Methods of Adjustment and Error Evaluation of Neutron Capture Cross Sections; Application to Fission Product Nuclides

In the analysis of fission product reactivity worths, measured in the fast reactor spectra of the STEK critical-experiments facility, extensive use is made of a statistical method of cross-section adjustment. The principle is that adjustments are applied to the evaluated cross sections, as much as possible within their error limits and taking into account the existing correlations, in such a way that a better agreement between calculated and measured integral data is obtained. The method is briefly summarized in general terms, with some special applications needed for the STEK project. Then, a description is given of the practical realization for capture cross-section adjustment on the basis of reactivity worths of samples of fission product mixtures in different thicknesses measured in several fast reactor spectra. Details are given on the way the various contributions to the covariance matrix of group cross sections, including resonance selfshielding, are calculated for the fission product nuclides and for the mixtures. The paper only outlines the methods used; examples of applications can be found elsewhere. Finally, some limitations of the method and possible extensions in connection with standard nuclear data error files are discussed.