Reactor Safety Division, Bhabha Atomic Research Center, Mumbai (INDIA)

1. Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000Sunil Ganju, B. Chatterjee, D. Mukhopadhyay, R. K. Singh, H. G. Lele and A. K. Ghosh Reactor Safety Division, Bhabha Atomic Research Center, Mumbai (INDIA)

2. OVERVIEW OF IRSN-BARC COOPERATION STUDIES BRIEF DESCRIPTION OF IPHWRs (Indian Pressurised Heavy Water Reactors) IPHWR SAFETY STUDIES WITH ASTECThermal-hydraulic and hydrogen distribution behaviour in the containment. Simulation of aerosol behaviour experiments in NATF facility at BARC. Aerosol and FP transport in the primary heat transport (PHT) system and containment.VVER SAFETY STUDIES WITH ASTECCONCLUSIONS AND FUTURE PLANS Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000

3. IRSN-BARC COOPERATION IN THE FIELD OF NUCLEAR REACTOR SAFETY ASTEC V1 CONSTITUTIVE MODULES CPA IODE SYSINT Thermalhydraulics & Aerosol behaviour In containment Iodine in Safety system management containment ISODOP Isotope treatment & Activity Isotope treatment & activity SOPHAEROS Aerosol & FP vapour behaviour in RCS CESAR Thermal. in RCS DIVA DIVA Core degradation ELSA FP release CORIUM Corium in containment MEDICIS RUPUICUV Corium/Concrete Interaction Corium ejection & Entrainment in cont. • COOPERATION ON CONTAINMENT SAFETY(STC-1, May 2000) • Area of Cooperation: Development and Validation of code ASTEC (jointly developed by IRSN, France, and GRS, Germany) for containment and PHT thermal hydraulics and aerosol behaviour. • ASTEC applications for • Containment thermal hydraulics and aerosol behaviour studies in IPHWRs • PHT system aerosol behaviour studies in IPHWRs • PHT system thermal-hydraulic studies in IPHWRs • Safety studies for VVER-1000

4. Salient Features of 220 MWe INDIAN PHWRs Pressurised Heavy Water Reactor system Figure ‘Figure-of-Eight’ layout used in PHT permits coolant in each circuit to make two passes through the core. Figure Calandria penetrated by 306 horizontal fuel channels (220 MWe) Figure Zircaloy pressure tube with stainless steel end fittings Figure Twelve fuel bundles in each channel with heavy water coolant Figure Pressure tube surrounded by a concentric calandria tube, which keeps the low-temperature heavy water moderator in the calandria thermally isolated from the hot fuel channels Figure   Presence of large heat sink provided by cold mass of moderator gives high assurance of fuel channel integrity and helps prevent fuel melting in case of accidents No large pressure vessel in the coolant loop. Largest rupture limited to the size of the reactor headers or a break in the main steam line

5. Salient Features of 220 MWe INDIAN PHWRs CONTAINMENT SYSTEM DOUBLE CONTAINMENT PRINCIPLE Primary Containment Pre-stressed concrete shell (42 m. diam.) Design pressure = 270.98 KPa Secondary Containment Reinforced concrete shell Design pressure = 108.483 KPa TWO ACCIDENT BASED VOLUMES ENGINEERED SAFETY FEATURES Containment Isolation, RB Coolers, PCFPB, PCCD, SCFRP

6. Salient Features of 220 MWe INDIAN PHWRs

7. Salient Features of 220 MWe INDIAN PHWRs

8. IPHWR SAFETY STUDIES WITH ASTEC • Thermal-hydraulic and hydrogen distribution behaviour in the containment Accident Sequence Hypothetical dual failure: LOCA in RIH with unavailability or loss of ECCS 46 kg H2 released over a period of about 8.5 hours in the transient 63,000 kg steam released within a period of 120 seconds CPA (containment module) application: Base Case 11 non-equilibrium zones for the various compartments and one DRASYS zone for suppression pool Variants: Influence of Safety Options on H2 Distribution Behaviour in containment: - Passive Ducting between the two Fuelling Machine Vaults - Forced Circulation of air at a defined flow rate from the break compartment into the Pump Room Vaults

9. IPHWR SAFETY STUDIES WITH ASTEC Thermal-hydraulic and hydrogen distribution behaviour in containment

10. IPHWR SAFETY STUDIES WITH ASTEC Thermal-hydraulic and hydrogen distribution behaviour in containment FURTHER STUDIES COMPLETED Progressive refinement in the nodalisation scheme to achieve Grid-insensitive nodalisation 31-volume nodalisation study completed Cases Studied Case 1: Total 14 zones, zone ZFMV1 divided into 2 sub zones. Case 2: Total 11 zones. The Dome and Boiler Room considered as one zone and the zone ZFMV1 divided in to 4 sub volumes as in Case 1. Case 3: Total 13 zones. ZFMV1 divided in 4 sub-compartments and only one zone considered for the Dome and Boiler Room. Case 4: Total 15 zones. ZFMV1 divided in 4 sub-compartments, three zones considered for the Dome and Boiler Room. 4 sub-cases considered by changing the inter- compartment junction definitions. Case5: Total 31 zones. All zones defined in the base case split into 4 sub- volumes except the Suppression Pool (ZSPOOL), Vent Shaft (ZSHFT) and the V2 volumes

11. IPHWR SAFETY STUDIES WITH ASTEC Thermal-hydraulic and hydrogen distribution behaviour in containment RESULTS, DISCUSSION • Results highly dependent on nodalisation refinement: -Affects peak H2 conc. in a volume. - Affects time required to homogenise hydrogen across various volumes. • More detailed nodalisation studies is imperative. 100-volume nodalisation scheme being developed for analysis.

12. IPHWR SAFETY STUDIES WITH ASTEC Simulation of aerosol experiments conducted in the ‘Nuclear Aerosol Test Facility’ (NATF). NATF • Objectives - Investigation of spatial and temporal behavior of metallic or oxide aerosols under simulated post-accident contt. atmospheres. - Study of aerosol scrubbing in a water pool simulating the suppression pool of nuclear power plants. - Validation of existing computer codes for aerosol behavior studies in the containment of NPPs. - Estimation of source term for atmosphere dispersion models. • Set-Up - Cylindrical test vessel, 2.25 m. diam., 2 m. height (Vol.= 10 m3). - Plasma Torch Aerosol Generator. - Instrumentation for aerosol characterisation and thermo-hydraulic parameters.

13. IPHWR SAFETY STUDIES WITH ASTEC Simulation of aerosol experiments conducted in the ‘Nuclear Aerosol Test Facility’ (NATF). NATF • CPA simulation of aerosol experiments in NATF - Nodalisation of the single-volume geometry into 1, 6, or 12 ‘zones’ and study of the spatial and temporal aerosol behaviour. - Reasonable agreement on peak average aerosol concentration. Good agreement on decay of aerosol concentration in the vessel with time. - Parametric studies on influence of various aerosol parameters viz. aerosol density, particle size range, and aerosol phenomena (thermophoresis, diffusiophoresis, etc.). - Total of about 15 cases analysed using various nodalisation schemes and varying the aerosol parameters.

14. IPHWR SAFETY STUDIES WITH ASTEC Aerosol and FP transport in IPHWR PHT system Accident Sequence, Nodalisation • Hypothetical dual failure: LOCA in RIH with unavailability / loss of ECCS • Limited core uncovery and release of a small amount of fission products besides the evolution and transport of steam (Cs is the predominant FP released) • T/H and FP data for the various volumes of the PHT system used as boundary conditions for ASTEC V1.3-SOPHAEROS calculations • PHT modelled using 76 volumes Figure Core region simulated using 4 parallel channels One pair represents maximum rated power channel of the core and the other pair represents a channel with average power corresponding to the remaining 152 pairs of channels Each of the 4 channels divided into ten volumes of equal length Large break in RIH 78 junctions to connect the various volumes

15. IPHWR SAFETY STUDIES WITH ASTEC Aerosol and FP transport in IPHWR PHT system Break

16. IPHWR SAFETY STUDIES WITH ASTEC Aerosol and FP transport in IPHWR PHT system SOPHAEROS results Retention Factors Deposited Aerosols Mass Ratio (%) by Mechanism

17. IPHWR SAFETY STUDIES WITH ASTEC Aerosol Behavior in IPHWR Containment Stand-alone CPA calculation performed with the assumption that the entire FP inventory released from the core directly released into the containment. Fuelling Machine Vault 1 (ZFMV1) in the containment defined as break compartment and the steam and FP releases assumed to be injected into this compartment only. FP calculations performed for two pre-dominant species Cs and I

18. SEVERE ACCIDENT ANALYSIS FOR VVER 1000 VVER 1000 ASTEC V1 plant model improvement ASTEC V1.2 calculation of the SBO scenario with PRZ valves stuck open. New Plant Model : 1. Added components • Grid Spacers • Lower Core Plate • Guide Tubes with and without B4C 2. Physical models • Heat transfer in new components • Gap heat transfer between baffle-barrel-vessel • B4C oxidation • B4C-SS reaction 3. Observations • Heat-up of barrel and vessel wall • Higher H2 & corium production • Vessel failure by one hour earlier than with former plant model. Prediction of final core state: former plant model Prediction of final core state: new plant model

19. SEVERE ACCIDENT ANALYSIS FOR VVER 1000 Influence of oxidation environment LOCA+SBO : Poor Oxidation Scenarios considered with ASTEC V1.2 calculations SBO with PRZ Stuck Open (steam rich) Large LOCA with SBO (steam starved) SBO +PRZ open : Strong Oxidation Hydrogen generation is 15.6 times more for SBO+PRZ open as compared to LBLOCA

20. SEVERE ACCIDENT ANALYSIS FOR VVER 1000 Influence of quenching (through hydro-accumulators) • SBO with PRZ Stuck Open with Hydro accumulators • Large LOCA with SBO with Hydro accumulators Prolonged core heat-up Higher hydrogen production Delayed failure of vessel bottom Case : SBO+HA Case :LOCA+HA

21. SEVERE ACCIDENT ANALYSIS FOR VVER 1000 Sensitivity studies with ASTEC V1.2 Parameters Selected • Candling velocity at grid • Clad candling velocity • UO2 and ZrO2 solidus/liquidus temperatures • Clad dislocation criteria Basis of Selection • FPT0,1,2 Analysis Results: Low candling velocity and multiple dislocation criteria show influence on • hydrogen production, corium composition (debris, metal and pool) • vessel failure time

22. CONCLUDING REMARKSIPHWRsApplicability of ASTEC for thermal hydraulic, hydrogen distribution and aerosol behaviour studies in the containment was demonstrated. Applicability for aerosol behaviour studies in the PHT system of IPHWRs was demonstrated.ASTEC proposed to be used for a detailed integral accident analysis for IPHWRs. Studies performed being continued further:Modelling of thermal hydraulic behaviour of the PHT of IPHWRs using CESAR Comparison against available results from other codes at BARC (RELAP) Performing coupled CESAR-SOPHAEROS calculation for a severe accident sequence for IPHWRs Performing a coupled CESAR-SOPHAEROS-CPA calculation for the accident sequence analysed in the above –mentioned studiesEfforts underway to simulate hydrogen distribution and aerosol tests in an upcoming multi-compartment experimental BARC facility ‘Containment Studies Facility’ (CSF).(Figure) Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000

23. CONCLUDING REMARKSVVER-1000Phase-wise severe core damage study for VVER-1000 with ASTEC.Improvements of the former plant model: Heat-up of barrel and vessel wall predicted, Higher hydrogen and corium production, Vessel failure occurs an hour earlier.Higher steam availability in the reactor core during the SBO with PRZ valve stuck open leads to higher hydrogen generation than the case for LBLOCA in the cold leg along with SBO. Prolonged core heat-up evident for accumulator injection cases leads to higher hydrogen production and delayed failure of vessel bottom. Sensitivity studies for SBP with PRZ valve stuck open show influence of low candling velocity and multiple dislocation criteria on hydrogen production, corium composition (debris, metal and pool) and vessel failure time. All these calculations gave reliable and consistent results. Work being continued for benchmark exercises with codes RELAP5/SCDAP, MELCOR and ICARE/CATHARE. Uncertainty analysis should be performed. Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000

24. Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000 PHT System Model Enclosure Control & Instrumentation Room Containment Model

25. Application of Computer Code ASTEC for Severe Accident Studies in IPHWRs and VVER-1000 ACKNOWLEDGEMENTS IRSN BARC FZK Mr. J. P. Van Dorsselaere, ASTEC Coordinator, IRSN Dr. T. Albiol, SARNET Coordinator Dr. Walter Tromm, FZK Dr. P. Grudev, INRNE, BAS THANK YOU