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Interpretation of fission product transport and chemistry in Vercors HT and Phebus tests

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. CONTENTS 1. Objectives 2. Approach - Modelling

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Interpretation of fission product transport and chemistry in Vercors HT and Phebus tests

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  1. 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 CONTENTS 1. Objectives 2. Approach - Modelling 3. Main Experimental findings 4. Calculation results 5. Discussion (sensitivity analyses) 6. Summary - Conclusions - - 1 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  2. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Source Term Aerosol behavior In Containment Containment out-leakage Isotope treatment & Activity Aerosol nucleation/ transport in primary circuit, break and release in containment FP release, conveyed by gas emission from degraded core Lower head failure, Collapse of corium, Core/concrete int. 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 - - 2 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  3. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Objectives and means • 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 H2 and H2O with SIC/B Vercors HT1/3 for FP speciation in TGT mostly under H2 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) - - 3 Fission Product Transport Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007 VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  4. Objectives Approach Exp. Findings Calc. results Discussion Conclusions What needs to be explained in Phebus and Vercors HT RCS ? KEY POINT = VAPOUR PHASE CHEMISTRY (TGT analyses with SOPHAEROS including ext. chemical database) - - 4 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  5. Objectives Approach Exp. Findings Calc. results Discussion Conclusions 150°C 200°C 200°C TGT 800°C Zone de T variable linear thermal Gradient transition zone heating power 600 W thermal gradient tube non heated transition zone 700°C 700°C fluid fluid PHEBUS FP VERCORS HT Δtsampling~150s Δtsampling~2-3 h in the non heated part of the oven where non stationnary thermal conditions prevailed and in upstream zone of TGT significant deposits can occur - - 5 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  6. Objectives Approach Exp. Findings Calc. results Discussion Conclusions cooler  WALL Sorbtion Deposition Condensation Inlet flow Supersaturated vapours Release Aerosol Agglomeration Nucléation Gas phasechemical reactions 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.) 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) CARRIER GAS H2O, H2, O2, N2, He,Xe, Kr, Ar - - 6 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  7. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Heterogeneous nucleation or evaporation Circuit inlet Sorption Condensation Homogeneous nucleation Carrier fluid transfer (including fall back/down) Chemical equilibrium and mass balance equations • The chemical equilibrium in the gas phase is computed using constant partition for each species i and element k • 5 physical states  vapour aerosols vapour condensed onwall •  deposited aerosols sorbed vapour • Case of state  : - - 7 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  8. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Phebus test Overview Significant H2 release main Zr oxidation phase max (FPT2) : 95% FPT1 FPT2 FPT3 Main characteristics  FP  SM releases very variable and depends on fuel degradation events (50g in FPT2 -150g in FPT1) H2/H2O ratio : main H2 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 131I at point G (cold leg of RCS) FPT1 FPT2 FPT3 - - 8 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  9. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Vercors test Overview HT 3 • Main characteristics • FP releases depend on Tfuel and H2O/H2 access to fuel (≠ HT1/HT3) • H2/H2O ratio : Zr oxid. lasts ~ 40 min but low amount of H2 (15% max) • SM releases : Ag, In vaporisation starts during oxidation plateau at 1800 K, Cd at 1250 K, B2O3 only in H2 rich phase (~10 µg/s) (constant mass flow rates in tests) • volatile FP releases (Mo excepted) starts before Zr oxid. in steam, when H2 phase starts ~50% of volatiles already released; Mo release in HT3 starts during steam phase (~ 30%) Iodine - - 9 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  10. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Results: Main differences between HT1/3 and FPT2 tests • H2/H2O ratio: in FPT2 no complete steam starvation while in HT1/3 pure H2 phase • volatile FP releases: in HT3 (Cs, I, Mo) release starts during steam rich phase while in FPT2 Cs and I release initiated under H2 (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 H2in HT3 (with low Mo release) while in FPT2 ~ constant (in excess/ I & Cs); constant Cd release in HT3 while inPhebus tests release «puffs» are suspected Cs/I, Mo/Cs similar ≠ B/Cs, Re/Cs, SIC/I • 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 ? - - 10 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  11. Objectives Approach Exp. Findings Calc. results Discussion Conclusions I  Cs TGT 700 HT-1 I : 6 10-7 mol Cs : 2 10-6 mol Main experimental findings 1)FP vapour speciation (FPT2/HT1-3) ) iodine species not only depends on oxido reducing conditions BUT on FP release kinetics (molar ratios : I/Cs, Cs/Mo-B-Re)  in H2O/H2 mixtures when Cs <<< I : volatile I not associated with Cs  when Mo release low (during H2 phase) : CsI  after H2 release : large increase of Mo (CsI with others volatile I species) evidence of volatile iodine not associated to Cs in FPT2 whatever H2O/H2 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 - - 11 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  12. Objectives Approach Exp. Findings Calc. results Discussion Conclusions FPT1 scram H2 peak Main experimental findings 2) Volatile iodine formation HT-1 HT-2 HT-3 < 0.05 • 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) 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) - - 12 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  13. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Main experimental findings 3) Retention in Phebus/Vercors loops • 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 case of iodine - - 13 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  14. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Results: Revaporisation phenomena FPT2 • Evidence of partial Cs revaporisation at high T (600-700°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 In HL In H2O (700°C) decrease in Cs deposited activity HT3 In TGT In H2O (600°C) - - 14 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  15. Objectives Approach Exp. Findings Calc. results Discussion Conclusions relative fraction Cs/I10 1) CdI2 relative fraction Phebus Vercors 2) CsI 3) Volatile I Results: 1.1) Calculated integrated I speciation in Phebus and Vercors loops • In FPT0 large impact of SIC on I speciation : I (CdI2)[becauseof Cs (CsReO4)] • 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  CdI2) 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 - - 15 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  16. Objectives Approach Exp. Findings Calc. results Discussion Conclusions relative fraction Vercors relative fraction 1) CsReO4 Phebus 2) Cs2MoO4 BCsO2 Results: 1.1) Calculated integrated Cs speciation in Phebus and Vercors loops • In FPT0 large impact of Re : Cs (CsReO4) main species • Impact of B (Mo) in FPT2/3 but CsI not preventedand(Cs2MoO4 = BCsO2) • In Vercors HT1 : large fraction of Cs remained unreacted (Cs/CsOH ~ 70 %) • In Vercors HT3, though Cs2MoO4 becomes significant Cs2Te and CsI formed in similar proportions/HT1 (decreased unreacted fraction of Cs) • Compared to FPT2, in HT3 Cs2MoO4 and above all BCsO2 are formed in lower amounts (BCsO2 < Cs2Te) - - 16 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  17. Objectives Approach Exp. Findings Calc. results Discussion Conclusions CdI2 275°C 475°C CsI Results: 1.2) Predicted I & Csspeciation in FPT2 HL samplings TL(H2) 10050s TGT (H2O) 15000s Mo/Cs < 1 early release phase Mo/Cs ~ 1 late release phase 210-450°C CsI Cs2I2 RbI SOPHAEROS mainly predicts CsI and CdI2 (minor amount of Cs2I2 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) Explained as following : • when low Mo release (under H2), large amount of CsOH and HI to form CsI • when high Mo release (late H2O phase) CsOH consumes by H2MoO4 to form Cs2MoO4, leaving large fraction of HI to react with Cd at lower T • limited impact of B (especially under H2 conditions) - - 17 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  18. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Cs2Te CsOH 650°C CsI CsI 450°C BaI2 Results: 1.2) Predicted I & Cs speciation in HT1 TGT (under H2) without B and SIC • SOPHAEROS mainly predicts Cs2Te 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, Cs2MoO4 is not favoured leaving large amount of CsOH to react with HI to form CsI (in HT1 Mo starts to release only during H2 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) - - 18 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  19. Objectives Approach Exp. Findings Calc. results Discussion Conclusions at end of steam injection at test end Mo/Cs  0.7 Mo/Cs  1.5 Cs2MoO4 Cs2MoO4 Results: 1.2) Predicted Cs speciation in HT3: Cs revaporisation • SOPHAEROS predicts partial vaporisation (~50%) of Cs2MoO4 deposited during the steam injection phase : explains by a decrease of Mo release kinetics during the subsequent H2 phase • in HT3 due to early release of Mo (under H2O : > 20 % i.i. is released) Mo has a large impact on Cs speciation in calculations : • totally preventing CsI formation during H2O phase • partly inhibiting its condensation (Cs2Te) in TGT during H2 phase • CsI is the only iodine species formed and predicted : no interaction of I with Cd - - 19 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  20. Objectives Approach Exp. Findings Calc. results Discussion Conclusions at test end Cs2Te CsI 650°C ~450°C Results: 1.2) Predicted FP speciation in HT3 TGT • Cs-Te species formed under H2 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 (In2Te) because In mainly transported as aerosols - - 20 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  21. Objectives Approach Exp. Findings Calc. results Discussion Conclusions measured in containment 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 H2O/H2 as measured  Some other volatile iodine species are also predicted but only if no Cd In HT1(~FPT3 but without B) no HI is predicted in agreement with exp.data (only CsI due to limited Mo (metallic) release under H2) - - 21 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  22. Objectives Approach Exp. Findings Calc. results Discussion Conclusions downstream furnace tube TGT HT3 Results: 3) FP retention in Phebus total iodine retention factor overestimated by  1.8 mainly due to overestimation in SG in Vercors : • 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 - - 22 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  23. Objectives Approach Exp. Findings Calc. results Discussion Conclusions 700°C CdI2 CsI H2MoO4 CsOH 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 factorin 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 • at 700°C, high volatility of H2MoO4that leads to significant formation of Cs2MoO4 (CsI prevented and large fraction of HI) • if Psat (H2MoO4) is decreased ( MoO3) • CsI (RbI) is formed (Cd do not compete • with CsOH to form CdI2) • in HT3 Cs2MoO4 predominant during H2O phase, CsI-Cs2Te then formed during H2 - - 23 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  24. Objectives Approach Exp. Findings Calc. results Discussion Conclusions  cooling VERY HIGH T up to 2800°C HIGH Temperature 2800 <T< 700°C Upper Plenum Hot leg CORE FP/SM RELEASES CORE FUSION CsI ↓ Cs2MoO4↓ CsReO4 ↓ BCsO2 ↓ Cs2Te ↓ HI  H2MoO4 ReO H3B3O6  CsOH+ Cd Overview of “FP” Chemistry in RCS gas phase COLD Temperature 700 <T< 150°C Steam Generator Cold leg FP/SM RETENTION  condensation   condensation  I HI, I, (CsI) Cs CsOH, Cs, (Cs2MoO4) Mo MoO3, H2MoO4 ReReO, CsReO4,(ReO2Re2O7) B BO2,H3B3O6 (BO HBO2 H3BO3) BH3, BH2,B (in H2) Cd Cd Te H2Te, SnTe ,CsTe, AgTe (in H2) Cd + HI CdI2↓ ---- gas----vapour ---- condensed state Potential kinetics limitations due locally to low residence time and high T - - 24 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  25. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Summary and Conclusions  Volatile iodine species at low T SOPHAEROS implies strong sensitivity to Cd, main species = HI (small amounts of SnI2, SnI4 and I2MoO2 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 H2 (Ag, In mainly under metallic aerosol forms, HI completely react with Cs) analysis of VERCORS HT2 test : impact of SIC under H20 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) - - 25 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

  26. Objectives Approach Exp. Findings Calc. results Discussion Conclusions Summary and Conclusions Iodine & Caesium vapour speciation good agreement between iodine exp. data and SOPHAEROS only when measured I is associated with Cs  Cs2MoO4 favoured to detriment of CsI with high Mo release (under H2O), CsI/Cs2Te with low Mo release under H2  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 CdI2 ?) : 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 ?) - - 26 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

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