Thorium - A Definition

Updated: Dec 2, 2022

What is Thorium?

Thorium (Th) is an element of nature, like Iron and Uranium. Like Uranium, Th is a fissile material and can fuel a nuclear chain reaction due to its properties and run a power plant to make electricity. Thorium itself will not split and release energy. Instead, it will undergo a series of nuclear reactions when exposed to neutrons until it eventually emerges as an isotope of uranium called U-233, which will readily split and release energy next time it absorbs a neutron. Due to this, the element is denoted as fertile, whereas U-233 is fissile.

Reactors that use Th are operating on what’s called the Thorium-Uranium (Th-U) fuel cycle. The vast majority of existing or proposed nuclear reactors, however, use enriched uranium (U-235) or reprocessed plutonium (Pu-239) as fuel (in the Uranium-Plutonium cycle). Current and exotic designs can theoretically accommodate thorium but only a handful have been developed.

The Th-U fuel cycle has some intriguing capabilities over the traditional U-Pu cycle. Of course, it has downsides as well. On this page, you’ll learn some details about these and leave with the ability to discuss and debate thorium with knowledge of the basics productively.

Up-and-coming nuclear reactor powerhouses, China and India, both have substantial Thorium-bearing minerals and not as much Uranium. So, expect this energy source to become a big deal.

KEY TAKEAWAYS
-  Thorium itself will not split and release energy. Instead, it will undergo a series of nuclear reactions when exposed to neutrons.
-  Only a handful have used thorium. Current and exotic designs can theoretically accommodate thorium.
-  These reactors could be exceedingly safe, proliferation-resistant, resource-efficient, environmentally superior, and even cheap.
-  The nuclear industry is quite conservative, and the biggest problem with Thorium is that we lack operational experience with it.

WHY THORIUM ROCKS VIDEO

What is a Thorium Reactor?

One incredibly cool possibility suitable for the Th-U fuel cycle's thermal-breeding capability is the molten salt reactor (MSR) or a derivative of MSR, the Liquid Fluoride Thorium Reactor (LFTR). In these, fuel is not cast into pellets but dissolved in a liquid salt vat. The chain reaction heats the salt, which naturally convects through a heat exchanger to bring the heat to a turbine and make electricity. Online chemical processing removes fission product neutron poisons and allows online refuelling (eliminating the need to shut down for fuel management, etc.). None of these reactors operates today, but Oak Ridge had a test reactor of this type in the 1960s called the Molten Salt Reactor Experiment [wikipedia] (MSRE). The MSRE will run for extended amounts of time following successful concept trials. It competed with the liquid metal-cooled fast breeder reactors (LMFBRs) for federal funding.

Alvin Weinberg discusses this project's history in detail in his autobiography, The First Nuclear Era, and more information is available on the internet. These reactors could be exceedingly safe, proliferation-resistant, resource-efficient, environmentally superior (to traditional nukes and fossil fuel, obviously), and maybe even cheap. Exotic but successfully tested. Who will start the startup on these? (Just kidding, there are already like four startups working on them, and China is starting to develop thorium as well).

The benefits of Thorium

Thorium cycles exclusively allow thermal breeder reactors (as opposed to fast breeders). In the fuel in a traditional (thermal) type of reactor, more neutrons are released per neutron absorbed. As such, an operator could fuel the reactors without mining any additional U-235 for reactivity boosts if reprocessing the fuel. If this is the case, we could extend the nuclear fuel resources on Earth by two orders of magnitude without some of the fast reactors' complications. Thermal breeding is perhaps best suited for Molten Salt Reactors.
The Th-U fuel cycle does not irradiate Uranium-238 and therefore does not produce transuranic (bigger than uranium) atoms like Plutonium, Americium, Curium, etc. These transuranic elements are the primary health concern of long-term nuclear waste. Thus, Th-U waste will be less toxic on the 10,000+ year timescale.
Thorium is more abundant in Earth’s crust than Uranium, at a concentration of 0.0006% vs 0.00018% for Uranium (factor of 3.3x). Although often cited as a real benefit, if you look at the known reserves of economically extractable Thorium vs Uranium, you’ll find that they are nearly identical. Also, we find substantial Uranium dissolved in seawater, whereas there is 86,000x less Th in there. If closed fuel cycles or breeding become mainstream, this benefit will be irrelevant because both the Th-U and the U-Pu fuel cycles will last us well into the tens of thousands of years, which is about as long as modern history.

Negatives of Thorium?

We don’t have as much experience with Th. The nuclear industry is quite conservative, and the biggest problem with Th is that we lack operational experience with it. When money is at stake, getting people to change from the norm is challenging.
Thorium fuel is a bit harder to prepare. Thorium dioxide melts at 550 degrees higher temperatures than traditional Uranium dioxide, so very high temperatures are required to produce high-quality solid fuel. Additionally, the fuel is relatively inert, making it challenging to process chemically. For fluid-fueled reactors, discussed below, this is irrelevant.
Irradiated Thorium is more dangerously radioactive in the short term. The Th-U cycle (the thorium fuel cycle) invariably produces some U-232, which decays to Tl-208, a 2.6 MeV gamma-ray decay mode. Bi-212 also causes problems. These gamma rays are tough to shield, requiring more expensive spent fuel handling and reprocessing.
Thorium doesn’t work as well as U-Pu in a fast reactor. While U-233 an excellent fuel in the thermal spectrum, it is between U-235 and Pu-239 in the fast spectrum. For reactors that require a superb neutron economy (such as breed-and-burn concepts), Th is not ideal.