Breeder reactors

Type of physical science: Nuclear physics

Field of study: Nuclear reactors

A breeder reactor is a nuclear reactor which produces more fuel than it consumes. Because of the "breeding" capability of this reactor, there are several important advantages that a breeder reactor possesses over both conventional power plants, involving the burning of fossil fuels, and the other types of nuclear reactors.

Overview

While chemical processes produce energy by releasing the energy stored in the chemical bonds that hold the individual atoms together in molecules, nuclear processes produce energy through the direct conversion of mass into energy. Nuclear processes involve the release of much greater energies per gram of fuel than combustion of chemical fuels. For example, the burning of 1 kilogram of gasoline produces about 5 x 10⁷ joules of heat, whereas 1 kilogram of uranium fuel could produce as much as 9 x 10¹⁹ joules of heat.

Thus, the nuclear fuel can release 1.8 x 10¹² times as much energy. With the impending exhaustion of fossil fuel reserves, nuclear reactors have become increasingly attractive alternatives to conventional means of energy production that employ fossil fuels.

A typical conversion reactor with uranium as the nuclear fuel uses less than 1 percent of the uranium present in a natural uranium ore. A breeder reactor, however, is capable of using the entire uranium ore sample, which gives the breeder reactor several distinct advantages over the conversion reactor. The breeder reactor can use very low-grade uranium ores in its operation and still be economically feasible, as the operation of such a reactor is essentially insensitive to the cost of the nuclear fuel. In addition, the breeder reactor can be used to produce fuel to operate another breeder or converter reactor. The time required for a breeder reactor to produce enough nuclear fuel to operate another reactor of equal capacity is known as the doubling time. Another definition of the doubling time is the time required for a breeder reactor to double the amount of its fuel.

Most nuclear reactors use uranium as fuel. The uranium atom undergoes fission to release energy. Fission is the process whereby a larger nucleus splits into two smaller nuclei, several neutrons, and energy. The nucleus or center of any atom consists of protons and neutrons.

The nucleus also contains a certain amount of nuclear binding energy, which holds the protons and neutrons together in the nucleus. It is the excess binding energy that is released during the fission process. Excess binding energy results because the smaller nuclei created from the fission need proportionately less energy to hold them together. All uranium atoms contain 92 protons in the nucleus. There are two forms or isotopes of uranium: uranium 235 and uranium 238. The difference between the two uranium isotopes is the number of neutrons contained in the nucleus in each case.

The uranium 235 atom contains 143 neutrons; the uranium 238 atom contains 146 neutrons. (Note that the number is equal to the sum number of protons and neutrons in the nucleus of the atom; it is known as the mass number.)

A typical ore of uranium contains about 99.3 percent uranium 238 and 0.7 percent uranium 235. Of these two types of uranium, only uranium 235 readily undergoes fission. In the fission process, the uranium 235 nucleus absorbs a neutron, forming the unstable uranium 236 nucleus. This unstable nucleus undergoes fission, splitting into two smaller nuclei, two or three neutrons, and energy. The neutrons produced can be absorbed by other uranium 235 nuclei, creating a chain reaction which sustains the fission process in a nuclear reactor. Conversion nuclear reactors use only 0.7 percent of the nuclear fuel present as uranium 235. A breeder reactor, because of the way it is designed, transforms fertile uranium 238 nuclei into the fissile plutonium 242 isotope through a series of neutron absorptions and nuclear decays. Thus, the breeder reactor can employ both uranium isotopes to produce energy.

It is also possible to design a breeder reactor system based on the utilization of fertile thorium 232 as the nuclear fuel. In this case, the fertile thorium 232 nuclei are converted to fissile uranium 236 nuclei. It is interesting to note that fertile isotopes in general have an even mass number, such as thorium 232, uranium 234, uranium 238, and plutonium 240. Fissile nuclei generally have an odd mass number, such as uranium 233, uranium 235, plutonium 239, and plutonium 241.

The conversion ratio of a nuclear reactor determines whether a reactor is classified as a converter or a breeder. The conversion ratio is defined as the ratio of fissile material produced to fissile material destroyed. A conversion ratio of one is the break-even point: A reactor with a conversion ratio of less than one is considered a conversion reactor, and a reactor is a breeder reactor if the conversion ratio is greater than one. The conversion ratio is also called the breeder ratio, in reference to a breeder reactor.

The heart of any nuclear breeder or conversion reactor is its core, which contains the nuclear fuel. The most common type of conversion reactor is the light-water nuclear reactor. The core of this reactor contains uranium ore, which has been processed to increase the percentage of uranium 235 to about 2 percent. The term "light-water" signifies that ordinary or "light" water is employed as the coolant, the vehicle for heat transport, and the moderator.

Water cools the core of a nuclear reactor in much the same way as the water in an automobile radiator cools the engine. The water is circulated through the core of the reactor to remove excess heat, thus preventing the thermal meltdown of the reactor core. In the process of cooling the reactor, the water transports heat from the core to other parts of the reactor, where it is used to generate power.

For fission to occur, the neutrons must be effectively absorbed into the nucleus of the fissile atom. If the neutrons are traveling too fast or are too "hot" (radioactive), then they will not be absorbed as effectively into fissile uranium nuclei. In a light-water nuclear reactor, the slower neutrons that are effectively absorbed are known as thermal neutrons. Thus, the third function of the water is to moderate or slow down the neutrons for easier absorption into the fissile uranium nuclei.

A breeder reactor contains no water to act as a coolant or moderator. The much higher temperatures generated in a breeder reactor preclude the use of water as a coolant, and most breeders employ liquid metal coolants such as sodium. The liquid metals also transport the heat more efficiently from the reactor core to the power-generation area of the nuclear power plant.

The slow or thermal neutrons that are employed by conversion reactors are not the most efficient type of neutrons to be absorbed into a fertile nucleus for conversion into a fissile nucleus. Therefore, in a typical breeder reactor, moderation of the neutrons would impede efficient reactor operation. Breeder reactors that absorb "fast" or higher-energy neutrons are known as "fast" breeder reactors.

It is also possible to use gases as coolants for breeder reactors. Helium, for example, has the advantage of absorbing and moderating the neutrons to a lesser degree than liquid metals, which leads to a higher conversion ratio. Another advantage of using helium as a coolant is that it eliminates the need of the secondary coolant loop that is present in liquid metal breeder reactors.

The secondary loop is necessary in the fast breeder because liquid metals tend to become radioactive upon contact with the core. Helium, on the other hand, does not become radioactive.

A disadvantage of using helium is that liquid metals are more efficient conductors of heat. The liquid metal can continue to cool the reactor by convection, even if the pumps should fail.

Another type of breeder is the light-water breeder reactor. This reactor uses uranium 233 and thorium 232 as fuel. The thorium is the fertile nucleus and is converted to fissile uranium fuel. In order to avoid neutron losses, the core is arranged in a seed-blanket arrangement, with small areas or "seeds" of the fissile uranium 233 embedded in "blankets" of the fertile thorium 232.

Applications

The main application of the breeder reactor is the generation of nuclear power. In 1991, it was estimated that nuclear power plants could generate five hundred times more power if all the reactors were fast breeders. Most of the breeder reactors in operation in the past can be broadly classified as either test reactors or power-generation reactors. All the breeder reactors in operation in the early 1990's were of the fast breeder type.

The core of some fast breeder reactors is designed differently from that in the typical conversion reactor. These fast breeder reactors employ an internal core consisting of the nuclear fuel or fissile nuclei surrounded by a blanket consisting of one or more types of fertile nuclei.

This arrangement is known as a heterogeneous core. Some breeder reactors also have a homogeneous core. In this case, the fertile and fissile nuclei are dispersed throughout the core, and a blanket area containing fertile nuclei may also be present. The core of a typical fast breeder reactor is smaller than the core of a conversion reactor which generates the same amount of power.

The first breeder reactor, the Clementine, was built in the United States at Los Alamos, New Mexico, in 1946. It was fueled by plutonium metal and utilized liquid mercury as the coolant and heat-transfer medium. The next fast breeder was the EBR-I, which was designed at the Argonne National Laboratory and built in Idaho. On December 20, 1951, this reactor became the first nuclear reactor in history to produce electricity. The successor to EBR-I was the EBR-II.

This reactor was used for testing advanced reactor fuels and reactor materials. A commercial breeder reactor called the Enrico Fermi Atomic Power Plant operated in Michigan from 1960 to 1972. After a fuel failure in 1966, the plant was shut down briefly. The plant was later reopened, but was finally shut down again in 1972 because of the high cost of fueling the reactor.

The Fast Flux Test Facility (FFTF) was opened at the Department of Energy Hanford Engineering Development Laboratory (HEDL) in Richland, Washington. This reactor was designed mainly as a test reactor for testing and irradiating fuels and materials for possible use in the liquid metal fast breeder reactor program.

Other countries besides the United States have been active in breeder reactor development. France, Germany, Great Britain, and the Soviet Union have operated commercial fast breeder reactors. Several more commercial breeders were planned for France, Germany, Japan, and the Soviet Union. One of the largest of the commercial breeder reactors was the Super Phenix breeder reactor in France, which began operating in April, 1986. The Super Phenix had double the power-generating capacity of the next largest breeder reactor, the BN-600 power plant in the Soviet Union.

Two different types of cooling systems were employed in these liquid metal fast breeder reactors: the pool design and the loop design. In the pool design, the core, pumps, and intermediate heat exchangers are built in a pool tank filled with liquid metal. This arrangement was employed in the PFR, Phenix, Super Phenix, and BN-600 reactors. The second type of cooling system employed in fast breeder reactors is the loop design. In this arrangement, only the reactor core is contained in the reactor vessel. The liquid metal is pumped to the intermediate heat exchanger through a series of pipes. The other reactors discussed in this article all use this type of system.

Another possible application of the breeder would actually help solve the problem of nuclear waste buildup from conversion reactors: A breeder reactor could use much of this waste as fertile material to help run the breeder reactor. A future application of breeder reactors is the so-called hybrid reactor. The hybrid reactor would utilize both fission and fusion processes. The fusion process would create neutrons, which could be used by the fertile material in the breeder segment of the reactor. Electric breeder reactor technology has also been explored. In an electric breeder reactor, protons would be accelerated into the fertile nuclei, converting them into fissile materials.

Context

The discovery of the atomic nucleus is attributed to Ernest Rutherford in 1909. The concept of isotopes followed soon after, as they were postulated by Joseph John Thomson in 1913. Rutherford discovered the existence of the proton in 1919. James Chadwick completed the basics of the modern understanding of the nucleus by postulating the existence of the neutron in 1932.

The realization that energy might be produced from nuclear sources occurred about six years later. The chemists Hahn and Strassman found that, if uranium were bombarded with neutrons, then some of the uranium atoms present would undergo fission into smaller nuclei.

Enrico Fermi constructed the first nuclear "pile" in 1942 and, in 1951, a breeder reactor became the first reactor to generate nuclear power successfully.

The future of breeder reactors remains uncertain. Certainly, the impending energy crisis makes the breeder reactors look very attractive. The breeder reactor could even be employed to produce nuclear fuel for conventional conversion reactors while generating nuclear power at the same time. This arrangement would extend nuclear fuel reserves almost indefinitely.

The public seems to be wary of nuclear power in general, a sentiment which may be attributed to the nuclear accidents occurring at Three Mile Island in 1979 and Chernobyl in 1986.

Like conversion reactors, breeder reactors produce nuclear waste that must be processed and stored. Breeder reactors produce considerable amounts of plutonium. Critics of breeder reactors argue that the availability of this plutonium would impair efforts to limit the proliferation of nuclear weapons. In addition, in breeder reactors, there is the fear that during a meltdown, the reactive part of the core could become supercritical and significant amounts of energy could be released. All evidence, however, indicates that the containment vessel could retain any radiation produced in such an accident. These concerns have hampered the speed with which breeder reactors have been developed in the United States and elsewhere.

An advantage of the breeder reactor is that it operates at a higher temperature than conversion reactors. This causes less damage to the environment from thermal pollution than conversion reactors. Breeder reactors would also require less mining of uranium ore, resulting in fewer environmental problems.

Principal terms

ATOMIC NUMBER: the number of protons present in the nucleus of an atom

CONVERTER REACTOR: any nuclear reactor having a conversion ratio of less than one

FERTILE: a nucleus capable of absorbing a neutron, which means that it can undergo fission

FISSILE: capable of undergoing fission to produce energy

FISSION: the splitting of a nucleus into two smaller nuclei; the process is usually accompanied by the absorption of a neutron and the emission of several more neutrons and energy

FUSION: the combining of two smaller nuclei to yield a larger nucleus and energy

ISOTOPES: nuclei of the same element that have the same number of protons but a different number of neutrons

LIGHT WATER: normal water composed of the lightest isotope of hydrogen, which contains one proton and no neutrons in its nucleus

MODERATOR: a chemical substance which slows down neutrons for better absorption into a fissile nucleus

NEUTRON: one of two basic particles that make up the nucleus of an atom

Bibliography

Dudenstadt, James J., and Chihiro Kikuchi. NUCLEAR POWER: TECHNOLOGY ON TRIAL. Ann Arbor: University of Michigan Press, 1979. The latest printing of this text appeared in 1983. The book is written by scientists in the nuclear power industry for the nonscientist or nontechnical reader. Its purpose is to better inform people about nuclear power so that they are able to make better decisions concerning this topic. A very readable discussion of nuclear power concepts and issues.

Hafele, Wolf. "Energy from Nuclear Power." SCIENTIFIC AMERICAN 263 (September, 1990): 136-142. This article appears in a special issue of SCIENTIFIC AMERICAN which considers energy choices in modern society. Places nuclear energy in the context of other alternative forms of energy. The section of the article on breeder reactors is very short, but it raises some interesting questions on future breeder reactor development and uses.

Kessler, G. NUCLEAR FISSION REACTORS. New York: Springer-Verlag, 1983. Contains a good chapter on the fast breeder reactor which provides numerous tables as well as a discussion of the Super Phenix fast breeder reactor as a case study. This reference also presents a brief discussion of near-breeder reactors, thermal breeders, and light-water breeder reactors.

Levine, Melvin M. FISSION REACTORS. College Park, Mass.: American Association of Physics Teachers, 1983. A very short monograph on fission reactors. The section on breeder reactors is also very short, but there is an extremely concise review of basic nuclear phenomena in the appendix. This review is treated in a fairly nontechnical manner.

Nero, Anthony V. A GUIDEBOOK TO NUCLEAR REACTORS. Berkeley: University of California Press, 1979. This book attempts to answer basic questions on reactor design as well as future directions, waste disposal, and reactor safety. Contains extensive information on breeder reactors in several chapters.

United States. Assistant Secretary for Nuclear Energy. Office of Support Programs. ATOMS TO ELECTRICITY. Washington, D.C.: U.S. Department of Energy, 1987. A fairly short (eighty-five-page) treatise concerning nuclear energy production in the United States. It is especially good for the person wanting a nontechnical treatment of nuclear power. This reference would be an excellent starting place to learn more about nuclear power in general.

Waltar, Alan E., and Albert B. Reynolds. FAST BREEDER REACTORS. Elmsford, N.Y.: Pergamon Press, 1981. A very extensive but somewhat technical book on breeder reactors. Highly recommended for any reader who wants in-depth coverage of the fast breeder. The reference treats not only technical and theoretical physics aspects of breeder reactors but economic and social aspects as well.

The Structure of the Atomic Nucleus

Nuclear Reactors: Design and Operation

Reactor Fuels and Waste Products