Multigroup covariance matrix self-shielding effects for thermal reactors fueled with slightly enriched uranium

A numerical approach has been successfully developed to treat the self-shielding effects in the multigroup cross section covariance matrices of thermal reactors fueled with slightly enriched uranium. The procedure employs the coupled NJOY/AMPX-II systems developed at IPEN and the ²³⁸U resonance parameter covariance data from JENDL 3.3. Only the first two most important ²³⁸U resonances are under analyses. The direct and indirect effects of the ²³⁸U resonance self-shielding is taken into account. The effect of the change in the cross section is called the direct effect and that of the neutron flux due to change in the cross section is called the indirect effect. The k_eff uncertainty analyses applied to the IPEN/MB-01 reveal that the self-shielding effects both direct and indirect have an important bearing on the multigroup covariance matrix as well as on the k_eff uncertainty. Also, the indirect effects account for nearly 44% of the total uncertainty. The ERRORR module of the NJOY system is in severe disagreement to the developed method because it considers only the direct effect in the multigroup cross section covariance matrix. Such results underline the application dependence of multi-group cross section covariance matrix, and that ENDF FILE 33 content must be corrected due to the resonance self-shielding effects mainly for applications in thermal reactor fueled with slightly enriched uranium.