DS528 - Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants

The IAEA Safety Guide SSG-4 “Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants” (2010) was developed to provide recommendations for the development and application of Level 2 PSA for nuclear power plants (NPPs) together with SSG-3 (on Level 1 PSA) and to meet the relevant requirements established in the IAEA Safety Standards existing at that time, in particular GSR Part 4 “Safety Assessment for Facilities and Activities” (2009), NS-R-1 “Safety of Nuclear Power Plants: Design” (2000), and NS-R-2 “Safety of Nuclear Power Plants: Operation” (2000). 

Since then, all these Safety Requirements publications have been revised, taking into account the latest developments and relevant practices in the Member States as well as the feedback from the Fukushima Daiichi nuclear accident in 2011.

Among the significant changes incorporated into the Safety Requirements publications GSR Part 4 (Rev. 1), SSR-2/1 (Rev. 1) and SSR-2/2 (Rev. 1) are those related to severe accidents, and the margins to both withstand extreme external events and avoid cliff edge effects.

Those changes have had an impact on safety provision incorporated, in the plant design as well as on the plant operation for all plant states, to cope with severe accidents which are modelled in Level 2 PSA. 

This Safety Guide addresses the necessary technical features of Level 2 PSA and applications for both existing and new NPPs. The revision of the Safety Guide expands the content to integrate updated considerations for Level 2 PSA in specific areas related to Level 2 PSA, such as:

• Modelling of additional safety features considered for design extension conditions;
• Modelling the use of non-permanent equipment;
• Multiunit and multi-source considerations;
• Severe accident management;
• More detailed information on current practices considering low power and shutdown states as well as internal and external hazards, and their combinations, in the scope of the Level 2 PSA;
• More detailed information on the latest update on strategies for dealing with the progression of core damage and with damage of fuel in the spent fuel pool and their related phenomena, and the results of experiments conducted in support of those strategies and improvements of code simulation capabilities; as the interface between Level 1 and Level 2 PSA is based on both CDF and FDF;
• More detailed guidance on human factors for Level 2 PSA;
• Expansion of the scope of the Safety Guide by providing practical examples of Level 2 PSA practice for different reactor technologies to the extent possible.
• Development and use of dynamic and parametric models

Hence, the Safety Guide will undergo a full revision to incorporate the above-mentioned aspects in the related sections.

The added value of the revised Safety Guide will be to provide Member States with comprehensive, consistent and up-to-date recommendations for the development and application of Level 2 PSA for NPPs.