To cut power generation costs by obtaining high-temperature superheated steam at a core outlet to heighten the thermal efficiency and improve the power generation efficiency and reduce the contamination due to exhaust heat and radioactive waste at the same time, in fuel assemblies and a control rod constituting a core of a boiling water reactor (BWR).

Fuel assemblies and a control rod are composed of: protective fuel rods 140 made of pellets of a sintered fertile material; high-temperature fuel assemblies 100 consisting of output fuel rods 150 made of multilayer sintered pellets; a guide tube 110 for the upper part of the control rod; and a high-temperature control rod 120. As to a material for high-temperature cladding tubes and others, the percentage of nickel , not including cobalt, in an iron-based alloy demonstrated by gas turbine blades is 26% or less. The high-temperature superheated steam is obtained at the core outlet by equalizing the flow rate at a core inlet with that of the steam at the core outlet.

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