The present work takes a Small Modular Reactor as a reference case study. One of its design features is that it has a grace period of 36 hours after an initiating event has occurred, during which no power supply for active systems nor human actions will be required to control the situation. During that time, safety functions will be satisfied by means of passive systems, which will lead the plant to a stable state. Once the grace period ends a post-grace period starts which is completely dependent on human actions and power supply.
The motivation of this paper was the discussion of including the post grace period into the PSA analysis, for which is necessary to extend the traditional PSA evaluation time. To evaluate needed operator actions and potential failures in the control strategy for a Station Black-out event in the post-grace period, diverse control strategies are analysed, developing a probabilistic model based on Event Tree technique, framed in the Defence in Depth principle. Core Damage Frequency is considered as a risk metric to compare the strategies.
Finally, it is concluded that the evaluation time extension is a complementary analysis pertinent to evaluate different strategies and to identify actions to maintain the stable state through time. It is observed that for the postgrace period stage, Human Reliability Analysis becomes relevant.