THERMAL SCATTERING LAW ENDF LIBRARIES FOR LIQUID FLIBE

Y Zhu,A I Hawari,C A Manring,M Margulis,P Blaise

doi:10.1051/epjconf/202124709023

Abstract

Several advanced nuclear reactor concepts have been proposed in the past few years where FLiBe molten salt represents a major constituent of the core. In this case, neutrons produced in fission slow down and moderate in FLiBe (a eutectic with a mixture of 2:1 ratio of LiF and BeF2) until they reach low energies (i.e, below 1 eV). At that stage, the thermalization process becomes dominant and the neutrons achieve a quasi-equilibrium energy state that is dependent on the temperature of the moderator. In neutronic simulations, the description of neutron thermalization is captured using the thermal scattering law (TSL), i.e., S(α,β), of the material in which low energy neutrons are interacting. S(α,β) defines the energy-momentum phase space that is available for an incoming low energy neutron. In addition, it is directly proportional to the double differential thermal neutron scattering cross section. In this work, the TSL of molten salt FLiBe is developed based on a generalized density of excitation states (GDOS) derived from atomic trajectories generated using classical molecular dynamics (MD) simulations that were performed with the LAMMPS code. The MD simulations utilized a Born-Mayer type atomic potential function that was verified to reproduce the properties of FLiBe including density and viscosity. The FLASSH code was used to evaluate the TSL’s ENDF File 7 in a temperature range extending from 773 K to 1673 K. In addition, ACE type cross section libraries are produced and tested with the objective of contributing the data to the National Nuclear Data Center for inclusion in the ENDF/B-VIII database.

Highlights

Neutron thermalization is the physical process that describes the evolution of the thermal neutron energy spectrum in the core of a nuclear reactor
The physics of thermalization is compactly represented by the thermal neutron scattering law (TSL), which is quantified by the S(D,E) data libraries for a given material
The generalized density of excitation states (GDOS) was generated as a function of a selected temperature set, which demonstrates that the FLiBe thermal scattering law (TSL) can be evaluated for the relevant temperatures of a given reactor design

Summary

INTRODUCTION

Neutron thermalization is the physical process that describes the evolution of the thermal neutron energy spectrum in the core of a nuclear reactor. Over the past 15 years, modern methods emerged that utilize atomistic and quantum mechanical techniques for calculating the fundamental data needed in generating S(D,E) [1]. The availability of such methods has facilitated the process of TSL generation and in many cases provided first-of-a-kind libraries in support of. Key technical improvements have been made in the process of calculating S(D,E) that surpassed the physics used in traditional tools such as the NJOY code [3]. The S(D,E) data are generated as ENDF/B File 7 libraries and supplied to the US National Nuclear Data Center (NNDC)

Molecular Dynamics Analysis of Molten Salt FLiBe

Thermal Scattering Law and Inelastic Cross Sections for Molten Salt FLiBe

CONCLUSIONS