A nuclear reactor is a device by which energy is produced as the result of a nuclear reaction, either fission or fusion. All commercially available nuclear reactors make use of fission reactions, in which the nuclei of large atoms such as uranium are broken apart into smaller nuclei with attendant release of energy. It is theoretically possible to construct reactors that operate on the principle of nuclear fusion, in which small nuclei are combined with each other with the release of energy. But after seventy years of research on fusion reactors, there is still no tangible evidence that this approach can lead to commercially feasible reactors.

When neutrons strike the nucleus of a large atom, they can cause that nucleus to split apart into two roughly equal pieces known as fission products. In that process, additional neutrons and very large amounts of energy are also released. Only three isotopes are known to be fissionable, uranium-235, uranium-233, and plutonium-239. Of these, only the first, uranium-235, occurs naturally. Plutonium-239 is produced synthetically when nuclei of uranium-238 are struck by neutrons and transformed into plutonium. Since uranium-238 always outweighs uranium-235 in commercial nuclear reactor fuel, plutonium-239 is made as a by-product in all commercial reactors now in operation. Uranium-233 can also be produced synthetically by the bombardment of thorium with neutrons. Thus far, however, this isotope has not been put to practical use in nuclear reactors.

The release of neutrons during fission makes it possible for a rapid and continuous repetition of the reaction. Suppose that a single neutron strikes a one-gram block of uranium-235. The fission of one uranium nucleus in that block releases, on an average, about two to three more neutrons. Each of those neutrons, then, is available for the fission of three more uranium nuclei. In the next stage, about nine neutrons (three from each of three fissioned uranium nuclei) are released. As long as more neutrons are being released, the fission of uranium nuclei can continue. (In practice, a single neutron cannot begin a chain reaction even in a pure 1-g block of uranium-235; too many neutrons will simply escape from the sample without causing a fission event. However, the chain-reaction principle is used in practice.)

A reaction of this type that continues on its own once underway is known as a chain reaction. During a nuclear chain reaction, many billions of uranium nuclei may fission in less than a second. Enormous amounts of energy are released in a very short time, a fact that becomes visible with the explosion of a nuclear weapon.

Arranging for the uncontrolled, large-scale release of energy produced during nuclear fission is a relatively simple task. Fission (atomic) bombs are essentially devices in which a chain reaction is initiated and then allowed to continue on its own. The problems of designing a system by which fission energy is released at a constant and usable rate, however, are much more difficult.

The heart of any nuclear reactor is the core, which contains the fuel, a moderator, and control rods. The fuel used in some reactors consists of uranium oxide, enriched with about 3–4 percent of uranium-235. In other reactors, the fuel consists of an alloy made of uranium and plutonium-239. In either case, the amount of fissionable material is actually only a small part of the entire fuel assembly.

The fuel elements in a reactor core consist of cylindrical pellets about 0.6 in (1.5 cm) thick and 0.4 in (1.0 cm) in diameter. These pellets are stacked one on top of another in a hollow cylindrical tube known as the fuel rod and then inserted into the reactor core. Fuel rods tend to be about 12 ft (3.7 m) long and about 0.5 in (1.3 cm) in diameter. They are arranged in a grid pattern containing more than 200 rods each at the center of the reactor. The materials that fuel these pellets must be replaced on a regular basis as the proportion of fissionable nuclei within them decreases.

A nuclear reactor containing only fuel elements would be unusable because a chain reaction could probably not be sustained within it. The reason is that nuclear fission occurs best with neutrons that move at relatively modest speeds, called thermal neutrons. But the neutrons released from fission reactions tend to be moving very rapidly, at about 1/15 the speed of light. In order to maintain a chain reaction, therefore, it is necessary to introduce some material that will slow down the neutrons released during fission. Such a material is known as a moderator.

The most common moderators are substances of low atomic weight such as heavy water (deuterium oxide) or graphite. Hydrides (binary compounds containing hydrogen), hydrocarbons, and beryllium and beryllium oxide have also been used as moderators in certain specialized kinds of reactors.

A chain reaction could easily be sustained in a reactor containing fuel elements and a moderator. In fact, the reaction might occur so quickly that the reactor would explode. In order to prevent such a disaster, the reactor core also contains control rods. Control rods are solid cylinders of metal constructed of some material that has an ability to absorb neutrons. One of the metals most commonly used in the manufacture of control rods is cadmium.

The purpose of control rods is to maintain the ratio of neutrons used up in fission compared to neutrons produced during fission at about 1:1. In such a case, for every one new neutron that is used up in causing a fission reaction, one new neutron becomes available to bring about the next fission reaction.

The problem is that the actual ratio of neutrons produced to neutrons used up in a fission reaction is closer to 2:1 or 3:1. That is, neutrons are produced so rapidly that the chain reaction goes very quickly and is soon out of control. By correctly positioning control rods in the reactor core, however, many of the excess neutrons produced by fission can be removed from the core and the reaction can be kept under control.

The control rods are, in a sense, the dial by which the rate of fission is maintained within the core. When the rods are inserted completely into the core, most neutrons released during fission are absorbed, and no chain reaction occurs. As the rods are slowly removed from the core, the rate at which fission occurs increases. At some point, the position of the control rods is such that the 1:1 ratio of produced to use up neutrons is achieved. At that point, the chain reaction goes forward, releasing energy, but under precise control of human operators.

In most cases, the purpose of a nuclear reactor is to capture the energy released from fission reactions and put it to some useful service. For example, the heat generated by a nuclear reactor in a nuclear power plant is used to boil water and make steam, which can then be used to generate electricity. The way that heat is removed from a reactor core is the basis for defining a number of different reactor types.

For example, one of the earliest types of nuclear reactors is the boiling water reactor (BWR) in which the reactor core is surrounded by ordinary water. As the reactor operates, the water is heated, begins to boil, and changes to steam. The steam produced is piped out of the reactor vessel and delivered (usually) to a turbine and generator, where electrical power is produced.

Another type of reactor is the pressurized water reactor (PWR). In a PWR, coolant water surrounding the reactor core is kept under high pressure, preventing it from boiling. This water is piped out of the reactor vessel into a second building, where it is used to heat a secondary set of pipes also containing ordinary water. The water in the secondary system is allowed to boil, and the steam formed is then transferred to a turbine and generator, as in the BWR.

Some efforts have been made to design nuclear reactors in which liquid metals are used as heat transfer agents. Liquid sodium is the metal most often suggested. Liquid sodium has many attractive properties as a heat transfer agent, but it has one serious drawback. It reacts violently with water and great care must be taken, therefore, to make sure that the two materials do not come into contact with each other.

At one time, there was also some enthusiasm for the use of gases as heat transfer agents. A group of reactors built in the United Kingdom, for example, were designed to use carbon dioxide to move heat from the reactor to the power generating station. Gas reactors have, however, proved technically troublesome and have not experienced much popularity in other nations.

At the end of World War II, great hopes were expressed for the use of nuclear reactors as a way of providing power for many human energy needs. For example, some optimists envisioned the use of small nuclear reactors as power sources in airplanes, ships, and automobiles. These hopes have been realized to only a limited extent. Nuclear powered submarines and aircraft carriers, for example, have become a practical reality. But other forms of transportation do not make use of this source of energy (although the National Aeronautics and Space Administration [NASA] continues research into nuclear-powered rockets).

Instead, the vast majority of nuclear reactors in use today are employed in nuclear power plants, where they supply the energy needed to manufacture electrical energy. In a power reactor, energy released within the reactor core is transferred by a coolant to an external building in which are housed a turbine and generator. Steam obtained from water boiled by reactor heat energy Page 3136  |  Top of Articleis used to drive the turbine and generator, thereby producing electrical energy.

Reactors with other functions are also in use. Reactors are also used to make plutonium and tritium for nuclear weapons. Enriched uranium—uranium with a high percentage of uranium-235—can also be used for making nuclear weapons. A breeder reactor is a type of reactor in which new reactor fuel (plutonium) is manufactured from uranium-238. By far the most common material in any kind of nuclear reactor is uranium-238. This isotope of uranium does not undergo fission and does not, therefore, make any direct contribution to the production of energy. But the vast numbers of neutrons produced in the reactor core do react with uranium-238 in a different way, producing plutonium-239 as a product. This plutonium-239 can then be removed from the reactor core and used as a fuel in other reactors. Reactors whose primary function is to generate plutonium-239 are known as breeder reactors.

Research reactors may have one or both of two functions. First, such reactors are often built simply to test new design concepts for the nuclear reactor. When the test of the design element has been completed, the primary purpose of the reactor has been accomplished.

Second, research reactors can also be used to take advantage of the various forms of radiation released during fission reactions. These forms of radiation can be used to bombard a variety of materials to study the effects of the radiation on the materials.

Nuclear reactors have experienced several notable failures to contain their radioactive contents, especially the Chernobyl disaster in Ukraine in 1986 and the Fukushima Daiichi disaster in Japan in 2011. Both incidents caused widespread criticism of nuclear power and its hazards. The shift in public perception following the partial meltdown in Fukushima caused several countries to change their plans regarding their nuclear programs and the development of new nuclear power plants, and even decide to phase out their currently operating reactors. Germany is expected to shut down all of its remaining nuclear power reactors by 2022. Proponents of nuclear power argue that reactor dangers are exaggerated by public fears; opponents of nuclear power argue that reactors remain intrinsically dangerous, and that the nuclear fuel cycle produces other dangers as well that make nuclear power an unwise option.

Although nuclear reactors have faced tough opposition due to the technical failures of fission reactors and fallout after each such accident, nuclear power is still accountable for 10 percent of energy generation worldwide, and there are active fronts of research and development aiming to harness the power of the atomic nucleus in safer, more sustainable ways.

A new generation of fission reactors (termed Generation IV) is being constructed by companies and governments around the world. These new reactors are expected to be connected to the grid in various stages in the 2020s. One type of modern fission reactor is the SMR (Small Modular Reactor), which is a significantly smaller version of a conventional fission reactor. Such reactors generate far less power but their smaller size, and use of modular components (manufactured elsewhere and assembled on site), allows for expedited construction, increased efficiency, and improved security. Another actively developed alternative to traditional fission reactors is the advanced fission reactor, which uses different coolants instead of the traditional water-based cooling of conventional reactors. These coolants include helium, light sodium, or molten salts. China is actively developing a helium-cooled advanced Page 3137  |  Top of Articlefission reactor, which is expected to be connected to the power grid sometime in the early 2020s.

Of course, the main driver of hope for nuclear energy in the future remains the successful attainment of commercial nuclear fusion. The global focus of research and development is ITER, the International Thermonuclear Experimental Reactor mega-project, under construction in the south of France by a 35 nation scientific and engineering collaboration. The first experiments are planned for 2025, and the feasibility of self-sustained fusion reactions (that would generate energy) would be proven with further experiments in the mid-2030s. If ITER is successful, it will pave the way for the development of globally available commercial solutions in the second half of the twenty-first century.