Abstract: A fuel assembly of the type having a plurality of a fuel material characterized in that a water rod disposed in a channel has a large diameter so that it has a space into which a plurality of fuel rods could be loaded and liquid water boil flows through the water rod and which has a section filled with a fuel material comprising a natural uranium section defined at least one of the upper and lower ends of each fuel rod and an enriched uranium section comprising the uppermost, intermediate and lowermost sections each having a different degree of average enrichment across a cross section of the fuel assembly and the upper and lowermost sections have the degree of enrichment across a section of the fuel assembly is lower than that of the intermediate section, and the enriched uranium section has two sections each of which has a different content of a burnable poison per unit length in the axial direction and the content of the burnable poison per unit of length in the axial direction in the uppermost section is

Abstract: A fuel assembly for a nuclear reactor includes a number of fuel rods filled with a fuel material. A plurality of fuel rods have a partial effective fuel area filled with a fuel material and has a portion in which enrichment of a fissile nuclide is significantly reduced or the fissile nuclide does not exist at all on an axial level including a reactor shut-down zone at which subcriticality becomes small during a reactor operation period. The other fuel rods are filled with the fuel material throughout the entire axial length thereof. The first mentioned fuel rod may be provided with a partially interposed zone or may be constructed by a fuel rod having a length shorter than that of the other fuel rod. The tube means may be arranged in the fuel assembly so as to pass the moderator therethrough.

Abstract: A fuel assembly for a nuclear reactor includes a number of fuel rods filled with a fuel material. A water rod having a cross sectional area larger than that of each fuel rod is disposed at substantially the central portion of the fuel assembly. The fuel rods include first fuel rods having a whole effective fuel zone filled with a fuel material throughout the entire length thereof and second fuel rods each having an interposed zone in which enrichment of a fissile nuclide is significantly reduced or the fissile nuclide does not exist at all. The second fuel rods are arranged so as to surround the water rod so that the interposed zones are positioned on an axial level including a portion at which subcriticality is made small at a period in which maintenance of reactor shut-down margine is made difficult during the reactor operation period.