The loss of offsite power transient without scram which is one of important ATWS events of a PWR was analyzed by the thermal-hydraulic transient analysis code RETRAN. Sensitivity studies on important parameters, the moderator density reactivity feedback coefficient and the heat transfer coefficient at SG secondary side, were included in the analysis. From the results, it has been found that DNB may occur in the core, saturation appears in the primary system since an early phase of transient, and natural circulation may be conditionally degraded, especially influenced by the heat transfer rate along uncovered part of SG U-tubes. These findings have not been reported in any of past analyses and therefore it is very necessary for further detailed investigations to be performed.
