Development of technical basis in the initiating and transition phases of unprotected events for Level-2 PSA methodology in sodium-cooled fast reactors

A probabilistic safety assessment (Level-2 PSA) methodology was developed for comprehensive risk evaluation of sodium-cooled fast reactors. As part of this development, in this paper, phenomenological event trees were developed as well as technical database to quantify the probability of event sequences in the Level-2 PSA, focusing on the initiating and transition phases of unprotected events. Typical and important accident categories were selected: unprotected loss of flow (ULOF), unprotected transient overpower (UTOP) and unprotected loss of heat sink (ULOHS). Based on the state-of-the-art knowledge, the headings of these event trees were selected so that dominant factors in accident consequences can be represented appropriately. For each of the headings, available information for the probability quantification were reviewed and integrated as the technical database for the Level-2 PSA. It was clarified that the headings of the ULOF category, for which experimental database and evaluation models have been reasonably established, can be commonly applied to certain part of the different accident categories except for some specific points, which were identified in this study. For the ULOHS category, an additional event tree is necessary before the core disruption providing various boundary conditions for the initiating phase. For the transition phase, dominant factors were also identified through parametric analyses. In the Japan sodium-cooled fast reactor, an inner duct is introduced into a fuel subassembly for enhancing molten fuel discharge from disrupted core in the transition phase. The parametric study showed that the analytical case without the fuel discharge through the inner duct resulted in an occurrence of recriticality regardless of the fuel discharge through control-rod guide tubes. This suggests that the fuel discharge through the inner duct is essential to avoid severe recriticality in the transition phase.