Application of covariances to fast reactor core analysis

Ishikawa, Makoto

In the present paper, the current status of covariance application to the fast reactor analysis and design is summarized with actual examples. The covariance application is classified into three fields. First, the covariance is used to quantify the uncertainty of nuclear core parameters such as criticality, control rod worth, reaction rate ratio, power distribution, sodium void reactivity, etc. The special features of Japanese case are to include both the burnup-related parameters such as burnup reactivity loss or fuel composition changes, and the temperature-related parameter, that is, the Doppler reactivity. Second, the covariance is used to select the important nuclides, reactions and energy ranges which are dominant to the uncertainty of core parameters, and to give nuclear scientists recommendations to improve the accuracy of the data. Finally, the covariance is used to improve the accuracy of core design values by adopting the integral data such as the critical experiments and the power reactor operation data. The way of the improvement is classified into the conventional E/C (Experiment/Calculation) bias method, and the more comprehensive cross-section adjustment based on the Bayesian theorem and the generalized least square method. In Japan, an adjusted group-constant set, ADJ2000R, is now being used in the design work for future fast reactors.