Core of Boiling Water Reactor

A core of a boiling water reactor in which multiple fuel assemblies are loaded in a square lattice shape,

• wherein, during fuel exchange, without loading fuel assemblies having a shorter loading period than a fuel assembly having the shortest loading period loaded into the core before the fuel exchange in the core after the fuel exchange, the multiple fuel assemblies are loaded so that a parameter X represented by the following equation (1) is 0.8 or more and 1.0 or less using the number of residence cycles, in the core, of fuel assemblies laterally adjacent and longitudinally adjacent to a fuel assembly having the shortest loading period and the number of residence cycles, in the core, of fuel assemblies diagonally adjacent thereto, among fuel assemblies loaded into the core after the fuel exchange,
• X=Ta/Tb (1)
• where,
• Ta:
• T in a case where fuel exchange is performed,
• Tb:
• T in a case where fuel exchange is not performed,
• T:
• an average value oft of a fuel assembly having the shortest loading period loaded into an inside-core region,
• t:
• (ΣTs+0.5×ΣTx)/(4+0.5×4),
• Ts:
• the number of residence cycles, in the core, of fuel assemblies laterally and longitudinally adjacent to the fuel assembly having a shortest loading period in core cross section, and
• Tx:
• the number of residence cycles, in the core, of fuel assemblies diagonally adjacent to fuel assembly having a shortest loading period in core cross section.