As the nuclear regulatory framework and licensing requirements transition from prescriptive to more goal-oriented requirements, it is important to develop best practices for the setting of the reliability goals. This is especially important for small modular reactors (SMR), where the simplified safety design and use of passive systems impose high expectations for the reliability of the systems.

This paper describes a probabilistic risk assessment (PRA) model developed for the LDR-lite SMR design concept. LDR-lite is a public research study version of the LDR-50 SMR design. The PRA model is applied to define and allocate reliability goals to individual LDR-lite systems. The influence of initial and boundary assumptions, such as risk criteria specific to different Defence-in-Depth (DiD) levels, on the goal setting is studied. Allocation of reliability goals based on PRA modelling provides sufficient flexibility to examine the possible impact of different assumptions. This flexibility has high importance when studying the reliability of SMR passive systems and determining if the reliability estimation method measures up to the allocated reliability goals.

The analysis method applied in the reliability estimation of the passive systems needs to be in line with the strictness of the reliability goals. For safety systems with lower reliability goals, the use of generic estimation methods and non-specific evidence for systems can be justified. For safety systems with higher reliability goals, such as a passive system responsible for a significant part of DiD protection of a plant, the reliability analysis methods need to be more sophisticated and developed for the application purpose, using case-specific evidence. It would be also beneficial if a graded approach can be applied in the method, as then the level of detail in the analysis can be set proportional to the reliability goals allocated for the passive systems.