Improvement of nuclear core design for boiling water reactor

In BWRs, the steam generated in nuclear reactors is supplied directly to turbines, and it is so-called direct cycle, accordingly BWRs have many features, for instance, steam generators are unnecessary, and the power output control with core flow rate is feasible. Recently, the improvement of BWR cores has been demanded with the accumulation of operational experiences as the reactors for power generation, aiming at increasing the flexibility of operation and improving the rate of utilization of plants by making the restriction due to the conditions limiting operation as small as possible, which are applied to maintain the soundness of fuel. In Hitachi Ltd., in order to cope with such demand, the basic features of BWR cores, i.e. the void distribution exists within the core and the peaking of thermal output arises in the lower part of core, and the accompanying characteristics were investigated, and the improved core called WNS core was developed, introducing new design concept and placing emphasis on the reduction of output peaking, the improvement of the rate of utilization of plant facilities, and the simplification of core operation. The basic concept about the improved core, the features of the improved core, the examples of design of the improved core, and the change of the cores used heretofore to the improved core are explained in this paper. (Kako, I.)