An investigation on the nuclear transmutations of elemental long-lived fission products (LLFPs) in a fast reactor is being conducted in Japan, focusing on the Se-79, Zr-93, Tc-99, Pd-107, I-129, and Cs-135, to reduce the environmental burden. With their high neutron flux and adequate excessive neutrons, fast reactors are considered strong candidates for transmuting those LLFPs into stable isotopes or shorter effective half-lives under the partition and transmutation (P/T) strategy. In this investigation, the LLFP subassembly is assumed to be loaded into the radial blanket region of a Japanese MONJU class sodium-cooled fast reactor (710 MWth). This work focuses on two LLFPs, namely, I-129 and Tc-99. The LLFPs are mixed with YD₂ or YH₂ moderator material to enhance the LLFP transmutation rate. The results include the optimal position of the LLFP assembly in the radial blanket and the optimal moderator volume fraction to optimize the transmutation performance indices, that is, the transmutation rate (TR, %/year), the support factor (SF is defined as the ratio of transmuted to produced LLFP), and the effective half-life (T-half) of the LLFPs. A countermeasure is also devised to resolve the expected higher power peak at the fuel assembly adjacent to the LLFP assembly. The neutronics and burnup analyses are conducted using the continuous-energy Monte Carlo SERPENT2 code with the JENDL-4.0 nuclear data library.

中文翻译：

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快堆长寿命裂变产物嬗变性能指标优化

日本正在对快堆中元素长寿命裂变产物 (LLFP) 的核嬗变进行调查，重点是 Se-79、Zr-93、Tc-99、Pd-107、I-129 和Cs-135，减少环境负担。凭借其高中子通量和充足的过量中子，快堆被认为是在分区和嬗变 (P/T) 策略下将这些 LLFP 转化为稳定同位素或更短的有效半衰期的有力候选者。在本次调查中，假设 LLFP 组件装载到日本 MONJU 级钠冷快堆（710 MWth）的径向覆盖区。这项工作的重点是两个 LLFP，即 I-129 和 Tc-99。LLFP 与 YD ₂或 YH _2混合慢化剂材料以提高 LLFP 嬗变率。结果包括径向覆盖层中 LLFP 组件的最佳位置和优化嬗变性能指标的最佳慢化剂体积分数，即嬗变率 (TR, %/year)、支持因子 (SF 定义为转化与产生的 LLFP 的比率）和 LLFP 的有效半衰期（T-half）。还设计了一种对策来解决与 LLFP 组件相邻的燃料组件的预期更高功率峰值。使用带有 JENDL-4.0 核数据库的连续能量 Monte Carlo SERPENT2 代码进行中子学和燃耗分析。