Study of an Irreversible Emergency Shutdown Concept for Molten Salt Reactors in Maritime Applications Through Liquid Poison Injection

Fast molten salt reactors (MSFRs) are one of the main advanced reactor designs being considered for maritime applications. For marine deployment, a shutdown system that is irreversible is highly desirable for safety and security due to unique maritime scenarios such as hijacking (piracy) and sinking. This paper evaluates the direct injection of a soluble neutron poison into the circulating NaCl-UCl3 fuel as a candidate irreversible shutdown method. A detailed two‑dimensional, axisymmetric computational fluid dynamics model of the primary circuit in STAR‑CCM+ was used to resolve buoyancy‑driven flow, frictional losses in a porous media heat exchanger, and passive scalar transport of injected poisons.Time‑dependent, spatially nonuniform poison fields were then transferred into an OpenMC model through a nine‑region core segmentation to compute reactivity insertion at different time steps. Candidate poisons were screened for chemical compatibility, melting/boiling points, hazards, diffusion coefficients, and fast capture cross sections; GdCl3 and 6LiCl emerged as most promising.The simulations showed the bottom cold leg/core inlet was the best injection location. At nominal power [180 MW(thermal)], GdCl3 achieved −2000 pcm within 15 s at 0.1 bar and required 186 kg for permanent shutdown; 6LiCl achieved −2000 pcm within 8 s at 0.01 bar with 59 kg for permanent shutdown.At residual heat conditions, reduced circulation increases both shutdown time and required mass, favoring lower injection pressures to limit total inventory. The results demonstrated the feasibility of irreversible shutdown via poison injection, highlighting design trade‑offs among pressure, time, and inventory, and identifying experimental and modeling needs for multispecies transport and three-dimensional effects.At 12.6-MW(thermal) residual heat, GdCl3 allowed the core to reach −2000 pcm in under 39 s at 0.01 bar and required 200 kg of poison, while 6LiCl achieved the same reactivity insertion in 18.5 s with 200 kg. At 1.8 MW(thermal), an injection pressure of 0.01 bar achieved −2000 pcm within 82 s and 38 s for GdCl3 and 6LiCl, respectively, requiring 546 kg of poison for permanent shutdown.These findings indicate the feasibility of an irreversible shutdown system based on pressurized injections of GdCl3 or 6LiCl for MSFRs, contingent on experimental verification of indicative chemical compatibilities.