Nuclear Engineering ETDs

Publication Date

Spring 4-14-2023


This research consists of two comparison studies: the first study employed standard practices to characterize Upper Subcritical Limit (USL) estimation methods. A series of 33 neutronic systems that used standardized nuclear data and benchmark libraries were studied to compare the Whisper, TSURFER, and USLSTATS methods relative to a stochastic USL. USLs were also estimated for these 20 systems using the Whisper 1.1 code. Sensitivity data files were produced using MCNP6.2 and then used with the ORNL TSURFER and USLSTATS methods to estimate USLs for a cross-method USL comparison. The results show that USLs for each of the loosely coupled system models were higher than the calculated stochastic USLs. The single system uranium models also displayed a lower stochastic USLs as compared to the USL calculational methods, while the single system plutonium models showed close agreement between the stochastic USLs and the other USL calculational methods.

The second USL comparison study was performed in hopes of comparing USL estimates using experimental benchmark results whose biases were reflective of the uncertainty in cross section covariance data. A total of 167 44-group ENDF/BVII.1 covariance data sets were constructed and used to construct three randomly perturbed the ENDF/BVII.1 cross section libraries. The TSURFER method was found to be generally the least conservative method but also generally the most accurate method with respect to a constructed reference USL while the efficacy of the USLSTATS method has no clear pattern over all neutronic systems. The Whisper method was found to produce reliably conservative USL estimates.


criticality safety, nuclear engineering, Whisper, Upper Subcritical Limit, USL, TSURFER


NNSA, Los Alamos National Laboratory, Criticality Safety Program

Document Type




Degree Name

Nuclear Engineering

Level of Degree


Department Name

Nuclear Engineering

First Committee Member (Chair)

Christopher Perfetti

Second Committee Member

Forrest Brown

Third Committee Member

Michael Rising

Fourth Committee Member

Brian Kiedrowski

Fifth Committee Member

Bradley Rearden


Here is the revised dissertation with the