Initially loaded reactor core and fuel assembly

In an initially loaded reactor core of a BWR type reactor, fuel assemblies having the lowest average enrichment degree (low enrichment degree fuel assemblies) are disposed to a control cell and the outermost circumference. Fuel assemblies having a higher average enrichment degree than the low enrichment degree fuel assemblies are disposed to other region. Four low enrichment degree fuel assemblies are arranged in a regular square in the control cell, and a cross-like shaped control rod is disposed at the center of the four low enrichment degree fuel assemblies. The control cell comprises fuel rods containing uranium fuels having a lower isotope ratio of uranium 235 than that of natural uranium at the corners on the side of the control rod. With such a constitution, a local peaking coefficient of fuel assemblies can be decreased. Accordingly, continuous operation can be conducted for a long period of time while controlling the maximum linear power density thereby capable of attaining high burn-up degree. (I.N.)