The impact of fast spectrum graphite moderated systems on thermal spectrum graphite moderated reactors through the resonance physics of n+12C

Abstract

Understanding the nuclear data used in predictive modeling and simulation of advanced reactors is vital for their development, licensing, and deployment. In this paper, we use the Kairos gFHR and the X-Energy Xe-100 as demonstration examples to test the scientific hypothesis that the resonance physics of n+12C allows for integral experiments with fast-neutron-spectra to be useful in helping to reduce nuclear data uncertainty on k-effective of new reactor systems with thermal-neutron spectra. This work hypothesizes that legacy criticality experiments hold value for these new reactors. HMF071 (highly enriched, metal-fuel, fast-spectrum critical experiment) (Parkey et al., 2014; Nuclear Science Committee of The Nuclear Energy Agency, 2016) is a series of legacy criticality benchmarks with graphite reflectors which were selected to test this scientific hypothesis by leveraging the large correlation visually identified in the covariance matrix of n+12C. This hypothesis will be tested through calculating the similarity and uncertainty-reduction provided from evaluating the effect of a fast-neutron-spectrum system, HFM071, on a thermal-neutron spectrum system. The sensitivity with respect to elastic scattering of carbon had greater than 70% similarity for one of the cases of HMF071 compared against each of the advanced reactor designs considered. This result quantifies the relevance that the fast spectrum experiments can have for thermal spectrum applications due to the resonance physics of 12C. With this connection quantified, the uncertainty reduction from the fast-spectrum experiments on these thermal-spectrum reactors was calculated using the SCALE module TSURFER (Rearden and Jessee, 2018; Wieselquist and Lefebvre, 2023). The HMF071 series of critical experiments were able to lower the nuclear data uncertainty associated with carbon on the Xe-100 and gFHR only by about 6 pcm using SCALE 6.2.4 and the ENDF/B-VII.1 nuclear data library. In addition, the HMF071 series of critical experiments were able to lower the nuclear data uncertainty associated with carbon on the Xe-100 and gFHR by about 11 pcm and 16 pcm respectively using SCALE 6.3.1 and the ENDF/B-VIII.0 nuclear data library. When all-materials were used in the TSURFER evaluation, the uncertainty reduction on the gFHR was about 39 pcm using SCALE 6.3.1 and the ENDF/B-VIII.0 nuclear data library. This is very small; however, due to the fact that carbon is such a well-studied element, any further reduction of uncertainty for thermal-spectrum systems is difficult (but useful) to obtain. These null results disproved our hypothesis and do not allow us to demonstrate this specific use case for evaluating legacy fast-neutron-spectrum experiments, which are otherwise often neglected, for modeling modern thermal-neutron spectrum graphite moderated/reflected reactors.