Patent Number: 
Section: claims

1. A core of a boiling water reactor comprising:multiple fuel assemblies arranged in a square lattice shape in the core,wherein an arrangement of fuel assemblies adjacent to a fuel assembly having a shortest loading period among the multiple fuel assemblies arranged in the core is arranged in the core so as to satisfy a relationship X=Ta/Tb, where X is a parameter representing a core residence period of the arrangement of fuel assemblies that is 0.8 or more and 1.0 or less, 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 arranged in an inside-core region of the core having the shortest loading period,t: (ΣTs+0.5×ΣTx)/(4+0.5×4),Ts: a number of residence cycles, in the core, of fuel assemblies laterally and longitudinally adjacent to the fuel assembly having the shortest loading period in core cross section, andTx: a number of residence cycles, in the core, of fuel assemblies diagonally adjacent to fuel assembly having the shortest loading period in core cross section. 2. The core of a boiling water reactor according to claim 1,wherein the parameter X is 0.8 or more and 0.9 or less. 3. The core of the boiling water reactor according to claim 1,wherein the core is configured by the inside-core region and an outside-core region. 4. The core of the boiling water reactor according to claim 1,wherein, in the core cross section, an inside of a circle having a radius of 70% of a radius of a circumscribed circle circumscribing the fuel assemblies arranged in an outermost periphery of the core is the inside-core region. 5. The core of the boiling water reactor according to claim 2,wherein, in the core cross section, an inside of a circle having a radius of 70% of a radius of a circumscribed circle circumscribing the fuel assemblies arranged in an outermost periphery of the core is the inside-core region. 6. The core of the boiling water reactor according to claim 3,wherein, in the core cross section, an inside of a circle having a radius of 70% of a radius of a circumscribed circle circumscribing the fuel assemblies arranged in an outermost periphery of the core is the inside-core region.