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A plurality of fuel assemblies 103, themselves obtained by arranging fuel rods 100 and water rods 107 in square lattices, are arranged in a square lattice at a certain pitch. The blades 102a of a cross-shaped (cruciform) control rod 102 in a cross section are inserted into four adjacent spaces forme

A plurality of fuel assemblies 103, themselves obtained by arranging fuel rods 100 and water rods 107 in square lattices, are arranged in a square lattice at a certain pitch. The blades 102a of a cross-shaped (cruciform) control rod 102 in a cross section are inserted into four adjacent spaces formed by four fuel assemblies 100 facing each other. A value of 0.06 cm?1 or greater is selected for the ratio (B/S) of the width (B) of each blade on the cruciform control rod 102 and the surface (S) of the fuel lattice defined by the surface area of a square whose side is equal to the pitch between the fuel assemblies 103.

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1. A reactor core capable of being mounted in a lower portion of a reactor pressure vessel comprising a core support plate mounted on the lower portion in the reactor pressure vessel and an upper grid disposed on and above the core support plate, said reactor core comprising:a plurality of fuel asse

1. A reactor core capable of being mounted in a lower portion of a reactor pressure vessel comprising a core support plate mounted on the lower portion in the reactor pressure vessel and an upper grid disposed on and above the core support plate, said reactor core comprising:a plurality of fuel assemblies configured to be supported by the core support plate and the upper grid so as to be arranged in a square grid form at a certain pitch; and a plurality of control rods having a cruciform cross-section comprising four blades each having a width (B), each of said control rods being adapted for insertion into four adjacent spaces between four fuel assemblies facing each other, wherein a ratio (B/S) of the width (B) of the control rod blades to a surface area (S) of a square having sides each being equal to the pitch between the fuel assemblies is set in a range of 0.065 to 0.08 cm?1. 2. A reactor core as claimed in claim 1, wherein said fuel assemblies comprise fuel rods and said fuel rods contain uranium, plutonium, or oxides or nitrides of the two elements as nuclear fuel material.3. A reactor core as claimed in claim 2, wherein fuel rods disposed around said fuel assemblies contain thorium as the nuclear fuel material.4. A reactor core as claimed in claim 1, wherein a fuel assembly has a plurality of fuel rods charged with a fissionable material thereinto, and a mean enrichment of said fissionable material is the same for all loaded fuel assemblies.