Fuel assemblies

Purpose: To decrease the power peaking thereby flatten the power distribution. Constitution: A plurality of fuel rods incorporated with burnable poisons in a channel are divided into two upper and lower portions, in which the content of the burnable poisons in the upper portion with less thermal neutrons is reduced as compared with the content in the lower portion. Further, the enrichment degree of fuel elements constituting the fuel rods containing the burnable poisons and other fuel rods not containing the burnable poisons are entirely divided into two types. Fuel rods constituted with fuel elements at higher enrichment degree are locallized the side of the outer layer, while fuel rods constituted with fuel elements at lower enrichment degree are locallized on the side of the inner layer in a concentrated manner. This can reduce the maximum value of the power in the direction of the height to flatten the power in that direction by which the power peaking can be suppressed to less than an allowable value even if the local power is increased, to thereby improve the combustion efficiency. (Sekiya, K.)