2021

Björkman, Kim; Tyrväinen, Tero

Small modular reactors (SMR) include special characteristics, such as multi-module issues and passive safety systems, that can pose challenges for probabilistic risk assessment (PRA) of SMRs. PRA method, tool and risk metric development is needed to account for the reliability and the uncertainties related to the different aspects of SMR risk. In this study, we focused on multi-module issues.<br/>Even though SMR modules are not equivalent to units of large reactors, the challenges considering modelling of multi-module accidents in PRA are similar to those of modelling of multi-unit accidents. Therefore, multi-unit PRA approaches could provide a good starting point for multi-module accident modelling.<br/>Most multi-unit PRA methods are based on a single-unit model (for each unit) developed conventionally using event trees and fault trees. If SMR PRA is developed using the conventional PRA approach for a single module, the multi-unit PRA methods seem in general quite well applicable to multi-module PRA. However, there are aspects in multi-module PRA of SMRs that can differ from multi-unit PRA of large reactors. These include risk metrics, plant operating states, initiating events, human dependencies and CCF groups. In summary, many analysis details may need to be reconsidered for SMRs compared to large reactors.<br/>Both for multi-unit and multi-module PRA dynamic methodologies have been proposed. However, it is a bit unclear why a conventional PRA approach would not be sufficient.