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WGRISK Site-Level PSA Project: Status Update and Preliminary Insights for the Risk Aggregation Focus Area

Abstract of the technical paper/presentation presented at:
International Topical Meeting on Probabilistic Safety Assessment and Analysis
September 24–28, 2017

Prepared by:
Smain Yalaoui, Yolande Akl
Canadian Nuclear Safety Commission
and Marina Röwekamp (GRS)


Since the Fukushima Daichi accident, the assessment of the "total" risk at a nuclear site has become a topic of interest. The assessment of total risk goes beyond the current practice in the conduct of probabilistic safety assessments (PSAs), which are developed on an individual unit basis, and separately for each internal and external hazard (internal events, internal fires, internal floods, seismic events, high winds, external floods, etc.). Although this practice has provided valuable insights and information about the single risk being considered (e.g., the seismic event PSA that identifies plant vulnerabilities with regard to the seismic event), it is recognized there is an increasing interest in assessing the total risk that encompasses the impacts of both internal and external hazards on all the nuclear facilities at a given site.

Assessing the total risk poses the question of the risk metrics to be used to characterize the risk at the site level. This also raises the question of multi-unit interactions and dependencies, as well as risk aggregation across all hazards, plant operational modes, and all radioactive sources at the site.

A project, aiming to collect information on how member countries address multi-unit and /or multi-source issues in PSAs of nuclear power plant sites that contain multiple radioactive sources, was initiated in March 2015, within the Organisation for Economic Co-operation and Development (OECD) / Nuclear Energy Agency (OECD/NEA), as the Committee on the Safety of Nuclear Installations activity proposal sheet "Status of Site-level PSA developments". As part of this effort, Canada is leading Phase 1 of this project which identifies three focus areas: risk aggregation, multisource interactions, and site-based risk metrics and safety goals.

This paper provides the results of the focus group on risk aggregation. This includes the international benchmarking regarding risk aggregation for the purpose of comparison with the national safety goals, the identification of the risk metrics, and a summary of the aggregation methods for both the unit and the site level, taking into consideration the different degrees of uncertainty and PSA maturity for the different hazards.

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