Theoretical and Numerical Analysis of Beryllium Reflector Poisoning in High-Flux Research Reactor
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摘要: 高通量研究堆广泛使用铍作为反射层，以提高中子利用率。为研究铍反射层内核素嬗变过程的中子毒物6Li、3He的积累特性与规律，而非仅对特定算例进行评估，本研究通过解析求解铍反射层内核素嬗变相关方程，获得各核素积累过程的相关规律，从而在理论层面上评估铍反射层中毒问题引入的负反应性，并获得了铍反射层中毒过程中6Li平衡浓度与中子注量率水平无关，3He积累速率上限与中子注量率水平无关等规律性结论。通过使用RMC对宽能谱超高通量研究堆（THFR）的铍反射层进行计算，并与理论预测结果进行对比，结果符合良好，验证了理论分析的正确性。相关结论可以省去长时间多步燃耗计算的资源消耗，仅需进行少数几次临界计算即可获得负反应性引入数值，从而高效准确地为高通量研究堆的反射层设计与更换频率、剩余反应性设计等提供重要依据。Abstract: Beryllium is widely used as the reflector in high flux research reactors to improve neutron economy. To investigate the accumulation characteristics and laws of neutron poisons 6Li and 3He during the transmutation process in the beryllium reflector—rather than merely evaluating specific test cases—this study analyzes and solves the equations of nuclide transmutation in the beryllium reflector, obtains the relevant laws of the accumulation process of each nuclide, and evaluates the negative reactivity introduced by the beryllium reflector poisoning theoretically. It is concluded that the equilibrium concentration of 6Li is independent of the neutron flux level, and the upper limit of 3He accumulation rate is independent of the neutron flux level. By using RMC to calculate the beryllium reflector of the Tsinghua High Flux Reactor (THFR) and comparing with theoretical predictions, the results are in good agreement, which validates the correctness of the theoretical analysis. The relevant conclusions can save the resource consumption of long and multi-step burnup calculations, and only a few critical calculations are needed to obtain the value of the negative reactivity, thus efficiently and accurately providing important basis for the design and replacement frequency of reflector, residual reactivity design, etc. of high flux research reactors.
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图 3 9Be和6Li的(n,α)反应截面随入射中子能量的变化曲线（ENDF Ⅷ.0）
Figure 3. (n,α) Cross Sections of 9Be and 6Li versus Incident Neutron Energy (ENDF Ⅷ.0)
图 7 3H和3He的数密度变化率随时间变化曲线
Figure 7. Changing Rates of Number Densities of 3H and 3He versus Time
图 8 THFR铍反射层燃耗条件下堆芯keff随时间变化曲线
Figure 8. THFR’s Core keff versus Time with Burnup of Beryllium Reflector
图 9 模拟计算和理论计算（$ \phi = $2.1×1014 cm−2·s−1）得到的6Li和3He数密度随时间变化曲线对比图
Figure 9. Comparison for the Number Densities of 6Li and 3He versus Time between Simulation and Theory ($ \phi = $2.1×1014 cm−2·s−1)
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