| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to reduce output peaking without impairing fuel economy and operation controllability by constituting a core of the first group of fuel assemblies where combustible poison almost burns out at the end of the first fuel cycle period and the second group where some of the combustible poison remains unburned till next cycle. SOLUTION: A reactor core 1 of a boiling water reactor consists of two groups A and B of fuel assemblies, for example. The group A of the fuel assemblies is composed of fuel assemblies 3 which are set up so that gadolinia, a combustible poison, will burn out before the end of the first cycle and is classified into A1 to A4, for example, according to the number of cycles of the stay in a reactor. The group B consists of fuel assemblies 3 which are set up so that gadolinia will remain unburned in and after the first cycle and is classified into B1 to B3 according to the number of cycles. The ratio of fuel rods containing gadolinia in the group B is set at a lower one than that in the group A, and the concentration of the gadolinia loaded for the first time in the group B is set at 1.2 or more times as much as that of the group A.
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