The mission of the Nuclear Energy Agency (NEA) Committee on the Safety of Nuclear Installations (CSNI) Working Group on Risk Assessment (WGRISK) is to advance understanding of probabilistic safety assessment (PSA) and to enhance its utilisation for: 1) improving the safety of nuclear installations; 2) improving the design and operation of nuclear installations; and 3) increasing regulatory effectiveness through risk-informed approaches. In support of this mission, WGRISK routinely collects and shares information among member countries regarding PSA methods and practices used to estimate nuclear accident risks and to identify significant risk contributors in support of nuclear installation applications.
A Level 3 PSA is an assessment of the off-site public risks attributable to a spectrum of possible accident scenarios involving a nuclear installation. In the traditional PSA framework for commercial nuclear power plants, a Level 3 PSA includes three progressive levels of analysis: 1) core damage accident or plant damage state frequency analysis; 2) accident progression, containment performance, and radiological release frequency analysis; and 3) off-site radiological consequence analysis. A Level 1 PSA includes only the first level of analysis and estimates the frequency of core damage accidents or plant damage states and identifies significant contributors to these frequencies. A Level 2 PSA includes the first and second levels of analysis and - in addition to Level 1 PSA risk metrics - estimates the frequencies of defined radiological release categories and identifies significant contributors to these frequencies. Finally, a Level 3 PSA includes all three levels of analysis and - in addition to Level 2 PSA metrics - estimates the frequencies of off-site public health, environmental, and economic consequences attributable to the set of radiological release categories and corresponding source terms defined in the Level 2 analysis. Compared to Level 1 and Level 2 PSAs, a Level 3 PSA thus represents a more complete characterisation of the off-site public risks attributable to a spectrum of possible accident scenarios involving a nuclear installation and provides an important input to cost-benefit analyses used to evaluate proposed risk management options. Although the term Level 3 PSA has been used in some cases to refer only to the off-site radiological consequence analysis element, the focus of this activity is on off-site radiological consequence analyses that are performed as part of an integrated PSA that includes all three analysis levels.