| 摘要 |
PURPOSE:To dissolve degrading of core characteristics, improve fuel economy and attempt efficient use of resources by distributing in axial direction the density of fissile material in fuel assembly containing MOX fuel rod charged with mixed fuel of plutonium and uranium. CONSTITUTION:By lowering the density of fissile material (U-235, Pu-239, 241 etc.) at upper end, lower end or both ends and enhancing it correspondingly in the middle region of the assembly, the thermal utilization factor enhanced in the middle of the core where thermal neutron flux is high, Hence, thermal utilization factor is improved in the core as a whole and the reactivity of core is also improved. Also by lowering the density and reactivity in the upper region of the core in advance to lower accumulation of plutonium, core reactivity in a cold state is decreased and core shut down margin is improved. Since the axial distribution method for the density of those fissile materials show the efficiency independently, all are not necessarily combined.
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