105 Use of the PSA in the Integrated Regulatory Safety Oversight in Switzerland: Status and Outlook (PSAM-0181)

In order to have a comprehensive, balanced and adequate decision-making process, the Swiss Federal Nuclear Safety Inspectorate (HSK) follows the approach of an integrated regulatory safety oversight. “Integrated oversight” is defined as a decision-making process consisting of three elements, namely (i) classical deterministic rules and methods; (ii) safety relevant aspects of the operational experience; and (iii) plant-specific insights of the Probabilistic Safety Assessment (PSA). The introduction of the integrated oversight process initiated a number of activities. This paper focuses on the role of PSA in the integrated safety oversight process. The legal and the technical basis (i.e. the status of the PSA in Switzerland) as well as the current and the planned use of the PSA are outlined:

Legal basis: In February 2005, a new Nuclear Energy Law and an accompanying ordinance were enacted in Switzerland. The ordinance anchors now a number of PSA applications in the law:

• For the construction permit of a new nuclear power plant (NPP), it has to be demonstrated that the core damage frequency is below 10⁻⁵ per year. This risk criterion is also expected to be fulfilled by the existing plants, to the extent that is reasonably achievable.

• PSA is an integral part of every Periodic Safety Review.

• The risk impact of plant modifications, findings and events is to be assessed systematically.

Technical basis: Full-scope (i.e. level-2 full power, low-power and shutdown, including all credible internal and external initiators), plant-specific PSA studies have been performed for all Swiss NPPs. These studies have been extensively reviewed, resulting in continuous updates and improvements over the last 15 years.

Current use of the PSA: The initial applications of PSA concentrated on the determination of the overall plant safety level, the assessment of the balance of the plant safety concept and the identification of procedural and hardware improvements. Other applications have included the assessment of the risk impact of power uprate, justification of reference scenarios for the emergency planning, categorization of accidents and the selection of risk-important components to be considered in aging surveillance programme.

Activities to implement further PSA-applications in the integrated oversight process: The implementation process of the integrated oversight strategy has initiated a number of activities where PSA is involved:

• Using the PSA as an element of the decision-making process premises, that the results of the various plant-specific PSA are comparable. Therefore, the quality and the scope of the Swiss PSAs shall be further harmonized with the help of a prescriptive guideline as far as possible.

• A guideline clarifying the general role of PSA in the integrated oversight with the help of a number of principles and the requirements for some specific PSA applications is to be developed. The relevant risk measures and (if sufficient experience is available) risk criteria will be defined for the considered PSA applications.

• Recently, a procedure for the systematic performance of a probabilistic event analysis has been introduced. The probabilistic event analysis will be used to monitor the operational safety. It is planned, to use the results of the probabilistic event analysis as an input for the event classification and as a basis for the introduction of a risk informed maintenance rule.

• A databank providing plant-specific risk information such as various importance lists as well as the results of the probabilistic event analysis is expected to support the oversight process.