Patent ID: 6925138

Claim:
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 .