Reactor types

Classifications:

Nuclear Reactors are classified by several methods; a brief outline of these classification schemes is provided.

Classification by type of nuclear reaction

• Nuclear fission. Most reactors, and all commercial ones, are based on nuclear fission. They generally use uranium as fuel, but research on using thorium is ongoing (an example is the Liquid fluoride reactor). This article assumes that the technology is nuclear fission unless otherwise stated. Fission reactors can be divided roughly into two classes, depending on the energy of the neutrons that are used to sustain the fission chain reaction:
• Thermal reactors use slow or thermal neutrons. Most power reactors are of this type. These are characterized by neutron moderator materials that slow neutrons until they approach the average kinetic energy of the surrounding particles, that is, until they are thermalized. Thermal neutrons have a far higher probability of fissioning uranium-235, and a lower probability of capture by uranium-238 than the faster neutrons that result from fission. As well as the moderator, thermal reactors have fuel (fissionable material), containments, pressure vessels, shielding, and instrumentation to monitor and control the reactor's systems.
• Neutrons of intermediate energies are less useful because plutonium-239 has a high ratio of capture cross section vs. fission cross section at these energies, impairing neutron economy. Uranium-233 has low capture/fission ratios across the neutron energy spectrum, so the thorium cycle can use intermediate neutron energies.
• Fast neutron reactors use fast neutrons to sustain the fission chain reaction. They are characterized by an absence of moderating material. Initiating the chain reaction requires enriched uranium (and/or enrichment with plutonium 239), due to the lower probability of fissioning U-235, and a higher probability of capture by U-238 (as compared to a moderated, thermal neutron). Fast reactors have the potential to produce less transuranic waste because all actinides are fissionable with fast neutrons, but they are more difficult to build and more expensive to operate. Overall, fast reactors are less common than thermal reactors in most applications. Some early power stations were fast reactors, as are some Russian naval propulsion units. Construction of prototypes is continuing (see fast breeder or generation IV reactors).
• Nuclear fusion. Fusion power is an experimental technology, generally with hydrogen as fuel. While not currently suitable for power production, Farnsworth-Hirsch fusors are used to produce neutron radiation.
• Radioactive decay. Examples include radioisotope thermoelectric generators and atomic batteries, which generate heat and power by exploiting passive radioactive decay.