Does ^135mXe cross-section uncertainty matter in nuclear thermal propulsion reactors?

The nuclear reactor in a nuclear thermal propulsion core operates at 100 s of MW s for 10 s of minutes at a time. Traditional burn-up analyses tend to neglect short-lived nuclides with low fission yields since these nuclides quickly reach equilibrium. However, in the nuclear thermal propulsion core, some of these short-lived nuclides cause a noticeable difference in neutron eigenvalue, namely ^135mXe. Cross-sections for ^135mXe are not known well, but computation methods allow for calculation of the cross-section and covariance information via the TENDL nuclear data library. This work utilizes the calculated covariance matrix for ^135mXe absorption cross-section and samples nuclear data many times to perform uncertainty quantification. A sampling strategy is presented for a single burn step on a unit cell is analyzed. It was found that uncertainties of the ^135mXe cross-section caused a 5.7 pcm error in eigenvalue. This is a very small error such that the absorption covariance could be neglected. It has been observed that the ^135mXe cross-section changes substantially between TENDL releases. These changes in cross-section cause 94.0 pcm error in eigenvalue. This means that the ^135mXe cross-section does not need to be well known, but the integral value is important and should be further investigated to better model operations. (Author)