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 that contains a small amount of radioactivity that will last a very short time—this is stuff like clothes or 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 material that creates a bunch of heat because of all the radioactive decay, so it requires both shield and cooling.
Some types of radioactive waste, particularly spent fuel of the kind used in nuclear power plants, can be reprocessed, which means separating it into other types of useful products, including another type of mixed nuclear fuel that can be used in lieu of uranium, though generally not economically 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 stuff, though, doesn’t have that kind of second-life purpose, and that, combined with the medium- and long-lived waste that is quite dangerous to have just sitting around, has to be stored somehow, shielded and maybe cooled, and 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 be 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 radioactive waste is stored in pools of spent fuel water, cooled in that water somewhere near the nuclear reactors where the waste originated. Other waste has been relocated into what’re called dry casks, which are big, barrel-like containers made of several layers of steel, concrete, and other materials, which surround a canister that holds the waste, and the canister is itself surrounded by inert gas. These casks hold and cool waste using natural air convection, so they don’t require any kind of external power or water sources, while other solutions, including storage in water, sometimes does—and often the fuel is initially stored in pools, and is then moved to casks for longer-term storage.
Most of the radioactive waste produced today comes in the form of spend fuel from nuclear reactors, which are typically small ceramic pellets made of low-enriched uranium oxide. These pellets are stacked on top of each other and encased in metal, and that creates what’s called a fuel rod.
In the US, alone, about 2,000 metric tons of spent nuclear fuel is created each year, which is just shy of half an olympic sized swimming pool in terms of volume, and in many countries, the non-reuseable stuff is eventually buried, near the surface for the low- to intermediate-level waste, and deeper for high-level waste—deeper, in this context, meaning something like 200-1000 m, which is about 650-3300 feet, beneath the surface.
The goal of such burying is to prevent potential leakage that might impact life on the surface, while also taking advantage of the inherent stability and cooler nature of underground spaces which are chosen for their isolation, natural barriers, and water impermeability, and which are also often reinforced with human-made supports and security, blocking everything off and protecting the surrounding area so nothing will access these spaces far into the future, and so that they won’t be broken open by future glaciation or other large-scale impacts, either.
What I’d like to talk about today is another potential use and way of dealing with 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, and back in 2023 the group achieved its first criticality, got 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 then creates steam and spins turbines, which generates electricity.
What’s special about this reactor, though, is that it is a thorium molten salt reactor, which 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, because thorium only contains trace amounts of fissile material, which isn’t enough to get a power-generating, nuclear chain reaction going.
This reactor was able to successfully perform what’s called in-core thorium-to-uranium conversion, which allows the operators to use thorium as fuel, and have that thorium converted into uranium, which is sufficiently fissile to produce nuclear power, inside the core of the reactor. This is an incredibly fiddly process, and requires that the thorium-232 used as fuel absorb a neutron, which turns it into thorium-233. Thorium-233 then decays into protactinium-233, and that, in turn, decays into uranium-233—the fuel that powers the reactor.
One innovation here is that this entire process happens inside the reactor, rather than occurring externally, which would require a bunch of supplementary infrastructure to handle fuel fabrication, increasing the amount of space and cost associated with the reactor.
Those neutrons required to start the thorium conversion process are provided by small amounts of more fissile material, like enriched uranium-235 or plutonium-239, and the thorium is dissolved in a fluoride salt and becomes a molten mixture that allows it to absorb that necessary neutron, and go through that multi-step decay process, turning into uranium-233. That end-point uranium then releases energy through nuclear fission, and this initiates what’s called a burn while breeding cycle, which means it goes on to produce its own neutrons moving forward, which obviates the need for those other, far more fissile materials that were used to start the chain reaction. All of which makes this process a lot more fuel efficient than other options, dramatically reduces the amount of radioactive waste produced, and allows reactors that use it to operate a lot longer without needing to refuel, 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 which have to be periodically shut down so operators can replace spent fuel rods. This new method instead allows the fissile materials to continuously circulate, enabling on-the-fly refueling—so no shut-down, no interruption of operations necessary.
This method also requires zero water, which could allow these reactors to be built in more and different locations, as 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, in part because it has vast thorium reserves it wanted to tap in its pursuit of energy independence, and in part because this approach to nuclear energy should, in theory at least, allow plant operators to use existing, spent fuel rods as part of its process, which could be very economically interesting, as they could use the waste from their existing plants to help fuel these new plants, but also take such waste off other governments’ hands, maybe even be paid for it, because those other governments would then no longer need to store the stuff, and China could use it as cheap fuel; win win.
Thinking further along, though, maybe the real killer application of this technology is that it allows for the dispersion of nuclear energy without the risk of nuclear weapons proliferation. The plants are smaller, they have a passive safety system that disallows the sorts of disasters that we saw in 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 just like oil, that availability is cyclical and controlled by relatively few governments. In the future, that resource could become more scarce, and this reactor setup may become even more valuable as a result, because thorium is a lot cheaper and more abundant, and it’s less tightly controlled because it’s useless from a nuclear weapons standpoint.
This is only the very first step on the way 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, showing that this type of reactor has promise and would probably work scaled-up, as well, and that means the 100MW demonstration reactor China is also building in the Gobi, hoping to 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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