ANALYSIS OF ASTEC CODE ADAPTABILITY TO SEVERE ACCIDENT SIMULATION FOR CANDU TYPE REACTORS

1. ANALYSIS OF ASTEC CODE ADAPTABILITY TO SEVERE ACCIDENT SIMULATION FOR CANDU TYPE REACTORS Marin CONSTANTIN, Andrei RIZOIU Institute for Nuclear Research, PO Box 78, Pitesti

2. INR -Severe Accident Research Activities started in FP5-PHEBEN2 -validation for some models (FPs transport, core degradation, H2 production etc.) of ASTEC code -comparison against calculation results and experimental data FP6- SARNET NoE Deterministic methods and computer codes -ASTEC adaptation to CANDU NPP -ASTECV1.1, V1.2 validation on different experiments (CORA13, CORA W2, FPT1&FPT2 integral -development of new algorithms and methods Experimental activities -Experiments: design and conducting; analysis of the results -Source term formation- FPs release from debris bed -Oxidation of Zy-4 and fuel pellets in steam condition PSA Level 2 - Harmonisation of Level-2 PSA methodologies at EU level - CANDU specificities

3. ASTEC Structure and Capabilities ASTEC code -initial developed for PWR design -adapted for VVER - adaptation in progress for CANDU, RBMK and BWR Starting point: exploratory calculations for CANDU phenomena evaluation. SOPHAEROS is actually one of the Canadian IST codes Some additional codes (e.g. CATHENA, ORIGEN, RELAP5) may be used in order to supply margin and initial conditions for ASTEC.

4. Exploratory Calculations

5. Calandria tube Pressure tube Cooling water Outer ring Middle ring Inside ring Steam Heated rods Un-heated rods Geometrical structure for oxidizing experiment- AECL DIVA – Cladding Oxidation Graphite rod coated with tungsten carbide – for electrical heating of FES (Fuel Elemet Simulator) Alumina pellets simulating fuel pellets Zr Cladding Different literature experiments with twenty-eight FESs (fuel element simulators- electrical heated), horizontal orientation, Tcladd.>1500 C, injection of 9 to 15 g/s steam.

6. DIVA- Calculation Results Comparison between DIVA results and experimental results. A good agreement between calculation and experimental results is observed. For the total hydrogen production the realtive difference is 1.23%. Conclusion: DIVA module is appropriate for oxidation simulation in CANDU NPP, but cannot be used for core degradation

7. BO1UP2 BO2UP2 BO2DN2 BO1DN2 BO1UP1 BO1DN1 BO2DN1 BO2UP1 OUTP1 OUTP2 INP1 OPMP1 BOP2 BIP2 BIP1 BOP1 OPMP2 INP2 RB1 RB2 P224 PAO1 PAI1 PAO2 PAI2 RA1 RA2 PA1 PA2 OHD1 IHD2 IHD4 OHD3 IFD2 OFD1 OFD3 IFD4 ENF2 ENF3 CHANL1 ENF1 ENF4 CHANL2 CESAR Simplified model for one loop in CANDU NPP Termal-hydraulics in a more complicated circuit than PWR. Problems of channels and feeders- different diameters and lengths. Solution: average diameter and length + sensitivity calculations + interpolation

8. CESAR- Calculation Results Pressure evolution in IHD2 (35% rupture) obtained by CESAR Total fluid mass evolution in the broken loop, obtained by CESAR

9. SOPHAEROS (1) Vapours (Gas) Chemical interaction (2) Suspended Aerosols (3) Condensed Vapours Vapour sorbtion Evaporation Coagulation of aerosols Condensation of vapours Transport vapours & Aerosols Deposition Mechanical Resuspension (4) Adsorbtion of vapours Back Transport (5) Deposited Aerosols Deposition by: - Gravitational settlement; -laminar or turbulent difusion; - impact; - termophoresis; -difusiophoresis; Fluid transportor • Coagulation of aerosols: • - gravitational; • Brownian difusion • turbulent difusion;

10. SOPHAEROS- Results

11. Dousing system 6 1 gas 2 aerosol 3 gasous phas on the surface 4 water 5 water on the surface 6 systems (filters, recombinators) Bfil&rec 1 Bwall 2 3 Bwashinge FPs Source Bfloor 5 4 Bdepos Bpool scr Transport and deposition phenomena in the containment CPA FPs injection by PHT rupture and fuel cladding failure FPs Distribution – regions and phases. Termalhydraulics Chemistry Releasing in the environment by containment failure

12. CPA- Containment Model R2= CV780 REZ= CV790 REZ= CV790 CV770 CV760 CV770 Transfer to environment R1= CV750 R3= CV740 CV730 PHT to Containment FPs Injection CV720 CV 700 CV710

14. Strontium Distribution

15. Cesium Distribution

17. The most complex calculation performed for CANDU NPP: CESAR-SOPHAEROS-CPA-IODE Coupled calculation -margin and initial conditions (inventory-ORIGEN, releasing factors-literature,…)

18. CANDU Specificities- Needs for Adaptation

19. CANDU – important features for SA m=130 t 650C M=227 t m=465 t

20. CANDU and PWR Comparison Fuel Channel Oriented Channel -CANDU fuel bundles lie inside the pressure tube -a Calandria tube separates the pressure tube from the moderator -a gas gap between pressure and Calandria tube provide thermal insulation Pressure tube Zircaloy-2.5wt%Nb 0.5% Cu;Dint=103.4 mm;  =4.2 mm Calandria tube Dint=129.0 mm,  =1.4 mm Moderator(D2O, 650C) Coolant(D2O, 3120C)

21. CANDU Specificities Normal operation: -4,5% of thermal energy is deposited in moderator, mainly by radiation; - this energy is removed by MCS (Moderator Cooling System) Severe accident: -MCS will remove decay heat from the fuel channel, even if they contain no coolant at all; -the fuel will be severely damaged, but the UO2 would not melt, and the channel would remain intact and retain the debris; (capability verified by tests at Whiteshell Laboratory); Heat rejection to Shield Tank Water: The passive water can absorb the decay heat in the event of a severe accident and slow down the progression of core melting.

22. Needs for Adaptation- SARNET2

23. Conclusions • ASTEC code is able to simulate an important part of CANDU SA phenomena: FPs transport in PHT and containment, FPs Chemistry, cladding oxidation, termalhydraulics in PHT and containment • SOPHAEROS, CPA, IODE and CESAR may be used in stand alone and coupled calculation for CANDU SA simulation • DIVA needs a drastic reconstruction, practically a new module DIVA for CANDU must be designed • An important experimental effort should support the soft development