Interpretation of fission product transport and chemistry in Vercors HT and Phebus tests N. Girault, C. Fiche Institut de Radioprotection et de Sûreté Nucléaire Direction de la Prévention des Accidents Majeurs VERCORS SEMINAR, Gréoux, October th, 2007 CONTENTS CONTENTS 1. Objectives 2. Approach - Modelling 3. Main Experimental findings 4. Calculation results 5. Discussion (sensitivity analyses) 6. Summary - Conclusions Fission Product Transport

Nuclear power plant context : stakes ? What importance ? FP transport/retention in primary circuit determines the source term  significant FP deposition  for containment by-pass sequences, FP retention in primary circuit is the only possibility to reduce radioactive releases in the environment  possible delayed FP releases (re- vaporisation is a source term factor in late phase)  at the break, aerosol/vapour-gas split for some FP (especially for Ru and I which poses main short-term radiological risk for human populations) Obtain a realistic assessment of possible releases in environment to optimise management of accident’s consequences ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

 Provide predictability of FP retention in primary circuit (I, Te, Cs, Ru)  Provide predictability of volatile iodine (and Ru) speciation exiting the RCS in 900/1300 PWR undergoing a severe accident through different scenarios Phebus containment chemistry analyses can not explain the early gaseous iodine fraction  Analyses of PHEBUS FP (integral) and VERCORS HT (analytical) tests with SOPHAEROS (equilibrium chemistry in gas) to investigate FP retention and speciation within different oxido-reducing conditions and SIC/B release kinetics Phebus FPT2 for FP speciation in TGT/TL under H 2 and H 2 O with SIC/B Vercors HT1/3 for FP speciation in TGT mostly under H 2 with SIC/B Earlier and on-going work  SOPHAEROS analyses continuously progressed with regards to thermo dynamic code MTDATA/SGTE (check of thermodynamic data of elements)  besides analysis of potential chemical kinetics limitations (because of low residence time, strong thermal gradient in some parts of RCS (CHIP) Objectives and means ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

What needs to be explained in Phebus and Vercors HT RCS ? KEY POINT = VAPOUR PHASE CHEMISTRY (TGT analyses with SOPHAEROS including ext. chemical database) ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

in the non heated part of the oven where non stationnary thermal conditions prevailed and in upstream zone of TGT significant deposits can occur VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport VERCORS HT ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings PHEBUS FP Δt sampling ~150s 200°C 700°C fluid 200°C 700°C fluid thermal gradient tube Δt sampling ~2-3 h transition zone TGT Zone de T variable heating power 600 W linear thermal Gradient non heated transition zone 800°C 150°C

Transport phenomenology Simultaneous occurrence of: chemical interactions (vapour-vapour, vapour-surfaces) vapour supersaturation  condensation on structures, aerosol formation aerosol agglomeration & deposition (phoretic effects, diffusion, etc.)  cooler  Gas phase chemical reactions Inlet flowSupersaturated vapours Condensation Deposition Release Agglomeration Sorbtion Nucléation Aerosol WALL ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport CARRIER GAS H 2 O, H 2, O 2, N 2, He,Xe, Kr, Ar Important assumption : time constant of gaseous reaction is sufficiently smaller than that of vapour condensation (gas species essentially in local chemical equilibrium but may be in non equilibrium with respect to the condensed species)

Chemical equilibrium and mass balance equations oThe chemical equilibrium in the gas phase is computed using constant partition for each species i and element k ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings Fission Product TransportVERCORS SEMINAR, Gréoux, October th, 2007 o 5 physical states   vapour  aerosols vapour condensed on wall  deposited aerosols sorbed vapour Case of state  : Heterogeneous nucleation or evaporation Circuit inlet Sorption Condensation Homogeneous nucleation Carrier fluid transfer (including fall back/down)

ApproachCalc. resultsConclusionsObjectivesDiscussion Phebus test Overview Main characteristics  FP  SM releases very variable and depends on fuel degradation events (50g in FPT2 -150g in FPT1)  H 2/ H 2 O ratio : main H 2 peak lasts ~ 2 (FPT1) to 20 min (FPT2/3) : never complete steam starvation  SM releases : most of SIC release in oxidising conditions, B constant in FPT2 (0.5 µg/s)  volatile FP releases (Mo excepted) initiated during Zr oxidation phase, Mo release starts after this phase Significant H 2 release main Zr oxidation phase max (FPT2) : 95% Exp. Findings FPT1 FPT2 FPT3 131 I at point G (cold leg of RCS) FPT1 FPT2 FPT VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

HT 3 Vercors test Overview Main characteristics  FP releases depend on T fuel and H 2 O/H 2 access to fuel (≠ HT1/HT3)  H 2 /H 2 O ratio : Zr oxid. lasts ~ 40 min but low amount of H 2 (15% max)  SM releases : Ag, In vaporisation starts during oxidation plateau at 1800 K, Cd at 1250 K, B 2 O 3 only in H 2 rich phase (~ 10 µg/s) (constant mass flow rates in tests)  volatile FP releases (Mo excepted) starts before Zr oxid. in steam, when H 2 phase starts ~50% of volatiles already released; Mo release in HT3 starts during steam phase (~ 30%) Iodine VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings

ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings Results: Main differences between HT1/3 and FPT2 tests  H 2 /H 2 O ratio: in FPT2 no complete steam starvation while in HT1/3 pure H 2 phase  volatile FP releases: in HT3 (Cs, I, Mo) release starts during steam rich phase while in FPT2 Cs and I release initiated under H 2 (with no Mo); lower Te/Cs in FPT2/3 TGT due to high its retention in hot zones  SM releases: Ag, In and B mainly released under H 2 in HT3 (with low Mo release) while in FPT2 ~ constant (in excess/ I & Cs); constant Cd release in HT3 while in Phebus tests release «puffs» are suspected  Others : fluid velocity ~2 times higher in FPT2/3 (≥1m/s) but conc. 50 times higher : impact on gas phase chemistry kinetics ? (no gas. iodine measured but upstream high capacity filter could trapped gaseous iodine if any (  res 3 s))  Res. times upstream TGT very low in Vercors: impact on aerosol/vapour split ? VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport Cs/I, Mo/Cs similar ≠ B/Cs, Re/Cs, SIC/I

 iodine species not only depends on oxido reducing conditions BUT on FP release kinetics (molar ratios : I/Cs, Cs/Mo-B-Re)  in H 2 O/H 2 mixtures when Cs <<< I : volatile I not associated with Cs  when Mo release low (during H 2 phase) : CsI  after H 2 release : large increase of Mo (CsI with others volatile I species)  evidence of volatile iodine not associated to Cs in FPT2 whatever H 2 O/H 2 and releases  in Vercors I always associated to Cs even in HT2-3 with no clear impact of SIC (B)  Te condensation in TGT suggests Cs- tellurides in both HT1-3 tests Main experimental findings 1) FP vapour speciation (FPT2/HT1-3) ) ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport I  Cs TGT 700 Cs : mol I : mol HT-1

Main experimental findings 2) Volatile iodine formation Total Volatile iodine (% i.b.i.) Experiment FPT-0  2 FPT-1< 1 FPT-2< 0.1 FPT-3  30 In Phebus tests :  no direct evidence of gaseous I in primary circuit (FPT3 excepted) BUT early detection of gaseous iodine in containment,  higher fraction in FPT0 (higher steam flow rate/lower conc.) and without SIC (FPT3) ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings FPT1 H 2 peak scram In Vercors HT tests (to be compared to FPT2 for fluid velocity):  no direct evidence of gaseous I in the gaseous bulb nor in maypack downstream TGT (<0.05 % detection limit) HT-1 HT-2 HT-3 < VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

 iodine mainly deposited in SG in Phebus and in TGT in Vercors (  25-40% )  in Vercors, I retention in TGT more significant in HT3 (under H2 + SIC)  low I retention in Phebus UP : 5-10%, no I retention in Vercors hot zones Other FP retention (Cs, Te, Ru)  high Te retention upstream SG in all Phebus tests (  20-40%); high Cs (Mo) retention in FPT1 UP (  40%)  in Vercors, Cs, Te retention favoured in hot zones in oxidising conditions (HT2 :  18% compared to 4% in HT3  Ru retention favoured in hot zones but in oxid. conditions 5% deposited in TGT Main experimental findings 3) Retention in Phebus/Vercors loops ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport case of iodine

Results: Revaporisation phenomena decrease in Cs deposited activity HT3 In TGT In H 2 O (600°C) In HL In H 2 O (700°C) FPT2  Evidence of partial Cs revaporisation at high T ( °C) from SS (HT-3) and inconel surfaces (FPT-2 after core shutdown) in steam rich conditions  no significant decrease of I, Te, Mo deposited activity in HT-3/FPT2  no significant revaporisation of Cs in HT1 ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

Results: 1.1) Calculated integrated I speciation in Phebus and Vercors loops  In FPT0 large impact of SIC on I speciation : I (CdI 2 ) [because of Cs (CsReO 4 )]  In FPT2/3 small/no AIC impact : I (CsI) (CsOH not fully consumed by Mo and B)  More volatile iodine species (HI) in FPT3 because no Cd (Cd + HI  CdI 2 ) and in FPT0 because less Cs/CsOH to react with (CsOH + HI  CsI)  In Vercors main predicted I species is CsI in HT1/3 (in agreement with FPT2/3) calculations and with exp. results ; no HI is predicted ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport 1) CdI 2 2) CsI 3) Volatile I Phebus relative fraction Vercors relative fraction Cs/I  10

ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport 1.1) Calculated integrated Cs speciation in Phebus and Vercors loops Results:  In FPT0 large impact of Re : Cs (CsReO 4 ) main species  Impact of B (Mo) in FPT2/3 but CsI not prevented and (Cs 2 MoO 4 = BCsO 2 )  In Vercors HT1 : large fraction of Cs remained unreacted (Cs/CsOH ~ 70 %)  In Vercors HT3, though Cs 2 MoO 4 becomes significant Cs 2 Te and CsI formed in similar proportions/HT1 (decreased unreacted fraction of Cs)  Compared to FPT2, in HT3 Cs 2 MoO 4 and above all BCsO 2 are formed in lower amounts (BCsO 2 < Cs 2 Te) relative fraction Vercors 1) CsReO 4 Phebus relative fraction 2) Cs 2 MoO 4 BCsO 2

Results: 1.2) Predicted I & Cs speciation in FPT2 HL samplings SOPHAEROS mainly predicts CsI and CdI 2 (minor amount of Cs 2 I 2 and RbI) :  in accordance with similar deposition profiles in 2/3 TGT for I and Cs  in contradiction with no/small Cs detection in some TL (trigerred during early release phase) ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings CsI Cs 2 I °C RbI TL(H 2 ) 10050s Explained as following :  when low Mo release (under H 2 ), large amount of CsOH and HI to form CsI  when high Mo release (late H 2 O phase) CsOH consumes by H 2 MoO 4 to form Cs 2 MoO 4, leaving large fraction of HI to react with Cd at lower T  limited impact of B (especially under H 2 conditions) CsI  475°C 275°C CdI 2 TGT (H 2 O) 15000s Mo/Cs < 1 early release phase Mo/Cs ~ 1 late release phase VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings Results: 1.2) Predicted I & Cs speciation in HT1 TGT (under H 2 ) without B and SIC  SOPHAEROS mainly predicts Cs 2 Te and CsI too a less extent in agreement with exp. condensation profiles of Cs, I and Te in TGT (small CsOH chemisorption at inlet tube)  As in reducing phases of Phebus, Cs 2 MoO 4 is not favoured leaving large amount of CsOH to react with HI to form CsI (in HT1 Mo starts to release only during H 2 release phase)  Main difference with Phebus tests is Cs-Te species condensation in TGT; in FPT2 these species were not evidenced in TGT (may be deposited upstream) VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport BaI 2 CsI 450°C Cs 2 Te CsI 650°C CsOH

Results: 1.2) Predicted Cs speciation in HT3: Cs revaporisation ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport  SOPHAEROS predicts partial vaporisation (~50%) of Cs 2 MoO 4 deposited during the steam injection phase : explains by a decrease of Mo release kinetics during the subsequent H 2 phase  in HT3 due to early release of Mo (under H 2 O : > 20 % i.i. is released) Mo has a large impact on Cs speciation in calculations : totally preventing CsI formation during H 2 O phase partly inhibiting its condensation (Cs 2 Te) in TGT during H 2 phase  CsI is the only iodine species formed and predicted : no interaction of I with Cd Cs 2 MoO 4 Mo/Cs  1.5 at end of steam injection Cs 2 MoO 4 Mo/Cs  0.7 at test end

Results: 1.2) Predicted FP speciation in HT3 TGT  Cs-Te species formed under H 2 release rapidly become dominant species (in calc. Cs-Te condensation in TGT disturbed by aerosol particles, mainly metallic B)  no predicted interaction of Cd with I because all iodine has reacted with Cs in TGT  similar calculated behaviour for B, Ag and In that nucleates as metallic particles (limiting their interaction with FP, while metallic Cd vapours predicted to condense at TGT outlet This explains :  small impact of B on Cs speciation : boron mainly deposited in furnace tube and in TGT by thermophoresis  only small interaction of In with Te (In 2 Te) because In mainly transported as aerosols ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport at test end Cs 2 Te CsI 650°C ~450°C

Results: 2) Volatile iodine at low T  According to base-case analysis, HI is the main candidate in FPT0/1/2  HI predicted amount doesn’t significantly depend on H 2 O/H 2 as measured  Some other volatile iodine species are also predicted but only if no Cd ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings  In HT1(~FPT3 but without B) no HI is predicted in agreement with exp.data (only CsI due to limited Mo (metallic) release under H 2 ) VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport measured in containment

Results: 3) FP retention ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport Basic Analysis Experiment I RCS (%) FPT-0 FPT-1 FPT-2 FPT-3 ~ 53 ~ 42 ~ 28 ~ 19 > 30 ~ 25 > 18 > 15 in Phebus total iodine retention factor overestimated by  1.8 mainly due to overestimation in SG  volatiles (I,Cs,Te) : low retention of in hot zones well reproduced while retention in TGT well calculated (40- 60%)  low volatiles (Ba, Ru) : underestimation of their retention in hot zones  metallic particles were found to disturb FP condensation in TGT in calculations aerosol /vapour split need to be investigated in Vercors : HT3 furnace tube TGT downstream

Discussion: Impact of Cd and Mo release kinetics  assuming a continuous Cd release leads to overprediction of I retention in SG AND low amount of volatile HI (cf FPT0/1)  if limited, a better agreement with I retention factor in SG BUT overprediction by 10 of HI formation (cf FPT3)  in HT3, complete I consumption by Cs prevents any reaction of Cd vapour with I Total volatile IBasic Analysis Exp. (% i.b.i.) FPT-2 non limited Cd FPT-2 limited Cd ~ 0.03 ~9 < 0.1 ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport  at 700°C, high volatility of H 2 MoO 4 that leads to significant formation of Cs 2 MoO 4 (CsI prevented and large fraction of HI)  if P sat (H 2 MoO 4 ) is decreased (  MoO 3 ) CsI (RbI) is formed (Cd do not compete with CsOH to form CdI 2 )  in HT3 Cs 2 MoO 4 predominant during H 2 O phase, CsI-Cs 2 Te then formed during H 2 CdI 2 CsOH CsI H 2 MoO 4 700°C

Overview of “FP” Chemistry in RCS gas phase Steam Generator Cold leg COLD Temperature 700 <T< 150°C  condensation  CORE VERY HIGH T up to 2800°C CORE FUSION Upper Plenum Hot leg HIGH Temperature 2800 <T< 700°C FP/SM RELEASES FP/SM RETENTION I HI, I, (CsI) Cs CsOH, Cs, (Cs 2 MoO 4 ) Mo MoO 3, H 2 MoO 4 Re ReO, CsReO 4,( ReO 2 Re 2 O 7 ) B BO 2,H 3 B 3 O 6 (BO HBO 2 H 3 BO 3 ) BH 3, BH 2, B (in H 2 ) Cd Te H 2 Te, SnTe,CsTe, AgTe (in H 2 ) Cd + HI  CdI 2 ↓ CsOH + Cd HI  H 2 MoO 4  ReO  H 3 B 3 O 6  CsI ↓ Cs 2 MoO 4 ↓ CsReO 4 ↓ BCsO 2 ↓ Cs 2 Te ↓ Potential kinetics limitations due locally to low residence time and high  T ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings  cooling  ---- gas ---- vapour ---- condensed state VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings Summary and Conclusions  Volatile iodine species at low T  SOPHAEROS implies strong sensitivity to Cd, main species = HI (small amounts of SnI 2, SnI 4 and I 2 MoO 2 in FPT3 with no Cd) in Phebus (FPT3 excepted) when volatile iodine correctly predicted, its condensation in SG is under estimated  FPT3 calc. results show a very high volatile I fraction (18% /i.b.i) when Cd is completely missing in accordance with exp. data (~30%) not observed in Vercors (HT1) because of Cs not completely consumed by Mo  no clear impact of SIC materials in Vercors HT tests under H 2 (Ag, In mainly under metallic aerosol forms, HI completely react with Cs) analysis of VERCORS HT2 test : impact of SIC under H 2 0  in Phebus more volatile iodine measured in FPT0 with high flow rate and low FP concentrations role of kinetics limitations still need to be investigated (CHIP) VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport

Summary and Conclusions  Iodine & Caesium vapour speciation  good agreement between iodine exp. data and SOPHAEROS only when measured I is associated with Cs  Cs 2 MoO 4 favoured to detriment of CsI with high Mo release (under H 2 O), CsI/Cs 2 Te with low Mo release under H 2  limited impact of B on Cs speciation (too much ?)  volatile iodine in Phebus only with low Cs or high Mo release not well predicted (only CdI 2 ?) : impact of others SM (Fe, Cr, Si…) VERCORS HT2 and FPT3 Sensitivity calculations  depending on Mo chemistry more or less CsI is calculated CHIP (investigation of simplified systems under equilibrium) Continuous check/development of MDB (polymolybdates ?) ApproachCalc. resultsConclusionsObjectivesDiscussionExp. Findings VERCORS SEMINAR, Gréoux, October th, 2007Fission Product Transport