The Reactor Zoo – Part 2: Thorium and Other Breeder Reactors
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The Reactor Zoo – Part 2: Thorium and Other Breeder Reactors

By Dr. Zoomie

Hey Dr. Z – I’ve seen some headlines about thorium reactors; it looks like China and India are starting to build them and the articles make them sound pretty cool. Are they really that good? And how come the US isn’t making them?

This is a really interesting topic, and thanks for bringing it up! Not only that, but it’s also related to the subject of “breeder” reactors as well – nuclear reactors that produce more fissionable materials than they produce. So let’s start with how breeder reactors (including thorium reactors) work and we’ll see where we go from there. We’ll start with how to get energy out of thorium.

We’ll start with something that sounds a little unexpected – thorium won’t fission. With very minor exceptions, all of the thorium on Earth is the isotope Th-232, and Th-232 won’t fission. And, given that, one might wonder how thorium reactors work at all. The thing is – they won’t if all we’ve got is thorium. But if we can expose the thorium to neutrons then it’ll capture some of those neutrons – Th-232 captures a neutron to become Th-233, which is unstable and decays to form U-233, and U-233 fissions quite nicely – very similar to the process by which breeder reactors can produce fissile material. So if we can only expose some thorium to a neutron flux, we can produce U-233, which makes a great reactor fuel. Yay! But…where do the neutrons come from?

Well…they come from fission – we start with a regular uranium fission reactor that’s surrounded by thorium and, as the uranium fissions, it produces scads of neutrons. Some of these neutrons go on to cause other uranium fissions, some of them escape the reactor or are absorbed by the reactor itself, and most of them are absorbed by Th-232 atoms to eventually form U-233. Once there’s enough U-233 formed, it starts to fission and the reaction can then proceed using U-233 and U-235 is no longer required. What makes this so great is that there’s about four times as many thorium atoms (the vast majority of which are Th-232) on Earth as there are atoms of uranium – and every single Th-232 atom can become fissionable (as opposed to only one of every 139 atoms of uranium). This means that it’ll take centuries to run out of thorium atoms, even though we could run out of uranium in just a century or two. Thorium reactors can power our civilization for centuries – and without emitting carbon dioxide.

Thorium reactors have other advantages as well. For instance, unlike uranium reactors, thorium plants don’t produce plutonium that could be used to make nuclear weapons, they don’t produce as much radioactive waste, and they’re more inherently resistant to meltdowns than are uranium reactors. This last point warrants a little more explanation.

What makes most nuclear reactor designs susceptible to melting down is the use of water to slow down neutrons (this is called moderation) as well as to transfer heat from the reactor fuel to form steam to produce power. Water is an excellent moderator, but it has to be kept at a high pressure to transfer heat effectively without boiling. And that’s the problem – if the water leaks away the reactor shuts down, but the fuel heats up and melts down and, at such high pressures, even small leaks can quickly become serious. Some of the common thorium reactor designs, however, use a molten salt to cool the core; the salt doesn’t need to be kept at high pressure to remain liquid, so it’s a lot easier to contain; instead of having to withstand a few thousand pounds of pressure per square inch it only has to withstand pressures just slightly higher than atmospheric. This means that the reactor plant itself doesn’t need to be as ruggedly built and that, if there is a leak, the coolant will come out more slowly owing to the low coolant pressures. This greatly reduces the risk of a meltdown if the reactor springs a leak. 

I mentioned earlier that there’s more thorium in the Earth than there is uranium; what I didn’t mention is that thorium is less likely to form minerals so there are fewer rich thorium deposits. On the other hand, thorium is commonly found in as a contaminant in ores for the rare earth elements that are so important for the electronics industry – the fact that rare earths are so valuable means that it’s economically viable to mine the rocks for the rare earths, and since the minerals have already been mined and brought to the processing plant, recovering the thorium is also economically viable.

With all of these advantages, one might wonder why the entire world hasn’t been using thorium instead of uranium from the start.

Part of the answer to that question is more surmise than documented fact, and it goes back to the very first years of nuclear power in the US. According to this line of thinking, it comes down to Admiral Hyman Rickover – the irascible founder and head of the Naval Nuclear Power program for over three decades. Rickover, the thinking goes, had decided on reactors using highly-enriched uranium for fuel, and thorium reactors didn’t fit into his vision of naval nuclear power. Rather than have thorium reactors competing with uranium for development dollars, Rickover settled on uranium and stifled funding for anything else. On top of that, uranium reactors produce plutonium (specifically, Pu-239) that could be used in nuclear weapons – here, too, thorium reactors were of no help, and their development is thought to have been suppressed (or, at least, discouraged).

Thinking has changed over the last few decades and a number of nations – India and China are leading the field – are beginning to take a serious look at thorium reactors. China has built a testbed reactor and is planning a full-sized plant after a few years of testing (assuming the testing is successful). A few other nations – the US, UK, and Japan among them – have dabbled with the idea a bit, but thus far, none have gone beyond the testing stage.

Right now it’s just too early to tell if thorium-based reactors will be as good as they appear – we’ll learn a lot from China’s small-scale testing over the next few years and we’ll learn even more as reactors come online and we get more experience with them. Let’s cross our fingers and hope they live up to their promise.