Nuclear reactor fuel assemblies

Purpose: To obtain fuel assemblies for BWR type reactors capable of increasing the operation period, as well as ensuring sufficient reactor shutdown allowance. Constitution: Fuel elements in a fuel assembly are grouped into a coaxial pattern such that first layer situates nearest to the shroud and second, third and fourth layers situate successively toward the inside. The average enrichment degree in the fission products of the second layer fuel elements situated inner side in adjacent with the first layer fuel elements is set to more than 1.5 times as high as that of the first layer fuel elements in adjacent with the channel box of the fuel assembly. This enables to decrease the power fluctuation in the first layer fuel elements in adjacent with gap water between high temperature and low temperature periods. In actual design, the distribution of the enrichment degree in each of the fuel elements is optimized so as to set the local power peaking within a reference value. The above performance can further be improved by decreasing the enrichment degree for the fourth layer fuel elements and increasing the enrichment degree for the fuel elements in the third layer remote from the water rod. (Ikeda, J.)