| 摘要 |
PURPOSE: To prolong the effective lifetime by constituting a fuel assembly of fuel rods having first specific diametral dimension arranged on the inside and fuel rods having second specific diametral dimension larger than the first diametral dimension arranged on the outside. CONSTITUTION: Fuel rods 42A in the inside arrangement 54 of each small fuel rod bundle have first specific diametral dimension while fuel rods 42B in the outer circumferential arrangement 56 have second specific diametral dimension larger than the first diametral dimension. Preferably, the diametral dimension of fuel rod 42A is smaller by about 10-12% than that of the fuel rod 42B. Furthermore, fuel rods 42 in the arrangement 54, 56 constitute a square arrangement as a whole. The fuel rods 42A form a substantially square inside pattern arranged in the center while the fuel rods 42B surround the arrangement 54 and form a substantially square outside pattern arranged on the outer circumference. Only the fuel rods in square outside pattern are the fuel rods 42B having second larger diametral dimension and the fuel rods 42A, 42B cooperate to constitute a square arrangement as a whole and arranged in row and column.
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