REACTOR FUEL ASSEMBLY AND UPPER TIE PLATE

PURPOSE:To reduce a void coefficient without reducing the degree of reaction and at the same time reduce a two-phase pressure drop by removing a partial boss located on the extension of a partial-length fuel rod of the grid part of an upper tie plate. CONSTITUTION:A grind part 32 positioned on the extension of a partial-length fuel rod 3 of an upper tie plate 7 is in the constitution where a first web 35 is connected in T shape. Then, the part located on the extension of a partial- length fuel rod 3a adjacent to a water rod is used as a hollow region 36 excluding a boss part 30 and the cooling material channel of the upper tie plate 7 is enlarged on the extension line of partial-length fuel rods 3 and 3a. The pressure drop ratio of the upper tie plate 7 occupying the pressure drop of a fuel assembly is relatively large and the W-phase pressure loss of liquid phase and gaseous phase exists at the part, thus further improving the effect of two-phase pressure drop with the combined effect of the partial-length fuel rods 3 and 3a and the cooling material channel enlargement of the upper tie plate 7 and hence further improving channel stability and reactor core stability.