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Proceedings of the Eighth International Conference on Probabilistic Safety Assessment & Management (PSAM)

Michael G. Stamatelatos
Michael G. Stamatelatos
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Harold S. Blackman
Harold S. Blackman
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ASME Press
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The paper describes a research project titled “The Validity of Safety Goals” recently initiated by NKS (Nordic Nuclear Safety Research) and NPSAG (Nordic PSA Group) with participants from Finland and Sweden.

Quantitative results from a probabilistic safety assessment (PSA) for a nuclear power plant are typically presented as core damage frequency (CDF) and frequency of radioactive release. In order to judge on the acceptability of results, criteria for the interpretation of results and for the assessment of their acceptability need to be defined.

The risk levels specified by the safety goals differ between organizations and between different countries. There may also be differences in the definition of safety goals. In most countries, safety goals started to be defined in the 1980s, i.e. at a time when PSA models were rather limited in scope. During the 1990s, the PSA models expanded considerably, both regarding operating states and classes of initiating events. In parallel, PSA:s have been expanded to level 2, making it possible to calculate release frequencies. In view of this development, and of current needs, the objective of the project includes looking back and mapping the evolvement of safety goals for PSA.

Defining quantitative goals for reactor safety may have a large impact on both the analysis burden and on requirements for safety improvements at nuclear power plants. It is therefore of great importance that these goals are effective and soundly based, that they can be effectively and unambiguously applied, and that they can be accepted and understood by all parties concerned (analysts, decision makers, the public, etc.). In connection with this, a number of specific issues related to the definition and use of safety goals will also be discussed. Issues, which may become involved in the project, include:

• Reasons for developing safety goals.

• Identification of conditions where safety goals can (and should) be applied.

• Definition of subject of safety goal, i.e., what exactly is meant by “core damage”, “large release”, etc.

• The background to the safety goals developed nationally (Sweden/Finland) and internationally.

• The organizations involved in defining the goals, and the process involved in the formulation of the goals.

• The basis for defining the actual numerical levels used in the safety goals (population risks, etc.)

• Relation of safety goals used for nuclear power plants to safety goals used for other man-made risks.

• The relationship between safety goals defined for level 1 PSA (CDF) and for level 2 PSA (release frequencies)

• Use of the safety goals on results from PSA:s with differing scopes

• Effects on the use of safety goals arising from major PSA uncertainties, including uncertainties related to completeness and the state of knowledge.

The project work was initiated in late 2005, and will be performed mainly during 2006. The paper describes in some detail the background to the initiation of the project, its contents and aims, and also gives some initial comments on the issues related to the use and interpretation of safety goals and on the applicability of safety goals for decision making.

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