A conceptual design study has been performed on upgrading the core performance of the Japanese fast breeder reactor (FBR) Monju. The main aim of this study is to investigate and demonstrate the feasibility of an upgraded core with an extended refueling interval of 365 equivalent full-power days and increased average fuel burnup of 150 GWd/t, which are expected in future commercial FBRs.

Two main design measures have been taken to accommodate the largely increased burnup reactivity loss and the reactivity control characteristics for the 1-yr cycle operation: (a) A modified fuel pin specification with increased pin diameter, pellet density, and fissile height has been chosen to improve the burnup reactivity loss per extended cycle, and (b) the control rod specification has been modified to enhance the reactivity worth by increasing the 10B content to ensure sufficient shutdown margin.

The major core characteristics that have been evaluated are the core power distribution, safety-related reactivities such as Doppler and sodium void effect, thermal hydraulics, and reactivity control characteristics. The results show that even a medium-sized upgraded core with a volume of [approximately]2.5 m3 could achieve the primary targeted performance of 1-yr cycle operation, without causing significant drawbacks to the core characteristics and safety aspects. The feasibility of the upgraded core concept has thus been demonstrated.