Reactors are conveniently classified according to the typical energies of the neutrons that cause fission. Neutrons emanating in fission are very energetic; their average energy is around two million electron volts (MeV), nearly 80 million times the energy of atoms in ordinary matter at room temperature. As neutrons scatter or collide with nuclei in a reactor, they lose energy. This action is referred to as down-scattering. The choice of reactor materials and of fissile material concentrations determines the rate at which neutrons are slowed through down-scattering before causing fission.

In a thermal reactor, most neutrons down-scatter in the moderator material before interacting with a fissile material. Down-scattering events take place until the neutrons have reached thermal equilibrium with the reactor at energies of a few hundredths of an electron volt. Neutrons lose energy most efficiently by colliding with light atoms such as hydrogen (mass 1), deuterium (mass 2), beryllium (mass 9), and carbon (mass 12). For this reason, materials that contain atoms of these elements—water, heavy water, beryllium metal and oxide, and graphite—are deliberately incorporated into a thermal reactor and are known as moderators. Since water and heavy water also can function as coolants, they perform a dual purpose in thermal reactors. (See below Coolants and moderators.)

One disadvantage of thermal reactors is that at low energies uranium-235 and plutonium-239 not only can be fissioned by thermal (or slow) neutrons but also can capture neutrons without undergoing fission. Neutron capture transforms these nuclides into, respectively, uranium-236 and plutonium-240, which are not fissile. The probability of neutron capture is much lower at higher energy levels than at thermal energies. To achieve higher energy levels and promote fission over neutron capture, a reactor can be built to operate without a moderator. Depending on the number of scattering events that take place with heavier atoms before fission occurs, the typical fission-causing neutrons may have energies in the range of 0.5 electron volt to thousands of electron volts (intermediate reactors) or several hundred thousand electron volts (fast reactors). Such reactors require higher concentrations of fissile material to reach criticality than do reactor designs that operate at thermal energy levels; however, they are more efficient at converting fertile material to fissile material. Fast reactors can be designed to produce more than one new fissile atom for each fissile atom destroyed. Such reactors are referred to as breeder reactors. Breeder reactors may become important if world demand for nuclear power turns out to be long-term and if access to naturally available sources of fissile material becomes limited.

Core

All reactors have a core, a central region that contains the fuel, fuel cladding, coolant, and (where separate from the latter) moderator. The fission energy in a nuclear reactor is produced in the core.

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The fuel is usually heterogeneous—i.e., it consists of elements containing fissile material along with a diluent. This diluting agent may be fertile material or simply material that has good mechanical and chemical properties and does not readily absorb neutrons. All diluents act as a matrix in which the fissile material can stably reside through its operable life. In solid fuels, the diluted fissile material is enclosed in a cladding—a substance that isolates the fuel from the coolant and minimizes the likelihood that radioactive fission products will be released. Cladding is often referred to as a reactor’s first fission product barrier, as it is the first barrier that fissile material contacts after nuclear fission.