A new loss of flow transient analysis method for OPR1000 (Optimized Power Reactor 1000, previously called KSNP: Korean Standard Nuclear Power Plant) based on RETRAN code were developed. The reference plant for the analysis is Ulchin Unit 3 and the transient analyzed is 4 pump coastdown. The current analysis for loss of RCS flow transient of OPR1000 uses COAST and CESEC codes. The new method uses RETRAN code to replace COAST and CESEC codes. Since the ability of RETRAN to replace CESEC has been studied in other non-LOCA transients, this paper will focus on COAST code and RCP coastdown flow rates. The results from simplified RETRAN nodalization corresponding to COAST show good agreement with RCS flow results from COAST code. The results are also compared with RETRAN basedeck for safety analysis which is more complex and show similar trends. Therefore, previous analysis method for loss of flow of OPR1000 using COAST code can be replaced with the new analysis method based on RETRAN.

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