An Evaluation of CDA event sequences in a gas-cooled fast reactor

Yamano, Hidemasa   ; Tobita, Yoshiharu

An event-sequence analysis of a core disruptive accident (CDA) has been performed for a carbon-dioxide-gas-cooled fast reactor in order to evaluate the safety design in a feasibility study on commercialized fast reactor cycle systems in Japan. An unprotected loss-of-flow accident has been chosen as a representative CDA initiator. The analysis has showed that molten cladding relocation out of the core caused a power burst, leading to the molten fuel dispersion mainly toward a downward direction, then the reactivity settled down under the sub-critical condition. In the present design, the measure for avoiding the energetic recriticality is not necessary because of the short lower axial blanket (LAB). For extending LAB length aiming at a higher breeding ratio, the fuel penetration length into the LAB region just after core disruption has been estimated by a validated fuel-freezing model. A design measure enhancing early fuel discharge has been proposed for the core adopting the longer LAB length.