Development study of high conversion PWR

As for high conversion type PWRs, the feasibility study has been advanced as one form of plutonium utilization. The basic feature of this core is to weaken the degree of moderation of neutrons by making the volume ratio of water moderator to fuel pellets less than 1.9 for current PWRs. By this means, the neutron resonance absorption of U-238 and Pu-240 is made large, and the conversion to nuclear fission substances Pu-239 and Pu-241 from their parent substances can be increased. As the volume ratio is smaller, the rate of conversion can be made larger, therefore, in the initial period of the study, the development of the concept of the core adopting the dense lattice by making the volume ratio less than 1 was carried out. But in the dense lattice type core, the problems that the amount of charging plutonium becomes too large, and the reactivity effect of boron in primary coolant becomes very small arose, accordingly in order to solve the problems, the design was changed to the semidense lattice of the volume ratio more than 1. The prospect that the core in which spectral shift and blanket are added to the semidense lattice, and also uranium fuel is used is effective for saving natural uranium resources, and also the safety of the core can be ensured was obtained. (K.I.)