Nuclear fuel assemblies for use in BWR type reactors

Purpose: To improve the fuel economy by making the thermal neutron flux distribution at the lateral plane uniform so that all of the fuel rods are burnt uniformly in fuel assemblies. Constitution: Fuel rods each comprising an elongate tube filled with fuel pellets and an elongate outer tube surrounding the inner tube while maintaining a gap thereto are disposed in a lattice-like pattern. When the fuel assembly is charged in the reactor core, boiling water flows through the gap between each of the inner tubes and the outer tubes while non-boiling water flows to the outside of the outer tubes. Accordingly, all of the fuel rods are put under the identical neutron moderation condition to unify the thermal neutron flux distribution in the fuel assembly. As a result, uniform burning can be attained even if the enrichment for all of the fuel rods is made equal to improve the fuel economy. Further, since it is possible to constitute the fuel assembly with the fuel pellets of a single concentration, the fuel pellet administration can be simplified. (Kamimura, M.)