On May 29, Yuwei Zhu successfully defended his PhD dissertation, Analysis of Neutron Thermalization in Liquid FLiBe. Yuwei’s committee consisted of his advisor, Ayman hawari, and members, Bernard Wehring, Michael Zerkle, Nam Dinh, and Paul Huffman.

Abstract

Zhu, Yuwei. Analysis of Neutron Thermalization in Liquid FLiBe. (Under the direction of Prof. Ayman I. Hawari).

FLiBe, a liquid formed by fusing crystalline LiF and BeF₂ salts at temperatures exceeding 732 K, has the chemical formula Li₂BeF₄. FLiBe demonstrates outstanding properties such as chemical stability at high temperatures, high moderating ratio, and high heat capacity. It has therefore been proposed as a coolant, moderator, and heat storage medium in thermal neutron driven nuclear reactors. In proposed reactor designs such as the thorium molten salt reactor (TMSR), and the small modular advanced high-temperature reactor (SmAHTR), liquid FLiBe can compose up to 70% volume of the fuel blanket. Neutronic simulations for the design of such thermal reactors require high fidelity thermal neutron scattering data for FLiBe. However, there is currently no available thermal neutron scattering law for liquid FLiBe. To date, proposed thermal neutron scattering cross sections for FLiBe are based on a crystalline solid model that fundamentally exhibits different thermal neutron scattering behavior from liquid FLiBe.

In this work, the thermal neutron scattering law (i.e. ) for liquid FLiBe is calculated. The process used a liquid FLiBe molecular dynamics (MD) model based on the Born-Mayer pair potential. Predicted physical properties from the MD model such as density, viscosity, heat capacity, and diffusion coefficient showed good agreement with experimental measurements. Velocity autocorrelation functions (VACF) as well as particle trajectories were generated from the model. With these data, two different methods were implemented to calculate the FLiBe thermal scattering law (TSL). The first method is used to evaluate the TSL of this molten salt liquid by separating the excitation DOS into the bound vibrational mode and the diffusional mode. The partial TSLs of the bound vibrational mode and the diffusional mode were evaluated separately, each using its respective partial DOS. The total TSL of FLiBe was calculated by convolving the partial TSLs. The differential and the total thermal scattering cross sections of FLiBe were subsequently evaluated.

In the second method, MD trajectories were used to evaluate the TSL. This required introducing a quantum correction to the classical width function that appears in the intermediate scattering function. Subsequently, the classical width functions for each element in FLiBe were evaluated and quantum corrected. With the quantum width function, the corresponding TSLs for each element in FLiBe were evaluated at the representative reactor core temperatures of 873K, 923K, and 973K. Generally, good agreement was found between the quantum corrected self scattering law and the DOS evaluated self scattering law.

However, in the MD based analysis, the distinct scattering law is included in the evaluation by enforcing the same quantum correction ratio as for the self scattering law. The inclusion of distinct scattering law, which is validated by thermal neutron scattering measurements using liquid lithium, relaxes the Gaussian and the incoherent approximations and allows capturing the angular aspects of the scattering cross section. Based on that, the total thermal scattering law for each element in FLiBe was generated at the temperatures listed above and all corresponding thermal scattering cross sections were evaluated in a form appropriate for addition to the Evaluated Nuclear Data File (ENDF) and for use in reactor design analysis.