Enhancing Realism in a Spent Fuel Pool PSA: Incorporating the I&AB Method for Dynamic Repair Modelling and Realistic Mission Time Analysis

The PSA (Probabilistic Safety Assessment) of a storage facility for spent nuclear fuel is evaluating the risk of boiling in the fuel storage pool. Repair of certain components in the cooling system is modelled, as crediting repair has a significant impact on the results. Looking at the interpretation of a MCS containing a repair event, it can be concluded that the static PSA representation does not capture the dynamic features associated with a repair process in real life. Consequently, the static PSA representation is associated with a great amount of conservatism.

Another issue in the PSA is the assigned mission time for the cooling system, which is derived from deterministic criteria. A sensitivity analysis shows that the results are highly driven by the choice of mission time. As the aim of the PSA is not to verify deterministic criteria, but rather to be a realistic representation of the spent fuel pool process and its safety functions, a question that consequently arises is; what is the appropriate realistic mission time that should be used?

To address these issues, the calculation method I&AB (Initiators & All Barriers) has been incorporated in the PSA. Additionally, estimation of repair times has been performed on a higher level of detail and including a broader scope of component types and failure modes compared to what was used in the static PSA. The repair time data together with the I&AB method has been implemented in the PSA model. With this new method the mission time is replaced by a repair time of the initiating event, which also corresponds to the time to reach a safe state. This new approach will not only enhance the realism of the model but also enable the extraction of valuable insights and information, such as importance measures of repair times for different components, which can inform decision-making and optimize repair and maintenance routines.

Keywords: PSA, Spent fuel pool, Repair, Mission time, Safe state, I&AB, PSA application.

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