Flux-Independent Uncertainty Propagation of Nuclear Cross Section Data Using the Windowed Multipole Formalism

Abstract

This thesis encompasses work in the propagation of resolved resonance range nuclear cross section uncertainties, primarily for eigenvalue calculations. The first portion explores the use of resonance parameter uncertainty data in the generation of multi-group cross section libraries. Multigroup cross section uncertainties are fundamentally flux and temperature-dependent, but are often used as general purpose across multiple applications. Traditional methods of handling resonance parameter uncertainty data do not account for the impact of the change in flux shape due to the change in resonance parameters. A single resonance model is developed to indicate the temperature and dilution regions where the approximate methods may not perform well. Additionally, a new approach for using the region-dependent multigroup uncertainties is proposed. The temperature-dependent multigroup covariances and region-wise propagation method are applied to a partial boiling water reactor (BWR) assembly calculation. The second portion involves the determination of Windowed Multipole (WMP) parameter uncertainty from continuous energy cross section uncertainty data. WMP parameters present a path forward for flux-independent uncertainty propagation in the resolved resonance region. While a library of WMP parameters exists that matches the mean value behavior of the Evaluated Nuclear Data File (ENDF) library, uncertainty information is only available for nuclides for which resonance parameter uncertainty is available. A least squares approach is developed to convert continuous energy cross section uncertainties to WMP parameter uncertainties. The approach is used to generate WMP covariances for various nuclides, highlighting nuclides for which no WMP covariance were previously available such as ¹⁶O and ⁵⁶Fe. These covariances are used along with sensitivities produced by the CLUTCH method in OpenMC to calculate resulting eigenvalue uncertainties in a partial BWR assembly as well as a plutonium nitrate solution criticality benchmark. This new method of producing WMP uncertainties enables the creation of a WMP parameter covariance library that includes all the cross section uncertainty information available in ENDF.