In-Vessel Retention Analysis for Pressurised Heavy Water Reactors (PHWR) under Severe Core Damage Accident (SCDA)

Transcription

1 A Presentation on In-Vessel Retention Analysis for Pressurised Heavy Water Reactors (PHWR) under Severe Core Damage Accident (SCDA) By Onkar Suresh Gokhale Reactor Safety Division Bhabha Atomic Research Center INDIA Contributors O.S. Gokhale, D. Mukhopadhyay, A. Ramarao

2 Objective Verification of Technical Basis for addition of Fire Fighting Water (FFW) under Severe Accident Management Guidelines (SAMG) for PHWR Suitable model development to address possible phenomena Analysis of small PHWR (220 MWe) and large PHWR (540 MWe) with the developed model with and without SAMG action 2

3 Reactor Block of PHWR CALANDRIA Ref: The Indian PHWR, Bajaj and Gore, Nuclear Engineering Design, 236 (2006),

4 EOP and SAMG actions for SBO EOP FFW Injection into Steam Generator (Secondary Side) Criteria for EOP to SAMG transition* Loss of Subcooling in Headers Loss of Moderator Cooling SAMG Actions FFW Injection into Calandria FFW injection into Calandria Vault FFW Injection (SAMG) FFW Injection (EOP) FFW Injection (SAMG) CALANDRIA VAULT EOP: Emergency Operating Procedure FFW: Fire Fighting Water SBO: Station Black Out * Ref: IAEA TECDOC

5 Postulated Station Black Out (SBO) and Accident Progression Postulated SBO Simultaneous failure of Class IV (Grid) and Class III (DG) power Reactor shuts down Moderator Cooling and End Shield Cooling is unavailable ECCS is assumed unavailable (although Hydro Accumulators will be available, they have not been credited) Accident Progression Boiloff of SG secondary inventory Pressurization of Primary Heat Transport System and inventory boil-off through Instrumented Relief Valves Burst of high power channels leading to moderator expulsion Top rows of channels exposed Moderator in Calandria starts boiling Exposed channel disassembly and formation of suspended debris Core Collapse and formation of terminal debris Complete moderator boil off Vault water heatup and boil off Exposure of Calandria and loss of integrity Limited Core Damage Accident (LCDA) Severe Core Damage Accident (SCDA) 5

6 LCDA and SCDA for PHWRs Vault Water Vault Water Vault Water Boil off Moderator Moderator LCDA: Limited Core Damage Accident Moderator boil off Channel Exposure Channel Diassembly --- > Core Diassembly SCDA: Severe Core Damage Accident Moderator boil off --- > Debris Exposure Debris melting to Magma Vault Boil off --- > Vessel Failure 6

7 Adaptation of ASTEC for PHWR SCDA Simulation ASTEC: Accident Source Term Evaluation Code Modified under bilateral collaboration: IRSN, France & BARC, India Facilitates: Modelling of Calandria (Horizontal Cylindrical Vessel) Modelling of ENDSHIELD Fuel-machine Side Tube Sheet (FSTS) Carbon Steel (CS) balls End Shield (ES) water Calandria Side Tube Sheet (CSTS) Inner Shell (IS) Outer Shell (OS) Octagonal Flange etc. Radiation heat exchange within the Calandria (RADPHWR model) Implementation of PHWR specific Ex-Vessel Heat Transfer Correlations 7

8 END SHIELD END SHIELD Modeling Details RADPHWR 5 4 Calandria discretization Radial direction: Azimuthal direction: Vertical symmetry 3 nodes 5 nodes 3 Channels assumed to be relocated into Calandria in the form of debris 1 2 RADPHWR model accounts for radiative heat exchanges between debris top layer, exposed inner surfaces of Calandria and end shield. PHWR specific heat transfer correlations are used for Cooling of Calandria External Surface CHF " q CHF = C 1 + C 2 θ + C 3 θ 2 + C 4 θ 3 + C 5 θ 4 Natural Convection to Single Phase McAdam s Correlation Churchill-Chu s Correlation RADPHWR Nucleate Boiling Forster-Zuber s Correlation DEBRIS Film Boiling Breen and Westwater s Correlation 8

9 Modeling Details In contact with Heavy Water / Debris / Magma CS Balls & Cooling Water Passage for channel In contact with Air in Fuelling Machine Vault Detailed model of End Shield Internal Structures CSTS FSTS Cooling Water inlet & outlet 9

10 Analysis of Unmitigated SBO: Small PHWR Scenario Plant : Unmitigated Station Blackout : Indian PHWR 220 MWe Initial Condition: Channel Collapsed to Calandria Vessel Bottom forming Debris Inventory Fuel Zircaloy Moderator End Shield Water Calandria Vault Water kg kg kg (13 % of initial inventory) kg kg 10

11 OPRD Flow (kg/s) Level(m) Results: PHWR 220 MWe Unmitigated SBO Moderator ES Water Vault Water Moderator Boil off followed by boil-off of End Shield water and Vault water 11

12 Temperature (K) Calandria Temperature (K) Results: PHWR 220 MWe Unmitigated SBO CSTS Baffle Plate CS Balls FSTS Inner Shell Outer Shell Vault Wall Inner Diaphragm (SS) Inner Diaphragm (CS) Outer Diaphragm (SS) Outer Diaphragm (CS) Octagonal Flange Node 1 Node 2 Node 3 Node 4 Node Complete Moderator Boil-off Complete Boil-off of ES Water Exposure of Calandria Begins Temperature escalation of End Shield components and Calandria Nodes 12

13 Debris Temperature (K) Results: PHWR 220 MWe Unmitigated SBO Rise in debris temperature after complete moderator boil-off Debris temperature remains below melting temperature Low power density in PHWR 220 MWe Large diameter of Calandria provides sufficient area for heat transfer 13

14 (m) Temperature (K) (m) Temperature (K) Results: PHWR 220 MWe Unmitigated SBO Calandria with Debris Calandria with Debris (m) (m) Debris become hot. But no melting is observed till complete boil-off of vault water 14

15 Analysis of SBO with SAMG: Small PHWR Scenario Plant : Station Blackout with Water Injection into Vault : Indian PHWR 220 MWe Initial Condition: Channel Collapsed to Calandria Vessel Bottom forming Debris Inventory Fuel Zircaloy Moderator End Shield Water Calandria Vault Water kg kg kg (13 % of initial inventory) kg kg 15

16 Level(m) Results: PHWR 220 MWe SBO with Vault Injection Calandria Temperature (K) Moderator ES Water Vault Water FFW injection begins Node 1 Node 2 Node 3 Node 4 Node Water injection to Vault started at the time of Calandria Exposure No rise in temperature of Calandria Time available for SAMG action > 2 days from SBO initiation 16

17 Temperature (K) Results: PHWR 220 MWe SBO with Vault Injection Debris Temperature (K) CSTS Baffle Plate CS Balls FSTS Inner Shell Outer Shell Vault Wall Octagonal Flange Inner Diaphragm (SS) Inner Diaphragm (CS) Outer Diaphragm (SS) Outer Diaphragm (CS) Slow reduction in CSTS and Baffle Plate Temperature Slow reduction in Debris Temperature Effective removal of decay heat through Calandria external cooling 17

18 Analysis of Unmitigated SBO: Large PHWR Scenario Plant : Unmitigated Station Blackout : Indian PHWR 540 MWe Initial Condition: Channel Collapsed to Calandria Vessel Bottom forming Debris Moderator Half Filled (IAEA TECDOC 1727) Vault Water Completely Filled (IAEA TECDOC 1727) Inventory Fuel Zircaloy Moderator End Shield Water Calandria Vault Water kg kg kg (47 % of initial inventory) kg kg 18

19 Results: PHWR 540 MWe Unmitigated SBO Moderator Boil off followed by boil-off of End Shield water and Vault water 19

20 Temperature (K) Results: PHWR 540 MWe Unmitigated SBO Temperature escalation of End Shield components and Calandria Nodes 20

21 Results: PHWR 540 MWe Unmitigated SBO Rise in debris temperature after complete moderator boil-off Debris temperature reaches melting temperature Conversion of solid debris to Molten Magma 21

22 (m) Temperature (K) (m) Temperature (K) Results: PHWR 540 MWe Unmitigated SBO Calandria with Debris Calandria with Debris (m) (m) Debris become hot. Melting leads to formation of Magma layer 22

23 Analysis of SBO with SAMG: Large PHWR Scenario Plant : Station Black Out with Water Injection into Vault : Indian PHWR 540 MWe Initial Condition: Channel Collapsed to Calandria Vessel Bottom forming Debris Moderator Half Filled (IAEA TECDOC 1727) Vault Water Completely Filled (IAEA TECDOC 1727) Inventory Fuel Zircaloy Moderator End Shield Water Calandria Vault Water kg kg kg (47 % of initial inventory) kg kg 23

24 Temperature (K) Results: PHWR 540 MWe with Vault Injection FFW Injection begins 1 2 Injection to Calandria Vault started at flow rate prescribed by utility Injection leads to complete fill-up of Calandria Vault No rise in Calandria Temperature Time available for SAMG action > 1 day from SBO initiation 24

25 (m) Temperature (K) Results: PHWR 540 MWe with Vault Injection Calandria with Debris (m) Debris, in the form of molten magma, are contained within the Calandria Slow reduction in magma temperature Effective removal of decay heat 25

26 Conclusion The ASTEC code is suitably adapted for PHWR to simulate SCDA phase Analysis carried out for 220 MWe and 540 MWe illustrates heatup of Calandria when exposed from outside Addition of FFW into Calandria Vault helps to stabilize the debris/magma temperature within Calandria Calandria integrity maintained. IVR of Debris/Magma is successful. A large time margin ( 2 days for PHWR 220 MWe and 1 day for PHWR 540 MWe) is available for FFW water injection into the Calandria Vault 26

27 THANKS