Patent Number: 
Section: claims

1. A fuel assembly configured to have a fuel rod, an upper tie plate, a lower tie plate, and a spacer, the fuel assembly comprising:a fuel support into which a lower portion of the lower tie plate as a part of the fuel assembly is inserted and which supports a lower portion of the fuel assembly;a core plate to which the fuel support is fixed;a Y-shape type control rod that is disposed in a space between some fuel assemblies when a plurality of fuel assemblies are arranged in a reactor pressure vessel; andan installation of a neutron instrumentation tube that is disposed at a wing of a distal end of the Y-shape type control rod, whereinthe lower portion of the lower tie plate that is inserted into a socket above the fuel support defines an extension portion, and a stable member is provided around the extension portion, the extension portion defining protrusions that are inserted into L-shaped cavities,the upper distal end of the installation of the neutron instrumentation tube is supported and fixed to the channel box in the vicinity of the upper end of the fuel assembly,the lower portion of the neutron instrumentation tube is disposed separate from the reactor pressure vessel, andthe stable members are provided on the periphery of the extension portion of the lower portions of the lower tie plate, and the stable members are disposed at positions at which the stable members are adjacent to but separated by a distance from the neutron instrumentation tube. 2. The fuel assembly according to claim 1, wherein the stable member is provided over the entire circumference of an outer circumferential portion of the extension portion extending from the lower portion of the lower tie plate. 3. The fuel assembly according to claim 1, wherein the lower end surface of the stable member is in contact with an upper surface of the core plate. 4. The fuel assembly according to claim 1, wherein the fuel assemblies are accommodated in a hexagonal channel box in which fuel rods are arranged to form a triangular lattice shape.