Covariance data for ENDF/B-VI [Evaluated Nuclear Data File]

For ENDF/B-V [Evaluated Nuclear Data File], the Cross Section Evaluation Working Group recognized the need to quantify the uncertainty of the cross sections most important to applications. Covariance (here includes variance) formats and procedures were developed for the uncertainties in smooth cross sections and in isolated Breit-Wigner resonances. The goal was to allow propagation of the differential data uncertainties to yield uncertainty estimates for parameters calculated from the data base for applications having a broad neutron spectrum. One cannot rely on the variances (diagonal elements) alone, in part because the propagated uncertainty would then be strongly dependent on the fineness of the input data energy mesh. For ENDF/B-VI, developments and new covariance formats have made it possible to include covariance data to meet uncertainty propagation needs for pressure vessel dosimetry, fusion reactor activation, fast reactor core physics. The ENDF/B-VI evaluated covariance files can meet the requirements for uncertainty propagation and data adjustment because of broadened format capabilities and because of the increased experience with covariance data that is now possessed by measurers, evaluators, and users. Since covariance evaluation is demanding work, however, the first release of ENDF/B-VI will not contain all the needed information