Thorium Reactors

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...