| 摘要 |
PURPOSE:To improve fuel economy by setting a minimum value of thermal neutron flux distributions in the diametrical direction and a minimum value of resonance neutron flux distributions in the diametrical direction to the magnitudes for the minimum values to be positioned in the outside regions in the diametrical direction. CONSTITUTION:Further, the fuel assembly 10 has short fuel rods 13 arranged in four corner parts in the region which is adjacent to the axial lower part 12a at the lower part of a larger-diameter part of the axial upper part 12b of a water rod 12, that is, in the central region of 3X3 including the water rod 12. The short fuel rods 13 are low enriched fuel rods containing low enriched uranium in which the weight-rate of uranium - 238, such as natural uranium, depleted uranium, recovered uranium, etc., is high. With the fuel assembly 10, the water cross-sectional area of the axial upper part 12b of the water rod 12 is taken to be larger than that of the lower part 12a to improve the axial distribution of a moderator to fuel ratio (H/U ratio). Further, when a reactor is operated, a space between a channel box 14 and an outside, adjacent fuel assembly is filled with saturated water (gap water), and the axial upper part 12b of the water rod 12 is formed into the shape as shown in Fig.3. Thus, the distribution of H/U-ratio in the diametrical direction at the upper part of the fuel assembly 10 can be improved.
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