Comparison of methods used for internal flood probabilistic safety analyses

An abstract of a technical paper presented at:
The Canadian Nuclear Society Conference
Saskatoon, Saskatechewan, Canada
June 10-12, 2011

Prepared by:
Robert A. Gibb
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

The use of probabilistic safety analysis has become a tool in the regulatory framework of the Canadian Nuclear Safety Commission (CNSC). It is now part of the safety analysis that is used to evaluate the safety of new nuclear power plants, is used in to safety management of plant life and is used to assist in the provisions for accident management.

The Commission issued regulatory document S-294 Probabilistic Safety Assessment (PSA) for Nuclear Power Plants in 2005. Canadian utilities were encouraged to develop these analyses tools. This was particularly emphasized for use in supporting an integrated safety assessment in support of plant life extension. The PSA results would be used to identify weaknesses in the overall plant design for defence‑in‑depth and means by which the core damage frequency and the large release frequencies could be reduced. The analysis tool would be used to evaluate the benefit of plant modifications. The scope of the analysis was intended to include all internal and external events and all pre-dominate plant states. 

The regulatory document required that a licensee methodology be accepted by the CNSC staff. This was a practical approach. All Canadian utilities have now submitted reports describing their methodologies for the various ranges of analyses. Note that many utilities refer to these reports as probabilistic risk assessments.

As CNSC staff review these submissions along with completed analyses reports there is a wide variation of techniques used in the preparation of input data, event identifications and descriptions, the analysis of this data and the reporting of results. Up to this time, CNSC staff has reviewed each utilities submission on a stand alone-basis.

To better understand the implications of variations in techniques among the utilities, a comparison study program was initiated. The first comparison was of the internal flood assessment methodologies. This paper reports on the work that was done and the possible implications. The comparison also includes a specific comparison to methods approved by the US regulator; and to International Atomic Energy Agency (IAEA) guides.

It is expected that the next step will be to perform a comparison on the internal flood analyses reports and findings themselves. The findings of this study will be discussed with interested industry members to seek a path forward of improving standardization and facilitating regulatory oversight.

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