Reactor core structure in BWR type reactor

Purpose: To suppress the bending in the outer enclosures of fuel assemblies thereby eliminate troubles to insertion performance of control rods in BWR type reactors. Constitution: Outer enclosure for a fuel assembly disposed to the outer circumferential portion of a reactor core is bent in such an arcuate shape that the side opposed to a control rod is bulged toward the reactor core outer circumference at an intermediate portion along the longitudinal direction thereof. In addition, the gap between the enclosure and the control rod is made greater at the intermediate portion along the longitudinal direction of the fuel assembly than at the both ends in the axial direction. Thus, even if the protruding bending toward the central portion of the reactor core is caused to the outer enclosure along with the operation, since the outer enclosure is previously shaped such that it expands toward the outside of the reactor core, it offsets with the protruding bending and no large protrusion to the control rod is formed in the absolute displacement amount thereof. That is, the gap between the control rod and the outer enclosure can always be at a substantially constant level at any positions of the reactor core and, accordingly, the insertion performance of the control rod is not damaged, the reactor safety can be improved and the restriction of the fuel arrangement is removed to improve the economical advantage. (Horiuchi, T.)