Study on In-Vessel Retention (IVR) of unprotected accident for fast reactor ((2) Assessment of PAMR/PAHR phase in ULOF)

Abstract

The achievement of In-Vessel Retention (IVR) of the accident consequences in an unprotected loss of flow (ULOF), which is one of the technically inconceivable events postulated beyond design basis, is effective and rational approach in enhancing the safety characteristics of sodium-cooled fast reactor (SFR). In the evaluation of the event progressions during ULOF, the whole sequence is categorized into several phases: initiating, transition, and post-accident-material-relocation/post-accident-heat-removal (PAMR/PAHR) phases. The following two points should be confirmed to achieve IVR: 1) there is no significant mechanical energy release, and 2) the decay heat generated from relocated fuels is stably removed. The assessments of the initiating and transition phases for a small-scale SFR showed that 1) there was no significant mechanical energy release. The objective of the present study is to show that 2) the decay heat generated from the relocated fuels would be stably removed in the PAMR/PAHR phase, where the relocated fuels mean fuel discharged from the core into the low-pressure plenum through control-rod guide tubes, and fuel remnant in the disrupted core region (non-discharged fuel). As a result of the present assessments, it should be concluded that the stable cooling of the relocated fuels was confirmed and the prospect of IVR was obtained.