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Application of idealized statistical methods to GRMA shows that variance among conventional RMA capture widths in extant RMA evaluations could be used to estimate variance among off-diagonal elements neglected by conventional RMA. Significant departure of capture widths from an idealized distribution may indicate the presence of underlying doorway states.

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Advanced reactor concepts being developed throughout the industry are significantly different from light-water reactor (LWR) designs with respect to geometry, materials, and operating conditions, and consequently, with respect to their reactor physics behavior. Given the limited operating experience with non-LWRs, the accurate simulation of reactor physics and the quantification of associated uncertainties are important for ensuring that the nuclear design for advanced reactor concepts include appropriate margins. Nuclear data are a major source of input uncertainties in reactor physics analysis. As part of a project sponsored by the US Nuclear Regulatory Commission at Oak Ridge National Laboratory (ORNL), key nuclear data relevant to reactor safety analysis in selected advanced reactor technologies 1 were identified, and their impacts on important key figures of merit were assessed based on (1) a review of available advanced reactor specifications, (2) analysis of previous studies performed at ORNL and other research institutions, and (3) sensitivity and uncertainty analyses performed for six selected benchmarks-three experimental and three computational-to quantify the impacts of the identified key nuclear data on several key metrics. This report summarizes the key nuclear data-nominal data and nuclear data uncertaintiesconsidering the most important nuclear reactions in the fuel and in various materials for the moderator, coolant, and structure of the considered advanced reactors.The major nominal missing data that were identified consist of thermal scattering data and 135m Xe cross section data for molten salt reactor (MSR) analysis. The identified major gaps with respect to nuclear data uncertainties are (1) the missing uncertainties in the thermal scattering data for high-temperature gas-cooled reactors and moderated MSR systems, and (2) the incomplete uncertainties on angular distributions, particularly for fast spectrum systems such as sodium-cooled fast reactors, fast molten salt reactors, and heat pipe reactors.Large uncertainties of reactions that are not commonly considered to be relevant in LWR studies were found to be significant for several advanced reactor systems. The large uncertainty of 238 U inelastic scattering in the fast energy range contributes significantly to large output uncertainties in all fast spectrum systems. The large uncertainty of 235 U (n,) in the fast energy range causes significant reactivity uncertainties in fast neutron spectrum systems that use 235 Uenriched fuel. A large uncertainty of 7 Li (n,) causes a large fraction of uncertainty in the output quantities investigated for MSR systems in which lithium is part of the salt. Special attention should be paid to differences in cross section and uncertainties of different evaluated nuclear data library releases. Significant differences were found in nuclear data that can lead to major differences in reactivity calculations, even for well-known nuclides. In particular, differences in 235 U, 238 U, and 239 Pu nominal and unce...