Benchmarking a first-principles thermal neutron scattering law for water ice with a diffusion experiment

Jesse Holmes,Michael Zerkle,David Heinrichs,A Plompen,P Siegler,S Kopecky,P Schillebeeckx,F.-J Hambsch,J Heyse,S Oberstedt,W Mondelaers

doi:10.1051/epjconf/201714613004

Abstract

The neutron scattering properties of water ice are of interest to the nuclear criticality safety community for the transport and storage of nuclear materials in cold environments. The common hexagonal phase ice Ih has locally ordered, but globally disordered, H2 O molecular orientations. A 96-molecule supercell is modeled using the VASP ab initio density functional theory code and PHONON lattice dynamics code to calculate the phonon vibrational spectra of H and O in ice Ih . These spectra are supplied to the LEAPR module of the NJOY2012 nuclear data processing code to generate thermal neutron scattering laws for H and O in ice Ih in the incoherent approximation. The predicted vibrational spectra are optimized to be representative of the globally averaged ice Ih structure by comparing theoretically calculated and experimentally measured total cross sections and inelastic neutron scattering spectra. The resulting scattering kernel is then supplied to the MC21 Monte Carlo transport code to calculate time eigenvalues for the fundamental mode decay in ice cylinders at various temperatures. Results are compared to experimental flux decay measurements for a pulsed-neutron die-away diffusion benchmark.

Highlights

IntroductionThe dynamics and thermal neutron scattering properties of liquid water have been extensively studied for decades
Water is the most common neutron moderator in nuclear reactor applications
The dynamics and thermal neutron scattering properties of liquid water have been extensively studied for decades

Summary

Introduction

The dynamics and thermal neutron scattering properties of liquid water have been extensively studied for decades. H2O molecules participate in hydrogen bonding with each other due to electrostatic dipole-dipole interactions [1]. These relatively weak hydrogen bonds are responsible for small variations in the H-O-H bond angle for different phases and temperatures. In the solid form, hydrogen bonds are responsible for the geometric flexibility of H2O alignments in various phases of ice [2]. The details of the particular interatomic structure affect the available vibrational modes with which thermal neutrons may interact

The structure and modeling of ice Ih

Comparison of theoretical and experimental total cross sections

Comparison of theoretical and experimental time eigenvalues

Conclusions