Reactor core and operation method therefor

Among fuel assemblies of a high enrichment degree facing to a water rod, those having greater number of burnable poison-incorporated fuel rods are defined as A, and those having smaller number of them are defined as B, and greater number of the fuel assemblies B are loaded to peripheral regions of the reactor core and greater number of the fuel assemblies A are loaded to the inner region of the reactor core. The infinite multiplication factor of the fuel assemblies B is greater than that of the fuel assemblies A at an initial burning stage, on the contrary, the infinite multiplication factor of the fuel assemblies A is made greater as the burning proceeds. Accordingly, the neutron absorbing effect of burnable poisons is smaller, and the lifetime of the burnable poisons is longer in the fuel assemblies B. The concentration of the burnable poisons is determined higher than that in a conventional case within an allowable range in view of fuel integrity, and sufficient reactor shut down margin can be ensured while controlling surplus reactivity to an appropriate range from the initial to the final burning stage. (N.H.)