + H2 He + 10% airLast plateau duration (min) c c c c c c c

Steam flow rate (g/min) 1.5 1.5 0.876 0.075 1.5-0 1.5Hydrogen flowrate (g/min) 0.027 0.027 0.024 0.075 0.012 0.027Airflowrate (g/min) 0.048

FP released fraction (%)Kr (via85Kr) ∼100 ∼100 ∼100 ∼100 ∼100 ∼100 ∼100Sr(via91Sr)Y (via 93Y)Zr (via 95Zr and 97Zr) <2 <2 <2 <2Nb (via95Nb) 40.3 3 30.4 10.9Mo (via 99Mo) 33.2 7 100 100Ru (via 106Ru and 103Ru) 9 5.4 7.7 1.5 2 28.1 17Rh (via 105Rh)Ag (via 110mAg) >90 >97 ∼100 ∼100 ∼100Sb(via125Sband127Sb) 95 77 89 60–95 86 90 84Te (via 132Te) 100 99 99.5 100I (via 131Iand 133I) 100 99 100 100Xe(via133Xe) 100 ∼100 100 100Cs(via 134Csand 137Cs) 100 100 96 99.8 98 100 100Ba(via 140Ba) 93.9 64 57.7 67.2La (via 140La) 12.5 3 7 10.6Ce (via 141Ce, 143Ce and 144Ce) 3.2 <3 2.5 14 16 1.8Eu(via 154Eu) 1.2 0.3 0.8 3.5 10 1.7 2.6Np (via 238Np and 239Np) 1.4 4 10.3 2.7Ud 7.8 3.8–4.9 9.9 2.3Pud 0.3 0.1 0.1 <0.1

a Detailed analysis of the programme led to re-evaluation of the maximum temperatures attained in the course of the different tests, for RT7 and RT3 a significant drop ofaround 200–300 ◦C was obtained.

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• For VERCORS 3–5, all carried out at 2300 ◦C on the same initialfuel, the released fractions were significantly higher: 70%, 93% and93% respectively in VERCORS 3–5 for caesium; 70%, 87% and 93%for iodine. These released fractions also depended on the dura-

b Test with intermediate temperature plateaus at 1200 ◦C for 15 min under mixedc Succession of 10-min plateaus every 100◦C from 2000 ◦C until “relocation” wa

0 min).d From chemical analysis.

Naturally, no retention of gaseous FP was ever measured on thearious elements in the experimental loop (except the cold traps athe end of the circuit).

.2. Caesium and iodine

These two volatile FPs are of great importance with regard tohe radiological consequences following a severe accident in a PWRore. They are composed of isotopes with very different half-lives:

Short half-life for iodine (from 1 h for 134I to 8 days for 131I); theshort-term radiological effects are very high in the first few daysfollowing an accident, but are negligible after 1 month. Iodinecarries 15% of the core’s decay heat 1 day after the emergencyshutdown;Long half-life for caesium (30 years for 137Cs); the radiologicaleffects, which are low in the short term (there are nevertheless138Cs and 136Cs with respective half-lives of 30 min and 13 days)stretch into long term over several decades.

The global release of these two elements was more or less equiv-lent. There was total release for iodine and almost total release foraesium in all the most severe VERCORS tests, i.e. from VERCORSonwards. It nevertheless seems that a low yet significant quan-

ity of caesium remained present in the corium following certain

sphere (H2O + H2) to simulate LOCA conditions.ined (after specific plateau for total cladding oxidation in H2O + H2 at 1500 ◦C for

tests, particularly during VERCORS 6 and RT7,6 where the fractionsreleased were respectively 97% and 98%. This was attributed topossible retention (Ducros et al., 1996) in the form of uranate orzirconate,7 or even of Cs2MoO4, as proposed in the case of RT7, fol-lowing distribution similarities for Cs and Mo within the coriumafter the test (Fig. 3). Caesium retention in the corium was alsoidentified as a result of the TMI2 accident, where associations withoxides stable at high temperatures were proposed: metal oxides ofchromium (Cs2Cr2O4) or iron (Cs2Fe2O4), or silicates (Cs2Si4O9) orcaesium borates (CsBO2) (Hobbins et al., 1993).

For the tests carried out at lower temperatures, and in a mannersimilar to the fission gases, the release depended on the level of thefinal temperature and the duration of the corresponding plateau:

• For VERCORS 1 and 2, whose end-of-test temperatures werewithin the 1860–1880 ◦C range, released fractions were in the30–40% range;

6 RT1 also showed this type of retention but with a lower fraction of caesium inthe corium, typically around 0.2–0.3%.

7 The tomography of VERCORS 5 shows that the residual caesium was in thecladding.

1858 Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866

Fig. 2. Release kinetics for active gases: (a) VERCORS RT7 (MOX 3 cycles) and (b) VERCORS RT6 (UO2 6 cycles).

Fig. 3. Distribution of 137Cs and 99Mo within the corium after the VERCORS RT7 test.

Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866 1859

Table 3VERCORS HT test conditions and summary of the fractions released at the end of the test.

Test VERCORSHT1a VERCORS HT3 VERCORS HT2

Date of test 06-1996 06-2001 04-2002

Fuel UO2 UO2 UO2

PWR irradiation Gravelines Gravelines GravelinesFuel burn-up (GWd/tU) 49.4 49.3 47.7Re-irradiation Yes Yes Yes

Test conditionsMax fuel temperature (◦C) 2630 2410 2150Atmosphere (end of test) Hydrogen Hydrogen SteamLast plateau duration (min) 7 b b

Steam flow rate (g/min) 1,5-0 1,5-0 1.5Hydrogen flowrate (g/min) 0.012 0.012 0Injection Ag, In, Cd, B No Yes Yes

FP released fraction (%)Kr (via 85Kr) ∼100 ∼100 ∼100Sr(via91Sr) <5Y (via 93Y)Zr (via 95Zr and 97Zr) <3 <2 <2Nb (via 95Nb) 9 18 9.7Mo (via 99Mo) 49 33 100Ru (via 106Ru and 103Ru) 8 6 65Rh (via105Rh)Ag (via 110mAg)Sb (via 125Sb and 127Sb) 100 98 71Te (via 132Te) 100 98 100I (via 131I and 133I) 100 100 100Xe (via 133Xe) ∼100 100 100Cs (via 134Cs and 137Cs) 100 100 100Ba (via 140Ba) 49 85 38.5La (via 140La) 8 13 5.5Ce(via141Ce, 143Ce and 144Ce) 5 0.8 1Eu (via 154Eu) 9 10.5 1

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it can be seen that the released fractions was higher during RT2 atany point in the experiment. This phenomenon can be assimilatedwith the effect of the burn-up, as the heterogeneous MOX fuel hasPu-rich agglomerates in its matrix, and the local burn-up of Pu is sig-

Np (via Np and Np) 7

a Test under hydrogen with intermediary plateau for total claddb Succession of 10-min plateaus of every 100 ◦C from 2000 ◦C un

oxidation in H2O + H2 at 1500 ◦C for 60 min).

tion of the high temperature plateau (15 min for VERCORS 3 and30 min for VERCORS 4 and 5).

In short, the behaviour in terms of global release of thesewo FP was homogenous under severe accident conditions andid not depend (or very little) on the test conditions (initialeometry, atmosphere, temperature, etc.) if a sufficiently high tem-erature was attained, typically more than around 2350 ◦C. Inhis case, there was almost total release. Below this threshold,he released fraction depended on the temperature level attained20–40% for 1860–1880 ◦C versus 70–90% for 2300 ◦C) and for aiven temperature, the time during which this temperature wasaintained.

Although the global release of these two FP did not depend (ornly very little) on the test conditions at temperatures of morehan 2300 ◦C, it was quite different for their release kinetics. Inter-omparison of all the tests in the HT/RT grid makes it possible tolearly demonstrate the influence of key parameters on the releaseinetics, particularly the burn-up, type of test atmosphere, type ofuel (MOX or UO2), and the initial fuel geometry (intact or debrised). It was also possible to identify the influence of one of thesearameters on another.

To illustrate the impact of the burn-up on the release kineticsf iodine and caesium, Fig. 4 compares tests RT6 (UO2 6 cycles)nd RT1 (UO2 4 cycles and end-of-test conditions comparable with

T6). An unequivocal effect on the rate of caesium’s release kineticsand assumed for the other isotopes) is visible. At the end of theladding oxidation plateau, caesium release was around four timesigher for RT6 than for RT1, and the fraction released was higher

or RT6 throughout the test.

5 <2

idation in H2O + H2 for 60 min.l relocation was obtained (after specific plateau for total cladding

The influence of the atmosphere (oxidising or reducing) is per-fectly highlighted by the comparison between tests HT2 and HT3(Fig. 5).8 These tests are effectively similar from the viewpoint ofboth the fuel used (identical father rods) and the temperature ramphistory. The comparisons are thus direct. As long as the atmospheresfor the two tests were identical, i.e. until the end of the claddingoxidation plateau, the kinetics could be superimposed. Then, afterthe change in atmosphere, a marked delay in the kinetics for HT3(reducing conditions, hydrogen) was measured in relation to HT2(oxidising conditions, steam).

At this point, it seems appropriate to compare these last twoeffects, i.e. burn-up with regard to the oxygen potential appliedduring the test. This comparison is proposed in Fig. 6 (superimpo-sition of the release kinetics of RT6 and HT2). As long as the testconditions were identical (cladding oxidation plateau at 1500 ◦C),the impact of burn-up could be felt and the release kinetics weregreater for RT6 than for HT2. Then the oxygen potential inducedduring HT2 (pure steam) seemed to gain the upper hand and therelease kinetics became greater for HT2 at the end of the test.

With regard to the impact of the nature of the fuel, by compar-ing the release kinetics for RT2 (MOX AUC 3 cycles, test conditionsidentical to RT1) and RT1 (UO2 pellets, 4 cycles), as detailed in Fig. 7,

8 It should be noted that the lanthanum signal shown on this figure is only givento visualise the fuel collapse in front of the gamma spectrometry view (relocationsignal) and does not represent any release of this FP.

1860 Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866

Fig. 4. Comparison of caesium emission kinetics between RT6 and RT1.

Fig. 5. Comparison of caesium and iodine emission kinetics between HT2 and HT3.

Fig. 6. Comparison of caesium emission kinetics between HT2 and RT6.

Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866 1861

ntptdfiR

bdw

••

•

emdnd

Table 4Comparison of manufacturing characteristics of the fuels used for RT2 and RT7.

RT2 RT7

Fuel MOX-MIMAS-AUC MOX-MIMAS-AUCPu/(U + Pu + Am) % 5.92 4.359Grain size (�m) 8.2 7.33Agglomerates size (�m) 18.6 16.3Gas release (F%) 4.36 2.71Burn-up (GWd/tM) 41.4 41.2

Fig. 7. Comparison of caesium emission kinetics between RT1 and RT2.

ificantly higher than the average burn-up on the pellet. That said,he behaviour of caesium during the two MOX tests (Fig. 8) deservesarticular attention. Although the releases started at approximatelyhe same time, the release kinetics during RT7 were then clearlyelayed in relation to RT2: at 1500 ◦C 10–15% for RT7 and 40–50%

or RT2. A release of 100% was obtained in both cases, but at rad-cally different temperatures (∼2200 ◦C for RT2 versus 2700 ◦C forT7) and experimental times (∼7 h for RT2 versus ∼10 h for RT7).

In order to draw clear conclusions from these different types ofehaviour, it would be necessary to do a detailed analysis of theifferent parameters involved in these release kinetics, especiallyith:

The respective irradiation conditions,The effect of Pu content and/or morphology and agglomeratesdistribution (4.36% for RT7 and 5.92% for RT2, Table 4),The difference in the thermal sequence between the two tests,where many temperature plateaus were carried out during RT7and not during RT2.

The last point that must be dealt with here concerns the influ-nce of the initial sample geometry. Once again, it was possible to

easure a clear-cut effect on the release kinetics. Caesium releases

uring RT1 (UO2 pellets, 4 cycles), RT3 (UO2 debris beds, so-calledeutral atmosphere) and RT4 (UO2/ZrO2 debris beds, oxidising con-itions) were systematically higher for the debris bed tests (Fig. 9).

Fig. 8. Comparison of caesium emission kinetics between RT2 and RT7.

Reactor GRAV 4 SLB 1Core outlet 31/10/92 17/01/91Re-irradiation No Yes (Osiris)

It should be noted that once again the role of atmosphere was con-firmed as the released fraction was also higher for RT4 (oxidising)in relation to RT3 (“neutral”).

As for the global released fractions, some of the effects high-lighted in this paper – particularly the influence of the atmosphereand burn-up – were already suspected from the analysis of the VER-CORS 1–6 grids and were thus well confirmed here (Ducros et al.,2001).

In summary, this analysis reveals that the release kineticsof iodine and caesium (which are generally highly comparable)increase with (1) the burn-up, (2) an initial debris bed configu-ration, (3) an oxidising atmosphere, and (4) MOX fuel, in relationto the standard fuel in RT1 (UO2 pellet, 4 cycles) used as the refer-ence.

In addition to the emission kinetics of FP released from thefuel, two other types of kinetics – deposit kinetics – were acces-sible thanks to the instrumentation used in the VERCORS loop: thedeposit kinetics on the filter and on the TGT (HT experiments only).With regard to the deposit kinetics on the filter, most informationcomes from the fact that there was little or no delay between emis-sion and deposit for either iodine or caesium, and that these twotypes of curves appeared to be very similar. An illustration of thisis given in Fig. 10 in the case of HT2.

For the deposits on the TGT, revaporisation of the depositswas highlighted for iodine and caesium during the cladding oxi-dation plateau for HT2 and HT3, the plateau during which theAg-In-Cd vapours started to be emitted within the test loop (seeFig. 11).

Transport of these two elements can initially be understood indetail thanks to the distributions obtained along the TGT during thethree HT tests (Pontillon et al., 2003a). This equipment (describedin detail in the preamble) made it possible to highlight preferential

temperature zones in relation to each FP and with regard to theirredeposition. This knowledge is essential for better understandingthe different chemical forms in which the radionuclides can befound.

Fig. 9. Comparison of caesium emission kinetics between RT1, RT3 and RT4.

1862 Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866

Table 5Fractions deposited on the top of the column, on the filters and on the TGT (where applicable) for all VERCORS HT/RT tests.

Released fraction (top of the sleeve) (%) Released fraction (TGT) (%) Released fraction (filter) (%)

I Cs Te Sb Ag I Cs Te Sb Ag I Cs Te Sb Ag

HT1 0 3 5 11 – 24 26 45 20 – 76 69 43 56 –HT2 0 22 20 66 – 16 17 36 5 – 84 61 41 0 –HT3 0 7 7 37 – 41 40 52 6 – 59 53 41 55 –

RT1 – 7 50 15 – 93 42 75RT2 – 8 28 19 – 92 47 78RT3 0 5 5 64 – 100 95 95 0 –RT4 – 14 – 34 27RT6 0,3 19 11 60 34RT7 0 24 2 32 –RT8 0 1 3 22 –

Fig. 10. Comparison of the emission and deposit kinetics of caesium and iodineduring HT2.

faTcT

Ca

e

Fig. 11. Measurement of FP activity on the TGT during HT3.

This illustration is given in Fig. 12 for HT1 and HT2, and in Fig. 13or HT3 (deposits of I, Cs, Te, Sb, Mo and Ba for the FP; and Agnd In deposits in the case of injection of Ag-In-Cd for the APs9).he corresponding total fractions deposited, as well as those asso-iated with the top of the column and the filters, are grouped in

able 5.

The first notable point – constant throughout the entire VER-ORS programme – was associated with the quantity of iodinend caesium deposits on the top of the column (high tempera-

9 APs: Activation products resulting from the re-irradiation of the correspondinglement in OSIRIS which were used to monitor these species.

– 82 – 44 7397 81 87 31 66100 73 97 54 –98 100 98 68 –

ture section of the loop): nil in the case of iodine and varyingby a few percent to approximately 20% for caesium. On thecontrary, the fractions found on the filter elements (low temper-ature section of the loop) were systematically higher for iodine.This clearly shows the more volatile nature of iodine in relationto caesium. This last point was also clearly highlighted by thelocation of deposits along the TGT, with a majority of iodine frac-tions deposited on the low temperature section (exit) of the TGT,whereas the caesium tended to redeposit along practically thewhole length, with several deposit peaks revealing multiple chem-ical forms.

It was also very clear that, as with the release kinetics ofthese FP, the test atmosphere had a considerable impact onthe nature of the species present on the TGT. The comparisonbetween HT2 and HT3, which were very similar excepted theend-of-test atmosphere, highlights a very significant change inthe deposits of both caesium (four main activity peaks for HT3,around 670, 580, 510 and 390 ◦C versus two for HT2 around700 ◦C and 130 ◦C) and iodine (two main peaks around 380 ◦Cand 300 ◦C for HT3 versus one for HT2 around 130 ◦C). The com-parison between HT1 and HT3 reveals the influence of Ag-In-Cdinjection during HT3, with the test atmospheres remaining com-parable. This “interaction” – clearly demonstrated with indium(co-location of peaks at 580 ◦C, 510 ◦C and 390 ◦C) during HT3– significantly changed the nature of the species redeposited atthis stage, just like the change in atmosphere between HT2 andHT3.

Finally, the three distributions still highlighted the co-locationof certain deposit peaks between caesium and iodine on the onehand, and between caesium and tellurium on the other.

Transport of these elements can also be studied from thedeposits on the top of the column measured throughout the entireVERCORS HT/RT programme. This approach is less detailed thanusing the TGT, particularly because the temperature gradient alongthe column evolved during the experiment, but it neverthelessmade it possible to draw a certain number of major conclusions,in particular (Table 5):

• Confirmation of non-existent iodine deposits regardless of thetype of test,

• The notable influence of both the test atmosphere and the fueltype on the quantity of caesium deposited at this level. Oxidis-ing conditions and high burn-up fuel effectively seem to increasethe fractions deposited. The relatively high quantity measured for

RT7 (test with reducing conditions) is certainly more similar to a“MOX” effect (and thus comparable, as mentioned above, with a“burn-up” effect),

• The injection of air seemed to “revaporise” the deposits or at leastbe responsible for a very low ratio between the top-of-columndeposits and the total releases.

Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866 1863

the TG

3

rtvi

aCticttVi

bdo

for caesium) respectively for VERCORS 2 and VERCORS 1 (whoseend-of-test temperatures were within the 1860–1880 ◦C range).The difference between the released fractions of these two testsno longer correlated with the duration of the final plateau (13 minfor VERCORS 2 versus 17 min for VERCORS 1), as was the case for

Fig. 12. Deposits along

.3. Tellurium, antimony and silver

From a general point of view, although they have very higheleased fractions justifying their presence in this category, thesehree FP clearly differ from iodine and caesium because of their indi-idual characteristics. In addition, these same characteristics maket possible to distinguish them from each other.

The global release of tellurium and silver10 was comparable andlmost total for all of the most severe VERCORS tests, i.e. from VER-ORS 6 onwards. The main difference between these two FP was inerms of the quantities redeposited in the hot zones of the exper-mental loop (see transport section). As in the case of iodine andaesium, releases depend on the final temperature level for bothellurium (and very likely for silver too) and lower temperature

ests. Nevertheless, the difference between VERCORS 1 and 2 andERCORS 3–5 was significantly greater for tellurium than for either

odine or caesium:

10 It should be noted that the number of tests in which the released silver coulde quantified was very low (RT1, RT2, RT4, RT6 and RT8) because of problems inetecting this element in the presence of a large number of FP with short half-livesbtained from re-irradiations.

T for (a) HT1, (b) HT2.

• There was a tellurium release of 18% and 4% (versus 30% and 42%

Fig. 13. Deposits along the TGT for HT3.

1864 Y. Pontillon, G. Ducros / Nuclear Engineering and Design 240 (2010) 1853–1866

CORS

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The more volatile nature of tellurium in relation to antimony waswell demonstrated by the location of deposits along the TGT, withthe majority of antimony fractions deposited at the TGT inlet, i.e. onthe section with a high temperature, whereas tellurium tended to

11 This last point was also highlighted during the HI and HEVA programmes.12 This made it impossible to monitor their release from the fuel over time.13 VERCORS 6 did not present any kinetics for tellurium because interference in

Fig. 14. Antimony distribution along the crucible for VER

iodine, caesium and the fission gases. Instead, it was linked toan interaction with the materials composing the cladding (seebelow).The released fractions were significantly higher in VERCORS 3–5,which were all carried out at 2300 ◦C on the same initial fuel: 76%,100% and 98% respectively. The released fractions thus dependedon the duration of the high temperature plateau (15 min for VER-CORS 3 and 30 min for VERCORS 4 and 5).

An interesting point should be mentioned here. Although theeleased fraction for tellurium was effectively much lower than thatf caesium and iodine in the VERCORS 1 and 2 tests, it became equiv-lent, not to say higher, for VERCORS 3–5. This probably illustratesow this element is kept in the cladding until it has been fully oxi-ised (Johnson et al., 1988; Bowsher et al., 1987; De Boer et al., 1995,997), a point that will be clearly highlighted below thanks to theuel release kinetics. This effect is attributed to the interaction ofe with Sn contained in the Zircaloy cladding and is certainly alsoesponsible for the difference between VERCORS 1 and 2, seeinghat the shortest plateau at 2300 ◦C was compensated by the inter-

ediary LOCA plateaus allowing total cladding oxidation in the casef VERCORS 2.

Antimony behaved in a slightly different manner. For the entireT grid, the released fractions were generally lower than thosebtained for VERCORS 4–6 (typically around 80–95% and 97–100%espectively for the RT grid and VERCORS 4–6). Of course, theseelease levels were still high, but it is surprising that they wereower for the RT grid tests. Given the type of test used, this behavioureems to have been the result of antimony retention in the coriumormed after fuel relocation when compared with a “long plateauime” effect at high temperature prior to fuel collapse. In addi-ion, this retention sometimes (tests VERCORS RT1, RT2 and RT7)nvolved the dissociation of this element in the solidified coriumnd a deposit under the initial position of the PWR pellets, as illus-

rated in Fig. 14.

The results obtained from the experiments in the VERCORS 1–6est grids made it possible to draw conclusions similar to thoseescribed for tellurium, i.e. the released fractions depended on thenal temperature level for tests carried out at lower temperatures,

RT7 (pink before the sequence, blue after the sequence).

with a difference in terms of release between VERCORS 1 and 2, andVERCORS 3–5, which was much higher than in the case of iodine andcaesium. Sb releases were equivalent to 2% and 7% respectively forVERCORS 1 and 2 versus 69%, 97% and 98% respectively for VERCORS3–5.

Still in line with tellurium, these results can also demonstrate theretention of this FP (Osborne et al., 1992; Lewis et al., 1996; Collinset al., 1987) in the cladding until the latter has fully oxidised.11 TheHI/VI and HEVA tests have also shown that oxidising atmosphereconditions encourage the release of this element at moderate tem-peratures.

In terms of the release kinetics, the results obtained for thesethree elements were relatively restricted because of (1) problemswith detecting antimony and silver in all the VERCORS tests12 and(2) the loss of detectability of 132Te (best tracer isotope for Te) withthe use of thoria in the furnace composition after VERCORS 6.13

The main fact worth noting from the small number of releasekinetics observations available for tellurium is the clear demon-stration of tellurium retention in the cladding until the latter wascompletely oxidised. The released fractions were then comparablewith those of caesium (Fig. 15).

As was the case for iodine and caesium, the transport ofthese three elements could be studied precisely by means of thedistributions measured all along the TGT during the three HTtests.14

the electronic chain which, more specifically, degraded the measurement of the lowenergy gamma lines.

14 NB: This illustration is shown in Fig. 12 for HT1 and HT2 and in Fig. 13 for HT3(deposits of I, Cs, Te, Sb, Mo and Ba for the FP and Ag and In deposits in the case ofinjection of Ag-In-Cd for the AP). The corresponding total fractions deposited, as wellas those associated with the top of the column and the filters, are listed in Table 5.

Y. Pontillon, G. Ducros / Nuclear Engineerin

rw

acattrmHadmtHt

c

btam

•

•

crvtt(h

which made it possible to extend the base of knowledge fromthe most volatile to least volatile FP. In addition, for a given tem-perature level, the plateau time at this temperature also had an

Fig. 15. Emission kinetics for Mo, Te and Cs during VERCORS 4.

edeposit down the length of the TGT a little like caesium (thoughith fewer activity peaks).

Furthermore, it was very clear that, as with Cs and I, the testtmosphere had a significant impact on the nature of the speciesoming into play on the TGT. In addition to the similar end-of-testtmosphere compositions, comparison between the HT2 and HT3ests highlighted: (i) a shift in the deposit peak towards the highemperatures for HT3 in relation to HT2 and (ii) larger quantitiesedeposited for tellurium. The equivalent between HT1 and HT3

ade it possible to study the influence of injecting Ag-In-Cd duringT3 (with comparable test atmospheres). The results did not revealny major impact for the Ag-In-Cd in terms of distribution. Theifference in terms of the fractions released within the TGT wasore certainly due to the difference in final temperature between

hese two tests: the high temperatures at the top of the column inT1 had the effect of transporting the FP from the top of the column

owards the TGT.Finally, the three distributions still revealed that the deposits of

aesium and tellurium coincided.As illustrated above, the transport of these elements can also

e studied via the deposits on the top of the column measuredhroughout the totality of the VERCORS HT/RT programme. Thispproach, which has been detailed above (for iodine and caesium),ade it possible to:

Confirm the greater volatility of tellurium in relation to silver, andof silver in relation to antimony (see the fractions deposited onthe top of the column, Table 5). For tellurium, these fractions werelow and comparable to those measured for caesium, or roughlya few percent, whereas they were considerably higher for silver,typically between 15% and 35%. Furthermore, the equivalent forantimony (from 30% to 60% for all the tests, excluding RT8) alsorevealed the less volatile nature of this element both in relationto tellurium and silver, and in relation to iodine and caesium. Thefractions deposited for antimony were similar to what is typicallymeasured for semi-volatile FP,Illustrate once again the effect of injecting air during RT8, whichseemed to “revaporise” the deposits or at least very clearly reducethe ratio between the deposits on the top of the column and thetotal releases.

In short, the general behaviour of these three FP was signifi-antly different under severe accident conditions. In terms of totaleleases, the levels attained (almost 100%) did not depend (or only

ery little) on the test conditions (initial geometry, atmosphere,emperature, etc.) if a sufficiently high temperature was attained,ypically more than 2300 ◦C and relocation did not occur too earlyparticularly for Sb). The corresponding fractions deposited in theot zones of the loop increased in the order Sb, Ag and Te. Below

g and Design 240 (2010) 1853–1866 1865

this temperature threshold, releases then depended very much onthe temperature level attained. For antimony, although the releasedfractions were high (greater than 90%) for a temperature typical ofa severe accident, they were limited by retention in the corium fol-lowing fuel relocation. Finally, a release delay was identified for Teand Sb and attributed to the two elements being trapped in thecladding by the tin until the cladding had been fully oxidised.

4. Conclusion

The VERCORS programme is an analytical experimental pro-gramme focusing on the release of FP and actinides from anirradiated fuel rod during a severe accident in a PWR. Followingon from the HEVA programme, VERCORS was conducted in a simi-lar manner in a high activity cell at the CEA Grenoble Centre over a14-year period between 1989 and 2002.

A total of 17 tests were carried out, in 3 experimental phases, ona plant that continued to evolve over time. The instrumentationprogressively become more sophisticated, thus making it possi-ble to extend the knowledge database to include the transport ofFP (HT loop), or on the contrary focus on the release objective byassociating the study of actinides (RT loop).

One of the main information revealed by the results obtainedfrom the VERCORS programme is that it is now possible to clas-sify FP into different behavioural categories in relation to theirreleased fractions and specific behaviour: (1) volatile FP, includ-ing fission gases, iodine, caesium, antimony, tellurium, cadmium,rubidium and silver; (2) semi-volatile FP, a category composed ofmolybdenum, rhodium, barium, palladium and technetium; (3)low-volatile FP, ruthenium, cerium, strontium, yttrium, europium,niobium and lanthanum; and (4) non-volatile FP composed of zirco-nium, neodymium and praseodymium. Actinides include uranium,plutonium, neptunium, americium and curium.

The cross-sectional analysis of the various experimentalsequences in the VERCORS grids has made it possible to highlightthe main physical parameters that affect release kinetics and ratesfor these different elements15:

(1) Temperature, which was the main parameter, at least until thesample lost its geometry;

(2) Oxidising–reducing conditions;(3) Burn-up;(4) Interactions with the cladding and/or structural elements;(5) Nature of the fuel;(6) Finally, the state of the fuel played an important role (transition

from a “rod” geometry to a “debris bed” geometry and/or moltenbath).

(1) Although its influence seems obvious, it is nevertheless impor-tant to remember that the relative consequences of thetemperature level attained – in terms of total releases – weresignificant in relation to all the other parameters. The temper-ature directly affected the release of all the FP, regardless oftheir degree of volatility; it was the amplitude of the release fora given temperature that differed. These different points wereperfectly corroborated by the four (including the HEVA series)major phases of the programme which explored increasinglyhigh temperatures (at least from HEVA to VERCORS HT1), and

effect on the released fraction, tending to increase them.

15 No notable effect was measured for the elements composing the control rods onFP release under the conditions studied here.

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866 Y. Pontillon, G. Ducros / Nuclear Engi

For example, the release of fission gases between VERCORS1–3 and VERCORS 4–5 perfectly illustrates this, with the frac-tions released from the fuel of 23%, 33%, 76% and more than 98%respectively for VERCORS 2 (final temperature of 1880 ◦C main-tained for 13 min), VERCORS 1 (final temperature of 1860 ◦Cmaintained 17 min), VERCORS 3 (final temperature of 2300 ◦Cmaintained for 15 min), VERCORS 4–5 (final temperature of2300 ◦C maintained for 30 min).

2) The oxidising–reducing conditions applied to the fuel played amajor role. The release kinetics of the volatile FP were particu-larly accelerated under oxidising conditions.

3) Generally speaking, a high burn-up aggravated the releases, asmuch with regard to the kinetics of volatile and semi-volatile FPand Ru, as to the amplitude of the release of species with littlevolatility (Pontillon et al., 2004b). A high burn-up effectivelyencouraged the global release of all the FP, with the exceptionof volatile FP (Te, I, Cs and Sb) which had almost total releasedfractions regardless of the test atmosphere conditions and theburn-up, provided that the temperatures were representativeof a severe accident.

4) The interactions with the cladding and/or structural elementscould have a non-negligible effect. The presence of tin in theZircaloy cladding especially resulted in a delay in the emissionof volatile elements such as tellurium and antimony.

5) The nature of the fuel also seemed to play a significant role:release from MOX tended to be higher (Pontillon et al., 2003c)than from UO2, a phenomenon probably associated with theheterogeneous micro-structure, with the presence of Pu-richagglomerates and the site of a very high local burn-up.

6) The state of the fuel also clearly played a major role: the transi-tion from a “degraded rod” geometry to a “debris bed” geometrywas accompanied by an increase in released fractions and therelease kinetics of volatile FP, by increasing the surface-volumeratio. However, the transition to a “molten pool” geometryslowed down the FP released fractions. This last point was cor-roborated by all tests where fuel degradation occurred earlyin relation to a long plateau period at a high temperature, andwhere the release was generally attenuated by retention of theelements in the molten corium.

It is not surprising to find these parameters more specificallys factors strongly influencing the release of FP as it is generallyccepted that release follows a double process: (i) diffusion of theP (in solution in the matrix) as far as the grain boundaries, or dif-usion of precipitates when the solubility limit has been reached,ii) followed by a mechanism of vapourisation/transfer of massrom the surface of the grains. This latter is coupled with physical

nd chemical aspects: potential formation of defined compoundsCsI, molybdates, zirconates, caesium uranates, barium, strontium,tc.) or oxidation/reduction of the precipitates by the steam and/orydrogen, or potential chemical reactions with the “external” ele-ents (structures, neutron absorber, etc.), with these chemical

g and Design 240 (2010) 1853–1866

reactions having a significant effect on the volatility of some ele-ments.

References

Bowsher, B.R., et al., 1987. The interaction of Zircaloy cladding with fission prod-uct tellurium released during a severe reactor accident. In: Proceedings ofthe Workshop Chemical Reactivity of Oxide Fuel and Fission Product Release,Gloucestershire, England, 7–9 April, Central Electricity Generating Board, p. 455.

Collins, J.L., et al., 1987. Fission-product tellurium release behavior under severe lightwater-reactor accident conditions. Nucl. Technol. 77, 18.

De Boer, R., et al., 1995. Reaction of tellurium with Zircaloy-4. J. Nucl. Mater. 223,103.

De Boer, R., et al., 1997. The chemical form of fission product tellurium during reactoraccident conditions. J. Nucl. Mater. 240, 124.

Ducros, G., et al., 2001. Fission Product release under severe accidental conditions;general presentation of the program and synthesis of VERCORS 1 to 6 results.Nucl. Eng. Des. 208, 191–203.

Ducros, G., et al., 1996. Atmosphere dependence of fission products release: the VER-CORS 4 and 5 experiments. In: Cooperative Severe Accident Research ProgramMeeting, Washington, U.S.A.

Hobbins, R.R., et al., 1993. Fission product release from fuel under severe accidentconditions. Nucl. Technol. 101, 270.

Johnson, I., et al., 1988. Mass spectrometry studies of fission product behavior: I.Fission products released from irradiated LWR fuel. J. Nucl. Mater. 154, 67.

Kudo, T., et al., 2005. VEGA; an experimental study of radionuclides release fromfuel under severe accident conditions. In: Proceedings of Water Reactor FuelPerformance Meeting, Kyoto, Japan.

Leveque, et al., 1994. The HEVA experimental programme. Nucl. Technol. 108, 33–44.Lewis, B.J., et al., 1996. A model for nonvolatile fission product release during reactor

accident conditions. Nucl. Technol. 116, 34.Lui Z. et al., 1994. A summary of CRL fission product release measurements from UO2

samples during post-irradiation annealing (1983–1992). Report COG-92-377.Lorenz R.A., Osborne M.F., 1995. A summary of ORNL fission product release tests

with recommended release rates and diffusion coefficients. Report ORNL/TM-12801 - Nureg/CR-6261.

Noirot, J., et al., 2004. Fission Gas Inventory in PWR high burnup fuel: Experimen-tal Characterisation and Modelling. In: Proc. of the 2004 Water Reactor FuelPerformance Meeting, September 19–24, Orlando, Florida, USA.

Noirot, L., 2005. MARGARET An advanced mechanistic model of fission gas behaviorin nuclear fuel. In: Proc. of the 2005 Water Reactor Fuel Performance Meeting,October 2–6, Kyoto, Japan.

Osborne, M.F., et al., 1992. Ornl studies of fission-product release under lwr severeaccident conditions. Nucl. Safety 33, 344.

von der Hardt, P., Tattegrain, A., 1992. The Phebus fission product project. J. Nucl.Mater. 188, 115.

Pontillon, Y., et al., 2003a. Fission product release and transport in severe-accidentconditions: comparison between VERCORS HT1/HT2/HT3 experiments concern-ing UO2 fuel in reducing and oxidizing conditions with and without controlrod components. In: Cooperative Severe Accident Research Program Meeting,Bethesda, Maryland, USA.

Pontillon, Y., et al., 2003b. Fission gas release from high burn-up UO2 fuels undersimulated out-of-pile LOCA conditions. In: IAEA Technical Meeting on ImprovedFuel Pellet Materials and Design, Brussels, Belgium, October 20–24.

Pontillon, Y., et al., 2003c. Effect of high burn-up and MOX fuel on fission productrelease from VERCORS tests. In: 5◦ PHEBUS-FP Seminar, Aix en Provence, France.

Pontillon, Y., et al., 2004a. Experimental and Theoretical Investigation of FissionGas Release from UO2 up to 70 GWd/t under Simulated LOCA Type Conditions:The GASPARD Program. In: Proc. of the 2004 Water Reactor Fuel PerformanceMeeting, September 19–24, Orlando, Florida, USA.

Pontillon, Y., et al., 2004b. Lessons learnt about fission product release from highburn-up UO2 fuels under severe accident conditions: the VERCORS 6 and RT6tests. In: CSARP meeting, Washington DC.

Pontillon Y., et al., Nucl. Eng. Des., submitted for publication.Schwarz, et al., 1999. PHEBUS FP: a severe accident research programme for current

and advanced light water reactors. Nucl. Eng. Des. 187, 47–69.