Office for Nuclear Regulation

1 Office for Nuclear RegulationAn agency of HSE PROTECTIVE MARKING IF APPLICABLE Generic Design Assessment New Civil Reactor Build Step 4 Probabilistic Safety analysis Assessment of the Westinghouse AP1000 Reactor Assessment Report: ONR-GDA-AR-11-003 Revision 0 10 November 2011 MARKING IF APPLICABLE Office for Nuclear Regulation An agency of HSE Report ONR-GDA-AR-11-003 Revision 0 Page (i) COPYRIGHT Crown copyright 2011 First published December 2011 You may reuse this information (excluding logos) free of charge in any format or medium, under the terms of the Open Government Licence.

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3 For published documents, the electronic copy on the ONR website remains the most current publically available version and copying or printing renders this document uncontrolled. MARKING IF APPLICABLE Office for Nuclear Regulation An agency of HSE Report ONR-GDA-AR-11-003 Revision 0 Page (ii) PREFACE The Office for Nuclear Regulation (ONR) was created on 1st April 2011 as an Agency of the Health and Safety Executive (HSE). It was formed from HSE's Nuclear Directorate (ND) and has the same role. Any references in this document to the Nuclear Directorate (ND) or the Nuclear Installations Inspectorate (NII) should be taken as references to ONR.

4 The assessments supporting this report, undertaken as part of our Generic Design Assessment (GDA) process, and the submissions made by Westinghouse relating to the AP1000 reactor design, were established prior to the events at Fukushima, Japan. Therefore, this report makes no reference to Fukushima in any of its findings or conclusions. However, ONR has raised a GDA Issue which requires Westinghouse to demonstrate how they will be taking account of the lessons learnt from the events at Fukushima, including those lessons and recommendations that are identified in the ONR Chief Inspector s interim and final reports.

5 The details of this GDA Issue can be found on the Joint Regulators new build website and in ONR s Step 4 Cross-cutting Topics Assessment of the Westinghouse AP1000 reactor. PROTECTIVE MARKING IF APPLICABLE Office for Nuclear Regulation An agency of HSE Report ONR-GDA-AR-11-003 Revision 0 Page (iii) EXECUTIVE SUMMARY This report presents the findings of the assessment of the Probabilistic Safety analysis (PSA) of the AP1000 reactor undertaken as part of Step 4 of the Health and Safety Executive s (HSE) Generic Design Assessment (GDA).

6 The assessment has been carried out on the PSA that supports the 2011 Pre-construction Safety Report (PCSR), (which is an update of the PSA that supports the AP1000 European Design Control Document EPS-GW-GL-700, Rev 1, and the 2009 PCSR) and the supporting documentation submitted by Westinghouse during GDA Step 4. To identify the scope of the GDA Step 4 assessment for the PSA, an assessment plan was set-out in advance. The GDA Step 4 review itself covered all the technical areas of the PSA, although it is important to note that the evidence supporting the PSA claims and arguments (assessed during GDA Steps 2 and 3) on how the PSA Safety Assessment Principles (SAP) are met, have been assessed on a sampling basis.

7 For PSA, evidence is interpreted as the details of the PSA models and data, and the underlying supporting analyses, with the assessment focussing mainly on detailed reviews of such evidence. The sampling has been done in a focused, targeted and structured manner with a view to revealing any specific or generic weaknesses in the PSA. The GDA Step 4 assessment has been conducted following the guidance and structure established in Appendix 1 of the Nuclear Directorate s PSA guide (T/AST/030 Issue 3, February 2009).

8 To help to reach a conclusion of whether an AP1000 can be constructed and operated safely in the UK, and to evaluate the importance of the findings in the various PSA technical areas, a Risk Gap analysis (RGA) was conducted. This was a complex task but it was essential for wrapping-up the results from the GDA Step 4 PSA assessment. The AP1000 PSA comprises a Level 1, a Level 2 and a simplified Level 3 PSA. The scope includes consideration of internal initiated events and internal hazards and includes low power and shutdown operating states. Westinghouse submitted a separate PSA for the Spent Fuel Pool, which has also been assessed.

9 In general, the methods and data used in the PSA are well known, although not always up-to-date or aligned with the latest international good practices. The high level review of all the PSA technical areas conducted during GDA Step 3 identified shortcomings in scope, methods and data. I indicated at that time that work would be required to complete and modernise the PSA so that it could provide a more adequate input into the as low as reasonably practicable (ALARP) demonstration. Despite that, I also indicated that the AP1000 Core Damage and Large Release Frequencies provided a degree of confidence that the Societal Risk Target (Target 9 from Numerical Target of the Health and Safety Executive s SAPs) would lie below the Basic Safety Level (BSL) and that I did not have any reason then to believe that this position would change once the PSA has been completed and updated.

10 The detailed GDA Step 4 review of the PSA, together with the results of the Risk Gap analysis , has confirmed that the conclusions from GDA Step 3 remain valid. Findings of greater or lesser importance were identified in all the technical areas of the PSA. These are listed in Annex 1 and will be carried forward as normal regulatory business. However, shortcomings in two particular areas give rise to more significant uncertainty in my understanding of the risk associated with the AP1000 design. These are PSA Success Criteria and Fire PSA . These are identified in this report as GDA Issues and each one has an associated Resolution Plan proposed by Westinghouse.