Subcritical Thorium Reactors: Dr. Rubbia’s Really Bad Idea

The Telegraph (hattip Insty) turned the hype level to max in a recent article about the potential of thorium reactors.  According to the headline, “Obama could kill fossil fuels overnight with a nuclear dash for thorium.”  Against all odds, this is to happen in three to five years with a “new Manhattan Project,” and a “silver bullet” in the form of a new generation of thorium reactors.  The author is so vague about the technologies he’s describing that it’s hard to avoid the conclusion that he simply doesn’t know what he’s talking about, and couldn’t be bothered to spend a few minutes with Google to find out.  I’ll try to translate.

It’s claimed that thorium “eats its own waste.”  In fact, thorium is very promising as a future source of energy, but this is nonsense.  Apparently it’s based on the fact that certain types of thorium reactors actually could burn their own fuel material, as well as plutonium scavenged from conventional reactor waste and other transuranics, much more completely than alternative designs.  This is certainly an advantage, but the fission products (lighter elements left over from the splitting of uranium and plutonium) would still be highly radioactive, and would certainly qualify as waste.  Such claims are so obviously spurious that they play into the hands of opponents of nuclear power.

It is also claimed that “all (thorium) is potentially usable as fuel, compared to just 0.7% for uranium.”  In fact, thorium is not a fissile material, meaning that, unlike uranium 235 (U235), which is the 0.7% of natural uranium the author is referring to, it cannot sustain a nuclear chain reaction on its own.  It must first be converted to a lighter isotope of uranium, U233, which is fissile.  In fact, the U238 that makes up most of the rest of the leftover 99.3% percent of natural uranium is “potentially usable as fuel” in that sense as well, by conversion to plutonium 239, also a fissile material.

The author is vague about exactly what kind of reactors he is referring to, lumping Dr. Carlo Rubbia’s subcritical design, which depends on a proton accelerator to provide enough neutrons to keep the fission process going, and molten fluoride salt reactors, which do not necessarily require such an accelerator.  He claims that, “Thorium-fluoride reactors can operate at atmospheric temperature,” which they certainly could not if the goal were to generate electric power.  I suspect that what he means here is that, unlike plutonium breeders, which require a high energy neutron spectrum to produce more fuel than they consume, thorium breeders could potentially use “thermal” neutrons that have been slowed to the point that their average energy, when converted to a “temperature,” would be much closer to that of the other material in the reactor core. 

In any case, the design he seems to be so excited about is Dr. Rubbia’s “energy amplifier,” which, as noted above, would be subcritical, requiring a powerful, high current proton accelerator to keep the fission process going.  It would do this via spallation, a process in which a copious source of the neutrons required to keep the reaction going would be provided via interaction of the protons with heavy nuclei such as lead, or thorium itself.  This is the process used to produce neutrons at the Oak Ridge Spallation Neutron Source.  Such reactors could easily be “turned off” by simply shutting down the source of neutrons.  However, the idea that they would be inherently “safer” is dangerously inaccurate.  In fact, they would be an ideal path to covert acquisition of nuclear weapons.  Thorium reactors work by transmuting thorium into U233, which is the isotope that fissions to produce the lion’s share of the energy.  It is also an isotope that, like U235 and Pu239, can be used to make nuclear bombs. 

The article downplays this risk as follows:

After the Manhattan Project, US physicists in the late 1940s were tempted by thorium for use in civil reactors. It has a higher neutron yield per neutron absorbed. It does not require isotope separation, a big cost saving. But by then America needed the plutonium residue from uranium to build bombs.

“They were really going after the weapons,” said Professor Egil Lillestol, a world authority on the thorium fuel-cycle at CERN. “It is almost impossible make nuclear weapons out of thorium because it is too difficult to handle. It wouldn’t be worth trying.” It emits too many high (energy) gamma rays.

What Lillestol is referring to is the fact that, in addition to U233, thorium reactors also produce a certain amount of U232, a highly radioactive isotope of uranium with a half life of 68.9 years whose decay does, indeed, release potentially deadly gamma rays.  It would be extremely difficult, if not impossible, to remove it from the U233, and, if enough of it were present, it would certainly complicate the task of building a bomb.  The key phrase here is “if enough of it were present.”  Thorium enthusiasts like Lillestol never seem to do the math.  In fact, as can be seen here, even conventional thorium breeders could be designed to produce U233 sufficiently free of U232 to allow workers to fabricate a weapon without serious danger of receiving a lethal dose of gamma rays.  However, large concentrations of highly radioactive fission products would make it very difficult to surreptitiously extract the uranium, and it would also be possible to mix the fuel material with natural or depleted uranium, reducing the isotopic concentration of U233 below that necessary to make a bomb.

With subcritical reactors of the type proposed by Rubbia, the problem of making a bomb gets a whole lot easier.  Rogue state actors, and even terrorists groups if we “succeed” in coming up with a sufficiently inexpensive design for high energy proton accelerators, could easily modify them to produce virtually pure U233, operating small facilities that it would be next to impossible for international monitors to detect.  There are two possible pathways for the production of U232 from thorium, both of which involve a reaction in which a neutron knocks two neutrons out of a heavy nucleus of Th232 or U233.  Those reactions can’t occur unless the initial neutron is carrying a lot of energy as can be seen in figure 8 of the article linked above, the threshold is around 6 million electron volts (MeV).  That means that, in order to produce virtually pure U233, all that’s necessary is to slow the incoming spallation neutrons below that energy.  That’s easily done.  Imagine two billiard balls on a table.  If you hit one as hard as you can at the other one, what happens when they collide?  If your aim was true, the first ball stops, transferring all its energy to the second one.  The same thing can be done with neutrons.  Pass the source neutrons through a layer of material full of light atoms such as paraffin or heavy water, and they will bounce off the light nuclei, losing energy in the process, until they eventually become “thermalized,” with virtually none of them having energies above 6 MeV.  If such low energy neutrons were then passed on to a subcritical core, they would produce U233 with almost no U232 contamination. 

It gets worse.  Unlike Pu239, U233 does not emit a lot of spontaneous neutrons.  That means it can be used to make a simple gun-type nuclear weapon with little fear that a stray neutron will cause it to fizzle before optimum criticality is reached.  And, by the way, a lot less of it would be needed than would be required for a similar weapon using U235, the fissile material in the bomb that destroyed Hiroshima. 

We’re quite capable of blowing ourselves up without Rubbia’s subcritical reactors.  Let’s not make it any easier than it already is.  Thorium reactors have many potential advantages over other potential sources of energy, including wind and solar.  However, if we’re going to do thorium, let’s do it right.

UPDATE:  Steven Den Beste gets it right at Hot Air.  His commenters throw out the usual red herrings about the US choosing U235 and Pu239 over U233 in the Manhattan Project (for good reasons that had nothing to do with U233’s suitability as a bomb material) and the grossly exaggerated and misunderstood problem with U232.  You don’t have to be a nuclear engineer to see through these fallacious arguments.  The relevant information is all out there on the web, it’s not classified, and it can be understood by any bright high school student who takes the time to get the facts.

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.

18 thoughts on “Subcritical Thorium Reactors: Dr. Rubbia’s Really Bad Idea”

  1. This is really interesting and is the first informed article I’ve read after reading the Telegraph’s too-good-to-be-true report. Thank you for explaining this.

    However, I’m left hanging at the end of your piece. “Thorium reactors have many potential advantages… However if we’re going to do thorium, let’s do it right.”

    Is there a “right” way to do it? Did I miss this explanation in the piece?

  2. As I said (pingback above) technical matters don’t seem to me as important as political ones.

    Opposition to nuclear power was the signature issue that brought the environmentalist movement together. That is and always was based on a committed core manipulating ignorant people with genuine (if overblown) concerns. The core can be divided into two groups: watermelons, originally financed by the USSR but now having achieved criticality, who know better or don’t give a damn but are violently opposed to any energy source that would allow the USA to keep working; and people who genuinely believe in and worry about the real, if overblown, concerns.

    Many of the latter group are also genuinely concerned about CO2 and global warming — that is, their worries are real to them, whatever the truth of the matter. They might well be convinced to follow Stewart Brand to the conclusion that nuclear power, however bad, is less bad than frying the penguins by emitting CO2, and that could be made easier if we could present a nuclear technology that can be seen as having fewer inherent dangers.

    Dr. Rubbia’s design is valuable for that political effort because it eliminates the possibility of “China Syndrome”, which I can tell you from experience is a genuine fear among ignorant people, both those who consider themselves “green” and those who don’t but like to worry. If something goes wrong, turn off the accelerator and the reaction stops. You and I both know that this is only technically true, but meltdown is only technically possible, too, and the failure modes support the “thorium is safe” myth — failure of the control rod mechanism gives meltdown; failure of the accelerator stops the process.

    Many of the saner environmentalists don’t follow Brand out of fear that changing their minds on something so basic to the movement’s origins would open them to challenges on other fronts, and that fear is justified. Thorium could give those people some cover: “Look, we won! We made them change the technology to something safer!” That could open a crack between them and the watermelons that a real nuclear program might be shoved through. Without ignoring the real dangers, objections should be, where possible, phrased in such a way as to allow that possibility. It’s political, not technical or scientific.


  3. Instead of a spallation source would a lower energy neutron source as a 2 Mev proton beam into Lithium target work as well? A dynamitron type resonant voltage source coupled with a reliable high current H- ion source is a potentially much cheaper souce of neutrons.

  4. Ric,
    Subcritical reactors seem a very risky way to pursue political goals. I would prefer to go with conventional breeders, while continuing to educate people concerning the health and environmental hazards of not going nuclear.

    Subcrits require a very high intensity neutron source, and you can get a very high gain with 1 GeV protons. I assume Rubbio and the rest of the subcrit promoters have looked at the alternatives and done the math. It may be they are counting on substantial direct fissioning of Th232 itself to increase the gain, in which case they would need a hotter neutron spectrum. I don’t know enough about what the fission cross section of thorium looks like above the threshold of around 1 MeV to say for sure.

  5. I notice you did not mention the much shorter radiation lifetime of the liquid fluoride thorium reactor. Do you consider Nuclear waste unimportant?

  6. I do agree that reporters are basically to be trusted with nothing 🙂 and that the subcritical reactor will not solve all of our problems overnight. Nonetheless I feel that your criticism of the actual concept is, in places, unfounded:

    1) “Ambient temperature” must be a misprint in your source. What is meant is ambient pressure, which is indeed a big advantage, as it eliminates the expensive pressure vessel and the looming risk of a steam explosion.

    2) Fuel cycle – The thorium cycle is more efficient, not because it can use burnt up fuel from other reactors, but because it has a better neutron economy. I will not go into specifics here, but it is expected that the thorium reactor will use about 10 times less U235 (actual fissile material) than a conventional LWR.

    3) Proliferation – One could use an accelerator to produce bomb material, but it’s in fact very inefficient. To make just one bomb, you would need a huge accelerator (a few hundred meters) which would cost a lot of money and have the heat signature of a small power station. Not something you can do unnoticed in your garage and on a shoestring budget.

    4) Proliferation 2 – If you try to steal bomb material from a civilian reactor that has been running for a couple of years, it would be contaminated with other uranium isotopes. This stuff will definitely not be useful for a gun-type device. Also, there will not be very much material, as the reactor doesn’t produce “surplus” fissile material. It does breed most of its own fuel, but it still needs a (small) net input of U235.

    All in all, i think the subcritical reactor is a good idea, though possibly uneconomical.

  7. While a LFTR would use thermal neutrons, that’s not what the Telegraph article meant: Michael is correct that the Telegraph article is simply in error about “atmospheric temperature” when they meant “atmospheric pressure”; you can see that it’s wrong because in the same paragraph it quotes Kirk Sorensen as saying “You wouldn’t need those huge containment domes because there’s no pressurized water in the reactor.”

  8. You’re misinterpreted a fair bit of the information. The 0.7% refers to the amount of U235 fuel in the fuel rods actually used in one fuel cycle, the rest requires that the fuel rod be reprocessed at a reprocessing plant similar to what France has. It’s actually 0.5% with US light water reactors and 0.7% with Canadian Candu reactors, it’s just a coincidence that it corresponds with the 0.7% of natural Uranium that happens to be U235.

    As the US never built their reprocessing plant, the remaining U235 in their spent fuel rods are transuranic nuclear wastes which must be stored for 100,000 years before being safe. What a molten salt reactor ( not necessarily a Thorium reactor ) brings is continuos fuel reprocessing allowing over 99% of the fuel to be used and the remaining nuclear waste is 1/10,000 the amount of the current transuranic waste and only needs to be stored for 300 years.

    I’m not certain if the UK and European proposals of a sub-critical Thorium reactor makes a lot of sense because criticality has never been a problem with nuclear reactors, it’s always been latent heat that’s a problem and a molten salt reactor avoid both issues by having passively cooled drain pans so that any loss of cooling results in not only a reduction of criticality but a shutdown of latent heat. A subcritical reactor requires a fair bit of power for the neutron bombardment for the purposes of a safe shutdown which already exists with a molten design. They seem to be bending over backwards to avoid molten reactors.

    I think your criticisms are misplaced.

  9. I think you’ve misinterpreted my post. I don’t have a problem with molten salt thorium reactors. My problem is with Dr. Rubbia’s subcrits. You’re right, thorium breeders are potentially an excellent solution to the transuranic problem, and, assuming efficient reprocessing, it makes sense to claim that, with such reactors, there will be less radiation from the residual waste than from the original ore after, say, 1000 years. Fission products will still be a problem for several hundred years, though, and any implication that thorium breeders would have no waste problem would be counterproductive. Molten salt reactors have many other potential advantages, and the main drawback at this point seems to be coming up with new alloys that can deal with the potential corrosion problems. That should hardly be a show stopper, though. In short, I don’t object to thorium breeders per se. I do object to the excessive hype that sometimes goes along with them.

    As far as the original topic of my post is concerned, the main point here is that, while “conventional” molten salt thorium breeders need not pose an excessive proliferation risk, the same is most definitely not true of the technology Dr. Rubbia is proposing if it becomes generally available.

  10. Here’s a much simpler and more feasible idea.

    Why don’t we just shut the fricking things down, RIGHT NOW and reduce our energy consumption to suit, and put some proper effort into finding better solutions to our energy needs, or political solutions to our unrealisticly profligate lifestyles?

    I bet you’d all rather carry on using the risky nuclear stuff wouldn’t you?

    Tahnks for providing a platform for my insane idea, I’ll go back to my quiet mumbling now.

  11. Ill say that people at LANL, where I worked in Neutron science, were trying to build one of these before it was “Rubbia’s idea,” long ago. They had plans up in the 70s. They built LANSCE, and they wanted to build another sister accelerator to drive to world’s first subcritical reactor. Everyone I talked to in neutron science thought the idea was very smart. They have all the transmutation tables, energy spectrums of every fissionable isotope, and every important isotope’s half life and decay methods in the back of their heads, and they thought this was a great idea.

    I think that says something.

  12. Regarding the part about atmospheric tempratures, he in all likelyhood was speaking about atmospheric pressure. LWR need to be kept at very high pressure inorder for the water to remain liquid. Metal cooled reactors can be operated at standard pressure.

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