Molten Salt Reactors: Requirement for Fissionable Starting Material

Thorium 232 is a fertile element in abundant supply within the Earth's crust. Thorium can be jump-started into a fissionable fuel, U 233, with enough neutron bombardment. Thorium needs a quantity of fissionables -- as neutron donors -- to start the initial fission reaction. More from Brian Wang:

Molten salt reactors require anywhere from 500 kg to upwards of 5 or more tonnes per GWe plant (whether one runs a more thermal neutron spectrum or a more fast one). 1500 kg is considered typical. Any molten salt reactor can start on bomb grade or reactor grade Plutonium (roughly the same amount needed as U233). Plutonium is expensive to isolate and of finite supply in spent fuel (600 to 1000 tonnes worldwide?) Any MSR can also start on highly enriched uranium U235 (i.e. old weapons supply). HEU (Highly enriched uranium) is also limited and would be very politically unattractive to be shipping around. Potentially the simplest startup fuel would be Low Enriched Uranium (LEU is under 20% U235 and useless for weapons) because we can produce this in great amounts (we already do) but this is where I have to qualify my answer.

There are two main classes of molten salt reactors that produce all their own fissile fuel after startup, the better known Single Fluid design has everything in one fluid has a very hard time starting on LEU but Oak Ridge National Labs proposed such a design in the late 1970s (DMSR Breeder). Another way to run is called Two Fluid which has separate salts for the fissile U233 and fertile Thorium (which greatly simplifies the removal of fission products). In this design, favored by many these days, startup on LEU is fairly straightforward. You simply run LEU in the central fuel salt for a few years while building up and saving U233 produced in the thorium blanket salt. Once you have enough U233 saved up, you remove (and sell) any remaining LEU in the fuel salt and replace it with your saved up U233 and from then on run on the pure Thorium to U233 cycle.

....startup fissile requirement is not a roadblock to building thousands of GWe of molten salt reactors like it is for something like metal cooled fast breeders that need ten to twenty times as much starting fissile material. _NextBigFuture_quoting_DavidLeblanc

David Leblanc's 11 page paper giving more details via Brian Wang