A Coupled Approach to Calculate Reactivity Coefficients for Liquid Metal Fast Breeder Reactors

Among Generation IV proposed reactor designs, liquid-metal based technologies are recognized as some of the most promising due to the possibility to employ a fast neutron spectrum, enhancing nuclear fuel utilization, achieving high power densities, and providing strong inherent safety. However, being an innovative technology still under development, operational experience remains limited, requiring a strong reliance on numerical simulations. This work presents a methodology for evaluating reactivity coefficients by coupling two different computational tools: the Monte Carlo code OpenMC and the system thermal-hydraulic code RELAP5/Mod3.3. The latter is employed in a version modified at the University of Pisa to include molten lead among the available working fluids. This integrated approach enables the determination of reactor-specific reactivity coefficients, which are typical of the considered core geometry. Consequently, the methodology can be applied to derive neutronic parameters for any LMFBR core design, given the core geometry and materials specification. In particular, effects such as fuel temperature feedback (Doppler effect) and coolant density variations with temperature depend on specific fuel bundle geometries, fuel enrichment, and pin configurations can be derived. The calculated neutronic parameters can subsequently be implemented in the point kinetics model included in the RELAP5 code. This allows to enhance the accuracy of transient simulations and to obtain a more detailed analysis of reactor behaviour under prescribed accidental conditions. In particular, the methodology supports the simulation of critical scenarios like the Unprotected Transient Over-Power (UTOP), providing insights into safety aspects related to reactor design. By implementing the derived neutronic parameters into the thermal-hydraulics code, the overall reliability of accident analysis is enhanced, due to the detailed understanding of inherent feedback mechanisms and core behaviour under accident conditions.