Although the operation of any nuclear reactor is based on the division of a radioactive substance, accompanied by the release of temperature, depending on the design features, two types are distinguished - a fast neutron reactor and a slow one, sometimes called thermal.
The neutrons released during the reaction have a very high initial velocity, theoretically breaking thousands of kilometers in a second. These are fast neutrons. In the process of displacement due to collisions with atoms of surrounding matter, their speed slows down. One of the simple and affordable ways to artificially reduce speed is to place water or graphite in their path. Thus, having learned to regulate the level of kinetic energy of these particles, a person got the opportunity to create two types of reactors. “Thermal” neutrons got their name due to the fact that the speed of their movement after deceleration practically corresponds to the natural rate of intra-atomic thermal motion. In numerical terms, it is up to 10 km per second. For the microworld, this value is relatively low, so the capture of particles by nuclei occurs very often, causing new rounds of fission (chain reaction). The consequence of this is the need for a much smaller amount of fissile material than fast neutron reactors cannot boast. In addition, some other overhead is reduced . This moment just explains why most of the operating nuclear plants use slow neutrons.
It would seem - if everything is calculated, then why do we need a fast neutron reactor? It turns out that not everything is so simple. The most important advantage of such plants is the ability to provide other reactors with nuclear fuel , as well as create an extended fission cycle. Let us dwell on this in more detail.
The fast neutron reactor more fully uses the fuel loaded into the core. Let's start in order. Theoretically, only two elements can be used as fuel: plutonium-239 and uranium (isotopes 233 and 235). In nature, only the U-235 isotope is found, but it is not enough to talk about the prospects of such a choice. These uranium and plutonium are derivatives of thorium-232 and uranium-238, which are formed as a result of exposure to a neutron flux. But already these two radioactive materials are much more common in their natural form. Thus, if it was possible to launch a self-sustaining chain reaction of fission of U-238 (or plutonium-232), then its result would be the emergence of new batches of fissile material - uranium-233 or plutonium-239. When neutrons are slowed down to thermal speed (classical reactors), such a process is impossible: U-233 and Pu-239 are the fuel in them, but the fast neutron reactor allows such an additional conversion.
The process is as follows: we load uranium-235 or thorium-232 (raw materials), as well as a portion of uranium-233 or plutonium-239 (fuel). The latter (any of them) provide the neutron flux needed to “ignite” the reaction in the first elements. In the process of decay, thermal energy is released, which is converted by the station's generators into electricity. Fast neutrons act on raw materials, converting these elements into ... new portions of fuel. Typically, the amounts of burnt and formed fuel are equal, but if more raw materials are loaded, then the generation of new portions of fissile material occurs even faster than the consumption. Hence the second name of such reactors is breeders. Excess fuel can be used in classic slow reactor varieties.
The disadvantage of fast neutron models is that uranium-235 must be enriched before loading, which requires additional financial investments. In addition, the core structure itself is more complex.