A Study on the reliability of the FBR reactor shutdown system; Design study for the large scale FBR

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A conventional type of RSS in a large scale FBR was designed and its unavailability was analyzed with fault-tree. Reliability of logic circuits of the reaetor protection system is relatively high when compared to that of the control rod insertion. Contributing factors to the unavailabity are multiple failures of detection systems, and failure to insert rods such as failure to deratch or rod jamming. Then the new concept of control rod release mechanism was introduced in the RSS design. The thermal-hydraulic characteristics of the mechanism was analyzed using computer codes SSC-L and AQUA. Further, qualitative analysis of the common cause failure for the RSS was tried with the generic cause approach. The reactor protection systems of the backup RSS are diversified by the self actuated control rod release mechanism. With such a mechanism, the number of common cause factors were decreased for postulated LOF event.