Abstract Small fast reactors usually exhibit high neutron leakage and low breeding capability, while modular design demands long cycle length and compact core. These characteristics raise the requirement of control rods with strong absorption ability and long operating lifetime. In this paper, control rods with 90% 10B enriched B4C, 90% 10B enriched HfB2-90, and HfH1.62 are designed and assessed in a small sodium fast reactor. Results show that the operating lifetime of high 10B enriched B4C is limited by its burnup and temperature. Even with small pin design, 90% 10B enriched B4C is not able to stay safe as long as the fuel in a no refueling scenario. HfB2 can tolerate a longer residence time compared to the fuel. The absorption ability of HfB2 can be higher than B4C if the geometry effect is accounted for. HfH1.62 absorber is feasible in normal situations, but its absorption ability is inferior to B4C or HfB2 and will be significantly reduced at high temperature due to the hydrogen desorption issue. The spatial self-shielding effect increases with the absorption ability of control rods. In order to mitigate the heterogeneous distribution, two solutions are investigated in this paper. The radially mixed designs, with HfB2 at the outermost pins and B4C in the inner pins, do not extend the operating lifetime but increase the manufacturing complicity and the cost of raw materials. The substitution of absorber with a hydride moderator is able to keep high absorption ability and long operating lifetime of control rods while saving the investment of expensive absorber. Moreover, the local effects and hydrogen desorption of hydride moderators are acceptable.

中文翻译：

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小型快堆强吸收控制棒设计

摘要 小型快堆通常存在中子泄漏高、增殖能力低的特点，而模块化设计则要求循环周期长、堆芯紧凑。这些特点对控制棒的吸收能力强、使用寿命长提出了要求。在本文中，在小型钠快堆中设计并评估了富含 90% 10B 的 B4C、富含 90% 10B 的 HfB2-90 和 HfH1.62 的控制棒。结果表明，高 10B 富集 B4C 的工作寿命受其燃耗和温度的限制。即使采用小针设计，90% 10B 浓缩的 B4C 也无法在不加油的情况下保持安全。与燃料相比，HfB2 可以承受更长的停留时间。如果考虑几何效应，HfB2 的吸收能力可能高于 B4C。HfH1。62吸收器在正常情况下是可行的，但其吸收能力不如B4C或HfB2，并且在高温下会因氢解吸问题而显着降低。空间自屏蔽效应随着控制棒吸收能力的增加而增强。为了减轻异构分布，本文研究了两种解决方案。径向混合设计，HfB2 位于最外销，B4C 位于内销，不会延长使用寿命，但会增加制造复杂性和原材料成本。用氢化物慢化剂替代吸收器，既能保持控制棒的高吸收能力和长使用寿命，又可节省昂贵的吸收器投资。此外，氢化物减速剂的局部效应和氢解吸是可以接受的。