Questions have also been raised by some nuclear scientists about the Thorium cycle, in particular the proposed one that the LFTR would use. I’m not a nuclear physicist so I’ll merely forward you on to the relevant paper here, and a rebuttal here. The crux of the argument seems to be the proliferation risk (I’ll come back to that one later), the fact that a number of its spend fuel outputs (such as Technetium-99) are “nasty stuff” with a long half life and the fact we’ll still need supplies of Uranium to get Thorium reactors going again whenever we have to turn it off (which will happen at least once a year or so during its annual maintenance shutdown). They also highlight a number of technical issues, which I discussed in the chapter on HTGR’s.
Certainly the fission products from a Thorium reactor are a worry, Technetium-99 has a half life of 220,000 years, uranium-232 produces thallium-208 (a nasty wee gamma emitter), Selenium-79 (another gamma emitter with a 327,000 year half-life), even Thorium-232 is a problem with its half life of 14 Billion years (and while the T-232 isn’t a major worry, all the time during this 14 Billion years it will be decaying and producing stuff that is!).
The UK based NNL (National Nuclear Laboratories) also pour cold water on the idea of Thorium fuelled reactors (see here). While the report is low on detail (they seem to be saying “trust us we’re scientists who work with nuke stuff… and we smoke pipes!”) they do highlight the major time delays it would take to establish and get working a Thorium fuel cycle (10-15 years with existing reactors, 30 with more advanced options), point out that under present market conditions its unlikely to be economically viable and will (as the points above raise) offer only a modest reduction in nuclear wastes.
MIT recently undertook a study of future nuclear fuel supplies. The Thorium cycle barely gets a mention, and even then its usually in relation to Fast Reactor programs (of which the US currently has none) and modifed LWR systems, rather than the MSR.
Obviously, once we exhaust the world’s U-235 stockpiles, LFTR’s and any other Thorium fuelled reactors will cease to function. Indeed long before then the spike in Uranium prices will have rendered MSR’s (and all other nuclear plants) uneconomically viable (of course there’s plenty who’d say that’s already the case!). The LFTR fans usually groan at this point and state that “all we need is a little plutonium”. Now while I’m quite sure that in the fantasy world which the LFTR fans inhabit Plutonium is available in any good hardware store but back in the real world, it’s a little harder to come by! As with the HTGR’s using Thorium (if its possible) would certainly help stretch things out….a bit! But not by nearly as much as the supporters of Thorium reactors would have you believe.