Speaker
Description
Covariance mmatrices play an important role in nuclear reactor safety, dessign and cost optimisation studies. The availability of covariance matrix data in several recent cross section evaluations, such as ENDF/B-VIII, JEFF-3.3, JENDL-4 etc., has been considerably improved which demonstrates an increased awareness of the importance of these data among the nuclear community. The complete sets of recent covariance matrices were processed and partly verified using the NJOY and ANGELO-LAMBDA codes to be included in the updated version of the XSUN-2021 cross section sensitivity and uncertainty code package.
Although covariance matrices cannot be measured directly, and are governed by probability theory and statistics, with the inherent difficulty of estimating systematic uncertainties, they must still satisfy some basic mathematical properties such as positive semi-definiteness. The correct implementation of covariance matrix methodology in nuclear data evaluation can be therefore judged only indirectly by mathematical verification of the matrices and by the consistency, in a statistical sense, between the observed C/E values and the calculated nuclear data uncertainties.
The LAMBDA code was used to identify the matrices containing negative eigenvalues and correlation coefficients exceeding the range between -1 and +1. Large majority of the processed matrices, in particular those relative to the important isotopes, were found to be mathematically correct, with a few exceptions which will be listed. Several integral experiments, ranging from critical, kinetics and shielding, were studied next using the SUSD3D code to verify if the matrices provide the statistical credible, and quantitatively accurate nuclear data uncertainties. Shielding cases include fusion relevant (FNG) and fission reactor (ASPIS) benchmarks. Differences between various covariance matrix evaluations, both recent and older, will be presented and evaluated, which offers the perspective on the progress made. Some unresolved problems and gaps still persist and will be addressed, such as for example the uncertainties in secondary energy and angular distributions, and correlations among nuclear data of different isotopes.
The JEFF-3.3 and ENDF/B-VIII cross section covariance matrix libraries, processed in different energy group structures (33-group ECCO and 175 Vitamin-J) will be distributed as part of the XSUN-2021 code package.