Lately I’ve heard some descriptions about the potential of “thorium” that were very specific to the form of thorium that you would use in solid-fueled, water-cooled reactors–thorium oxide.
Thorium oxide, or specifically thorium dioxide (ThO2) is the solid form of thorium that one would use in a light-water reactor or in a pebble-bed reactor, perhaps even in a sodium-cooled fast breeder. Thorium dioxide is VERY chemically stable and has the highest melting point of any oxide–3300 degrees Celsius. This is why it is sometimes said that “thorium” can’t have a meltdown in a nuclear reactor–they’re talking about thorium dioxide fuel.
The problem with thorium dioxide is that it is extremely difficult to reprocess the fuel and extract any uranium that has been bred into the fuel from being in a reactor. To reprocess thorium oxide fuel, you first have to convert it into another chemical form (a nitrate) and do the chemical separations in this form. In fact, thorium oxide is so difficult to reprocess that this has been a major disincentive to use thorium dioxide as a nuclear fuel.
But there’s a much better option: thorium in a fluoride form, instead of an oxide. Specifically, thorium tetrafluoride (ThF4) is a form of thorium that’s EVEN MORE chemically stable than the oxide, but unlike the oxide it has a major advantage:
You can reprocess thorium fluoride without changing it into something else!
The reason for this magic has to do with the chemistry of uranium fluoride as well. There’s two basic forms of uranium fluoride–one with four fluoride ions (the tetrafluoride) and one with six fluoride ions (the hexafluoride). The tetrafluoride of uranium is stable in a salt solution, like sugar dissolved in water, but the hexafluoride is a gas and will come out of a solution.
This is the perfect trick for a thorium reactor, because you want to be able to separate the bred fissile product (uranium) from the parent material (thorium). In oxide form, this involves pulling solid rods out of a reactor and changing their chemical form to achieve reprocessing. In fluoride form, you don’t have to change the chemical form–you can process the fuel just the way it is, by shifting the small amount of uranium in the thorium from a tetra- to a hexafluoride.
It’s a trick that’s so SWEET that one has to wonder if we aren’t meant to build fluoride reactors that use thorium as a fuel!