Estimation of influence of implicit effect due to multi-group cross-section perturbations on uncertainty analysis in PWR-UO2 and -MOX lattice calculations

This study estimated the influence of implicit effect due to multi-group cross-section (XS) perturbations on the k-infinity uncertainty in the PWR-UO2 and -MOX lattice calculations. Firstly, the preliminary investigation was performed, where the influence of the implicit effect was roughly estimated based on the sandwich formula using the XS covariance matrix and the sensitivity coefficient. It was confirmed that the influence of implicit effect became large in the fission and $\left({n,\gamma } \right)$n,gamma reactions of heavy nuclides and the change of this dependence was small for the burnup of UO2 and MOX fuel assemblies. Then, focussing on the heavy nuclides, the influence of implicit effect was compared under several energy group conditions of neutron transport calculation and XS covariance matrix. For 239Pu and 240Pu, the noticeable influence of implicit effect was observed in the MOX lattice calculation. However, the increase in the number of energy groups for neutron transport calculations and that of the XS covariance matrix can reduce the influence of the implicit effect. Consequently, by appropriately setting the number of energy groups for neutron transport calculations and that of the XS covariance matrix, it became practically possible not to explicitly consider the implicit effect during the random sampling.