Investigation of Covariance Data in General Purpose Nuclear Data Libraries

The understanding of nuclear physics and the associated uncertainties is essential to the modelling of all nuclear systems. Nuclear reaction probabilities, emitted particle energies and angles, fission and other phenomena vary by many orders of magnitude over the energy ranges that neutrons experience within nuclear reactors and the correlations between the uncertainties over these energy ranges are indispensable to the useful estimation of uncertainty for operational parameters.

The Working Party on International Nuclear Data Evaluation Co-operation (WPEC), under the auspices of the Nuclear Energy Agency (NEA) Nuclear Science Committee (NSC), was established in 1989 to promote the exchange of information on nuclear data evaluations, validation and related topics. Its aim is also to provide a framework for co-operative activities among members of the major nuclear data evaluation projects. This framework includes the possible exchange of scientists in order to encourage co-operation. The WPEC determines common criteria for evaluated nuclear data files with a view to assessing and improving the quality and completeness of evaluated data.

This report gives an overview of the activities undertaken by WPEC Subgroup 44 (SG-44) on covariances for general-purpose nuclear data libraries. The SG-44 has studied the state of the art in evaluation techniques for nuclear data covariances, methodologies for using integral experiments to generate and update covariance matrices, investigations into general cross-correlations between different isotopes and physics types, and data format extensions to accommodate advanced covariance data. An intercomparison study was also performed to draw conclusions on covariance generation and explore the possibility of application-independent covariance data. This report summarises those studies and draws conclusions for the future evaluation of covariance data for general-purpose data libraries.