New Exploration on TMSR: Redesign of the TMSR Lattice

Molten Salt Reactor (MSR) has been recognized as one of the Next Generation Nuclear Power systems. Most MSR concepts are the variants evolved from the ORNL’s Molten-Salt Breeder Reactor (MSBR) which employs Molten-Salt as both fuel and coolant, and normally graphite is used as moderator. Many evaluations have revealed that such concepts have low breeding ratio and might present positive power coefficient. Facing these impediments, TMSR (Thorium Molten Salt Reactor) with redesigned lattice is proposed in this paper. Based on comprehensive investigation and screening, important lattice parameters including molten salt fuel composition, solid moderator material, lattice size, structure and lattice P/D ratio (lattice pitch to channel diameter) are redesigned. In this paper, new composition of fuel salt without BeF2, which is also recommend for Molten Salt Fast Reactor (MSFR), is employed instead of LiF-BeF2-ThF4-UF4 adopted in the design of single fluid MSBR. The new fuel composition makes TMSR to benefit from the increased solubility for actinides (e.g. Th4, UF4). Moreover, due to the decent slowing-down power and neutron multiplication effect by (n,2n) reaction of beryllium, BeO is employed as moderator to improve neutron economy instead of graphite. To avoid corrosion on the one hand, Ceramic cladding (e.g. SiC) is introduced to separate the flowing liquid fuel and fixed solid moderator. More importantly, ceramic cladding is capable of maintaining a stable flow channel and supporting the core structure on the other hand. Concerning neutron spectrum, P/D ratio is an important parameter indicating the volume fraction of fuel in the lattice. In order to obtain a suitable spectrum for better breeding and safety features, lattice size and P/D ratio have been optimized for TMSR. Furthermore, since online reprocessing capability and refueling control are key parameters influencing depletion behavior which concerns the sustainability of the reactor system, these issues are also discussed in this paper. Simulation of the redesigned TMSR system is performed to evaluate the outcomes of the lattice parameters optimization. SONG/TANG-MSR codes system is applied in the simulation, which is independently developed by Shanghai Nuclear Engineering Research & Design Institute (SNERDI). A traditional core model with LiF-BeF2-ThF4-UF4 fuel and graphite moderator is also evaluated by the codes for reference. Thanks to the optimized lattice parameters and as consequences of the redesigned lattice, TMSR has achieved a high breeding ratio close to 1.13. With a proper reprocessing and refueling strategy, the doubling time of TMSR can be shortened to about 15 years. Meanwhile a negative power coefficient is still maintained. Based on this lattice design, TMSR will have excellent performance on safety and sustainability.