| 摘要 |
A fuel assembly comprises a plurality of fuel rods bundled in grid pattern, a part of the fuel rods containing gadolinium as a burnable poison. At least one of the fuel rod having gadolinium contains gadolinium enriched in at least one kind of isotope of odd mass number more than an isotopic abundance of natural gadolinium. In the enriched gadolinium, a ratio of a content of Gd-155 to that of Gd-157 is 0.1 or less. An average concentration (wt %) G0 of enriched gadolinia is, with M denoting the number of month under rated power operation per one cycle of an equilibrium core, P power density of a nuclear reactor (kw/l unit) and W a sum of isotopic composition, is set in the range shown by the following expression.Thereby, in the fuel assembly, residual reactivity of a burnable poison at a cycle end can be decreased and thermal performance can be improved.In another aspect of the fuel assembly, a gadolinia fuel rod disposed in a region on a control rod side contains gadolinium enriched in an isotope of odd mass number more than natural isotopic abundance. A concentration of enriched gadolinia is set at for instance one half or less that of natural gadolinia. Thus, enrichment of fissile material (uranium) in the gadolinia fuel rods can be larger than that in the existing fuel assembly, uranium enrichment averaged over a bundle being increased.
|
