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Whisper: Sensitivity/Uncertainty-Based Computational Methods and Software for Determining Baseline Upper Subcritical Limits

Brian C. Kiedrowski, Forrest B. Brown, Jeremy L. Conlin, Jeffrey A. Favorite, Albert C. Kahler, Alyssa R. Kersting, D. Kent Parsons, Jessie L. Walker

Nuclear Science and Engineering / Volume 181 / Number 1 / September 2015 / Pages 17-47

Technical Paper /

First Online Publication:June 29, 2015
Updated:August 20, 2015

Nuclear criticality safety analysis using computational methods such as a Monte Carlo method must establish, for a defined area of applicability, an upper subcritical limit (USL), which is a calculated multiplication factor k that can be treated as actually subcritical and is derived from a calculational margin (combination of bias and bias uncertainty) and a margin of subcriticality. Whisper, a nonparametric, extreme-value method based on sensitivity/uncertainty techniques and the associated software are presented. Whisper uses benchmark critical experiments, nuclear data sensitivities from the continuous-energy Monte Carlo transport software MCNP, and nuclear covariance data to set a baseline USL. Comparisons with a traditional parametric approach for validation, which requires benchmark data to be normally distributed, show that Whisper typically obtains similar or more conservative calculational margins; comparisons with a rank-order nonparametric approach show that Whisper obtains less stringent calculational margins.

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