Over on Hot Air, Steven Den Beste claims that thorium reactors are a huge proliferation risk.
- oversimplifies and gets it wrong, and
- is going to be taken as gospel by lots of people anyway
Here's the real dope, in a nutshell:
- Thorium (Th-232) can be bred to fissile uranium 233 (U-233) by hitting it with a neutron, allowing the Th-233 to decay to protactinium (Pa-233) and then to uranium.
- Separation of pure U-233 requires not just a liquid-fuel reactor, but either something called a "two-fluid reactor", or chemical separation of Pa-233. Pa-233 is a strong neutron absorber and will go to Pa-234, which decays to U-234 which is not fissile (it takes yet another neutron to make U-235, which is the natural fissile isotope).
- The likelihood of Pa-233 grabbing a neutron before it beta-decays to U-233 is proportional to the neutron flux, so reactors with low power density don't need separation of the breeding and power sections. They can do it all with one uniform fluid.
- In a one-fluid reactor, U-233 sometimes gets hit with a neutron and, instead of fissioning, loses another neutron. This is the (n, 2n) reaction and it makes another isotope of uranium, U-232.
- U-232 is not fissionable, but another neutron turns it back to U-233.
- U-232 has a decay path which creates thallium 208 (Tl-208). Tl-208 is a very powerful gamma emitter, which will degrade explosives and fry electronics unless they are behind heavy shields.
- Because U-232 cannot be separated from U-233 by chemical means, any uranium removed from a one-fluid thorium reactor will soon have a growing amount of Tl-208 frying anything nearby and shining like a beacon saying "Here I am!". You can remove the thallium chemically, but as long as there's still U-232 you'll have more as soon as you stop.
- A bomb built to fit on a missile can't have heavy shielding, and a "stealth bomb" to be sneaked into an enemy city isn't going to work if its core can be detected though a foot of lead.
- If that's not good enough, there's a scheme called a "denatured molten-salt reactor" (DMSR) which has a mix of stuff that's even more useless for bombs and doesn't even breed to breakeven. You have to add small amounts of enriched fuel to keep it going (but you can run 20 years or so without removing anything, making it cheap and trouble-free).
- This isn't a problem with solid fuels. The thorium-uranium rods being developed by Lightbridge are even more of a headache for proliferators than straight uranium.
That's why nobody's ever tried to base a weapons program on thorium; if it was so easy, Kim Il Sung, A.Q. Khan and Saddam Hussein would have gone that way. Even the USA realized that neither a weapons program nor a plutonium economy could come from thorium reactors (which we now know is a good thing). Nobody did because they know more about nuclear technology than Steven Den Beste.
For more information, start with this lecture by Dr. David LeBlanc and the supporting materials.