Reactor operation method

In a reactor core of a BWR type reactor, fuel assemblies taken out after the completion of the operation of a first cycle has a lower burnup degree compared with that of other fuel assemblies and provide lower generation energy. In a method of operating a reactor loaded with a plurality of first fuel assemblies having the lowest average enrichment degree and second fuel assemblies having higher average enrichment degree than that of the first fuel assemblies, the reactor is operated while inserting a control rod to a first control cell constituted by four first fuel assemblies during a first cycle. In the most term of a second cycle, the reactor is operated while inserting a control rod to a second control cell constituted by four second fuel assemblies without inserting a control rod to the first control cell. Then, the burnup degree of a fuel assembly having low enrichment degree is increased thereby enabling to ensure thermal margin. (N.H.)