An Evaluation of Depletion Bias and Bias Uncertainty of the GBC cask with PLUS7 Fuels

A nuclear criticality safety evaluation that applies burnup credit (BUC) to a DSC is performed mainly through a two-step process: (1) the determination of isotopic compositions within UNFs to be loaded into a DSC by a depletion analysis and (2) the determination of the k_eff value with respect to the DSC by a criticality analysis. In particular, the isotopic compositions by a depletion analysis should be estimated accurately because the concentrations of the nuclides contained in a UNF have a significant influence on the accuracies of depletion analysis and its subsequent criticality analysis. However, since no depletion computer code can calculate exactly nuclide compositions contained in a used nuclear fuel assembly (UNFA), it requires bias and bias uncertainty in terms of a reactivity difference, Δk_eff, by a depletion code for burnup credit criticality safety analyses. In this work, the bias and bias uncertainty in k_eff resulting from biases and bias uncertainties in the calculated nuclide concentrations were determined for the GBC-32 DSC system with 32 PLUS7 16X16 UNFAs. First, the new one-group cross section libraries of the ORIGEN code were generated with respect to the PLUS7 16X16 NFA using the SCALE 6.1/TRITON code. Second, the appropriate initial enrichment values for which the k_eff-REF value of the DSC system was to be 0.94 were searched as a function of specific burnup using the SCALE 6.1/STARBUCS code