Technical Meeting on Experience in the Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants

Objectives

The objective of the event is to provide the participants with an opportunity to contribute to the revision of Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants (IAEA Safety Standards Series No. SSG-4) by collecting experiences in the application and development of Level 2 probabilistic safety assessment in Member States.

Target Audience

The event is targeted at professionals from NPP design organizations, operating organizations, nuclear regulatory authorities, technical support organizations and research institutions who are engaged in activities related to or in support to the development and application of Level 2 PSA for nuclear power plants in any of the particular areas listed in the Topics section, here below.

The event is open to representatives of all Member States with an active nuclear programme, including those from embarking countries in Phase 3 of their nuclear programme.

Topics

The event will address recent experiences in Member States in specific areas related to the development and the application of Level 2 PSA for nuclear power plants, such as:

• Assessment of the safety level improvements related to

             - the incorporation in the NPP design of additional safety features considered
               for design extension conditions.
             - the use of non-permanent equipment.
             - Multiunit considerations.

• Assessment of advantages and disadvantages of strategies for dealing with core melt and with damage of fuel stored in the spent fuel pool inside of the reactor containment and their related phenomena.
• Evaluation of considerations and factors related to modelling of human performance in Level 2 PSA.
• Prioritization of research conducted in support of severe accident strategies and for improvements of severe accident simulation codes capabilities.
• Enhancement and update of severe accident management guidelines using insights from Level 2 PSA.
• Current practices considering low power and shutdown states as well as internal and external hazards, and their combinations in Level 2 PSA.
• Development and application of Level 2 PSA for nuclear power plants with advanced reactor technologies (e.g. SMRs, non-LWR).

Outputs of the Technical Meeting 2021

Despite the virtual format, the Technical Meeting had a large participation of Member States and experts with many presentations leading to the publication of the web story (see link above).  The fruitful discussions highlighted the key subjects for the development and application of Level 2 PSA on:

• Other sources of potential radioactive release than reactor and spent fuel pool need to be considered in the source term calculation using a simplified model;
• Key issues from examples of application of the Level 2 PSA methodology for Small Modular Reactors and Sodium cooled Fast neutron Reactors;
• Harmonization in the definition of risk metrics and safety goals for Level 2 PSA;
• Need for further explanation on integral and separate models for the development of the interface with Level 1 PSA;
• Considerations related to the reassessment of the list of internal and external hazards and their combinations for the Level 2 PSA model;
• Considerations related to the development of Level 2 PSA model for multi-unit sites, including advantages and disadvantages;
• Key issues related to the modelling of Human Reliability Assessment (HRA) in Level 2 PSA, including main differences with HRA for Level 1 PSA model;
• Considerations related to the development of the Level 2 PSA model for the fuel in the spent fuel pool based on the damage state resulting from the Level 1 PSA;
• Examples of applications of Level 2 PSA, their limitations and advantages;
• Examples of research programmes conducted to support the development of Level 2 PSA model, such as in-vessel melt retention.

The result of the contributions and the discussion during the Technical Meeting will be compiled in a TECDOC, which is currently in progress towards publication in order to reach a larger audience to further disseminate the experiences, practices and challenges in the development of the Level 2 PSA model in Member States. The results of discussions and experiences will be considered for incorporation in the revision of the IAEA Safety Guide “Development and Application of Level 2 Probabilistic Safety Assessment for Nuclear Power Plants” (SSG-4).