Glad to see some thorium-bearing salt being irradiated in a conventionally-fueled test reactor. That's a big step to getting back on the road to fluid-fueled re

Glad to see some thorium-bearing salt being irradiated in a conventionally-fueled test reactor. That's a big step to getting back on the road to fluid-fueled reactors. Here are some reminders for everyone on the technical info about Thorium. First of all, Thorium is found in nature as a single isotope, Th-232, which is fertile like Uranium-238 (not fissile like U-235 or Plutonium-239). This means that you have to irradiate it first (using conventional fuel). Th-232 absorbs a neutron and becomes Protactinium-233, which naturally decays to Uranium-233, a fissile nuclide and good nuclear fuel. This is called breeding. Thorium is unique in that it can breed more fuel than it consumes using slow neutrons, whereas the Uranium-Plutonium breeder cycles require fast neutrons (which in turn require highly radiation-resistant materials, higher fissile inventory, and moderately exotic coolants like sodium metal or high-pressure gas). Any kind of breeder reactor (Th-U or U-Pu) can provide world-scale energy for hundreds of thousands of years using known resources and billions of years using uranium dissolved in seawater (not yet economical). Great, so Thorium can do thermal breeding, so what? Well to actually breed in slow neutrons, you have to continuously remove neutron-absorbing fission products as they're created (lest they spoil the chain reaction), so you really can only do this with fluid fuel. This leads to an interesting reactor design called the Molten Salt Reactor (MSR). Fun facts about this kind of reactor are that it can run at high temperatures (for process heat/thermal efficiency), can run continuously (high capacity factor), is passively safe (can shut down and cool itself without external power or human intervention in accident scenarios), and doesn't require precision fuel fabrication. Downsides are that the radionuclides (including radioactive volatiles) are not contained in little pins and cans like in solid fueled reactors so you get radiation all over your pumps, your heat exchangers, and your reactor vessel. This is a solvable radiological containment issue (use good seals and double-walled vessels) but is a challenge (the MSRE in the 1960s lost almost half of its iodine; no one knows where it went!!) U-Pu fuel can work in MSRs as well, getting those nice safety benefits, but it can't breed unless you have fast neutrons. People on the internet may tell you that Thorium can't be used to make bombs and that it's extremely cheap, etc. These are not necessarily true. You can make bombs with a Th-U fuel cycle (just separate the Pa-233 before it decays), and nuclear system costs are unknown until you build and operate a few. There are reasons to hope it could be cheaper due to simplicity, but there are major additional complexities over traditional plants or other advanced reactors in the chemistry department that add a lot of uncertainty. Fluid fueled reactors are probably ~100x or more safer than traditional water-cooled reactors, on par with sodium-cooled fast reactors and other Gen-IV concepts with passive decay heat removal capabilities.