Analysis of the coolant density reactivity coefficient in LFRs and SFRs via Monte Carlo perturbation/sensitivity

The coolant density coefficient represents one of the main reactivity feedback in Lead-cooled Fast Reactors (LFRs) and Sodium-cooled Fast Rectors (SFRs), and its accurate calculation is important for a correct evaluation of the dynamic of these systems. Coolant density reactivity maps have been calculated in the past adopting perturbation theory in deterministic codes. Usually, full-core simulations employed multi-group diffusion codes or 2D (r, z) geometrical approximations. Nowadays, Monte Carlo neutron transport simulations are commonly adopted for the study of LFR and SFR. Nonetheless, reactivity feedback is usually calculated via direct perturbations, i.e., comparing the effective multiplication factor of two separate Monte Carlo runs. When small effects are to be investigated via the direct perturbation approach, the adoption of either large system perturbations or a large number of simulated particles is required, in order to reduce the statistical errors. Moreover, if spatial maps of coolant density reactivity coefficient are to be generated via direct perturbation, one criticality source Monte Carlo simulation is required for each spatial region. In this view, the sensitivity/perturbation method offer the advantage of producing a large number of sensitivity coefficients is a single calculation. More important, this approach allows decomposing reactivity effects by energy and reaction for a deeper investigation of the feedback. In this work, two LFR and SFR core designs are considered, focusing on the calculation and analysis of the coolant density reactivity coefficient. The space-dependent lead and sodium density reactivity worth are calculated adopting the sensitivity/perturbation capabilities recently implemented in an extended version of the Serpent-2 code , previously adopted for the calculation of coolant void maps . The present work focuses on the validation of the sensitivity/perturbation results against direct perturbation calculations, on the analysis of the optimal parameters to be adopted for the simulations and on the discussion of the peculiar results obtained for the two considered cases.