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.