Speaker
Description
With the rise of interest in thermal neutron scattering data for advanced reactor, criticality safety, and shielding applications, new experimental data is required both for evaluations of new materials or for re-evaluations (or validations) of previously evaluated materials. New experimental data is evaluated in a 3-step process: 1) computing the phonon characteristics (e.g. eigenvalues/vectors of the material’s dynamical matrix, molecular dynamics trajectories, etc.), 2) computing the dynamic structure factor (DSF) from it, and, 3) using the experimental setup to simulate the experimental data. All three steps have challenges, from needing a sufficiently general material simulation code, a processing code that can compute the corresponding DSF, to having a detailed layout of the instrument/beamline/facility where the data were measured. Whereas phonon characteristics of materials can be computed using various methods (molecular dynamics, density functional theory, etc.), a high-fidelity computation of the DSF and the simulation of the experiment based on the DSF is vital to the accuracy of evaluation. The latter 2 steps can be achieved by using the two corresponding code systems developed by instrument scientists at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory: 1) OCLIMAX (a program that calculates the dynamic structure factor from DFT and MD simulation results), and 2) MCViNE (a Monte Carlo neutron ray-tracing program designed to simulate neutron scattering experiments). Recently, polyethylene and yttrium hydride were measured at the ARCS, SEQUOIA, and VISION instrument stations of the SNS. These experiments are simulated using the density functional theory code CASTEP to compute its phonon characteristics (eigenvalues/vectors and PDOS), which is then processed using OCLIMAX to yield the DSF, and finally the data at each instrument station is simulated by the MCViNE for comparison to the measured for evaluation. For comparison to conventional evaluation methods, results processed from OCLIMAX are compared against those processed from the LEAPR module of NJOY, and the results from MCViNE simulations are compared against previously used simplified beamline models implemented in MCNP.
Acknowledgments
This work was supported by the U.S. Department of Energy (DOE) Nuclear Criticality Safety Program, which is funded and managed by the National Nuclear Security Administration for DOE and the Transformational Challenge Reactor Program of the US Department of Energy Office of Nuclear Energy.
- This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for US government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).}