Nuclear reactors

Purpose: To improve the uranium fuel economy in water cooled reactors. Constitution: Spectrum shift rods are inserted to accumulate plutonium 239 fissible nuclides at the initial burning stage of fuels and solid moderator rods of greater neutron moderating effect are inserted instead of the spectrum shift rods at the latter-half period of burning thereby effectively utilize the accumulated plutonium. That is, at the initial period of burning of the fuel assembly, the hydrogen/uranium ratio in the bundle is reduced by the insertion of depleted uranium portions to increase the accumulation amount of plutonium 239 fissible nuclides. At the latter-half period of the fuel burning, depleted uranium rods are withdrawn for preventing the reduction in the reactivity accompanying the burning of uranium 235. Further, when solid moderators are inserted instead of depleted uranium rods, since the hydrogen density is higher, the hydrogen-uranium ratio is increased thereby enabling to effectively utilize the accumulated plutonium to improve the fuel economy. (Kamimura, M.)