Patent Number: 
Section: claims

1. A design method for a fuel assembly of a light-water reactor, which includes a plurality of fuel rods arranged in parallel separated by a distance in a direction perpendicular to a longitudinal axis of the fuel rods, each of the fuel rods including a fuel clad and a fuel in the fuel clad, the fuel containing material based on uranium dioxide containing enriched uranium 235, some of the fuel rods including a burnable poison in the fuel, the design method comprising:accumulating core feasibility determination data investigated by analyses or experiments, showing whether or not each one of a burnable poison average mass ratio and an average enrichment of the uranium 235 contained in all of the fuel rods in the fuel assembly is within an acceptable range for a core of the fuel assembly;formulating a criterion formula which determines whether the burnable poison average mass ratio and the enrichment value are within acceptable ranges for the core based on the core feasibility determination data;setting a tentative composition of the fuel assembly;determining whether or not the tentative composition of the fuel assembly is acceptable for the core based on the criterion formula, wherein the average enrichment is greater than or equal to 5%; anddesigning the fuel assembly of the light-water reactor based on the tentative composition of the fuel assembly determined to be acceptable for the core. 2. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the criterion formula is (a1·e)−b<p·n/N<(a2·e)−c, wherein N is an integer equal to or greater than 2 and N is a number of the fuel rods in the fuel assembly, n is a number of the fuel rods containing the burnable poison and an integer equal to or greater than 1 and less than N, p is a ratio wt % of the burnable poison in the fuel, and e is the average enrichment wt % of the uranium 235 contained in all of the fuel rods in the fuel assembly, each of a 1, a2, b and c is a positive constant and a1 is equal to or greater than a2. 3. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the criterion formula is (a1·e)−b<p·n/N<(a2·e)−c, wherein N is an integer equal to or greater than 2 and N is a number of the fuel rods in the fuel assembly, n is a number of the fuel rods containing the burnable poison and an integer equal to or greater than 1 and less than N, p is a ratio wt % of the burnable poison in the fuel, and e is the average enrichment wt % of the uranium 235 contained in all of the fuel rods in the fuel assembly, each of a1, a2, b and c is a positive constant and a1 is equal to or greater than a2. 4. The design method for the fuel assembly of the light-water reactor according to claim 3, wherein each of a1 and a2 is 0.57, b is 1.8, and c is 0.8. 5. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the average enrichment of the enriched uranium 235 in the fuel containing the burnable poison is less than the maximum enrichment of the enriched uranium 235 in the fuel in the fuel assembly. 6. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the fuel rods are arranged into a square lattice array and at least one of the fuel rods containing the burnable poison does not face other fuel rods. 7. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the fuel rods are arranged into a square lattice array and at least one of the fuel rods containing the burnable poison does face other fuel rods containing the burnable poison with at least one side of the four sides of the fuel rods arranged into the square lattice array. 8. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the burnable poison contains compounds containing gadolinium, erbium, or boron. 9. The design method for the fuel assembly of the light-water reactor according to claim 1, wherein the burnable poison is gadolinia and maximum concentration of the gadolinia in the fuel is less than 20 wt %. 10. The design method for the fuel assembly of the light-water reactor according to claim 1,wherein the burnable poison is gadolinia wherein gadolinia of odd mass is more concentrated than gadolinia of even mass.