Thorium is a promising candidate as a future source of energy. I just wonder what it is about the stuff that inspires so many people to write nonsense about it. It doesn’t take a Ph.D. in physics to spot the mistakes. Most of them should be obvious to anyone who’s taken the trouble to read a high school science book. Another piece of misinformation has just turned up at the website of Popular Mechanics, dubiously titled The Truth about Thorium and Nuclear Power.
The byline claims that, “Thorium has nearly 200 times the energy content of uranium,” a statement I will assume reflects the ignorance of the writer rather than any outright attempt to deceive. She cites physicist Carlo Rubbia as the source, but if he ever said anything of the sort, he was making some very “special” assumptions about the energy conversion process that she didn’t quite understand. I assume it must have had something to do with his insanely dangerous subcritical reactor scheme, in which case the necessary assumptions to get a factor of 200 would have necessarily been very “special” indeed. Thorium cannot sustain the nuclear chain reaction needed to produce energy on its own. It must first be transmuted to an isotope of uranium with the atomic weight of 233 (U233) by absorbing a neutron. Strictly speaking, then, the above statement is nonsense, because the “energy content” of thorium actually comes from a form of uranium, U233, which can sustain a chain reaction on its own. However, let’s be charitable and compare natural thorium and natural uranium as both come out of the ground when mined.
As I’ve already pointed out, thorium cannot be directly used in a nuclear reactor on its own. Natural uranium actually can. It consists mostly of an isotope of uranium with an atomic weight of 238 (U238), but also a bit over 0.7% of a lighter isotope with an atomic weight of 235 (U235). U238, like thorium, is unable to support a nuclear chain reaction on its own, but U235, like U233, can. Technically speaking, what that means is that, when the nucleus of an atom of U233 or U235 absorbs a neutron, enough energy is released to cause the nucleus to split, or fission. When U238 or natural thorium (Th232) absorbs a neutron, energy is also released, but not enough to cause fission. Instead, they become U239 and Th233, which eventually decay to produce U233 and plutonium 239 (Pu239) respectively.
Let’s try to compare apples and apples, and assume that enough neutrons are around to convert all the Th232 to U233, and all the U238 to Pu239. In that case we are left with a lump of pure U233 derived from the natural thorium and a mixture of about 99.3% Pu239 and 0.7% U235 from the natural uranium. In the first case, the fission of each atom of U233 will release, on average, 200.1 million electron volts (MeV) of energy that can potentially be converted to heat in a nuclear reactor. In the second, each atom of U235 will release, on average, 202.5 Mev, and each atom of Pu239 211.5 Mev of energy. In other words, the potential energy release from natural thorium is actually about equal to that of natural uranium.
Unfortunately, the “factor of 200” isn’t the only glaring mistake in the paper. The author repeats the familiar yarn about how uranium was chosen over thorium for power production because it produced plutonium needed for nuclear weapons as a byproduct. In fact, uranium would have been the obvious choice even if weapons production had not been a factor. As pointed out earlier, natural uranium can sustain a chain reaction in a reactor on its own, and thorium can’t. Natural uranium can be enriched in U235 to make more efficient and smaller reactors. Thorium can’t be “enriched” in that way at all. Thorium breeders produce U232, a highly radioactive and dangerous isotope, which can’t be conveniently separated from U233, complicating the thorium fuel cycle. Finally, the plutonium that comes out of nuclear reactors designed for power production, known as “reactor grade” plutonium, contains significant quantities of heavier isotopes of plutonium in addition to Pu239, making it unsuitable for weapons production.
Apparently the author gleaned some further disinformation for Seth Grae, CEO of Lightbridge, a Virginia-based company promoting thorium power. He supposedly told her that U233 produced in thorium breeders “fissions almost instantaneously.” In fact, the probability that it will fission is entirely comparable to that of U235 or Pu239, and it will not fission any more “instantaneously” than other isotopes. Why Grae felt compelled to feed her this fable is beyond me, as “instantaneous” fission isn’t necessary to prevent diversion of U233 as a weapons material. Unlike plutonium, it can be “denatured” by mixing it with U238, from which it cannot be chemically separated.
It’s a mystery to me why so much nonsense is persistently associated with discussions of thorium, a potential source of energy that has a lot going for it. It has several very significant advantages over the alternative uranium/plutonium breeder technology, such as not producing significant quantities of plutonium and other heavy actinides, less danger that materials produced in the fuel cycle will be diverted for weapons purposes if the technology is done right, and the ability to operate in a more easily controlled “thermal” neutron environment. I can only suggest that people who write popular science articles about nuclear energy take the time to educate themselves about the subject. Tried and true old textbooks like Introduction to Nuclear Engineering and Introduction to Nuclear Reactor Theory by John Lamarsh have been around for years, don’t require an advanced math background, and should be readable by any intelligent person with a high school education.