Fuel assembly

Burnable poison-incorporated fuel rods are used as fuel rods to be loaded to the outermost layer having high neutron fluxes among fuel loading areas of a fuel assembly. The fuel pellet of this fuel rod is divided into two parts of an inner region and an outer region, and gadolinium is incorporated to the inner region. As fuel rods to be loaded to the innermost layer of low neutron fluxes, a fuel rod incorporated with gadolinium in the outer region is used. In addition, ordinary fuel rod not incorporated with gadolinium is loaded to an intermediate layer. A fuel ingredient of the fuel rods comprises plutonium and uranium mixed oxide fuels. A power peaking coefficient is suppressed by the mixing of gadolinium to the outer region of the fuel rod at the innermost layer. This can improve thermal integrity when a predetermined excessive reactivity is to be suppressed by using the burnable poison-incorporating fuel rods. (I.N.)