Patent ID: 6141396
Filing Date: 2000-10-31
Classification: G21C,Y02E

Abstract:
An initial core comprising a plurality of fuel assemblies having different average enrichment and a plurality of control rods, each fuel assembly having a square-shaped cross section and each control rod having a cross-shaped cross section,wherein a unit cell of a square-form is composed of one first fuel assembly with the lowest average enrichment and three second fuel assemblies with higher average enrichment than the first fuel assembly,a plurality of unit loading patterns arranged in a central area of the initial core, each unit loading pattern being composed of one unit cell and four control rods arranged at four corners of said one unit cell,the second fuel assembly obliquely adjoining the first fuel assembly in each unit loading pattern is divided into one side area near to the control rod and another side area far from the control rod by a diagonal line, a number of fuel rods containing gadolinia in said one side area being 2 or more than the number of fuel rods containing gadolinia in said another side area of the second fuel assembly obliquely adjoining the first fuel assembly.