| 摘要 |
A core disposed in a reactor pressure vessel includes one layer (an outermost region) at an outermost side of the core, two-three layers (an outer region) inside the outermost region and other layers (an inner region) inside the outer region. Fuel assemblies arranged in the core are supported by fuel supports having orifice. Orifice pressure loss coefficient of the orifice in the outermost region is set to be maximum and the orifice pressure loss coefficient of the orifice in the outer region is set to be minimum such that the flow rate of the coolant W for each fuel assembly in the outermost region is lowest and that for each fuel assembly in the outer region is highest. In the core of the natural circulation boiling water reactor, the reactor power distribution in a radial direction is flattened, and it is possible to increase the thermal margin.
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