Analysis of implicit and explicit lattice sensitivities using DRAGON

Deterministic lattice physics transport calculations are used extensively within the context of operational and safety analysis of nuclear power plants. As such the sensitivity and uncertainty in the evaluated nuclear data used to predict neutronic interactions and other key transport phenomena are critical topics for research. Sensitivity analysis of nuclear systems with respect to fundamental nuclear data using multi-energy-group discretization is complicated by the dilution dependency of multi-group macroscopic cross-sections as a result of resonance self-shielding. It has become common to group sensitivities into implicit and explicit effects to aid in the understanding of the nature of the sensitivities involved in the calculations, however the overall sensitivity is an integral of these effects. Explicit effects stem from perturbations performed for a specific nuclear data for a given isotope and at a specific energy, and their direct impact on the end figure of merit. Implicit effects stem from resonance self-shielding effects and can change the nature of their own sensitivities at other energies, or that for other reactions or even other isotopes. Quantification of the implicit sensitivity component involves some manner of treatment of resonance parameters in a way that is self-consistent with perturbations occurring in associated multi-group cross-sections. A procedure for assessing these implicit effects is described in the context of the Bondarenko method of self-shielding and implemented using a WIMS-D4 multi-group nuclear library and the lattice solver DRAGON. The resulting sensitivity results were compared to those calculated by TSUNAMI-1D, which computes implicit sensitivities using a different methodology consisting of a combination of linear perturbation theory and automatic differentiation. Energy-dependent sensitivity profiles and integrated sensitivity coefficients are presented, as well as a comparison of calculated sensitivities for different energy group structures and geometry dimensionalities.