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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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Licensing decision making in nuclear regulation has evolved in many countries towards new approaches where probability and frequency considerations have growing importance, giving rise to the so-called risk-informed regulation.

Risk-informed approaches may help reducing unnecessary conservatisms in safety requirements, allowing for more flexibility and better economic performance in plant operation but, at the same time, maintenance of enough safety margins must be demonstrated. Typically, safety margins are evaluated as a part of Design Basis Accident Analyses (usually called deterministic analysis), subject to strict regulatory requirements, but risk-informing is based on the use of PSA where the concept of safety margin is not addressed. Therefore, the assessment of safety margins in risk-informed applications of plant changes, requires a consistent use of deterministic and probabilistic arguments, i.e., the harmonization of Design Basis Accident Analysis and Probabilistic Safety Assessment principles. Some international initiatives have been promoted to solve this issue, among them the Safety Margins Action Plan (SMAP) of the CSNI where the CSN is actively participating.

The approach proposed by CSN is based on the extension of the concept of safety margin from the Design Basis Space (i.e., the set of Design Basis Accidents) to the Risk Space defined as the set of every credible occurrence in a plant, where the credibility threshold is set to a very low frequency value. Being the risk a two-dimensional magnitude with consequences and frequency components, extended safety margins are also two-dimensional and have the same components.

Any safety objective can and should be analyzed. This includes safety limits and barrier integrity, maximum allowed source terms or radiological consequences considered in DBA analysis or severe core damage and large early release assessment typically addressed by PSA.

The assessment of extended safety margins is based on the use of event tree / fault tree techniques, to some extent similar to those of PSA, but with important differences due to the variety of success criteria that should be considered. In addition, acceptance criteria for the exceedance frequency of every limit being analyzed should be defined.

The frequency margin to any selected limit is given by the distance from the exceedance frequency of that limit, due to any cause, to the corresponding acceptance criterion. Frequency margins can be used as the primary decision element. Consequence margins of sequences not reaching the limit should be quantified and aggregated in some way in order to get an indication of collective proximity to the limit. Consequence margins can be used for optimization, following ALARP-like principles.

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