Consideration on rationalization of reactor safety systems, Study on passive safety in ATWS events

Chikazawa, Yoshitaka  ; Hori, Toru; Konomura, Mamoru; Kurisaka, Kenichi 

Improvement of safety features for prevention of core disruptive accident is studied in the feasibility study on commercialized fast breeder reactor systems. From FY1999 to FY2001, natural circulated decay heat removal system was studied. In this report, a study results about passive safety in ATWS events in FY2002 and FY2003 are shown. The core and plant features of promising concepts of the feasibility study are compared. Possibility of passive safety in ATWS events are found at metal fuel sodium cooled reactors, gas cooled reactors and lead-bismuth cooled reactors. A study on a small metal fuel sodium cooled reactor is carried out to specify design requirements for passive safety in ATWS events. A reactor structure with which the control rod drive line expansion reactivity is reliable is designed. The delay constant can be sufficiently reduced with the improved design of the upper core structure and the control rod guide lines. Thermal stress with the extended EMP Flow coast-down is evaluated. Though the damage exceed in pessimistic assumption, there is a possibility to satisfy limits in detailed evaluation considering core decay heat. A reliable electric source for EMP coast-down is designed with synchronized motor. without active signal or control and its reliability is evaluated to be sufficiently high.