1. OECD_L2PSA_SZB/mla/dmj/02039.ppt OECD/NEA International Workshop on L2 PSA and SAM, 29 th -31 st March 2004, Koln Approach in the PSR for Sizewell B LPSA M Ang, E Grindon, K Peers (NNC) N E Buttery (Sizewell B, BE) P Lightfoot (BE)

2. OECD_L2PSA_SZB/mla/dmj/02039.ppt Contents Development of Sizewell B PSA POSR Level 2 PSA approach Approach to PSR Conclusions

3. OECD_L2PSA_SZB/mla/dmj/02039.ppt Development of Sizewell B PSA PSA has been used in early days to: Confirm adequacy of final design Identify further design refinements PSA’s carried out at various stages in process L1: internal initiators at power (1982) L3: internal initiators at power (for public enquiry, 1982) L3: all faults at all power levels (for POSR, 1992) LPSA based on POSR model with revised L1 LPSA formed part of PSR (submitted to NII Dec 2003) IAEA IPSART on LPSA for Risk Informed Management (Jan 2004)

4. OECD_L2PSA_SZB/mla/dmj/02039.ppt Key Features of SZB Level 2 PSA (1) Core damage PDSs (14) faults at power (10) faults at shutdown (4) quantification via CET Reactor building bypass PDSs (7) SG (4) interfacing systems (3) MAAP analysis CET 20 nodes mainly phenomenological issues conservative approach, no claim on SAM -> RPV failure is assumed SAM actions treated in sensitivity studies based on expanded CET (22 nodes) to support ALARP reviews

5. OECD_L2PSA_SZB/mla/dmj/02039.ppt Key features of SZB Level 2 PSA (2) CET quantification Probabilistic containment structural integrity model, leakage pathways by gross failure/enhanced leakage/DB leakage SA and ST analyses provided by MAAP3.0B Supporting analysis for key sequences by other codes (e.g. SCDAP/RELAP5, CORCON, VICTORIA, CONTAIN - CORDE) expert reviews on SA and ST issues FCI addressed by expert group UK and international R&D programmes NUREG-1150 and other PSAs 2 stage radiological categorisation process Structured sensitivity analyses (code and CET) -> statistical uncertainty analysis was judged inappropriate

6. OECD_L2PSA_SZB/mla/dmj/02039.ppt Issue quantification approach in CET Discrete Probability DETMonte Carlo CET Sensitivity Study Creep rupture failure of RCS XX In-vessel steam explosion XX H2 burn HPME DCH XXXX X XXXX Ex-vessel steam explosion X CCIXX Source Term Issues X AM IssuesX

7. OECD_L2PSA_SZB/mla/dmj/02039.ppt Containment Failure Modes Early 3% Late 22 % Basemat 20 % Bypass 12 % Intact 43%

8. OECD_L2PSA_SZB/mla/dmj/02039.ppt Use of Level 2 PSA in SAM application Use of fire suppression system reduce likelihood of basemat failure ex-vessel cooling uncertain Other SOI 8.8 measures (e.g. depressurisation) reinforces likely outcome Level 2 PSA results an input to ALARP reviews Provided justification for not installing FV

9. OECD_L2PSA_SZB/mla/dmj/02039.ppt Level 2 PSA activities post licensing PSA study Maintained a watching brief on international studies on SA phenomenological issues (e.g. USNRC issue resolution studies, OECD reviews, MACE, RASPLAV) and L2 PSA development (IAEA, OECD) Implementation of MAAP4 and analysis provided for key sequences Re-assessment of shutdown sequences, assessment of accident progression and ST behaviour from air entrainment phenomenon Participation in a number of EU FW programmes, e.g. H 2 combustion: H2DDT ST issues: STU, OPTSAM, IC, ASTERISM SAM issues (e.g. core cooling recovery, IVR strategy): OPTSAM, SAMEM containment leakage: CONT-LEAK, CESA L2 PSA methodology: SAMEM, BEEJT Participation allowed access to cluster information Feasibility of WOG SAMG implementation

10. OECD_L2PSA_SZB/mla/dmj/02039.ppt PSR of Level 2 PSA Undertaken as part of PSR (chapter on ‘Safety Analysis Based Review of PSA and HF - Development and Issues) Process proceeded in 2 stages: (i) significant issue review and (ii) detailed review Significant issue review no ‘significant’ issues identified further L2 PSA quantification not necessary identified scope and issues for detailed review Issues for detailed review SA phenomenological issues treated in CET other key issues not treated in CET, including SAM issues ST issues status of MAAP code, models and validation SAM issues SAMG development and implications Instrumentation survivability in SA conditions Review based on information derived from listed activities following POSR

11. OECD_L2PSA_SZB/mla/dmj/02039.ppt Example of review: In-vessel fuel coolant interaction POSR assessment SERG-1 and Theofanous’ ROAAM analysis UK study based on ROAAM methodology, 6 x 10 -4 - 4 x 10 -4 calculated for pressure ranges PSR assessment SERG-2 study (1996) OECD study (1999), including a review of KROTOS experiments Theofanous’ study (1995) POSR results in accordance with more recent studies

12. OECD_L2PSA_SZB/mla/dmj/02039.ppt Example of review: ST issues Review considered: status of key issues (group 1) status of key issues not explicitly covered in MAAP analysis (group 2) implications for POSR results Review of Group 1 issues based on STU project results. 13 issues included: e.g. aerosol behaviour in containment, iodine chemistry, Te chemistry, late revaporisation and others More detailed review of Group 2 issues: speciation of releases, iodine behaviour in containment, containment leakage, SAM actions on fp behaviour POSR approach remains a conservative interpretation of fp behaviour

13. OECD_L2PSA_SZB/mla/dmj/02039.ppt Results of review SA phenomenological issues recent research evidence in concurrence with assumptions/conclusions derived in POSR no single issue has been identified which would contradict POSR conclusions derived from a conservative L2 PSA ST issues assumptions affecting fp behaviour remain bounding and in some cases remain conservative interpretation of current data MAAP status future analysis to be based on MAAP4 impact of re-analysis of POSR model based on MAAP4 is unlikely to provide a major impact on the overall L2 PSA results O verall, the conclusions derived from a conservative L2 PSA model are judged to remain valid. The model remains valid for licensing application

14. OECD_L2PSA_SZB/mla/dmj/02039.ppt Where do we go from here? Future direction of the L2 PSA will depend on: SZB NPP’s requirement on LPSA for RI applications Outcome of NII review of PSR Outcome of IAEA IPSART results

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