Fuel assemblies

Purpose: To improve the fuel economy by increasing the reactivity at the latter burning stage of fuel assemblies and thereby increasing the burn-up degree. Constitution: At the later stage of the burning where the infinite multiplication factor of a fuel assembly is lowered, fuel rods are partially discharged to increase the fuel-moderator volume ratio in the fuel assembly. Then, plutonium is positively burnt by bringing the ratio near to an optimum point where the infinite multiplication factor becomes maximum and the reactivity of the fuel assembly is increased by utilizing the spectral shift effect. The number of the fuel rods to be removed is selected so as to approach the fuel-moderator atom number ratio where the infinite multiplication factor is maximum. Further, the positions where the thermal neutron fluxes are low are most effective for removing the rods and those positions between which no fuel rods are present and which are adjacent with neither the channel box nor the water rods are preferred. The rods should be removed at the time when the burning is proceeded at lest for one cycle. The reactivity is thus increased and the burn-up degree of fuels upon taking-out can be improved. (Kamimura, M.)