Towards a better understanding of iodine chemistry in RCS of nuclear reactors

1. Towards a better understanding of iodine chemistry in RCS of nuclear reactors N. Girault, C. Fiche, A. Bujan and J. Dienstbier CONTENTS 1. Objectives 2. Approach 3. Main Experimental findings 4. Calculation results 5. Discussion (sensitivity analyses) 6. Summary - Conclusions - - 1 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007

2. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 2 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Objectives - Context HITEMP circle objectives and means  provide predictability of iodine species exiting the RCS in 900/1300 PWR undergoing a severe accident through different scenarios containment chemistry analyses can not explain the early gaseous iodine fraction measured nearly after the main hydrogen release  analyses of PHEBUS FP and VERCORS HT tests with SOPHAEROS (equilibrium chemistry in gas)to investigate iodine speciation within different oxido-reducing conditions and release kinetics of SIC (or B4C for FPT3 test) 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. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 3 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 What needs to be explained in Phebus RCS ? KEY POINT = VAPOUR PHASE CHEMISTRY

4. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 4 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Approach : analyses of Phebus tests with SOPHAEROS Impact of H2O/H2 Impact of Ag/In/Cd and B  investigation of iodine behaviour in Phebus RCS :  total iodine retention in RCS (in vertical line and SG hot leg)  volatile iodine formation at low T (cold leg)  iodine vapour speciation between 700 and 150°C in FPT2 TGT and TL (FPT3)

5. Objectives Approach Exp. Findings Calc. results Discussion Conclusions 200°C fluid 700°C fluid - - 5 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Thermal Gradient Tubes (3) Transition Lines (4) 700°C filters 150°C oven transition lines t =180s t0 150°C filters Ins.

6. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 6 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Phebus test Overview Significant H2 release main Zr oxidation phase FPT1 FPT2 FPT3 Main characteristics  AIC rod bursts at ~8700-9700s B4C rod bursts at ~ 9550 s  main H2peak lasts ~2 (FPT1) to 20 min (FPT2/3),  release phase initiated during Zr oxidation phase, lasts ~2 hr  FP  SM release rates are very variable and depends on fuel degradation events (release 50g in FPT2 -150g in FPT1/0) 131I at point G (cold leg of RCS) FPT1 FPT2 FPT3

7. Objectives Approach Exp. Findings Calc. results Discussion Conclusions I  Cs - - 7 - - 7 Source Term Issues Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 ERMSAR 2007, Karlsrühe, 12-14 June 2007 Main experimental findings iodine 1)Iodine vapour speciation (FPT2/3) iodine species not only depends on oxido reducing conditions BUT on FP release kinetics (molar ratios : I/Cs, Cs/Mo-B-Re)  before H2 release : Cs <<< I  during H2 release : large increase of Cs release; Mo remaining low  after H2 release : large increase of Mo evidence of volatile iodine not associated to Cs in FPT2 whatever H2O/H2 and releases (also seen in FPT3 with evenmore iodine condensation peaks )

8. Objectives Approach Exp. Findings Calc. results Discussion Conclusions 325-450°C 190-210°C - - 8 - - 8 Source Term Issues Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 ERMSAR 2007, Karlsrühe, 12-14 June 2007 …. during late steam-rich phase In transition Line I  Cs 700°C I - Cs 450-550°C  despite large Mo release (Cs/Mo  1), CsI is formed

9. Objectives Approach Exp. Findings Calc. results Discussion Conclusions FPT1 scram H2 peak - - 9 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Main experimental findings 2) Volatile iodine formation no direct evidence of volatile I in primary circuit BUT early detection of gaseous iodine in containment higher fraction in tests with higher steam flow rate (FPT0/1) or without SIC (FPT3)

10. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 10 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Main experimental findings 3) Iodine retention in RCS  quitesimilar iodine deposits in FPT1/2 may be less in FPT3 due to large fraction of gaseous iodine in FPT1/2/3 iodine mainly deposited in SG (taking into account upstream part) : 25-30% of deposited iodine  5-10% of iodine deposited in UP+VL : in FPT2 and FPT3 iodine deposited in UP rather than in VL  significant deposition of iodine in cold leg in FPT2 could be due to aerosol sedimentation in lower steam flow rate

11. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 11 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Results: 1.1) Integrated I  Cs speciation predicted in RCS Caesium Iodine 1) CdI2 1) CsReO4 2) CsI 2) Cs2MoO4 BCsO2 3) Volatile I In FPT0/1 large impact of Re and SIC : I (CdI2)andCs (CsReO4) Impact of B in FPT2/3 and small/no AIC: I (CsI)andCs (Cs2MoO4 = BCsO2) More volatile iodine species in FPT3 because no Cd (Cd + HI  CdI2) and in FPT0 because less Cs/CsOH (CsOH + HI  CsI)

12. Objectives Approach Exp. Findings Calc. results Discussion Conclusions CdI2 275°C 475°C CsI - - 12 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Results: 1.2) Predicted I speciation in HL samplings TL(H2) 10050s TGT (H2O) 15000s 210-450°C CsI Cs2I2 RbI SOPHAEROS mainly predict 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 TL 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 H2MoO4to form Cs2MoO4, leaving large fraction of HI to react with Cd

13. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 13 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 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

14. Objectives Approach Exp. Findings Calc. results Discussion Conclusions FPT2 G steam generator upper plenum vertical line C - - 14 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Results: 3) I retention in RCS in VLisunderestimated  underestimation of CsI condensation  improved calculations by considering non developped flows in SGis overestimated by 2-3 overestimation of CsI thermophoresis uncertainties on Cd release kinetics Total iodine retention factor overestimated by  1.8

15. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 15 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Sensitivity analyses Discussion: Impact of Cd release kinetics Large uncertainties on Cd release + potential release in puffs steam generator 0.6 0.2 FPT0 Assuming a continuous Cd release leads to overprediction of I retention in SG AND low amount of volatile HI mainly in FPT0/1 tests in which CdI2 prevailed on CsI When limited Cd release kinetics is assumed better agreement with I retention factorin SG BUT overprediction by 10 of HI formation (consistent with a higher volatile I fraction measured in FPT3)

16. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 16 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Sensitivity analyses Discussion: Impact of Mo chemistry 700°C  at 700°C, high volatility of H2MoO4 :  significant formation of Cs2MoO4  formation of CsI prevented large fraction of volatile HI if Psat (H2MoO4) is decreased ( MoO3) CsI (RbI) is formed (Cd do not compete with CsOH to form CdI2) CdI2 CsI CsOH H2MoO4 depending on Mo species more or less CsI is calculated Cs2MoO4 is generally too much favoured to the detriment of CsI

17. Objectives Approach Exp. Findings Calc. results Discussion Conclusions ALFA (1/0.1)  1.75 - - 17 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Sensitivity analyses Discussion: Impact of aerosol thermophoresis modelling Talbot formulation function of (fluid & partic. properties) is used to calculate particle THP deposition velocity Fluidprop. depend on “boundary layer” T  All “fluid properties” & Talbot correlation coded to separate “talbot.f” to evaluate influence of boundary layer temperature : Tfluid properties = ALFA * Tf + (1.0 – ALFA) * Tw with 0.1 ≤ ALFA ≤ 1.0 (near to wall T) ALFA (1/0.1)< 1.2 with ALFA = 0.1 in SG inlet (high T) slightly decreased iodine aerosol deposition (NOT sufficiently impact total retention in SG because predominant phenomenon is CdI2 condensation)

18. 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 ↓ HI  H2MoO4 ReO H3B3O6  CsOH+ Cd - - 18 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Overview of Iodine 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) Cd Cd Cd + HI CdI2↓ ---- gas----vapour ---- condensed state Potential kinetics limitations due locally to low residence time and high T

19. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 19 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Summary and Conclusions Iodine retention in primary circuit : Base-case calculations overall retention factor overestimated by about 2  nearly no retention of I predicted in UP+VL whereas measured RF is about 0.09 in FPT2 (0.07-0.04 in FPT1/3) retention in SG overestimated by 2-3, mainly CdI2condensation Improved calculations by considering non fully developed flows in UP/VL near to wall temperature in SG for THP modelling

20. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 20 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 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 tests (FPT3 excepted) when volatile iodine amount is correctly predicted, iodine condensation in SG is under estimated and vice-versa FPT3 calc. results shows a very high volatile I fraction (18% /i.b.i) when Cd is completely missing in accordance with exp. data (27%) CHIP programme (investigation of non–equilibrium effects) Analysis of VERCORS HT tests : impact of SIC materials

21. Objectives Approach Exp. Findings Calc. results Discussion Conclusions - - 21 Source Term Issues ERMSAR 2007, Karlsrühe, 12-14 June 2007 Summary and Conclusions Iodine vapour speciation Base-case calculations good agreement between exp. data and SOPHAEROS only when measured I is associated with Cs  Cs2MoO4 too much favoured to detriment of CsI with high Mo release,CsI with low Mo release volatile iodine measured with low Cs or high Mo release not well predicted (only CdI2 ?) : impact of others SIC/SM (In, Fe, Cr, Si…) VERCORS HT1,2,3 and FPT3 Sensitivity calculations  depending on Mo chemistry ode more or less CsI is calculated CHIP (investigation of simplified systems under equilibrium) Continuous check/development of MDB (polymolybdates ?)