| 摘要 |
PROBLEM TO BE SOLVED: To prevent the occurrence output of a fuel assembly from decreasing by arranging the same number of fuel rods as that of center water channels by changing a pitch to store the center water channels whose traverse area and/or shape change. SOLUTION: In each fuel rod 12, a nuclear fuel pellet 12b is filled into zirconium alloy pipe 12a and is supported and positioned by a lower tie plate 14 and a higher tie plate 16. A spacer 18 supports and positions the middle of the fuel rod 12 over the entire length of a fuel assembly 10 and a square outer channel 11 is provided around the fuel rod 12 and the spacer 18. A center water channel 44 is provided at the center of the arrangement of the fuel rod 12 but the square traverse area changes along the height of an assembly 10. For absorbing the amount of change while the diameter of the fuel rod 12 and the dimension of the channel 11 are maintained as they are, the position and pitch of the fuel rod 12 are changed along the height of the assembly 10, thus retaining the same number of fuel rods 12 and preventing the amount of generated output from decreasing.
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