Abstract

Yttrium hydride is being considered as a moderator material for microreactor concepts because of its excellent hydrogen retainment capacity at high temperatures. These types of reactors, operating at thermal to epithermal neutron energies, require accurate thermal scattering laws (TSLs) for yttrium hydride to predict and optimize moderator performance. Currently, TSL evaluations exist only for stoichiometric YH₂. To perform high-certainty neutronics calculations and to improve the criticality safety of yttrium hydride–moderated reactors, evaluations of substoichiometric yttrium dihydride TSLs are necessary. Ab initio density functional theory (DFT) was used to generate the phonon density of states for yttrium and hydrogen under harmonic approximation in yttrium hydride ($xin\delta -Y{H}_{2-x}$). To obtain substoichiometric yttrium dihydride, vacancies in the YH₂ crystal were created using special quasi-random structures (SQS). Using NJOY2016, the TSLs for yttrium hydride were constructed from the DFT results as a function of stoichiometry and temperature. Our TSLs for the stoichiometric composition YH₂ were in excellent agreement with the ENDF/B-VIII.0 evaluations. As such, this study extends the yttrium hydride TSLs for compositions between YH_1.31 to YH_1.91 with the interval of H/Y ≈ 0.1 for use in the MCNP code. The substoichiometric yttrium hydride scattering cross sections deviated by as much as 30% (elastic) and 60% (inelastic) when compared to the YH₂ TSLs, underlining the necessity to have the TSLs presented here available, e.g., for safety-related reactor calculations. For the validation of the underlying DFT results of our model, quasi-harmonic approximation was used to compute the thermal lattice strain and constant pressure heat capacity for YH₂. Neutron diffraction experiments were also carried out to characterize thermophysical properties that were adopted for stoichiometric and substoichiometric model validation. Additional properties such as heat capacity c_v, and thermal displacement parameters were also computed for yttrium hydride ($xin\delta -Y{H}_{2-x}$) and compared to experimental results. Neutron diffraction validation of the YH_2-x material properties and ENDF/B-VIII.0 verification of YH₂ TSLs provide a very strong basis on the accuracy of the extended yttrium hydride TSL evaluations at thermal energies.

摘要

氢化钇由于在高温下具有出色的氢保持能力，因此被认为是微反应器概念的减速剂。这些类型的反应堆在中子热至超热能下运行，它们要求氢化钇钇具有准确的热散射定律（TSL），以预测和优化减速剂性能。当前，仅对化学计量YH ₂存在TSL评估。为了执行高确定性的中子学计算并提高氢化钇减慢反应堆的临界安全性，必须对亚化学计量的二氧化钇TSL进行评估。使用从头算密度函数理论（DFT）来生成氢化钇（谐波近似）下钇和氢态的声子密度$X\mathrm{一世}ñ\delta -ÿH_{\mathrm{2个}-X}$）。为了获得亚化学计量的二氧化钇，使用特殊的准随机结构（SQS）在YH ₂晶体中产生空位。使用NJOY2016，根据DFT结果作为化学计量和温度的函数，构造了氢化钇的TSL。我们的化学计量组成YH ₂的TSL与ENDF / B-VIII.0评估非常吻合。因此，本研究将YH _1.31至YH _1.91之间的组成的氢化钇TSL扩展为H / Y≈0.1，以用于MCNP代码。与YH ₂相比，亚化学计量的氢化钇散射截面变化多达30％（弹性）和60％（非弹性）TSL，强调有必要提供此处介绍的TSL，例如，用于安全相关的反应堆计算。为了验证我们模型的基本DFT结果，使用准谐波近似来计算YH ₂的热晶格应变和恒压热容。还进行了中子衍射实验以表征用于化学计量和亚化学计量模型验证的热物理性质。还计算了氢化钇的其他性质，例如热容c _v和热位移参数（$X\mathrm{一世}ñ\delta -ÿH_{\mathrm{2个}-X}$）并与实验结果进行比较。该YH的中子衍射验证_2-X材料性质和YH的ENDF / B-VIII.0验证_2个TSLS提供在延伸的钇的氢化TSL评价在热能精度非常强的基础。