Assessment of thermal neutron scattering in a heavy paraffinic molecular material


Manring C.A., Hawari A.I.

Annals of Nuclear Energy

Base oil lubricants are prevalent in many mechanical systems and can have a wide range of chemical compositions. Their carbon numbers generally span from C20–40, and they are often classified by their predominant molecular species (e.g., paraffinic, naphthenic). These heavy, viscous fluids are of interest when used in or around nuclear fuel cycle facilities, where they may influence the local neutronic environment. Furthermore, variations in molecular structure impact the oil’s dynamic properties and render the modeling process a challenging endeavor. In this work, a model for a specific paraffinic oil was developed for obtaining the vibrational/translational density of states (DOS) for hydrogen and carbon, the primary and secondary scattering species, respectively. The DOS was subsequently used to generate the thermal scattering law (TSL). The molecular ensemble, constructed using the MedeA material design platform, was benchmarked using available static and dynamic properties (i.e., density, viscosity, viscosity index, diffusivity). All simulations were performed using the LAMMPS code, and the COMPASS force field, a semi-empirical molecular potential, was selected as a suitable representation of the system dynamics. Additionally, a modified version of the nuclear data processing code, NJOY, was employed to more accurately treat a viscous fluid. Through this investigation, the viscous behavior characteristic of a heavy paraffinic oil was captured and was found to minimally impact the TSL when compared to the solid approximation.