氮化铀热中子截面的第一性原理计算

氮化铀（UN）因其较好的热物性和耐事故容错性成为先进动力堆的候选燃料，但目前热能区缺少可靠的UN热中子截面数据，这对于热中子反应堆物理计算是很不利的.本文基于量子力学的第一性原理，利用VASP/PHONON软件模拟计算了UN的声子态密度，以此为积分得到UN的定容比热容，并基于新制作的声子态密度，采用核截面处理程序NJOY/LEAPR，利用热中子散射理论，得到UN的S（α，β）数据，进而研究UN的热中子散射截面，并与传统压水堆的二氧化铀（UO₂）进行对比.结果表明：优化的晶格参数与数据库符合较好，UN声子态密度的声子项和光子项较UO₂的分隔更加明显，定容比热容计算结果与实验值一致，基于该声子态密度计算得到的UN中²³⁸U的非弹性散射和弹性散射截面比相同温度下UO₂中²³⁸U小，UN中N仅考虑了非相干散射部分，随着温度升高，UN弹性散射截面变小，非弹性散射变大，并在高能段趋于自由核散射截面.本文的研究结果填补了UN热中子截面数据的缺失，为下一步系统研究UN燃料在轻水堆中的中子学性能奠定了基础.

Ab initio calculation of the thermal neutron scattering cross sections of uranium mononitride

Nuclear design and neutronic analysis of thermal neutron reactor need high reliable thermal neutron cross sections. Uranium mononitride (UN) is a candidate fuel material for advanced power reactor with its better thermodynamics and accident tolerance. However, in thermal neutron region, reliable thermal neutron scattering cross sections are lacked for UN, which is disadvantageous to reactor physics simulations. The scattering law of the UN fuel material may impact the thermal neutron spectrum and criticality of the reactor systems. Neutron cross sections in thermal range are correlated with energy, temperature, physical and chemical properties of the scattering medium, reflecting the phonon spectra of material itself. In this paper, based on the ab initio method of quantum mechanics, phonon density of states in UN are calculated by VASP/PHONON code, and used for integral to obtain UN heat capacity at a constant volume. Adopting this new phonon density of states, NJOY/LEAPR code is used to generate S (α, β) data by thermal neutron scattering theory and NJOY/THERMR utilizes these data to produce thermal scattering matrix in order to investigate thermal kernel effect of UN. Subsequently, thermal neutron scattering cross sections of UN are generated with NJOY code system. Comparison with uranium dioxide (UO₂) in the traditional PWR is done. Results indicate that optimized lattice parameter are in good agreement with the database; the optical modes are well separated from the acoustic modes compared with UO₂; heat capacity at a constant volume is consistent with experimental value; the inelastic and elastic cross sections of ²³⁸U in UN are lower than those of ²³⁸U in UO₂. N in UN only deals with incoherent part in elastic cross sections. As the temperature increases, elastic cross sections of UN decrease while inelastic ones increase, and cross sections approach to free atom cross section at high energies. Considering the limitations of ¹⁴N, the scattering law and inelastic scattering cross sections are also under investigation using ¹⁵N in UN compound. This paper's conclusion fulfill the vacancy of thermal neutron scattering cross sections of UN, which laid a foundation for systematic study on the neutronics properties of UN fuel in the light water reactors as well as for the design of new neutron moderators and neutron filer.