Abstract: In a light water moderation type nuclear reactor with the once-through method, the reactor core is divided into a central area and a peripheral area by a partition member, a first fuel assembly is arranged in the central area (high conversion area) and a second fuel assembly is arranged in the peripheral area. The ratio (r.sub.H/U) of the number of hydrogen atoms to that of uranium atoms in the central area is smaller than that of the ratio in the peripheral area and the second fuel assembly in the peripheral area is formed of fuel rods of the first fuel assembly having been previously burned in the central area and moved into the peripheral area. The plutonium production increases and uranium consumption is reduced during the first half of the lifetime of the fuel rods in the high conversion area with the take-up burn up increasing during the second half of the lifetime of the fuel rods in the burner area.

Abstract: The axial power distribution of a nuclear reactor in a region of high burn-up degrees should desirably be flat. In consideration of the fact that void coefficients have a distribution in the axial direction, a mean infinite multiplication factor in an axially upper region of a reactor core is made lower than a mean infinite multiplication factor in a lower region. Especially, this becomes possible in such a way that a mean enrichment factor of fuel located in the uppr region is made lower than a mean enrichment factor of fuel located in the lower region.

Abstract: A construction of boiling water reactor core having a plurality of fuel assemblies. Some of fuel assemblies have an infinite multiplication factor which is larger at their upper regions than at their lower portions, so that the effective multiplication factor of the reactor core is larger at the upper portion than at the lower portion of the reactor core under an equal condition. Consequently, a relatively flat axial power distribution is established even under an uneven axial void distribution.

Abstract: A fuel assembly suitable for use in boiling water reactor. The fuel assembly has a specific pattern of distribution of enrichment, so that the upper region of the assembly has a larger infinite multiplication factor than that of the lower region. In an example, the distribution of enrichment is such that the upper region has a mean enrichment larger than that of the lower region, while, in another example, the mean enrichment is maintained constant over the length of the assembly but the difference of enrichments between the central and peripheral portions of a plane normal to the axis of the assembly is made smaller at the upper region than at the lower region.