This article looks at the classification schemes for nuclear reactors, including the type of nuclear reaction, moderator material, coolant, generation, fuel phase, fuel type, and use. It focuses on fission reactors, which have been in use for over 60 years, and distinguishes between thermal and fast neutron reactors. Other classification schemes include generation, fuel phase, fuel type, and use. The article concludes by noting that there are many specific ways to characterize nuclear reactors and numerous projects in various stages of development.
Nuclear reactors can be classified in several ways: according to the type of nuclear reaction, the moderator material used, the coolant used, the generation of the reactor, the fuel phase, the fuel type and the use. Counting research reactors, there are thousands of them around the world, which fall into many different categories. In this article, I will look at the nuclear reactor classification schemes one at a time.
In this article we are only looking at nuclear fission reactors, i.e. reactors that break up nuclei, rather than fusion reactors, which fuse them together. Fusion reactors are still a highly experimental technology in the early stages of development, while fission reactors have been in use for over 60 years.
The type of nuclear reaction generally refers to whether the nuclear reactor uses slow (thermal) neutrons or fast neutrons. Most of the reactors that employ fast neutrons fall into the category of fast breeder reactors, while most of the reactors that use slow neutrons are called thermal reactors. Thermal reactors are the cheapest and most common, mainly because they can use unenriched natural uranium. Neutrons in thermal reactors are referred to as “slow” because the reactor uses a moderator material to decelerate the neutrons from their natural speed as they are ejected from broken atomic nuclei, which is fast enough, closer to the speed and heat of the surrounding fuel medium . Fast neutron reactors are more expensive and require more fuel enrichment, making them less popular. On the other hand, they create more fuel than they consume, making them attractive in the long run.
Moderator material is the second classification scheme for nuclear reactors. As stated earlier, only thermal nuclear reactors use moderators, so this only covers those. Graphite, heavy water, and plain water are all used as moderators. Graphite and heavy water reactors are more popular because these moderate materials thermalize neutrons better, ensuring that natural uranium can be used and no enrichment is needed.
The next classification scheme is based on generation. Generation I reactors were early prototype reactors, typically one-of-a-kind. Generation II reactors were built for commercial use and based on standard designs. These came into use during the 1950s. Generation III reactors are more modern, coming into use in the late 1990s. They are lighter and more efficient than the previous generation. The next generation, Generation IV reactors are currently in the research stage and are not expected to be launched until the late 2020s or early 2030s. These reactors will be very cheap and produce minimal waste.
Another type of classification is the phase of the fuel: liquid, solid or gas. The solid is the most typical. Along with the phase comes the type of fuel: uranium or thorium. These are the only two reactor-ready elements available in substantial quantities on Earth.
The last classification is based on the use: for power plants, propulsion, nuclear fuel production (breeder reactors), or research reactors. Radioisotope thermoelectric generators (RTGs) are also sometimes put into nuclear reactors, although they are somewhat different. RTGs generate energy from the decay of a radioactive isotope.
And this is all. There are more specific ways to characterize nuclear reactors and numerous projects in various stages of development, but the amount of material written on nuclear reactor types could probably fill a small library.
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