Abstract
The implementation of various nuclear technologies largely depends on computer simulations using nuclear data. The reliability of such calculations cannot be determined without precise knowledge of the uncertainties and related correlations associated with the basic nuclear data reported in the evaluated cross-section libraries. Scarcity of such information motivated establishment of the two international projects within the framework of the Working Party on International Nuclear Data Evaluation Cooperation (WPEC). The first project, WPEC Subgroup 24, is dedicated to the development of new methods for covariance data evaluation in the fast neutron region and the second project, WPEC Subgroup 28, on resonance region covariance processing methods to prepare covariance data libraries for sensitivity/uncertainty analyses. The goals and current achievements of both projects are reviewed.