Probabilistic Safety Assessment (PSA)

 

Objective

The Technical Safety Review of Probabilistic Safety Assessment (TSR-PSA) provides a comprehensive framework for PSA peer review against the IAEA Safety Standards. The objective of the TSR-PSA is to assist in the review of the technological and methodological aspects modelled in the probabilistic safety assessment, as well as probabilistic safety assessment applications to enhance safety.

Based on past reviews it is noted that the TSR-PSA can provide important  insights and recommendations on improvement of regarding  the quality of the PSA, thereby strengthening the credibility of the PSA and its use in making safety related plant enhancements, and its use in the risk informed decision making (RIDM).

Scope of the review

The quality of PSA models plays critical role in PSA applications and is to be verified through the proper peer review. The independent peer review provides a degree of assurance that the scope, modelling and data used in PSA are adequate. Therefore, it is a widely accepted practice that peer review by independent and experienced PSA practitioners should be an integral part of PSA programme.

The scope of the review is defined according to the following major considerations:

1)      Undesired end-state: The undesired end-state defines the main focus of the PSA and may vary, e.g. reactor core damage or fuel damage (Level 1 PSA), releases of radioactive material to the environment (Level 2 PSA) and adverse effects on public health and other societal consequences (Level 3).

2)      Initiating events and hazards: The range of initiating events and hazards that are considered in PSA may include (a) internal initiating events caused by random component failures and human error, (b) internal hazards (e.g. internal fires and floods, turbine missiles) and (c) external hazards, both natural (e.g. earthquake, high winds, external floods) and of human-induced (e.g. airplane crash, accidents at nearby industrial facilities).

3)      Operating modes: Various operating modes of the reactor and the spent fuel pool (e.g. power operation, shutdown).

The scope of the review is defined by the Requesting Party in consultation with the IAEA and is reflected in the terms of reference of TSR-PSA (e.g. Level 1 PSA for internal events, internal and external hazards for full power operation or Level 1 and Level 2 PSA for internal events and all operating modes). Full scope PSA review includes review of PSA for internal events, external and internal hazards at all operating modes (both for single-unit and multi-unit PSA).

In addition, the scope of the TSR-PSA may include specific PSA applications. The scope of TSR-PSA may include the PSA applications for the following areas:

-        Design (risk-informed design solutions, equipment reliability targets, human factor engineering at design stage, etc.)

-        NPP operation (maintenance optimization, risk-informed online maintenance, development of EOPs, improvement of operator training program, risk-informed techspecs)

-        Permanent changes to the operating plant (e.g. risk-informed in-service inspections, risk-informed SSC classification)

-        Oversight activities (e.g. risk-informed inspections, evaluation and ranking of operational events)

TSR-PSA is typically conducted for various designs of nuclear power plants, but can also be conducted for PSA studies for other nuclear installations (e.g. research reactors).

The following documentation is typically required for TSR-PSA from the Requesting Party:

-        PSA report relevant to the scope of TSR-PSA.

-        Detailed information on plant design focused on the SSCs considered in PSA study (e.g. system design descriptions including system flow diagrams, cable routing, layout information, drawings, etc.).

-        Supporting deterministic calculations (e.g. thermal hydraulic, severe accident analyses, neutron physics, etc.).

-        PSA electronic model

The framework for the TSR-PSA includes (but not limited to) the following set of applicable IAEA Safety Standards:

-        Safety Requirements on Safety Assessment for Facilities and Activities (IAEA Safety Standard Series No. GSR Part 4 (rev.1))

-        Safety Requirements Safety on Safety of Nuclear Power Plants: Design (Specific Safety Requirements No. SSR-2/1(Rev. 1))

-        Specific Safety Guide on Development and Application of Level 1 Probabilistic Safety Assessment for Nuclear Power Plants (SSG-3)

-        Specific Safety Guide on Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants (SSG-4)

-        Evaluation of seismic safety for existing nuclear installations, Safety Standard series No. NS-G-2.13.

Benefits

The TSR-PSA offers interested Member States a number of benefits including:

-        Identification of concrete recommendations for improvement of PSA model and/or PSA applications based on IAEA Safety Standards.

-        Verification of the quality of PSA model and/or PSA applications by thorough peer review by independent and experienced PSA practitioners.

-        Support of the development of regulatory review practices and expertise and increase in the regulatory authority’s capacities and expertise in PSA review process.

-        Brings in top-level experience into the PSA work and provides communication and exchange of views with experienced specialists

PSA Reviews Conducted