About a century ago, an adventurous Scandinavian discovered the first black thorium rock on a remote island in the Norwegian Sea. Now thorium is slowly heating up debates about the future of nuclear power, energy independence, and global warming.
I wanted to get to the bottom of this surge in nuclear enthusiasm, and I was inspired by Peter Thiel’s lecture on energy markets. This post encapsulates the surprising things I learned about thorium and nuclear reactors.
Thorium’s Bold Claim
The only naturally occurring radioactive elements that can be used to fuel nuclear reactors are uranium and thorium. uranium reactors are commonplace, but thorium reactors have been pretty experimental since the first one started up at Indian Point Energy Center, New York in 1962. Nearly all of the world’s power-producing reactors run on uranium or its derivatives.
This is where the debate heats up. Proponents of thorium argue that thorium reactors have big advantages over uranium-fueled reactors:
- Thorium is much more abundant on Earth than uranium.
- Thorium is cheaper to extract and prepare than uranium.
- Thorium isn’t very useful for making nuclear weapons.
- Thorium reactors are inherently stable, so “nuclear meltdowns” can’t happen.
- Thorium reactors produce dramatically less radioactive waste.
Opponents point out that thorium reactors are technically challenging, are still unproven, and still produce radioactive waste.
But these broad strokes paint thorium reactors as extremely promising, so it’s no surprise that India, China and the US are actively building experimental research reactors.
This all sounds tremendously exciting, but honestly it seems too good to be true. We need to answer a lot of questions about the apparent advantages of thorium.
Black Rocks, Red Sand, and Plain Old Dirt
The first black rock picked up by that Norwegian was composed largely of a mysterious new element, and so in honor of the Norse god Thor it was dubbed thorium. Marie Curie would later discover that it was radioactive.
But it turns out that thorium isn’t exclusive to snow-covered islands in Scandinavia. It’s actually surprisingly common: plain old dirt in your back yard has 6 parts per million (6 ppm) thorium. Large tracts of granite found throughout the world contain 13 ppm. And reddish Monazite sand, which contains 2.5% thorium, is abundant in specific places.
The important point is that thorium is ridiculously more abundant than uranium. And abundance matters when we’re talking about providing energy to the world. In fact, there’s so much thorium on Earth that the easily extractable reserves in the United States (10% of the world’s) could supposedly power the entire United States at current energy levels for the next 10,000 years. It’s not exactly renewable, but it’s a much longer lifeline than oil. And it can be mined safely within US borders: most of the US reserves are concentrated in a 25 square-kilometer pileup of mountains straddling the border of Idaho and Montana.
The rest of the world’s extractable thorium reserves are concentrated in India (25%) and Australia (10%), with the remaining 55% spread around nicely. It’s immediately obvious that the geopolitics of thorium are dramatically different than the drama surrounding oil in the Middle East.
Got Sand, Want Fuel
Once you’ve got some thorium-rich Monazite sand, there’s a straight-forward and well-known process for extracting high-purity thorium. You mix the sand with sulfuric acid, which gives a nice gray mud; wash that with cold water and treat with a couple more common chemical solutions, and boom your thorium precipitates right out as a nice sludge.
If you want fuel for a Molten Salt Reactor, you mix fluorine with the thorium to produce thorium tetrafluoride. Or if you want fuel for a solid fuel reactor, you have to sinter the thorium at very high temperature to produce thorium dioxide. The sintering process is complicated and expensive, but the fluorine process for Molten Salt Reactors is fast and relatively cheap.
The wonderful thing about thorium is that this pretty much wraps up the entire preparation process.
Let’s compare that to uranium.
Uranium is either extracted from the ground as ore and then leached for uranium metal, or the leaching can happen while the ore is still in the ground. The leached uranium solution goes through a similar chemical process to extract high-purity uranium oxide. But then things get complicated. uranium-235 and uranium-238 are chemically identical, but are different isotopes with correspondingly different nuclear properties.
Uranium-235 is fissile, which means that if a neutron comes near it, an atom of u-235 will split in two, releasing a bunch of energy and two additional neutrons. If you have more u-235 nearby, those two neutrons will create a nuclear chain-reaction and tada! you have runaway nuclear reaction… sometimes escalating to a nuclear meltdown or explosion depending on the situation.
But uranium-238 isn’t quite so excitable. u-238 is fissionable but not fissile, which means that you have to really slam a neutron into u-238 to split it apart. u-238 also releases a bunch of energy and a two neutrons, but those neutrons aren’t energetic enough to keep the reaction going. So, long story short: you need concentrated uranium-235 to keep the energy flowing in a reactor. Too much uranium-238 and your reactor will just cool down and stop.
And this is where nature gets us vexed. Naturally occurring uranium is 99.3% u-238 (not fissile) and 0.7% u-235 (fissile, very useful). In order to run a reactor, you need at least 3-5% u-235 and to make weapons you need 95% u-235. Separating u-235 and u-238 isotopes is enormously expensive, because they’re so subtly different. Large numbers of expensive, high-speed centrifuges or gas diffusion systems must be used to separate and remove uranium-238 (which is ever-so-slightly heavier) from the mixture. During this process, the uranium oxide is converted to uranium hexafluoride, and large amounts of u-238 hexafluoride (Depleted uranium) are produced. Depleted uranium is widely used in ammunition, armor, etc. but we have another 700,000 tons stockpiled and unused.
So preparing thorium is significantly easier and cheaper than preparing uranium, because you completely avoid the isotope separation step.
Now that we have our raw nuclear fuel, either thorium or uranium, we need to get a nuclear reaction going and convert the resulting energy into electricity. That requires a nuclear reactor, which is basically a carefully designed clustering of nuclear fuel that outputs lots of heat. That heat is used to create high-pressure steam, which drives massive turbines that output electricity.
There are many types of reactors, and each has a unique fuel cycle. But at its core, each reactor’s fuel cycle must include a fissile material, since that’s what keeps a nuclear reaction going.
The primary fissile materials are uranium-233, uranium-235, plutonium-239. What happened to thorium? Well, uranium-233 is bred from thorium. And where’d this plutonium come from? plutonium-239 is bred from uranium-238.
So this gives rise to the idea of breeder reactors. Breeder reactors are loaded with lots of fertile fuel like u-238 and thorium, and then get an initial boost from u-235. The thorium and u-238 then each get morphed into fissile elements, which generate the nuclear reaction that keeps the whole thing running.
Holy Smokes, Serious Engineering Starts Here
And this is where things start to get really murky and complicated. There are dozens of proposed or tested reactors of all different types, and each one has a different fuel cycle, waste profile, and cost. The waste profiles seem particularly complex to predict, and the costs associated even harder. After reading about many of the different designs I came to the unsurprising conclusion that nuclear engineering is, in fact, extremely complicated.
The goal, of course, is very cheap power. Where cheap takes into account the whole process, from initial conception, building the reactor, operating the reactor, all the way to properly handling the long-term waste produced.
How do these costs break down? And does thorium really reduce those costs? According to world-nuclear.org 14% of the cost of nuclear reactors is the fuel and 10% is waste disposal. The remaining 76% are construction and operating costs, mostly associated with high levels of training, safety and security required for the reactors. The cost of redundant safety systems in nuclear reactors is staggering. For example, a 400 MW coal fired plant in Arizona cost $1B (2010 dollars) to build, while a 2260MW nuclear station in South Carolina cost $15.6B (2010 dollars). In terms of construction costs, the nuclear plant was nearly 3 times as expensive. Typically this is due to financing overhead and the intense requirements for redundant safety and security. Keep in mind that the coal plants subsequently spend way more $$ on the coal… 78% of total coal plant costs are the fuel.
Let’s summarize how thorium affects the cost breakdown: 1. Thorium fuel would cost much less than the current 14% of total costs, but the fuel is not really the main expense for nuclear reactors. 2. Waste disposal is a hot topic, but doesn’t appear to be a primary expense either. Most thorium reactor designs are expected to produce less waste, particularly waste that isn’t as long-lived. So waste disposal for a thorium reactor is definitely cheaper, plus it would reduce environmental concerns overall. 3. Security and proliferation issues surrounding nuclear power increase costs significantly. Although proponents of thorium claim that thorium reactors are proliferation-proof (see top), a recent article in Nature suggests that thorium can actually be used quite easily to produce weaponizable uranium-233. So the cost savings on security is questionable at the moment. 4. Safety features of nuclear plants seem to dominate the cost. There are many claims about the inherent safety features of thorium Molten Salt Reactors. But those claims have yet to be proven in working prototypes. If thorium reactor designs and prototypes could prove the claims of inherent safety mechanisms, then thorium could dramatically reduce the cost of nuclear power.
While the debate goes on, we’re really just waiting for someone to buck up, build some thorium reactor prototypes and see what’s what. Here are some people making moves: - The Weinberg Foundation is actively financing Molten Salt Reactor experiments - Bill Gates and others have invested in TerraPower which is developing a traveling-wave reactor - Jim Kennedy has a novel idea to create a Thorium Bank. Thorium is often a nasty byproduct of rare-earth mining, and they have to figure out what to do with it. The Thorium Bank would take it off their hands and look for other uses… pending congressional approval. - Norway and Westinghouse are trying to use thorium fuel in a traditional reactor.
Do you know of other serious efforts to experiment with thorium power? Super interesting topic, would love to hear what everyone thinks.