Patent Number: 
Section: claims

1. A reactor core comprising:an outermost region;a core region surrounded by said outermost region;a plurality of fuel support members, each of which is disposed at a lower end portion of said outermost region and said core region; anda plurality of fuel assemblies loaded in said outermost region and said core region and supported by said fuel support members;wherein a plurality of said fuel assemblies disposed in said core region include a plurality of first fuel assemblies, each of which is inserted into a first coolant passage which is formed in said fuel support member and has a first resistor having an opening, and a plurality of second fuel assemblies, each of which is individually inserted into each of a second coolant passage which is formed in said fuel support member and has a second resistor having an opening and a larger pressure loss than that of said first resistor; andfour fuel assemblies, each of which is adjacent to each of four lateral sides of each of a plurality of first fuel assemblies in said core region, include at least three second fuel assemblies;wherein when a distance between a center of said core region and an axis of said fuel assembly located at a furthest position from said center of said core region among all of said fuel assemblies adjacent to said outermost region and disposed in said core region is represented as L, said core region includes an inner region wherein said fuel assemblies, each of which has said axis on an inner side of a L/√{square root over (2)} position in a radial direction of said core region from said center of said core region, are loaded, and an outer region surrounding said inner region; andwherein said first fuel assemblies and said second fuel assemblies are arranged in said inner region, and said second fuel assemblies are arranged in said outermost region which has no said first fuel assemblies therein. 2. The reactor core according to claim 1,wherein when said four fuel assemblies adjacent to said four lateral sides of said first fuel assembly include three said second fuel assemblies adjacent to three of said four lateral sides of said first fuel assembly, and an other fuel assembly among said four fuel assemblies which is one said first fuel assembly adjacent to a fourth of said four lateral sides of said first fuel assembly. 3. The reactor core according to claim 1,wherein said first fuel assemblies, each of which is adjacent to said at least three of said second fuel assemblies, are all of said first fuel assemblies arranged in said inner region. 4. The reactor core according to claim 1,wherein said first fuel assemblies, each of which is adjacent to said at least three of said second fuel assemblies, are all of said first fuel assemblies arranged in said inner region. 5. The reactor core according to claim 1,wherein said first fuel assembly is a fuel assembly having an in-core fuel dwelling time which is shorter than that of a core average. 6. The reactor core according to claim 1, wherein each of said four fuel assemblies adjacent to said four lateral sides of said first fuel assembly is said second fuel assembly. * 7. A reactor core , comprising:an outermost region;a core region surrounded by said outermost region;a plurality of fuel support members, each of which is disposed at a lower end portion of said outermost region and said core region; anda plurality of fuel assemblies loaded in said outermost region and said core region and supported by said fuel support members;wherein a plurality of said fuel assemblies disposed in said core region include a plurality of first fuel assemblies, each of which is inserted a first coolant passage which is formed in said fuel support member and has a first resistor having an opening, and a plurality of second fuel assemblies, each of which is inserted into each of a second coolant passage which is formed in said fuel support members and has a second resistor having an opening and a larger pressure loss than that of said first resistor;wherein when a distance between a center of said core region and an axis of said fuel assembly located at a furthest position from said center of said core region among all of said fuel assemblies adjacent to said outermost region and disposed in said core region is represented as L, said core region includes an inner region wherein said fuel assemblies, each of which has said axis on an inner side of a L/√{square root over (2)} position in a radial direction of said core region from said center of said core region, are loaded, and an outer region surrounding said inner region;wherein when a number of said second fuel assemblies among four of said fuel assemblies separately adjacent to four lateral sides of each of said first fuel assemblies is represented as α, an average number α in said inner region of said core region is no greater than 0; andwherein said second fuel assemblies are arranged in said outermost region which has no said first fuel assemblies therein.