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 WangTranscriptRadioactive 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.—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 ...