More Thorium Silliness

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.

Author: Helian

I am Doug Drake, and I live in Maryland, not far from Washington, DC. I am a graduate of West Point, and I hold a Ph.D. in nuclear engineering from the University of Wisconsin. My blog reflects my enduring fascination with human nature and human morality.

8 thoughts on “More Thorium Silliness”

  1. I did a piece on my blog about the new push to use thorium in nuclear power plants in which I cited your earlier post on the subject.

    I was taken aback by the response that I received from most of the thorium advocates in America. A post referencing my article on the Energy From Thorium’s Facebook page and a tweet from Kirk Sorensen put my hit count into high orbit.

    Lots of visitors made their way to Helian Unbound from blog in the first ten days of September if you noticed an uptick.

    The difference between their take and yours seems to be the reactor itself. They insist that liquid fluoride technology was perfected at Oak Ridge in the ’50s.

    I have already pleaded “over my paygrade” but you are an engineer and I count beans.

  2. The source of the inaccuracy is Thorium’s promise and for those who have a better understanding of the Molten Salt Reactor the mistakes are made from being new to nuclear physics. But the 200 figure is low if you compare Uranium in Light Water Reactors to the Molten Salt Reactor process that uses up nearly all of the fuel that is put as opposed to 5%. Much of the success of LFTR design is due to reprocessing which is inherently part of the design and entropy in a liquid state goes a long way

  3. If I wanted to compare apples and oranges the way you do, I could make the figure 500 to 1, or 1000 to 1. I could also make the numbers just as proportionate in the other direction. Recall what the article actually said. I quote: “Thorium has nearly 200 times the energy content of uranium without creating plutonium—an ingredient for nuclear weapons.” That statement, which makes no reference to any specific technology, is false. It makes as much sense as saying that, because I can build a diesel fueled turbine with an efficiency of 30%, and a poorly designed gasoline piston engine with an efficiency of 5%, therefore the energy content of diesel is six times that of gasoline.

  4. @gadfly

    “The difference between their take and yours seems to be the reactor itself. They insist that liquid fluoride technology was perfected at Oak Ridge in the ’50s.”

    Thanks for sending some traffic my way. As I’ve always said on my blog, I support the reasonable development of thorium technology. I don’t have a problem with thorium, but with factually innacurate statements about it, and with the marked tendency of some of its advocates to pretend that there are no proliferation issues with U233. The weapons issue is not a show stopper, but it would be very unwise to ignore it.

  5. Helian, I sincerely hope you never have to make a living selling anything.

    We don’t need thorium. There is plenty of uranium, and the techniques for refining, enriching, and using it, and getting rid of the waste, are well understood. They need a lot of engineering refinement, which they are not getting because nuclear power is not widely deployed because it is politically impossible due to demagogues preying on the emotions of the people who have to pay and vote for it.

    But if thorium is the new New NEW IMPROVED nuclear power! Now with less proliferation! Burns its own waste and the old waste too! And ABSOLUTELY NO CARBON DIOXIDE! Only $14.95 plus $5.95 shipping and handling, and you too… oops, sorry, got carried away. Anyway, you get the drift. Yes, thorium power has some new problems, but that’s the whole point — it solves some of the old problems, or can be sold that way, and the new problems will be taken care of in due time. There’s no need to dwell on them, and in fact it’s a bad idea to dwell on them. The point is to re-sell nuclear power, so we don’t freeze in the dark to make the Greenies feel good.


  6. It would be nice if we could find a salesman good enough to get the industry back on its feet.

  7. Helian,

    I would like to know what you think regarding Seth Grae’s recent proclamation before the National Press Club that Lightbridge (formerly Thorium Power) has made a “transformational” discovery wherein their all metal fuel rods can produce 30% more output in a conventional reactor. I am starting to think that this guy, Seth Grae, is a snake oil salesmen.


  8. It may be that the term “transformational” is a bit of a stretch. However, its use by someone like Grae might be excusable as “poetic license.” Keep in mind that government is a potential source of funding for Lightbridge. Research proposals to federal agencies are unlikely to be funded if the work suggested therein is described as “incremental” or “workmanlike.” A plausible source of funding for Lightbridge might be ARPA-E, DOE’s version of DARPA, which specializes in funding high-risk, high-payoff research. Research announcements from such agencies often positively demand that any proposed work be potentially transformational. In other words, occasionally such hyperbole is excusable if your chances of getting funded, and therefore your survival, depend on it.

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