Blending deterministic and probabilistic arguments in the regulatory decision making: The Canadian approach

Abstract of the technical presented to:
Probabilistic Safety Assessment and Management Conference 2010 (PSAM)
Seattle, Washington, United States
June 7-11, 2010

Prepared by:
Philippe Hessel
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada

Abstract

A safety assessment is a systematic process to verify that applicable safety requirements are met in all phases of the life cycle of a nuclear facility. Safety analysis is a key component of a safety assessment. Safety analysis incorporates both probabilistic and deterministic approaches, which complement each other.

Deterministic safety analysis is the principal means of demonstrating that the dose acceptance criteria are met with a high degree of confidence for all accidents within the design basis.

Probabilistic safety analysis is the principal means of demonstrating that the safety goals are met for potential accidents both within and beyond the design basis. It identifies vulnerabilities not necessarily accessible through deterministic safety analysis alone.

With the development of probabilistic analysis techniques, nuclear facilities licensees and applicants have introduced probabilistic arguments in support of applications for a licence to operate as well as submissions aimed at obtaining approval for facility modifications, for closure of action items or for temporary licence exemptions.

Probabilistic analysis can complement the deterministic analysis in any of the following areas:

1. supporting applications for licensing new or existing nuclear facilities
2. supporting submissions for modifications of the facility configuration, maintenance or operating procedures
3. framing the decisions to be made following reportable events as required by the Canadian Nuclear Safety Commission (CNSC) regulatory standard S-99 Reporting Requirements for Operating Nuclear Power Plants
4. supporting submissions proposing revisions to deterministic criteria

The integration of probabilistic and deterministic safety analyses and the degree of use of probabilistic criteria is considered on a case-by-case basis.

The depth and scope of the review of a submission is in proportion to its impact on safety. When a submission is supported by both deterministic and probabilistic arguments, the review is conducted as a multidisciplinary project.

The goal of the review is to check that the following general principles are met:

1. the status of the facility after implementation of the decision meets the relevant regulations and the current licence requirements
2. defence-in-depth is maintained
3. sufficient safety margins are maintained

The approach used for evaluating the safety impact of the submission is to ensure that all aspects of the submission have been addressed. Specialists of both deterministic and probabilistic safety analysis, facility operation, systems, maintenance, management, training, radiation protection, human factors, organizational factors and software reliability are consulted as appropriate.

The review of the submission takes into account probabilistic and deterministic considerations, current regulatory requirements and licence conditions. Information and insights from the probabilistic and deterministic analyses are considered, together with quantitative sensitivity studies, operational experience, historical facility performance and engineering judgment.

The scope and quality of the analyses conducted to justify the submission are assessed for the appropriateness for the nature and scope of the proposal(s) contained therein and whether it is based on the as-built and as-operated facility. The assumptions and elements of the models used are assessed for whether they are correct and adequate for resolving the issue. Furthermore, it must be clear that there is a commitment to perform the diverse activities (monitoring, surveillance, operating and maintenance procedures, etc.) that are credited in the submission.

The probabilistic arguments supporting the licensee's submissions include an evaluation of the absolute or relative change in risk metrics. The complexity and depth of this evaluation depends on the magnitude of the potential risk impact.

The results from the PSA, after modelling the proposal(s), are evaluated against the safety goals. The margin between the presented results (for example in terms of core damage or releases) and the safety goal limits is used to weigh the probabilistic and deterministic arguments.

It is important to realize that there is a difference in weighing the probabilistic and deterministic arguments which leads to either risk-based or risk-informed regulatory concepts. The first relies on the PSA in a stringent and rigid fashion whereas the latter uses a combination of the PSA, deterministic requirements, prudence and operating experience to assess relative importance of various safety issues and to confirm the adequacy of the design. As a result, the risk-informed approach gives a more holistic view of risk scenarios as opposed to more specific probabilities of initiating events.

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