Verification of the modified point kinetics model in the MARS code for molten salt reactors

This study presents the implementation and verification of a modified point kinetics model (PKM) within the thermal-hydraulic system code MARS for application to liquid-fueled molten salt reactors (MSRs). Traditional PKMs for MSRs establish the delayed neutron precursor (DNP) balance equation by simplifying the reactor system into two regions: a reactor core and an external loop only. This approach is inadequate for realistic MSR systems with complex flow paths. To overcome this limitation, a three-dimensional DNP transport equation was introduced into the PKM instead of DNP balance equation, enabling a more accurate treatment of precursor distribution throughout the reactor system. The model was numerically solved in the MARS code using the upwind scheme, and the effective delayed neutron fraction was evaluated from steady-state precursor distributions. Verification of the modified PKM in the MARS code was performed through comparison with exact analytical solutions and the independently developed GAMMA + code. The results confirm the correctness and numerical accuracy of the modified PKM and demonstrate the MARS code's capability for both steady-state and transient analysis of liquid-fueled MSRs.