Impact of fission yield correlations on burnup problems

Uncertainties and correlations of independent fission yields play a major role in the uncertainty quantification of several burnup responses-e.g. isotopic inventory, k-eigenvalue. However, nuclear data libraries generally provide only the best-estimates and uncertainties of such independent fission yields, but no correlation. In addition, at the current stage the current independent fission yield evaluated uncertainties do not comply to many physical constraints, like conservation equations and relations between different types of yield. This incongruity could have a large impact on uncertainty quantification (UQ) studies. As a part of this work, we sorted out the data inconsistency found in the JEFF-3.1.1 library introducing fission yield correlations. Such correlations were produced using a generalised least square updating approach, with conservation equations acting as fitting models. The process was iterative and fission yield estimates and covariances were revised, each time introducing specific sets of measured values, when available, or evaluated conservation criteria. The new uncertainties and correlations between fission yields appeared to comply to the physical constraints. We conveyed the information of the new covariance dataset into randomly perturbed files, ready for random sampling calculations. The number of samples was large enough to grant convergence of the mean values and the standard deviations. Then, we quantified the uncertainty of the isotopic inventory and keff of the PWR fuel rod sample of the REBUS international program, first using updated and then original data. This procedure included data sampling followed by depletion calculations using ALEPH, the SCK-CEN burnup code, which simulated the irradiation history. The response uncertainty estimate, obtained through a statistical analysis of the results, showed a sharp drop when using correlated fission yields.