Molten salt reactor
A molten salt reactor or MSR is a nuclear reactor that uses a molten salt mixture as the primary coolant or as the solvent for the nuclear fuel itself. The concept of a nuclear reactor with liquid fuel was first explored with the Aircraft Reactor Experiment and the Molten Salt Reactor Experiment at Oak Ridge National Laboratory, but because of poor political support compared to the fast breeder reactor, further research in the United States was terminated in 1969.
MSRs can be designed for thermal or fast spectrum fission depending on the salt used. Lithium and beryllium fluoride salts are well-suited towards thermal spectrum, while chloride salts are theoretically suited for fast spectrum.
In a thermal spectrum design, uranium tetrafluoride is dissolved in a molten mixture of lithium and beryllium fluorides. The fission from fissile isotopes of the uranium generates heat, which is transferred to a heat exchanger. Depending on the design of the reactor, the heat transferred away from the core can be used to boil water for steam turbines, to heat gas for Brayton cycle turbines, or as raw process heat for industrial applications or desalination.
MSRs operate at atmospheric pressure, meaning a heavy reactor vessel and the large containment building are not necessary, as they would be for a light water reactor. If the reactor design dissolves the fuel in the salt, the reactor can utilize a greater percentage of the fissile material than in reactors that use oxide fuel rods because the fuel is impervious to structural damage by the fission product xenon. Additionally, refueling and waste processing can be performed without shutting down the reactor, and greatly reduce the amount of long-lived radioactive waste.
However, because of the lapse in research, many engineering challenges remain, from tritium production to graphite contamination.
Liquid fluoride thorium reactor
A liquid fluoride thorium reactor or LFTR is a variant of the MSR that breeds fissile uranium-233 fuel from thorium in its operation. As the name indicates, the reactor uses molten fluoride salts to both contain the nuclear fuel and to transfer heat from the core. In the proposed designs for LFTR, molten fluoride salts of lithium and beryllium (called FLiBe) serve as solvent for fissile uranium and fertile thorium salts, neutron moderator to promote fission of uranium-233, and heat transfer medium in a separate coolant loop.
A single fluid LFTR uses a large fluoride-resistant reactor vessel with graphite moderator rods. Although relatively simple to build, the low rate of breeding means that fissile uranium may need to be added periodically.
A two fluid LFTR physically separates the core salt containing fissile uranium-233 from the blanket salt containing fertile thorium, but uses the core salt's neutron emissions to breed uranium-233 from the thorium. This has a better breeding rate and can theoretically sustain itself on thorium fluoride once started, but adds complexity to the reactor design and potentially opens up a way to create very pure uranium-233, which raises proliferation concerns. In order to mitigate complexity, a hybrid of the single and two fluid LFTRs, or a "1.5 fluid" LFTR, was suggested, which simplifies the reactor design and allows for the higher breeding ratio of the two fluid LFTR, at the cost of more complex fuel processing systems.
While fissile material such as uranium-235 would be ideal in starting the nuclear reactions in the core salt, some designs (among them cited by Dr. Takashi Kamei of Japan) can prepare the reactor's uranium fuel by irradiating thorium with a particle accelerator.