Comment by chasil
12 days ago
Uranium 235 as a fissile fuel is a very small percentage of the total uranium available (0.720% from the wiki).
https://en.wikipedia.org/wiki/Uranium
In order to make reactor fuel, this percentage must be increased, via conversion of the metal to uranium hexafloride gas, which is purified via gas centrifuges.
As opposed to this, thorium does not require difficult purification steps, but it does require a neutron source to start a reaction that converts a small portion of it to uranium 233.
From the uranium 235 purification perspective, uranium is an awkward fuel for commercial use.
I think it's a shame that we don't breed more fuel. The majority of mined Uranium 238 can be made fissile in a breeder reactor (fast neutrons), just like with Thorium. It does require reprocessing, which is a taboo topic due to proliferation concerns.
Plutonium doesn't build up in a breeder reactor, the fast neutrons split it. Our existing light water reactors have a build up of plutonium over time. I guess that's desirable when your objective is bombs.
A lot of the long lived radioisotopes get broken down in a breeder reactor, so the waste degrades to safe levels much quicker (a few hundred years).
The Gates backed reactor (Terrapower) in Wyoming is using fast neutrons.
There is a vast supply of thorium 232, produced as waste in rare earth mining. Why breed uranium?
"Natural thorium is usually almost pure 232-Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its radioactive decay is the largest single contributor to the Earth's internal heat; the other major contributors are the shorter-lived primordial radionuclides, which are 238U, 40K, and 235U in descending order of their contribution.
"[Thorium] is the 37th most abundant element in the Earth’s crust with an abundance of 12 parts per million.
"The low demand makes working mines for extraction of thorium alone not profitable, and it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals."
https://en.wikipedia.org/wiki/Thorium
Thorium 232 is barely fissile - it has a net negative neutron balance during fission. Uranium-233 is similarly neutron absorbing (half the time).
So you can’t run a reactor with just Thorium 232, and maintaining a decent fuel balance can be tricky once you start due to odd fission neutron ratios between the parent fuels and daughter products and long time delays (half life wise) until you get Uranium-233. About 30 days half life.
Certainly not impossible, but pretty awkward compared to other options.
> In order to make reactor fuel, this percentage must be increased, via conversion of the metal to uranium hexafloride gas, which is purified via gas centrifuges.
Unless you're using CANDU reactors which can use unenriched uranium (the trade-off is you need heavy water (deuterium, D2O) as a moderator, and producing it is an up-front cost).