Speaker
Description
Hydrogen fluoride (HF) is a material commonly used in nuclear fuel processing and manufacturing. As HF is commonly used directly in conjunction with various forms of uranium, an accurate representation of the thermal cross sections is vital to criticality safety. To generate the thermal scattering cross sections for HF, a classical molecular dynamics (MD) simulation of the flexible HF system was developed. The first, two-site model was developed using the LAMMPS simulation package. LAMMPS however is unable to capture the HF-HF interactions that result from intermolecular hydrogen bonding. Therefore, a second, three-site model was developed in GROMACS (GROningen MAchine for Chemical Simulations) to accurately capture the intermolecular effects. The hydrogen bonds were treated in MD by attaching a massless charge to a dummy site within the molecule. Liquid anhydrous HF was then simulated across typical temperatures and pressures.
In LAMMPS, a traditional two-site model with 4096 molecules was developed using a newly reparametrized potential with the addition of 12-6 Lennard Jones (LJ), Coulombic, and bond potential term between the hydrogen and fluorine atoms. The system was equilibrated for 200,000 timesteps (0.1 fs timesteps) using NPT. After equilibration, an additional 2000 loops of 5000 fs were calculated to generate the diffusional coefficient and normalized velocity autocorrelation function (VACF).
In order to capture the HF-HF intermolecular bonding, a three-site model of liquid anhydrous HF was developed for high temperature and pressure and implemented in the GROMACS simulation. For this model, the initial structure of 1000 molecules was produced using fftool and PACMOL. Both the Nose-Hoover thermostat and the Parrinello-Rahman barostat were applied. At 0.90 nm, the Lennard-Jones interactions were terminated, and the long-range electrostatic forces were calculated using the particle mesh Ewald technique. Initially, the system was equilibrated for 1000 ps using a 0.01 fs timestep. After the equilibration, the liquid HF system was simulated for a total of 2500 ps. The 2500 ps was split into 500 loops (5 ps each) to find the exact time-averaged normalized VACF.
From these two models, key parameters were compared against experimental data to validate the methodology and models. The bond length of hydrogen and fluorine atoms, potential energy, diffusion coefficient, and density were used to benchmark the model before producing the normalized VACF.
The phonon DOSs for both cases were then calculated as the Fourier transform of the normalized VACF. These DOSs were then used as the primary input for the evaluation of two thermal scattering laws (TSL, i.e. S($\alpha$,$\beta$)) and associated neutron thermal scattering cross sections for hydrogen in HF. The TSLs were analyzed using the Full Law Analysis Scattering System Hub (FLASSH) using the Schofield diffusional model. The inclusion of the hydrogen bonding results in significant change to the diffusional contributions of the TSL. This is reflected in the low energy cross section where the inclusions of the HF-HF intermolecular binding effects significantly shifts the scattering cross section.