Back to thorium – the thorium cycle and non-proliferation

University of New Mexico Center for Nuclear Non-Proliferation Science and Technology (

At one point John Kennedy predicted there might be over 20 nuclear powers by the mid-1970s – one of the triumphs of the Non-Proliferation Treaty is the fact that, as of the year 2000, there were fewer than 10 and only North Korea was added to the total since then. But we know that the A Q Khan network was selling nuclear weapons technology to anyone with a checkbook – we’re still not sure exactly who his clients were, but even one would be too many. And we also know that the US developed a nuclear weapon with 1940s-era technology – every nation on Earth now has access to the level of knowledge and technology adequate to design their own nuclear weapons.

Uranium enrichment is one way to produce fissionable materials, but it’s not the only method – plutonium also explodes quite nicely and plutonium production is not very hard to do. In fact, every operating nuclear reactor produces plutonium; a significant fraction of the power produced by our nuclear reactors comes from the fission of plutonium that’s produced in the core during normal operations. This means that, with very few exceptions, every nuclear reactor on Earth produces plutonium and the spent fuel from these reactors contains this plutonium – with some chemical processing this plutonium can in theory be extracted and made into a nuclear weapon. This is one of the downsides of nuclear energy – the spent fuel is not only intensely radioactive, but the plutonium it contains must also be safeguarded. This is one of the trade-offs of nuclear energy – carbon-free baseload power and plutonium. One of the advantages of the thorium fuel cycle is that is it more proliferation-resistant than the more typical uranium cycle – let’s see why.

A quick recap – in a “conventional” nuclear reactor the uranium fuel holds in the neighborhood of 5% fissionable U-235 and the other 95% or so is U-238. In the neutron-rich environment of the reactor core the U-238 atoms capture a neutron to become U-239 and, a few days to weeks later, the U-239 decays to form Pu-239 – the stuff of which bombs can be made. This means that 95% of “conventional” reactor fuel has the potential to become plutonium and the plutonium can be chemically separated from the uranium to be made into weapons. By comparison, a thorium-powered reactor uses neutron capture to turn Th-232 into U-233, which is what fissions. And this is where things get a little interesting.

First, U-233 is about as fissile as Pu-239 – there’s no getting around the fact that a thorium-cycle nuclear reactor produces material that can be made into nuclear weapons. What makes the thorium cycle more proliferation-resistant is that there are some kickers.

One of these is that the thorium cycle not only produces U-233, but also U-232 and over time U-232 decays to stability through a slew of other nuclides. Some of these nuclides emit gamma radiation and one, the thallium-208 gamma – is a whopper with an energy of 2.6 million electron volts (by comparison, visible light photons have energies of several electron volts, x-ray energies are typically in the tens of thousands of eV (keV), and even most gamma rays have energies of in the hundreds of keV). As the U-232 ages, the radiation from its progeny will increase – it can actually become increasingly dangerous to work with as time goes on. Not only that, but these high-energy gammas are hard to hide – they are so penetrating that they’ll punch through standard radiation shielding.

OK – so why not just separate the U-233 a nuclear weapons program would want from the U-232 that they don’t want? The big reason is that U-232 and U-233 are chemically identical (unlike plutonium) so removing the U-232 poses the same challenges as uranium enrichment – in effect, a nation trying to use the thorium cycle to produce nuclear weapons would have to face the technical challenges of both uranium enrichment and running nuclear reactors. It just doesn’t make sense to pursue this route to a nuclear weapon. It’s possible, of course, to chemically remove the decay products that produce the gamma radiation, but it’s just going to keep coming back as long as there’s any U-232 present; with a half-life of nearly 70 years the U-232 is just not going to go away anytime soon. Another easy-to-take step can help to reduce the proliferation threat even further – adding some U-238 to the mix to make it even more difficult to produce something that will go boom. And, again, the fact that U-232, U-233, and U-238 are both chemically identical means that separating the U-238 and U-232 from the U-233 still requires uranium enrichment. The bottom line is that using the thorium cycle to produce the material for nuclear weapons is dangerous and difficult, it’s easy to thwart, and it’s hard to hide the weapons that are produced.

Of course there’s another route from thorium to a nuclear weapon – trying to breed U-235 or Pu-239 by successive neutron capture. The problem here is that a single neutron capture is not necessarily a likely event; the odds that an atom to capture the six neutrons required to turn into Pu-239 is vanishingly small. Of course it’s easier (and more plausible) to capture two neutrons to become U-235 but, again, there’s the same problem with separating U-235 from the rest of the uranium. So this route is also a non-starter.

So let’s put this together with some other things that have been happening. In spite of the concerns raised by the Fukushima accident, many nations are continuing to go forward with their nuclear energy plans, in addition to the reactors being built by Iran and North Korea. To some extent it doesn’t matter whether these nations are friendly or not – conventional nuclear reactors produce plutonium as a byproduct of normal operation. Nations we don’t trust (e.g. Iran and North Korea) can separate the plutonium from their spent fuel (and terrorist groups can try to seize the spent fuel to separate the plutonium). The bottom line is that any reactor fueled with low-enriched uranium poses a potential proliferation risk and that the risk from reactors fueled with U-233 that has been bred from Th-232 is far lower.

Finally, I have to admit that when I first started looking into this particular topic I was somewhat dubious that thorium would live up to the claims of the pro-thorium crowd in this particular area. I should add that I wasn’t necessarily dubious that thorium posed a lower proliferation hazard than uranium, I just wasn’t sure that it would live up to the hype. But as I looked into it – especially as I dug into the likelihood of multiple neutron capture and the gamma radiation emitted by the U-232 decay series nuclides – I realized that thorium-cycle reactors are every bit as proliferation-resistant as claimed. In a world in which we worry about both nuclear weapons detonated in anger and about global warming it seems that thorium-cycle reactors offer a viable approach to addressing both of these concerns.

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8 Responses to “Back to thorium – the thorium cycle and non-proliferation”

  1. Martin Burkle August 31, 2012 at 8:44 AM #

    Here is the real proliferation problem with LFTR –
    However looking at the aspects of protactinium separation, I’m wondering if this could be a hole in the process which would allow for much lower U-232. U-232 is the daughter product of Pa-232 just as U-233 is the daugher of Pa-233. Pa-233 has a half-life of 26.9 days but Pa-232 is only 1.3 days.

    This seems as if it could cause a problem. Basically if you separate the protactinium and let it decay for about eleven days, for example, you’ve gone through eight half-lives of Pa-232 but less one half of a halflife cycle of Pa-233. Thus you still retain about three quarters of the Pa-233 you started out with but the Pa-232 has been diminished to less than half a percent of what you started with. You could do it for even longer before you start to loose a lot of the Pa-233.

    Thus, at this point you could do the process over again, removing the uranium and retaining the protactinium and you would have a very high concentration of Pa-233 and very little Pa-232, which is where the U-232 would come from. This is not very difficult and could easily be done with what is available. The result is basically an easy source of weapons grade U-233.
    From –

    • Toby September 1, 2012 at 3:32 PM #

      Good point, though the use of a single salt design would eliminate the chemical processing of freshly irradiated thorium where Pa233 could be extracted. Another option would be to configure the reactor to a narrow breed/fission ratio causing reactor failure should any fissile material be diverted for nefarious reasons.
      A rogue state would always be able to produce a bomb if determined enough but then again, that risk is now out there anyway as a result of 60 years of uranium power.

  2. george August 31, 2012 at 12:34 PM #

    After reading the article, my question about thorium: is the handling and disposal of the u232 byproducts of thorium less costly and toxic; to house, maintain, and store that theby products of a traditional u239 reactor?

    • Toby September 1, 2012 at 3:10 PM #

      In a molten salt reactor, the U232 byproducts along with any other actinides (the long lived ‘nasties’ of conventional waste) need never be removed from the reactor, the liquid fuel allowing the removal of fission products only. The actinides will successively undergo neutron captures until fissioning themselves. Therefore the only waste to deal with would be fission products which decay rapidly to stable matter

    • Kim L Johnson September 2, 2012 at 3:04 AM #

      The “actinide” elements like Th, Pa, U, etc are *not* the “waste” products in an MSR (Molten salt reactor). An MSR’s products are Energy (tons of Heat, internal radiation that get converted to heat) and Fission Products. FPs are lighter atoms like Strontium, Xe, I, Cs, etc and in a fluid MSR most FPs are easy to remove on the fly (w/o shutting down).
      MSRs turn Th-232 into U-233, which in turns fissions >92% of the time. Those <8% of 233 atoms that fail to split become U-234 and rapidly absorb n's to turn in U-235, which fissions 85% of the time. The ~1% that make it to U-236 eventually turn into Neptune-237, and Np is easy to remove chemically from MSRs and it the only precursor that exists for NASA-invaluable Pu-238.

      The gamma-nasty U-232 is simply left in the MSR (along all other atom weighing 232, 233, 234, etc), where it has a strong affinity to absorb an n, become U-233 and then fission. U232 is only a prob. when you take U233 out of the reactor, which would promptly starve it for fuel and shut it down.

      Hopes this helps! (for more on Molten Salts, check out

  3. chris August 31, 2012 at 9:34 PM #

    The issue that causes nuclear waster from traditional Light Water Reactors to need to be stored is the presence of Transuranic elements, elements heavier that Uranium that have very long half-lives.
    Since most of the neutrons in a LFTR are used to make U-233, and most of the remainder to add 2 more to get U-235, the amount of stuff heavier than Pu-238 is very, very small.

    One thing the article failed to mention is the different isotopes of Plutonium. Pu-239 is the fissile (bomb-worthy) material but unless the reactor is made very precisely, most of the Plutonium eats another neutron to make Pu-240 which isn’t fissile. Weapons grade Plutonium is about 95% Pu-239 content. There’s a theoretical chance that low grade Plutonium could be enriched much like Uranium is (different process, same concept) that drives this concern over proliferation.

  4. Justin Alexander September 1, 2012 at 3:36 PM #


    Short of it is, that removing the 232Pa is harder than it sounds, and even if you do even small traces of 232U are going to be a big problem in a bomb.

  5. Cavan Stone September 2, 2012 at 5:15 PM #

    Hi Martin,

    To touch on your protactinium comment, preventing this method of proliferation has already been addressed in the original Oak Ridge documents. 1st it is important to realize that Thorium MSR reactors are not a single design but a class of designs with various options to tailor to specific applications. That being said, these are the two design options that would make protactinium diversion impractical.

    First, protactinium separation is an option, NOT a requirement for MSR designs. It’s nice to have for neutron economy but you can easily run designs without it. Furthermore a little bit of U-232 goes a long way in frustrating weapon construction efforts. Without the needed chemical separation equipment on site, an adversary would have to either covertly divert the Pa or import on a closely monitored site a large amount of equipment needed with the purity required under the time constraints.

    Second, and even more crucial, is in at-risk countries, running designs with Iso-breeding. You can run designs tuned such that nearly every bit of Pa-233 is needed to keep the reactor running, converting that Th-232 into Pa-233. If an adversary where to divert even just a little Pa-233, the breeding reaction and thus the reactor itself would automatically shut down, quite loudly announcing the diversion effort. The only way around this is to feed into the reactor the very fissile material our hypothetical proliferator is trying to extract.

    Finally a note about where I think the goal posts lie. Given a hypothetical, non-existent adversary with unlimited resources any anti-proliferation system will fail and any technology nuclear AND non-nuclear can be converted into a weapon. Absolute safety is impossible and people get into there car everyday in spite of the risk of an accident.

    Now, if you review the literature, I am confident you’ll find that the science behind Thorium MSRs clearly demonstrates a highly significant reduction in proliferation risk relative to our present situation and the amount of effort required to make a weapon out of Thorium MSRs very clearly dwarfs the present methods countries like Iran and North Korea are using to make weapons in spite of the dedicated efforts of the world community to prevent them from doing so.

    My prime criticism of proliferation-based MSR opponents is that they let a non-existent perfect be an enemy of the magnitudes better. This especially true when after reviewing the literature, it is quite apparent that Thorium MSRs can eliminate the number one reason for modern era wars in the first place, intense competition over limited and diminishing natural resources.

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