Physics-based Uncertainty Quantification for ZrHx Thermal Scattering Law

Abstract

The thermal neutron scattering cross sections of ZrHx are heavily affected by the solid frequency distributions, also called “phonon spectra”, of Zr and H in ZrHx. The phonon spectra are different for ZrHx with different x. While current reference data files, e.g. ENDF, are based on the spectra of ZrH_(2). This may induce unnegligible errors in the simulations for TRIGA reactor. We, therefore, proposed parameterized phonon spectra that can explore the effects of changing the spectra by varying the parameters. For example, we can shift the phonon positions in the spectra. The ul- timate goal of this type of work is to calibrate appropriate parameter sets to improve the simulation accuracy via comparing the simulation results and experimental data.

In this thesis, a code has been developed to process the thermal scattering data for transport codes to use. Inputs of the code are basically the proposed parameters. The accuracy of the code processing Legendre moments of scatteirng was demonstrated. NJOY and MCNP were used to carry out the data processing and neutronic simulations, respectively. The phonon spectra were generated with the parameters produced in Latin Hypercube sampling designs. Quantities, like reactivity (ρ), fission rate density (FRD), neutron mean generation time (Λ), fuel temperature feedback coefficient (αFuel), effective delayed neutron fraction (βeff ) and ex-core detector material absorption rate (Rabs), were analyzed. Analyses indicate that ρ, Λ and αFuel are sensitive to the variations of parameters. Explicit relationships were established for those quantities and the parameters. However, FRD and Rabs is insensitive to any parameters. βeff are sensitive to the parameterized models, however, no explicit relationship could be built due to the unrecognized nonlinearities.

Ongoing work will perform these analyses for the state near critical. Furthermore, time-dependent behavior could be investigated and when combined with experimental data the reasonably accurate phonon spectrum models and therefore S (α, β) tables for the TRIGA reactor at Texas A&M University would be produced.