Go to table of reactor types

Nuclear reactors serve three general purposes. Civilian reactorsare used to generate energy for electricity and sometimes also steam for district heating; military reactors create materials that can be used in nuclear weapons; and research reactors are used to develop weapons or energy production technology, for training purposes, for nuclear physics experimentation, and for producing radioisotopes for medicine and research. The chemical composition of the fuel, the type of coolant, and other details important to reactor operation depend on reactor design. Most designs have some flexibility as to the type of fuel that can be used (see Dear Arjun). Some reactors are dual-purpose in that they are used for civilian power and military materials production. The technoweenie centerfold below gives information about civilian and military reactors.

A nuclear reactor is a vessel designed to contain and harness the large amount of energy generated when the nuclei of heavy elements are fissioned (split). In the fission process, a small amount of the mass of the nucleus of the heavy elements (like uranium and plutonium) is converted into energy. This conversion is described by Einstein's famous expression E=mc2, where E is the energy released, m is the mass of converted energy, and c is the speed of light. The expression c2, (pronounced "c squared"), means the speed of light multiplied by itself. Since the speed of light is a very large number (300 million meters per second), a small mass is converted into a large amount of energy.

Nuclear reactors fission heavy nuclei by bombarding them with neutrons. Fission of some isotopes of heavy elements such as uranium and plutonium produces enough free neutrons to enable reactors to be designed so that each fission produces exactly one additional fission. In this way, once a nuclear reaction starts, it can go on without an additional external source of neutrons, a process called a chain reaction.

Reactors differ from each other mainly based on three criteria: 1) the types of coolant used, 2) the type of moderator employed, and 3) whether the reactor is "thermal" or "fast".

The energy produced by nuclear fissions is carried away from the reactor vessel by a coolant. This transport of heat out of the reactor vessel is necessary both for converting the energy into usable heat and/or electricity, and to keep the reactor cool. A failure to carry away a sufficient amount of the heat generated by the fission reactions in a reactor will result in overheating the reactor and, in extreme cases, a melt-down of the fuel. A considerable amount of heat is also generated by the radioactive decay of the fission products. This heat must also be transported out of the reactor vessel by the coolant.

The neutrons generated by fission reactions are energetic, or "fast" neutrons. Since they tend to escape from a container too rapidly to be able to sustain a chain reaction, most reactors are designed to slow down or "moderate" neutrons in order to increase the probability of fission reactions. The process of slowing down neutrons is accomplished by a moderator, which causes fission neutrons to collide with atoms of another material, such as hydrogen or carbon. The slowing down of the neutrons in such collisions occurs much in the same way that a billiard ball slows down when it strikes another billiard ball. Since the weight of a neutron is about the same as that of a hydrogen atom, only light elements like hydrogen can be used to moderate neutrons, for the same reason that a billiard ball can be slowed down significantly by another ball of comparable weight. If the other ball is heavier, the lighter ball will tend to bounce off without slowing down very much. Reactors that use slow neutrons to sustain chain reactions are called thermal reactors because the distribution is about the same as that of the atoms or molecules of the surrounding medium.

Some reactors are designed to use fast neutrons for sustaining chain reactions, and so operate without moderators. Such reactors, called fast reactors generally use liquid metal coolants, usually liquid sodium. Fast reactors are usually designed to convert non-fissile isotopes of elements into fissile elements in such a way that the amount of fissile material produced in the reactors is larger than the amount of fissile material loaded as initial fuel. Therefore, they are also called breeder reactors. The most common design involves converting non-fissile uranium-238 into fissile plutonium-239 (see Dear Arjun). Another design, which has not yet been made into a large-scale power reactor, converts non-fissile thorium-232, found in nature, into fissile uranium-233, which does not occur naturally in significant quantities.

Pressurized water reactors that use regular water as both moderator and coolant are by far the most common reactors used for electricity generation.

Institute for Energy and Environmental Research

Comments to Outreach Coordinator: ieer@ieer.org
Takoma Park, Maryland, USA

Revised September, 1996