Major questions about derivation of variance-covariance information for nuclear data evaluations

The uncertainties in and correlations among some nuclear data are now evaluated to permit estimation of data-related uncertainties in the outputs of neutronic calculations and to focus data improvement efforts. Questions are discussed that arise in trying to obtain adequate numerical files of variance-covariance uncertainty information. These involve 1) discrepant data, 2) experimental data with incompletely reported uncertainties, 3) uncertainties in nuclear model results, 4) uncertainty data for the resonance regions and for angle and energy distributions, and 5) the role of integral data in nuclear data evaluation. The question also arises whether files of uncertainty data designed for technological applications can suffice to represent past knowledge in an evaluation that includes new data. Directions are indicated toward resolving these questions.