Characteristics of severe accidents of Reduced-Moderation Water Reactor (RMWR)

Yonomoto, Taisuke ; Akie, Hiroshi  ; Kobayashi, Noboru; Okubo, Tsutomu; Uchikawa, Sadao; Iwamura, Takamichi

Reduced-Moderation Water Reactor (RMWR) is a light-water cooled high-conversion reactor that is being developed by JAERI with collaboration from the Japanese industries. Since RMWR utilizes the highly enriched plutonium, the safety concern for RMWR includes the possibility of recriticality during severe accidents as is the case with the liquid metal cooled fast breeder reactor. In order to clarify this concern, characteristics of severe accidents of RMWR are analyzed in this study. The results obtained so far indicate that (1) the mechanical impact of recriticality in the core, if occurs, is supposed to be insignificant due to the absence of water, (2) the mixture of the fuel and cladding debris in the lower plenum does not cause recriticality when they are well mixed and distributed flatly, and (3) if requires, the installation of neutron-absorption material with realistic geometry can effectively prevent recriticality in the lower plenum even for the conservatively-assumed spherical accumulation of core debris.