DESIGN OF SMALL REDUCED-MODERATION WATER REACTOR (RMWR) WITH NATURAL CIRCULATION COOLING

A small scale around 300 MWe reduced-moderation water reactor (RMWR) concept has been developed at JAERI. In order to overcome the scale demerit for a small reactor, the passive safety features has been intended to be introduced to simplify the system, and hence, to reduce the plant cost. For the core design, a tight-lattice fuel rod arrangement under the high core average void fraction BWR type concept is introduced to attain a high conversion ratio over 1.0. The negative void reactivity coefficients are also required for the design, and the very flat short core concept is adopted. This short core design is also essential to make the natural circulation cooling possible. The core average discharge burn-up of 60 GWd/t and the operation cycle length of 24 months are also attained in the present core design. For the system design, simplification of the system introducing the passive safety features is the basic approach to reduce the plant cost, in addition to the natural circulation core cooling concept. For example, the high pressure coolant injection system using pumps is replaced with the passive accumulator system and the emergency diesel generators can be removed, resulting in the effective cost reduction. The cost evaluation of the nuclear steam supply system gives about 20 % reduction. The MOX fuel in the RMWR contains Pu around 30 wt% and is irradiated to a high burn-up. Therefore, the fuel safety evaluation has been performed and the acceptable results have been obtained from the thermal feasibility point of view.