The liquid fluoride thorium reactor (LFTR; often pronounced lifter) is a type of molten salt reactor. LFTRs use the thorium fuel cycle with a fluoride-based molten (liquid) salt for fuel. In a typical design, the liquid is pumped between a critical core and an external heat exchanger where the heat is transferred to a nonradioactive secondary salt. The secondary salt then transfers its heat to a steam turbine or closed-cycle gas turbine. Molten-salt-fueled reactors (MSRs) supply the nuclear fuel mixed into a molten salt. They should not be confused with designs that use a molten salt for cooling only (fluoride high-temperature reactors, FHRs) and still have a solid fuel. Molten salt reactors, as a class, include both burners and breeders in fast or thermal spectra, using fluoride or chloride salt-based fuels and a range of fissile or fertile consumables. LFTRs are defined by the use of fluoride fuel salts and the breeding of thorium into uranium-233 in the thermal neutron spectrum. The LFTR concept was first investigated at the Oak Ridge National Laboratory Molten-Salt Reactor Experiment in the 1960s, though the MSRE did not use thorium. The LFTR has recently been the subject of a renewed interest worldwide. Japan, China, the UK and private US, Czech, Canadian and Australian companies have expressed the intent to develop, and commercialize the technology. LFTRs differ from other power reactors in almost every aspect: they use thorium that is turned into uranium, instead of using uranium directly; they are refueled by pumping without shutdown. Their liquid salt coolant allows higher operating temperature and much lower pressure in the primary cooling loop. These distinctive characteristics give rise to many potential advantages, as well as design challenges. Background By 1946, eight years after the discovery of nuclear fission, three fissile isotopes had been publicly identified for use as nuclear fuel: Uranium-235, which is already fissile, and occurs as 0.72% of natural uranium Plutonium-239, which can be bred from non-fissile uranium-238 (>99% of natural uranium) Uranium-233, which can be bred from non-fissile thorium-232 (~100% of natural thorium; which has about four times greater abundance in the Earth's crust than uranium) Th-232, U-235 and U-238 are primordial nuclides, having existed in their current form for over 4.5 billion years, predating the formation of the Earth; they were forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovas. Their radioactive decay produces