I've read somewhere that one problem with Thorium is it can easily be converted to a fissionable isotope (of Uranium?) which can be used to build bombs. So there's that.
One of the anti-proliferation points is that U233 (which is the isotope you're thinking of) is mixed with U232, which decays to products with high gamma output, meaning that it's more dangerous to work with than the more typical fissionable materials, and much easier to detect. For a state this probably wouldn't be a significant barrier, but it's a fair point with respect to terrorist bombs. Of course, we haven't seen any of those, which implies that there's some non-obvious factor at work, here.
You will hear that we can’t make bombs out of U-233 because it is a virulent gamma-ray emitter. This is not true, and I find it curious that it is used as a reason. U-233 has a 158,000-year half-life. What they are referring to is the protactinium-233 contaminant, which has a 27-day half-life, beta-decaying into U-233 with gamma-ray involvement. Chemically scrub the protactinium, of course, or just wait a year and it will be gone.
No, that's wrong. The claim is that the bred U-233 will be contaminated with U-232, which is very hard to get rid of, and has the hard gamma emitter Thallium-208 in its decay chain.
Sometimes the thorium, when it absorbs a neutron, will emit two neutrons and turn into thorium-231, which decays (though a couple more isotopes) into U-232. It can also be formed when U-233 absorbs a neutron and emits two, forming U-232.
How hard this would be to separate depends on the type of LFTR design you're using. However, you could also include some thorium-230 in the fuel mix to denature any protactinium produced, so there's really no way around having U-232 mixed with the bred U-233. Here's a blog post with more details on the entire process:
No. The problem is the other way around: It _can't_ easily be converted. That's why is was not investigated in the 40s and during the cold war too much: People were interested in bombs, too.
That's true. We had two reactor options during the cold war that were choices to extend our Uranium supply. One was the Fast Breeder reactor and the other was the liquid thorium reactor. My dad worked on the Fast breeder which would use liquid sodium as a coolant to allow the reactor to operate at much higher temperatures and high neutron flux so as to burn actinides more efficiently and convert uranium into plutonium to be either burned or taken out and used for weapons. Although, it's silly, American commercial fast breeders wouldn't really be used for plutonium production even if they got off the ground because U.S. regulations wouldn't allow anyone to access the fuel very easily. Unlike Chernobyl, which was a weapons/energy plant, designed for easy access to it's fuel. Hence the lack of containment which all American plants are required to have.
Realistic breeder fuel cycles also use a mix of actinides that would be unattractive for bomb makers: you're going to start with fuel that's been through an LWR once or twice, so contamination with Pu240 and Pu241/Am241 will be high to begin with and will just get worse the more cycles you spend in a fast spectrum.
Now, hypothetically, a country like Iran could develop a fast reactor that's optimized for producing plutonium for military purposes, but practically this would be much more difficult than developing a heavy water reactor like the U.S. used at Savannah River.
The isotope which is most problematic in commercial fuel cycles would actually be Np237. It's easy to separate Np chemically from other elements, and Np237 is longer lived than other isotopes of Np, so it can be prepared in a very pure form. Np237 has a low spontaneous fission cross section and a critical mass close to pure U235 -- it would be an attractive material for primitive gun-type bombs.
Note that Np237 is produced from U235 by the chain of absorbing two neutrons, making U237, and then beta decay to Np237. A plutonium-fueled or thorium-fueled reactor isn't going to make as much of it as our current reactors do.
Add a neutron to Thorium-232 and it becomes protactinium for 30 days which then decays to Uranium-233. Uranium-233 is great bomb material and perfect reactor fuel, although as mentioned it is quite hot and difficult to work with. The key with thorium reactors, is that apart from the U-233 seed to start the process of converting fertile thorium into fissile material, no more needs to be added.
Thorium is all around us. Adding a neutron to it to create uranium is no simple matter if one doesn't have a high flux neutron supply (such as a reactor).
