Development of safety assessment methodology on fuel element failure propagation in SFR and its application to Monju

Fukano, Yoshitaka  

Probabilistic and deterministic safety assessments and experimental studies on local fault (LF) propagation in sodium cooled fast reactors (SFRs) have been performed in many countries because LFs have been historically considered as one of the possible causes of severe accidents. Adventitious fuel pin failures were considered to be the most dominant initiators of LFs in these probabilistic assessments because of high frequency of occurrence during reactor operation and possibility of subsequent pin-to-pin failure propagation. Four possible mechanisms of fuel element failure propagation from adventitious fuel pin failure (FEFPA) are extracted from the state-of-the-art review of open papers. All the methods for FEFPA including thermal, mechanical, chemical propagation are modeled into a safety assessment code which is applicable to all the SFRs with the development of some methods. Furthermore the assessment on FEFPA in Monju was performed using this methodology. It was clarified that FEFPA was highly unlikely and limited within at most one sub-assembly in Monju by the multiple and diverse detection and shutdown systems for FEFPA even assuming the propagation. This result also suggests future possibility of run beyond cladding breach which enhances the economic efficiency in Monju.