Development of Thermal Scattering Law for Hydrogen Fluoride for Criticality Safety Applications

Precise estimation of thermalization of neutrons in moderators relies on high-fidelity thermal scattering cross-sections, often termed as thermal scattering law (TSL). TSL libraries for several moderator materials like light water, heavy water, polyethylene, etc., are available in standard cross-section data libraries in ENDF format. However, cross-section data for moderators like Hydrogen Fluoride (HF) is still missing in libraries like ENDF/B-VIII.0, JEFF-3.3, and others. Some discrepancies between modeling and experimental data were observed for benchmarks involving HF as a moderator. This difference is believed to be partly linked to the lack of TSL data for HF. Development of new TSL data for HF may help improve criticality safety calculations. This paper presents the newly developed TSL data for HF based on molecular dynamics (MD) simulations. A bibliographical study of the available experimental and atomistic simulation data suitable for our application was carried out. MD simulations using the GROMACS code have been performed to generate phonon spectrum for a series of temperatures required for criticality safety applications. The phonon spectrum at different temperatures is then used by the LEAPR module of the NJOY code to generate TSL for H bound in HF and F bound in HF. The HEU-SOL-THERM-039 benchmark available in the ICSBEP database and a hypothetical critical benchmark using the CRISTAL V2 package is used to verify and validate the newly generated TSL and evaluate its impact on keff results for criticality-safety studies. Significant improvement in the C/E of the neutron multiplication factor is observed for the benchmarks using the newly generated TSL data.