Fuel assembly

Purpose: To unify the U-235 content for each of fuel rods arranged in a square pattern and unify the reactivity for each of the rods, with respect to a fuel assembly for use in a BWR type reactor. Constitution: All of the fuel rods charged in a fuel assmbly have an identical U-235 content. The fuel rods each of an elongate cylindrical configuration are arranged in a square pattern within a fuel assembly and four or more water rods are disposed at the central area of the square pattern. This improves the neutron decelerating effect at the central area and increases the thermal neutrons at the central area. A plurality of fuel rods incorporated with burnable poisons are disposed at the outer circumference of the square pattern. Since this decreases the thermal neutrons at the outer periphery and reduced the reactivity of the fuel rods at the periphery, the abnormal power rise in the fuel rods disposed at the outer periphery can be prevented even if the U-235 content for each of the fuel rods is identical. In this way, the kind of fuel rods to be used i s reduced to one, whereby the manufacture of the fuel rods and the assembling of the fuel assembly can be facilitated. (Ikeda, J.)