Reactor Fuels And Waste Products

Type of physical science: Nuclear physics

Field of study: Nuclear reactors

A material which is capable of undergoing a self-supporting chain reaction may be used as the fuel for a nuclear reactor. There is only one naturally occurring isotope, uranium 235, that is capable of sustaining such a reaction. Two other isotopes, uranium 238 and thorium 232, may become fissile if they are irradiated by neutrons in a reactor environment.

Overview

The nuclear power stations that were in operation in 1991 generated energy by a process called nuclear fission. Another type of nuclear reaction, fusion, was in the experimental stage and was not used in the commercial production of electric power. Fission involves the splitting apart of nuclei of atoms of heavy elements, such as uranium or plutonium, while fusion is the joining together of the nuclei of light elements, such as various isotopes of hydrogen. Both reactions produce an enormous amount of energy.

During a typical fission reaction, a fuel such as uranium 235 is bombarded with slow-moving neutrons. As a neutron passes through a uranium-235 nucleus, it is absorbed and forms the isotope uranium 236. For this absorption to take place, it is essential that the neutron have a low energy or be "slow-moving." Such a reaction is called induced fission. Because the uranium-236 nucleus is very unstable, it breaks up shortly after its formation, forming two new elements. For example, a nucleus of uranium 236 may fragment to form the nuclei of strontium 90 and the inert gas xenon 143. If the total mass of the products of one fission is added and compared to the mass of the original nucleus of uranium 235, then the numbers do not balance.

This result might be expected because some of the mass of the uranium 235 was converted into energy. This reaction is dramatic proof of a theory first stated in 1905 by Albert Einstein.

Einstein proposed that matter and energy were equivalent, that matter was in fact energy in a frozen state. He concluded that if 1 kilogram of matter were entirely converted into energy, then 25 billion kilowatt hours of energy would be generated. Einstein's famous equation, E = mc², states that the energy equivalent (E) of some given quantity of mass (m) is equal to the amount of mass multiplied by the square of the speed of light (c).

Along with producing new elements and energy, additional neutrons are freed from the uranium-235 nucleus. Exactly how many neutrons are released depends on the energy of the bombarding neutrons and the type of fuel being used. For uranium 235, the average yield is 2.43 neutrons per fission. These high-energy neutrons may escape from the material or become absorbed by another nucleus, subsequently causing that nucleus to fission. Before the latter fission can occur, the neutron must lose some of its energy by bouncing off other nuclei. In doing so, it transfers some of its energy to these nuclei in much the same way as a moving billiard ball transfers energy to a stationary one by striking it.

What happens to the escaping neutron is determined by its energy and the type of nucleus that it eventually strikes. If the material in question contains a high percentage of uranium 235, then the neutrons will have a better chance of striking other uranium-235 nuclei and causing these nuclei to fragment. In turn, more neutrons are released. These neutrons are then absorbed by other nuclei, which fission and release still more neutrons. If enough neutrons are released by subsequent fissions, then the reaction will be self-sustaining, or "critical." A chain reaction is taking place. As the process repeats, the rate of fissions increases dramatically. If such a reaction is allowed to proceed without controls, known as a runaway chain reaction, then a nuclear explosion could result under certain conditions. The conditions for such an explosion, however, do not exist at a nuclear power-generating station.

The only element found in nature that will undergo a fission reaction is the 235 isotope of uranium, which is known as a fissile fuel. This particlar isotope of uranium is also quite rare in nature, making up only 0.7 percent of all natural uranium. The most common form of uranium is the 238 isotope. Uranium is generally found in sandstones, conglomerate rocks, and sometimes vein deposits. There are significant uranium ore deposits in the western United States, the Sudbury basin of Ontario, South Africa, and Zaire. Some high-grade ores may contain as much as 4 percent uranium.

The uranium ore is generally removed by subsurface mining techniques. From the mine, the ore is sent to the mill for crushing. A series of stamp mills reduces the ore to a fine sand. The sand is then treated with a reagent which dissolves out the uranium. The dried product is known as "yellow cake" and contains between 70 percent and 90 percent uranium. At a uranium refining plant, the yellow cake is purified to a form known as uranium trioxide. Further steps in the purification process convert the uranium trioxide into uranium dioxide and then, by a reaction with hydrogen fluoride gas, into uranium tetrafluoride. This substance is referred to as "green salt." The green salt is then reacted with fluorine gas, which produces uranium hexafluoride.

If reactor fuel was made from uranium hexafluoride, then a chain reaction would be impossible. Since uranium 235 makes up such a small portion of the total amount of uranium, the 235 nuclei would be so far apart that neutrons emitted during the fission of a single nuclei would be absorbed by a uranium-238 nuclei, breaking the chain. In order to produce a substance that is capable of supporting a chain reaction, the fuel must be enriched. During the enrichment process, the uranium isotopes are separated from each other. The final product is a uranium compound that has a much higher concentration of the 235 isotope than is found in nature.

Since uranium 235 and uranium 238 are identical chemically, they must be separated by other than chemical means. The process used to accomplish this separation is known as gaseous diffusion. In the first step of this process, the uranium hexafluoride is heated to the gaseous state. The gas is then forced into a partially pressurized container which is divided into high- and low-pressure zones by a finely porous membrane. Since the uranium-235 nuclei are lighter than the 238 nuclei, they tend to move at greater speeds in the gaseous mixture. Because they move faster, the uranium-235 nuclei tend to strike the barrier more often and have a greater chance of penetrating the membrane and reaching the low-pressure side of the container. The gas that reaches the lower-pressure zone is slightly enriched with the uranium-235 isotope. The gas from the low-pressure zone is then repressurized and introduced into a second container, and the process is repeated. The further-enriched gas is then forced into a third container, and so on. This method is known as a diffusion cascade. The final product can be withdrawn from the process at any stage, depending on the degree of enrichment that is required.

Before fuel elements can be made from the enriched product, the uranium hexafluoride is chemically changed into uranium dioxide. This change is accomplished by reacting uranium hexafluoride with water and a hydroxide salt. The resulting precipitate is reacted with hydrogen to form uranium dioxide. When the uranium dioxide is to be used as fuel in a reactor which uses boiling water, the compound is first formed into small cylindrical pellets. The pellets are then loaded into tubes that are made of stainless steel or zirconium. After the tubes have been sealed, several of them are bundled together to form a fuel element. Openings through the element allow cooling water to pass through. Finally, hundreds of these fuel elements are set into a grid pattern, which makes up the core of the nuclear reactor.

Because the fissile fuel in a reactor is constantly undergoing fission, there is a constant accumulation of fission products. Some of these products absorb neutrons and make the operation of the reactor more difficult and less efficient. Other products may alter or cause some distortions in the fuel itself. Accordingly, it becomes necessary to remove fuel from a reactor after certain periods of time in order to extract fission products and reclaim unused fuel. In most cases, reactor cores have a useful life of about three or four years. When removed from the core, the remains of the fuel elements are intensely radioactive. In order to allow some of the radioactivity and the residual heat to subside, the elements are placed in underwater storage tanks for several months before any reprocessing of the spent fuel is attempted.

More than eighty fission products have been identified. During the fission process, elements such as uranium 238 and thorium 232 are altered to such a state that they may become fissile themselves. Metals that may be altered into fissionable fuels are known as fertile materials. For example, when uranium 238 is bombarded by radiation within the reactor, it becomes plutonium 239, which is a fissile fuel. When thorium 232 is bombarded by radiation, it becomes uranium 233, which is also fissile. Some of this newly formed fuel begins to fission in place immediately after its formation, thereby adding to the output of the reactor. The remaining portions will be reclaimed during the reprocessing procedure.

Applications

In the process of fission, the nucleus of a fissile fuel such as uranium 233, uranium 235, or plutonium 239 splits into two lighter nuclei. When this reaction occurs, a small amount of matter is converted into a tremendous amount of energy. The energy released per atom is approximately one hundred million times greater in nuclear reactions than in chemical reactions.

Within the nucleus of an atom, two of nature's four forces compete with each other.

The strong nuclear force, which operates only over tiny "nucleus-sized" distances, binds the nucleons (protons and neutrons) together. Since the protons are positively charged and objects of similar charge tend to repel one another, the electromagnetic force tends to offset the nuclear force. In most nuclei, the strong nuclear force dominates the electromagnetic force. In heavy nuclei, however, there is a delicate balance between the two. As a result, the nucleus is somewhat elongated in shape. When the nucleus of a fissile material absorbs a neutron, it begins to vibrate.

These vibrations cause the shape of the nucleus to change back and forth from a nearly spherical shape to one that is highly elongated. Since the strong nuclear force is effective over very short distances, it is somewhat diminished when the nucleus is in an elongated form. If the shape becomes distorted enough, the forces of electromagnetic repulsion overcome the ability of the strong nuclear force to keep the nucleus together, and the nucleus splits into two separate pieces.

The sizes of the two fragments may vary somewhat, but, from the fission of a uranium-235 nucleus, one of the fragments will have a mass number of about 140 and the other of about 90.

Varying numbers of neutrons can also be released during a fission reaction. Each of these neutrons may also cause an additional fission, which in turn releases more neutrons. This chain reaction may occur under very rapid and uncontrolled conditions, such as in a nuclear weapon, or under carefully controlled conditions, such as in a nuclear reactor. It is the ability to produce a chain reaction that makes the fission of uranium so useful as a means of generating electrical energy.

Of the total energy released during a fission reaction, about 80 percent appears in the form of the kinetic energy of the two major fission fragments. Kinetic energy is the energy of motion, and it is determined by multiplying one-half of the mass of a moving particle by the square of its velocity. The remaining 20 percent of the reaction's energy appears as decay products such as β and γ radiation and the kinetic energy of other neutrons emitted during the fission process.

In a nuclear reactor, the kinetic energy of the fission fragments is rapidly lost as the particles travel within the reactor fuel element. The kinetic energy is converted into heat. If there were only a few nuclei undergoing fission, then the heat would dissipate rapidly. If there were billions of reactions going on simultaneously (as in a nuclear reactor), then the fuel would soon begin to melt. To prevent this melting from happening, reactors have some type of circulating, cooling material. Usually, this material is ordinary water, but it may be heavy water, liquid sodium or lithium, or gases such as helium or carbon dioxide.

In the production of electric power, heat is the primary product of the reactor. In a boiling-water reactor, cooling water is contained within a closed loop which includes the reactor core itself. As the water passes between the fuel elements within the core, the heat is transferred to the water. The boiling water then flows to a heat exchanger, where it transfers its heat to water in another closed loop. The steam generated by such a process is used to turn a turbine, which produces electricity.

The major purpose for a nuclear reactor is the generation of power, but there are other uses for these devices. Many universities maintain reactors for the purpose of scientific research.

In addition, reactors may be used to produce new elements by neutron irradiation or to produce fissile fuels from fertile elements. In nature, ninety-two elements exist. With the coming of the atomic age, however, scientists were able to create new heavy elements artificially by bombarding various materials with neutrons.

Fertile materials such as uranium 238 and thorium 232 can be altered to produce fissile fuels. In the reactor environment, a uranium-238 nucleus may absorb a stray neutron and become first neptunium 239 and then plutonium 239. If a thorium-232 nucleus absorbs a neutron, then it becomes thorium 233. This isotope is unstable and will release two β particles and become first protactinium 233 and then uranium 233. If a shell of uranium 238 or thorium 232 were placed around an operating nuclear reactor, then new fuel could be made simultaneously with the production of energy. This type of reactor is known as a breeder reactor, and many scientists believe that it is the wave of the future in fission technology.

Context

In the early 1930's, Enrico Fermi began a series of experiments in which he bombarded uranium samples with neutrons. He had found that slow neutrons (those with little energy) were easily absorbed by uranium nuclei. It was the goal of Fermi's research to produce an element heavier than uranium, element number 93. His results were somewhat confusing. After uranium was bombarded with neutrons, he noted that the resulting substance gave off several beta particles of different energies. He was unable to identify positively any atoms of element number 93.

After Fermi published the results of his work, other physicists attempted to duplicate his experiments. Among these scientists were the team of Otto Hahn, Lise Meitner, and later, Fritz Strassmann. Their experiments revealed the presence of the element barium after uranium had been irradiated with neutrons. It was concluded by Hahn and Meitner that the atomic nucleus was actually splitting apart. Meitner and her nephew, Otto Frisch, prepared a paper describing the experiment and suggesting a reason for the observations. The paper was published in January, 1939, in Copenhagen, Denmark. The American biologist William Arnold, who was working in Copenhagen at the time, suggested that this splitting of the uranium atom be called "fission" after the term used for the division of living cells.

The first scientist to consider the possibility of a nuclear chain reaction seriously was the Hungarian physicist Leo Szilard (1898-1964). Szilard's idea would not have worked, however, because he was considering the use of highly energetic neutrons. He did attempt to obtain a patent on a device with which he hoped to initiate a chain reaction. He also hoped to prevent Adolf Hitler and the Nazis from obtaining the device and somehow using it as a weapon of war. Eventually, Szilard, Fermi, and other highly regarded nuclear scientists fled from Europe and came to the United States. Continued research on nuclear fission and controlled chain reactions led to the construction and operation of the first self-sustaining atomic reactor on December 2, 1942, at Stagg Field in Chicago.

During World War II, the Manhattan Project was initiated for the purpose of developing nuclear weapons. On July 16, 1945, the first runaway chain reaction in a mass of uranium 235 produced the world's first nuclear explosion. In 1949, the Soviets exploded their first nuclear bomb, with the British following in 1952 and the French in 1960. Later, China and Israel also developed nuclear weapons.

Yet, the development of the nuclear chain reaction was not only for the production of weapons. Nuclear reactors designed for the commercial production of energy rapidly multiplied in both number and efficiency. Many nations developed nuclear reactors and use them for a wide variety of reasons.

Principal terms

ATOMIC MASS: the total number of protons and neutrons in the nucleus of an atom

ELEMENT: a pure substance

FERTILE FUEL: a heavy material which may become fissionable when bombarded by neutrons

FISSILE FUEL: a heavy nucleus which is capable of undergoing a fission reaction

FISSION: the breaking apart of the nucleus of an atom

FUSION: the joining together of the nuclei of light elements; a thermonuclear reaction

ISOTOPES: atoms that are of the same element but have different atomic masses

NUCLEUS: the center portion of the atom, which contains most of its mass; it consists of protons and neutrons

Bibliography

Asimov, Isaac. WORLDS WITHIN WORLDS: THE STORY OF NUCLEAR ENERGY. Oak Ridge, Tenn.: U.S. Atomic Energy Commission, Office of Information Services, 1972. A very readable booklet which introduces the reader to the basic concepts of atomic physics and the history of developments in the field. Suitable for the layperson.

Hausner, Henry H., and Stanley B. Roboff. MATERIALS FOR NUCLEAR POWER REACTORS. New York: Reinhold, 1955. This volume introduces the reader to nuclear reactors, their parts, fuels, coolants, control elements, and their construction. The book is suitable for the informed reader.

Hogerton, John F. ATOMIC FUEL. Oak Ridge, Tenn.: U.S. Atomic Energy Commission, Division of Technical Information, 1963. This booklet describes the types of fuels used in nuclear reactors and how uranium is mined, milled, refined, and enriched for use in reactors. Included are chapters on fuel reprocessing and types of nuclear reactors. Suitable for the layperson.

Hogerton, John F. NUCLEAR REACTORS. Oak Ridge, Tenn.: U.S. Atomic Energy Commission, Division of Technical Information, 1970. This work introduces the lay reader to the various types of nuclear reactors and the various systems within the operating reactor.

Krane, Kenneth. MODERN PHYSICS. New York: John Wiley & Sons, 1983. A highly technical work covering such topics in modern physics as nuclear physics, quantum mechanics, and relativity theory. The reader should have some college-level background in physics and mathematics.

Murray, Raymond I. NUCLEAR ENERGY. New York: Pergamon Press, 1975. A technical volume dealing with topics such as atomic physics, radioactivity, nuclear reactions, and reactor concepts. A background in college-level physics and mathematics is suggested.

Patterson, Walter C. NUCLEAR POWER. New York: Penguin Books, 1983. Topics covered in this volume include nuclear reactions, reactors, fuel cycles, and some historical background on the development of nuclear power. Suitable for students.

Electrons and Atoms

Nuclear Reactors: Design and Operation

The Effects of Nuclear Weapons

Radioactive Nuclear Decay and Nuclear Excited States