Fuel assembly and reactor core

A fuel assembly comprising

• a plurality of first fuel rods,
• a means for moderating material surrounding with said first fuel rods, and
• a plurality of second fuel rods arranged at a location adjacent to said means for moderating material,
• the enrichment of said second fuel rod is set at most 0.7 of a horizontal cross sectional average enrichment of said fuel assembly.

A fuel assembly comprising:

• a plurality of first fuel rods,
• a means for moderating material having a larger horizontal cross sectional area at an upper region in an axial direction than the area at a lower region and being surrounded with said first fuel rods, and
• a plurality of second fuel rods having lower enrichment than a horizontal cross sectional average enrichment of said fuel assembly and being arranged at a location adjacent to said means for moderating material,
• wherein said means for moderating material comprises a water rod having a cruciform horizontal cross section occupying an area equivalent to five fuel unit cells at the lower region, and said second fuel rods are arranged at four denting locations formed by the cruciform horizontal cross section at the lower region of the water rod, and
• wherein the enrichment of said second fuel rods is at most 0.7 of the horizontal cross sectional average enrichment of said fuel assembly.

A fuel assembly comprising:

• a plurality of first fuel rods,
• a means for moderating material having a larger cross sectional area in an upper region than a lower region in an axial direction and being surrounded with the first fuel rods,
• a plurality of second fuel rods having a lower enrichment than a cross sectional average enrichment of the fuel assembly and being arranged in a location adjacent to the lower region of the means for he moderating material, and
• a channel box surrounding said first fuel rods, said second fuel rods, and said means moderating material,
• wherein a width of the horizontal cross sectional area at the lower region of said means for moderating material is so set that minimum values of both thermal neutron flux and resonance neutron flux in a vertical direction to the axis of the fuel assembly are located in an outer region with respect to the location of said second fuel rods in the vertical direction to the axis of the fuel assembly, and the wall thickness of said channel box is selected as thinner at the upper region in the axial direction than at the lower region, and as thicker at corners than any other location, and
• wherein the enrichment of said second fuel rods is at most 0.7 of the horizontal cross sectional average enrichment of said fuel assembly.

A fuel assembly comprising:

• a plurality of first fuel rods,
• a means for moderating material having a larger cross sectional area in an upper region than in a lower region in an axial direction and being surrounded with the first fuel rods,
• at least one second fuel rod having a lower enrichment than a cross sectional average enrichment of the fuel assembly an being arranged in a location adjacent to the lower region in the axial direction of the means for the moderating material, and
• a means for controlling neutron flux installed in a region, wherein the lower region of said means for moderating material is located, in order to locate minimum values of both thermal neutron flux and resonance neutron flux in a vertical direction to the axis of the fuel assembly at an outer region with respect to the location of said second fuel rod in the vertical direction to the axis of the fuel assembly, and
• wherein the enrichment of said second fuel rods at most 0.7 of the horizontal cross-sectional average enrichment of said fuel assembly.

A reactor core of a nuclear reactor loaded with a plurality of fuel assemblies, wherein said fuel assembly comprises:

• a plurality of first fuel rods;
• a means for moderating material having a larger cross sectional area in an upper region than in a lower region in an axial direction and being surrounded with the first fuel rods, and
• at least one second fuel rod having a lower enrichment than a cross sectional average enrichment of the fuel assembly and being arranged in a location adjacent to the lower region of the means for the moderating material,
• wherein the horizontal cross sectional area at the lower region of the means for moderating material is set so as to locate minimum values of both thermal neutron flux and resonance neutron flux in a vertical direction to the axis of the fuel assembly at an outer region with respect to the location of said second fuel rod in the vertical direction to the axis of the fuel assembly, and
• wherein the enrichment of said second fuel rod is at most 0.7 of the horizontal cross sectional average enrichment of said fuel assembly.

A loading method of fuel assemblies in a reactor core of a nuclear reactor having at least a central region and a peripheral region, wherein respective ones of said fuel assemblies comprise:

• a plurality of first fuel rods,
• a means for moderating material having a larger cross sectional area in upper region than in a lower region in an axial direction and being surrounded with the first fuel rods, and
• at least one second fuel rod having a lower enrichment than a cross sectional average enrichment of the fuel assembly and being arranged in a location adjacent to the lower region of the means for the moderating material,
• wherein the horizontal cross sectional area at the lower region of the means for moderating material is set so as to locate minimum values of both thermal neutron flux and resonance neutron flux in a vertical direction to the axis of the fuel assembly at an outer region with respect to the location of said second fuel rod in the vertical direction to the axis of the fuel assembly,
• wherein said fuel assemblies are loaded more in the central region than in the peripheral region, and
• wherein the enrichment of said second fuel rod is at most 0.7 of the horizontal cross sectional average enrichment of said fuel assembly.