The implementation of passive safety systems in the design of advanced small modular reactors presents many challenges. In particular, as a consequence of depending on natural phenomena rather than active safety systems, performing a reliability assessment is not straightforward in the context of safety studies for licensing or design feedback. For instance, larger uncertainties than with active systems, unique failure mechanisms, and limited operational experience are a few of the issues to be conveniently addressed in order to accurately model the reliability of passive safety systems.

In addition to the challenges presented in these kinds of assessments, as long as the success of passive systems strongly depends on their integration into the reactor, it is important to keep the results updated, as systems engineering evolves, through the engineering design phases, as well as during the whole plant life.

This paper presents an example involving an update of a previous assessment because the engineering phase has evolved and more information has become available. The assessment presented in this paper analyzes the impact of uncertainties on the performance of the medium pressure injection system of the iPWR during a small break loss-of-coolant accident combined with a station blackout.