This week we talk about radioactive waste, neutrons, and burn while breeding cycles.
We also discuss dry casks, radioactive decay, and uranium.
Recommended Book: Breakneck by Dan Wang
Transcript
Radioactive waste, often called nuclear waste, typically falls into one of three categories: low-level waste, which contains a small amount of radioactivity that will last a very short time—this includes things like clothes, tools, or rags that have been contaminated; intermediate-level waste, which has been contaminated enough that it requires shielding; and high-level waste, which is very radioactive and creates a great deal of heat because of radioactive decay, so it requires both shielding and cooling.
Some types of radioactive waste, particularly spent fuel of the kind used in nuclear power plants, can be reprocessed. This means separating it into other useful products, including another type of mixed nuclear fuel that can be used in lieu of uranium, though this is generally not economical unless uranium supplies are low. About a third of all spent nuclear fuel has already been reprocessed in some way.
About 4% of even the recyclable material, though, doesn’t have that kind of second-life purpose. That material, combined with the medium- and long-lived waste that is dangerous to have sitting around, has to be stored somehow, shielded and perhaps cooled, in some cases for a very long time. Some especially long-lived fission products have half-lives that stretch into the hundreds of thousands or millions of years, which means they will remain radioactive deep into the future, many times longer than humans have existed as a species.
According to the International Atomic Energy Agency, something like 490,000 metric tons of radioactive spent fuel is currently being stored on a temporary basis at hundreds of specialized sites around the world. The majority of this waste is stored in pools of water near the nuclear reactors where it originated.
Other waste has been relocated into what are called dry casks. These are large, barrel-like containers made of several layers of steel, concrete, and other materials surrounding a canister that holds the waste. The canister itself is surrounded by inert gas. These casks contain and cool waste using natural air convection, so they don’t require any external power or water sources, while other solutions, including storage in water, sometimes do. Fuel is often initially stored in pools, then moved to casks for longer-term storage.
Most of the radioactive waste produced today comes in the form of spent fuel from nuclear reactors, typically small ceramic pellets made of low-enriched uranium oxide. These pellets are stacked on top of each other and encased in metal, creating what’s called a fuel rod.
In the US alone, about 2,000 metric tons of spent nuclear fuel are created each year. By volume, that’s just shy of half an Olympic-sized swimming pool. In many countries, the non-reusable material is eventually buried: near the surface for low- to intermediate-level waste and deeper for high-level waste. “Deeper,” in this context, means something like 200–1,000 meters, or about 650–3,300 feet, beneath the surface.
The goal of such burial is to prevent potential leakage that could affect life on the surface, while also taking advantage of the stability and cooler temperatures of underground spaces. These locations are chosen for their isolation, natural barriers, and impermeability to water. They are also often reinforced with human-made supports and security measures, blocking off and protecting the surrounding area so nothing will access these spaces far into the future, and so they won’t be broken open by future glaciation or other large-scale impacts.
What I’d like to talk about today is another potential use for this type of waste, and why a recent, related development in China is being heralded as such a big deal.
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An experimental nuclear reactor was built in the Gobi Desert by the Chinese Academy of Sciences’ Shanghai Institute of Applied Physics. Back in 2023, the group achieved its first criticality—got it started up, basically—and it has been generating heat through nuclear fission ever since.
What that means is that the nuclear reactor did what a nuclear reactor is supposed to do. Most such reactors exist to generate heat, which creates steam that spins turbines, generating electricity.
What’s special about this reactor, though, is that it is a thorium molten-salt reactor. This means it uses thorium instead of uranium as a fuel source, and the thorium is processed into uranium as part of the energy-making process. Thorium contains only trace amounts of fissile material, which isn’t enough to get a power-generating nuclear chain reaction going.
This reactor successfully performed what’s called in-core thorium-to-uranium conversion. This allows operators to use thorium as fuel and have that thorium converted into uranium—which is sufficiently fissile to produce nuclear power—inside the reactor’s core.
This is an incredibly fiddly process. It requires the thorium-232 used as fuel to absorb a neutron, which turns it into thorium-233. Thorium-233 then decays into protactinium-233, which in turn decays into uranium-233, the fuel that powers the reactor.
One innovation here is that this entire process happens inside the reactor. Otherwise, it would have to occur externally, requiring a bunch of supplementary infrastructure to handle fuel fabrication and increasing the reactor’s cost and physical footprint.
The neutrons required to start the thorium-conversion process are provided by small amounts of more fissile material, such as enriched uranium-235 or plutonium-239. The thorium is dissolved in a fluoride salt and becomes a molten mixture, allowing it to absorb that necessary neutron and go through the multi-step decay process that turns it into uranium-233.
That end-point uranium then releases energy through nuclear fission, initiating what’s called a burn-while-breeding cycle. This means the process goes on to produce its own neutrons, obviating the need for the other, far more fissile materials used to start the chain reaction.
All of this makes the process far more fuel-efficient than other options, dramatically reduces the amount of radioactive waste produced, and allows reactors that use it to operate much longer without refueling, which also extends a reactor’s functional life.
On that last point, many typical nuclear power plants built over the past handful of decades use pressurized-water reactors. These have to be periodically shut down so operators can replace spent fuel rods. This new method instead allows the fissile materials to circulate continuously, enabling on-the-fly refueling, with no shutdown or interruption of operations required.
This method also requires no water, which could allow these reactors to be built in more and different locations. Conventional nuclear power plants have typically been built near large water sources, like oceans, because of their cooling needs.
China initiated the program that led to the development of this experimental reactor back in 2011. It did so partly because the country has vast thorium reserves it wanted to tap in its pursuit of energy independence, and partly because this approach to nuclear energy should, in theory at least, allow plant operators to use existing spent fuel rods as part of the process.
That could be economically interesting. China could use waste from its existing plants to help fuel these new plants, but it could also take such waste off other governments’ hands—maybe even be paid to do so—because those governments would no longer need to store the stuff, and China could use it as cheap fuel. Win-win.
Thinking further ahead, though, perhaps the real killer application of this technology is that it allows for the dispersal of nuclear energy without the same risk of nuclear-weapons proliferation. The plants are smaller, and they have a passive safety system that disallows the sorts of disasters we saw at Chernobyl and Three Mile Island. That sort of thing just can’t happen with this setup. And the fissile materials, aside from those starter materials used to get the initial cycle going, can’t be used to make nuclear weapons.
Right now, there’s a fair amount of uranium on the market. But like oil, that availability is cyclical and controlled by relatively few governments. In the future, that resource could become scarcer, making this reactor setup even more valuable. Thorium is much cheaper and more abundant, and it’s less tightly controlled because it’s useless from a nuclear-weapons standpoint.
This is only the first step toward a potentially thorium-reactor-dominated nuclear-power industry, and the conversion rate on this experimental model was meager.
That said, it is a big step in the right direction and a solid proof of concept. It shows that this type of reactor has promise and would probably work when scaled up. That means the 100 MW demonstration reactor China is also building in the Gobi, which it hopes will prove the concept’s full value by 2035, stands a pretty decent chance of having a good showing.
Show Notes
https://www.deepisolation.com/about-nuclear-waste/where-is-nuclear-waste-now
https://www.energy.gov/ne/articles/5-fast-facts-about-spent-nuclear-fuel
https://www.energy.gov/ne/articles/3-advanced-reactor-systems-watch-2030
https://world-nuclear.org/information-library/nuclear-fuel-cycle/nuclear-waste/radioactive-wastes-myths-and-realities
https://www.visualcapitalist.com/visualizing-all-the-nuclear-waste-in-the-world/
https://en.wikipedia.org/wiki/High-level_radioactive_waste_management
https://en.wikipedia.org/wiki/Radioactive_waste
https://en.wikipedia.org/wiki/Nuclear_reprocessing
https://en.wikipedia.org/wiki/Dry_cask_storage
https://en.wikipedia.org/wiki/Deep_geological_repository
https://onlinelibrary.wiley.com/doi/abs/10.1002/er.3854
https://archive.is/DQpXM
https://en.wikipedia.org/wiki/Thorium-based_nuclear_power
https://en.wikipedia.org/wiki/Thorium_fuel_cycle
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