In the MSR reactor fission occurs in a circulating molten salt fuel mixture in a neutron stingy reactor that permits a full actinide burn fuel cycle. In the MSR system, the fuel, a circulating liquid mixture of sodium, zirconium and uranium fluorides, flows through graphite core channels in the reactor chamber where it undergoes fission. The hot molten salt is then continuously pumped through a secondary coolant system, through an intermediate heat exchanger, and then through a tertiary heat exchanger where energy is extracted. MSR’s operate at high temperatures. They typically employ salts with melting points in excess of 650C. This means that the core containment vessel and heat exchangers must be made of high-temperature materials (such as nickel alloys). While the salt is continually circulating, fusion only occurs in the reactor proper in the presence of the graphite moderator. Molten fluoride salts have excellent heat transfer characteristics and a very low vapour pressure, which reduces stress on the vessel and plumbing.

The MSR’s liquid condition allows the direct addition of fuel as required and avoids the need for fuel fabrication. The MSR can be operated as a breeder reactor on a 232Th-233U fuel cycle that generates a waste with almost no long-lived actinides. The closed fuel cycle can be tailored to the efficient burnup of plutonium and minor actinides. Actinides and most fission products form fluorides in the liquid coolant. MSR’s also have the flexibility to utilise any fissile fuel mix in continuous operation with no special modification of the core.

One of the features of the MSR is that the fuel must undergo on-line reprocessing. The on-line reprocessing is necessary to keep the reactor in operation, clearing away typical reactor poisons like xenon and krypton. Also when operating as a breeder protactinium is produced as well, unfortunately, 233Pa has a moderately large absorption cross section and a half-life of 27 days. If it is left in the reactor, parasitic capture of neutrons by 233Pa will occur, resulting in a significant reduction in the breeding ratio.  To avoid this, on-line processing must also remove the 233Pa and store outside of the reactor until it decays to 233U.

The characteristics of the MSR also offer the potential for large economics of scale in very large reactors compared to other designs, yet it can also be produced in a low volume design, in fact the first of this type was developed to power aircraft, although this never came to fruition. Like any nuclear reactor, thorium reactors will be hot and radioactive, necessitating shielding. The amount of radioactivity scales with the size of the plant. It so happens that thorium itself is an excellent radiation shield, but lead and depleted uranium are also suitable.

More information can be found at these pages of the excellent website, Energy from Thorium:

Introduction and Basic Principles

Introducing the Liquid-Fluoride Reactor

A Brief History of the Liquid-Fluoride Reactor