| 摘要 |
PROBLEM TO BE SOLVED: To even the decelerating operation by surrounding a reactor core with a control rod void gap, in which a narrow void gap and a control rod are housed, and equally dividing a fuel assembly to four, and forming small diameter fuel rods at a different specified ratio in one part thereof, of which two sides face the narrow void gap, and other parts thereof. SOLUTION: A control rod gap 9a and a narrow void gap 9b to be respectively filled with water are provided between fuel assemblies 1, and control rods 11 having a cross-shaped section are inserted in the gap 9a. Each assembly 1 is housed in a fuel channel 2 having a hollow supporting member 3 having the cross-shaped section, and the assembly is divided to four auxiliary assemblies 6a-6d by the member 3. The assembly 1 has two kinds of fuel rods 7a, 7b at a large diameter and a small diameter. In an auxiliary assembly 6b inside of the assembly 1, of which two sides are arranged close to the gap 9b, the fuel rod 7b at 1/3 or more of the whole is formed, and in the auxiliary assemblies 6a-6c, the fuel rod 7b at 1/5 of the whole is formed. Decelerating ability inside of the assembly is thereby evened.
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