Fluid-fueled Molten Salt Reactors (MSRs) pose unique safeguarding challenges. The use of circulating fuel salts complicates tracking and accounting processes, while the capability for online reprocessing introduces additional risks of unauthorized material diversion. This research aims to enhance nuclear safeguards for MSRs by leveraging real-time gamma spectroscopy to monitor non-gaseous fission products within reactor cores. Using Monte Carlo N-Particle (MCNP) simulations, the study analyzed the burnup behavior of four fissile driver nuclides – ²³³U, ²³⁵U, ²³⁹Pu, and ²⁴⁰Pu – modeled on a representative MSR lattice element. The results identified the 14 gamma-emitting fission products most capable of differentiating reactor fuel compositions based on their relative levels. These findings demonstrate the feasibility of deducing driver nuclide(s) present using gamma spectroscopy, providing a more comprehensive monitoring solution than existing off-gas detection methods. This approach helps address critical gaps in MSR safeguarding, assisting regulators and operators to verify compliance with declared fuel cycles. By improving transparency and security in nuclear materials management, this methodology helps support the safe deployment of MSRs as a sustainable energy technology. Future work should validate these findings through experimental reactor studies and extend the methodology to full-core analyses and advanced detection technologies.
