Energy is produced
when the nuclei of atoms are either split (fission) or united (fusion).
Fission occurs when a heavy nucleus splits into two or more lighter
segments. Fusion is the combining of two light nuclei to form a single,
heavier nucleus. The energy released by the fission or the fusion of
nuclei, in the form of heat, light, or other radiation, is called
nuclear energy. Like energy from any other source, nuclear energy can
be used to do work.
Another reason for identifying atoms by atomic number is the existence of isotopes. Every kind of atom (including hydrogen) can have different numbers of neutrons in its nucleus, making different isotopes of the element. The isotopes are distinguished by using the mass number, but the element is identified by the atomic number.
Because of isotopes these weights are not the actual relative weights of single atoms. In nature the various isotopes of each element are found in a certain proportional abundance. They differ only in their mass numbers. The atomic weight of the element is the average weight of the mixture of the element's isotopes. Hydrogen is a good example.
The natural abundance of normal hydrogen atoms without neutrons is about 99.985 per cent; the abundance of hydrogen atoms containing one neutron (atoms of the isotope deuterium) is about 0.015 per cent. The isotope tritium, containing two neutrons, occurs too rarely to consider. The weights of the two main isotopes in atomic mass units are 1.0078252 and 2.0141022, respectively. The two weights, considering relative abundance, give the average atomic weight of hydrogen, 1.00797.
Atoms of elements heavier than lead or bismuth (atomic numbers 82 and 83) are radioactive, because the nucleus is unstable. Atoms of light elements such as carbon can also be made unstable by bombardment with nuclear particles. Carbon, for example, has two stable isotopes, each with six protons and six or seven neutrons (6C12 and 6C13). Atoms of unstable carbon 14 (6C14) can be produced by bombardment, either in nature with cosmic rays or with high-powered laboratory devices.
The nucleus cannot, however, hold the second excess neutron indefinitely. It gains a more normal neutron-proton balance by expelling an electron. The electron's charge apparently comes from changing a neutron to a proton. This changes carbon 14 to an isotope of nitrogen (6C14 becomes 7N14). The atomic number of the new element changes from 6 to 7. The mass number (14) is the same, because the loss of a neutron is balanced by the gain of a proton.
An opposite change occurs if bombardment drives out one or two neutrons from normal carbon atoms, making carbon with mass number 11 or 10. The remaining neutrons can hold all six protons for only a few minutes or seconds. The nucleus, however, does not expel protons. It expels a particle called a positron. This has the mass of an electron but a positive charge. The charge seems to come from changing a proton to a neutron. The expulsion changes carbon 11 and 10 to isotopes of boron (5B11 and 5B10).
The structure and chemical activity of electrons were revealed by studies of light with spectroscopes and of crystals of solid matter with X rays. Scientists who used X rays named the shells with letters beginning with K, to avoid confusion with other systems of labeling. Since chemical properties arise from an atom's outer electrons, they vary as the different shells are built up. The periodic table of the elements used in chemistry corresponds roughly to the shells. The principal periods end exactly where the X-ray shells do, with an atom that has four paired electrons in its outer shell
There are only certain energy states in which the hydrogen atom can exist. If the atom absorbs more energy from an outside source, it is then considered to be in an excited state. An excited state, compared to the ground state, is less stable but possesses more energy. The reverse action also takes place. When an atom in an excited state falls to the ground state, energy is lost in the form of radiation. There can be only certain excited states because of the quantities in which energy is exchanged. Emission or absorption of energy occurs in specific amounts called quanta. A quantum is a specific amount of energy. Every energy exchange must involve at least one quantum of energy or some whole number multiple of one quantum. If the amount of energy available for exchange is less than one quantum, no exchange can occur.
The mass defect or binding energy per nucleon is different for various elements. It has the least value for light elements, such as deuterium, or heavy hydrogen. The deuterium mass is less than the mass of its constituents of one proton and one neutron by about 2 MeV. For heavier elements, the mass defect per nucleon first increases to a maximum and then decreases. Thus if a heavy nucleus, such as uranium-235 (U-235) is split into fragments consisting of lighter elements, the sum of the mass defects of the fragments is found to be larger than that of the original U-235 nucleus. The difference is the energy released during the fission process. In the fusion process two light nuclei are joined to form a heavier element, also releasing larger amounts of energy. The final mass of the element is less than the mass of the two original nuclei.
The tracks of nuclear particles are also studied with the Wilson cloud chamber or Geiger counters. A new instrument designed especially for nuclear track recording is called the liquid-hydrogen bubble chamber. Its successful operation depends upon this fact: if a liquid is heated under pressure to a temperature that is above its atmospheric boiling point, it will not boil when pressure is reduced unless nuclei are present on which bubbles can form. Charged particles act as such nuclei. Thus, if a fast particle passes through the superheated liquid, boiling begins along the track. The track is photographed and studied.
The main purpose of an accelerator is to produce particle collisions. When these take place at high speeds, new particles are often formed and new phenomena are detected. This helps to explain the nature of elementary particles and their interactions.
The first accelerators were cyclotrons, in which a magnetic field is used to move charged particles, such as protons, in a spiral path as they are accelerated by an alternating electric field. A cyclotron is limited in its acceleration to a few hundred MeV. As particle velocities approach the speed of light, their relativistic masses increase and the alternating electric fields fall out of synchronization with the particles. In synchrocyclotrons, specialized cyclotrons, the frequency of the alternating field is changed to synchronize it with the particle motion. These machines can produce energy levels just below one gigaelectron volt (GeV) (1 GeV = 1,000 MeV).
The next advance in accelerators came in the late 1940s with the development of synchrotrons. These maintain charged proton particles in a nearly circular trajectory by increasing the magnetic field when the frequency of the electric field is increased. Particles are preaccelerated and then injected from the outside. The operation of a synchrotron can be compared to a person repeatedly using a bat to strike a ball that is tied with a string to a post so that it revolves around the post. The string represents the magnetic field that holds the particle motion in a nearly circular orbit. The bat corresponds to the electric accelerating field. As the speed and the energy of the particles increase, the magnetic field and the frequency of the electric field are also increased, similar to an increased frequency in striking the ball as it speeds up.
The latest particle accelerators are the alternating-gradient synchrotrons (AGS). Here a varying magnetic field is applied to the particle beam alternately in the horizontal and vertical directions to prevent the particles from drifting apart. This keeps the beam in focus, thus overcoming a serious problem encountered in the earlier machines. The location and energy levels of some powerful AGS devices are:
Brookhaven National Laboratory, Upton, N.Y., 33 GeV
Serpukhov Institute for High Energy Physics, near Moscow, Russia, 76 GeV
European Organization for Nuclear Research, or CERN, near Geneva, Switzerland, 500 GeV
Fermi National Accelerator Laboratory, or Fermilab, near Chicago, Ill., 1,000 GeV
Colliders accelerate electrons and positrons, protons and antiprotons, heavier ionized atoms, or combinations of other particles. The world's most powerful accelerator is the Tevatron Collider (proton-antiproton collider) of the Fermi National Accelerator Laboratory. CERN's Large Hadron Collider (LHC), scheduled to begin operation in 2005, is expected to accelerate beams of protons to 7,000 GeV and beams of heavy ions such as lead to even greater energies.
The particles that make up a nucleus were once considered the most elementary of matter. They are now thought to be made up of even smaller components known as quarks. Quarks, leptons, and bosons are believed to be some of the fundamental particles of matter. Leptons are electrons and similar particles that, unlike quarks, do not experience nuclear interactions. Another class of particles, called bosons, is believed to be responsible for transmitting the forces of nature by mediating the interaction between all other kinds of particles. Gluons, for example, are bosons that exhibit a strong interactive force and bind quark particles together
The significance of this reaction is that if one pound of uranium undergoes fission, it will release energy equal to that released by the burning of 3,000 tons of coal or 9,000 tons of TNT. Later experiments proved that plutonium and thorium could also undergo nuclear fission. Plutonium is an element made artificially by bombarding uranium 238 with neutrons.
First, huge plants were built to separate the fissionable uranium isotope U-235 from the isotope U-238, which is more abundant. Second, plutonium had to be manufactured by neutron bombardment of U-238. Production of these materials was a very slow process. Continuous production for many months was necessary to produce the quantities required for an atomic bomb.
The third task was the design and construction of the bomb. In a small bomb neutrons would fly out of the material without splitting enough nuclei to sustain the chain reaction. After much calculation and experiment, a sufficient, or “critical,” size was decided upon and construction was begun. On July 16, 1945, a test bomb was exploded near Alamogordo, N.M. The energy released by the blast proved to be as great as had been predicted theoretically. In August 1945 two atomic bombs were dropped on the cities of Hiroshima and Nagasaki in Japan. The explosive power of each bomb was that of approximately 20,000 tons of TNT.
1H2 + 1H3 → 2He4 + 0n1
It is the basic mechanism by which the sun and other stars generate energy as light and heat.
To make maximum application of nuclear energy, its release must be controlled exactly. In an atomic bomb explosion only a small percentage of the maximum possible number of fissions occurs. By using a moderator material to convert fast neutrons to slow neutrons, the number of nuclear fissions is greatly increased. A nuclear reactor controls the release of nuclear energy for practical use.
Compounds of uranium are also used as fuel. These include uranium oxide, uranyl sulfate, and uranyl nitrate. Depending on the construction and purpose of the reactor, the uranium fuel is made into a number of different elements. Solid uranium fuels are used in the shape of rods, slugs, cylinders, flat plates, curved plates, and pellets. Plutonium, separated by special chemical means, is sometimes used in the form of a pure metal.
The second essential part of a nuclear reactor is the moderator material. It must be relatively light in mass and must not absorb neutrons. Fast neutrons, produced by fission, are slowed by a series of collisions with the nuclei of the moderator atoms. A light nucleus is most effective because its weight is relatively close to that of a neutron. Energy lost by the neutrons in collision is absorbed by the nuclei of the moderator. Among the moderators that have been used in nuclear reactors are carbon in the form of graphite, the light metal beryllium, heavy water (having a deuterium nucleus), and ordinary water.One other essential part of the reactor is the coolant. The main coolant is a liquid or gas that is pumped or blown through the reactor core to remove heat given off mainly by the fuel. If this circulation is not maintained, the temperature of the fuel assembly can become high enough to melt the fuel elements.
The commonest method of controlling fission is by the insertion and withdrawal of a neutron-absorbing material such as a cadmium rod. As the rod is moved into the reactor, more and more neutrons are absorbed, and the fission reaction is slowed down. As it is withdrawn, and less and less of its surface is exposed for neutron absorption, the reaction rate increases. Safety, or shut-off, rods may also be provided in a reactor. These rods are made from boron or other good neutron absorbers. Any number of them can be inserted into the reactor simultaneously to bring the fission reaction to a complete stop.A layer of material called the reflector surrounds the core containing the fuel and moderator. The reflector may be ordinary water, heavy water, graphite, or beryllium. The purpose of the reflector is to reflect escaping neutrons back into the core. In turn the reflector is surrounded by a thermal shield. The shield reflects neutrons and absorbs some radiation that is produced by the fission reaction. The thermal shield, as its name indicates, also absorbs heat created by radiation.
Finally, all internal parts of the reactor that have been described are surrounded by a biological shield. The biological shield stops radiation and neutrons that pass through the thermal shield. It is made of concrete, which may contain a heavy material such as iron and steel punchings. The biological shield is usually many feet in thickness. Its main purpose is to protect personnel working near the reactor.
In 1954 the nuclear-powered submarine Nautilus was successfully launched. It became fully operational a year later, and more submarines with atomic reactors followed. The United States Navy then began construction of nuclear-powered aircraft carriers and guided-missile cruisers. By the late 1970s the Navy had more than 75 nuclear-powered craft. A nuclear-powered merchant vessel, the Savannah, was launched in 1959. The success of the United States Navy nuclear submarine program was largely due to the efforts of Adm. H. G. Rickover.
The first commercial nuclear power plant that generated electricity in the United States was actually an outgrowth of the United States Navy reactor program. The pressurized water reactor (PWR) began production (capacity, 60,000 kilowatts) at Shippingport, Pa., in 1957. In this type of reactor, ordinary water functions both as moderator and coolant. The reactor vessel has steel walls several inches thick, and the reactor core is 3.7 meters (12 feet) high. The water enters the reactor vessel, rises through the core, and absorbs heat released by nuclear fission.
This water, under pressure of 140 kilograms per square centimeter (2,000 pounds per square inch) and at a temperature in excess of 260° C (500° F) travels through a closed coil system to a heat exchanger that also contains water. Heat is transferred from the hot reactor water through the coils to the water in the heat exchanger. The temperature and pressure of the reactor water fall. As relatively cool water, it passes out of the heat exchanger to pumps that return it to the nuclear reactor to be reheated and begin the cycle again. The reactor vessel, coolant pumps, heat exchangers, and piping are all enclosed in a concrete containment building.
The water originally in the exchanger absorbs the heat given up by the reactor water and is changed to steam. The steam is piped to a turbineSteam pressure turns the turbine, which drives a generator . The steam then goes to a condenser, where it is changed back to water. Another cycle begins when the water returns to the heat exchanger. The electricity produced goes to an electric power station for distribution.The pressurized water reactor is one of two basic light (or ordinary) water reactor (LWR) designs that have been approved and are in use or under construction in the United States. The other basic design is the boiling water reactor (BWR). The major difference between the PWR and the BWR is that the latter converts water to steam directly in the reactor core. The steam then turns the turbine, which drives the electricity generator. Thus the steam generator circuit of the PWR can be eliminated in the BWR.
Canada's commercial heavy water reactor (HWR), the CANDU (Canadian-Deuterium-Uranium) reactor, replaces ordinary water (H2O) with heavy water (D2O) in the pressurized loop to remove heat from the core. The deuterium in D2O is twice as heavy as the hydrogen in H2O. Since heavy water absorbs fewer neutrons than ordinary water, more thermal neutrons survive in the reactor, increasing the chance that they will hit the fissionable U-235. This permits the use of natural uranium fuel, which consists of (99.3 percent) U-238 and (0.7 percent) fissionable U-235. Because of this advantage, an HWR does not require expensive U-235 fuel enrichment.
Gas-cooled reactors (GCR) employ either carbon dioxide or helium as the coolant instead of water. Carbon dioxide is used in commercial nuclear plants in the United Kingdom and France. The use of helium is under development in the United States, where nuclear power plants supply about 18 percent of the electricity generated. More than 70 percent of the electricity in France comes from nuclear plants.
The small EBR-1 breeder reactor first produced electric power in the United States in 1952. Other breeder reactors have since been developed in the United States, France, Germany, Italy, the United Kingdom, Japan, Russia, and India. The world's only operating commercial plant is the Super-Phoenix, a fast-breeder reactor in France.
In the operation of nuclear reactors, great care is exercised to protect personnel in the area and to safeguard instruments that are sensitive. The thermal shield and biological shield are examples of this built-in protection. Precise tools have been designed to handle radioactive material by remote control. When objects or areas have been irradiated, they are decontaminated by special methods. Radiation-resistant clothing has been designed for those who work in dangerous areas.
One form of reactor fuel is natural uranium. It is mined as an ore in the usual manner. To produce fissionable material the ore is first crushed and ground to a powder. The powder is mixed with water to form a slurry, which is dissolved in acid. Addition of barium carbonate precipitates radium and other metallic impurities. The uranium solution remaining is mixed with hydrogen peroxide, and uranium oxide is precipitated. The oxide is separated by filtration. It is dissolved in nitric acid, purified with ether, and precipitated as ammonium diuranate (NH4U2O7), a bright yellow solid. The ammonium diuranate is heated in an electric furnace and passed over hydrogen fluoride to produce solid uranium tetrafluoride. This compound is reduced at a high temperature to produce pure uranium metal. The metal is machined, enclosed in aluminum containers, and is then ready for use in a reactor. In a second method of uranium ore treatment, natural uranium is changed to a compound known as uranium hexafluoride. This compound, though corrosive and very active, is the only compound of uranium that is gaseous at moderate temperatures (around 60° C, or 140° F). Uranium hexafluoride is treated in several ways to separate the U-235 isotope from the U-238 isotope. It can be separated by gaseous diffusion through porous barriers. The compound uranium hexafluoride is most suitable because fluorine has only one isotope. Thus the course of diffusion is determined by the uranium and is not influenced by the fluorine. The process consists of passing the gas through barriers that contain billions of holes smaller than two millionths of an inch. The U-235 passes through the barriers more rapidly and goes on to the next higher stage for further concentration. The weaker portion is recycled through a lower stage. The U-235 isotope is gradually separated and concentrated. This method is utilized by the light-water reactor industry in the United States. The enriched material is converted chemically into uranium dioxide and fabricated into pellets for use in the reactor.
After plutonium is produced in a reactor, it must be separated from all other fission products that are present. This is a hazardous procedure because of radiation. The irradiated uranium slugs containing plutonium go to a primary separating plant. From this separation come four liquids, including one of impure plutonium. The plutonium solution is separated chemically and converted to pure plutonium metal. This plutonium may be used as reactor fuel and thus renew the fuel cycle. In every ton of slugs there are only a few ounces of plutonium.
Radioactive isotopes emit radiation in the form of beta and gamma rays. The intensity of the radiation is proportional to the rate at which the radioactive material decays. Thus the different radioisotopes can be used for special purposes and processes.
An example of this technique is an experiment with radioactive sodium 24. A very small amount of the isotope is added to a salt solution that is injected into the body. Instruments that are sensitive to radioactivity follow the tracer in the salt as it travels through the body. Scientists have learned that it passes through the walls of the veins, is carried to the sweat glands, changes into sweat, and appears on the surface of the body in less than a minute. Tracers are also being used to study the flying habits and travel patterns of insects. Agricultural and botanical research has benefited from the use of radioactive isotopes. Scientists have determined how plants absorb chemicals as they grow. With radioactive cobalt, botanists can produce new types of plants. Structural variations that normally take years of selective breeding to develop can be made to occur in a few months. Industrial operations often use radioactive tracers instead of X rays or radium in the detection of flaws in cast or welded metal. A few dollars' worth of cobalt can replace thousands of dollars' worth of radium in such work. The petroleum industry employs radioelements in checking almost every kind of operation, from the drilling of wells to the distribution and use of finished products.
The radioisotope of phosphorus is another important diagnostic aid. If a solution containing radiophosphorus is injected into the veins of a patient, it concentrates in the cells of certain brain tumors. A specially designed Geiger counter is then passed over the surface of the head. It accurately locates the tumor by recording the radiation that is emitted from the radiophosphorus lodged in the tumor. The thyroid gland strongly attracts iodine. Hence radioactive iodine is used both in diagnosing and in treating diseases of the thyroid.
In present experiments a magnetic field instead of a container is used to hold the material. Ionized deuterium (heavy water) is placed inside the coil of a powerful electromagnet. The magnetic field of the coil confines the material, called plasma in this state, to the axis of the coil. This arrangement is often called a “magnetic bottle.” The plasma is heated by shooting a tremendous electric charge through it.
Experiments with laboratory-size “bottles” have generated heats of from 20 to 30 million degrees Centigrade (70 to 85 million degrees Fahrenheit) and perhaps some fusion. Further development is retarded by the enormous amount of power needed for the coils and the charge. This obstacle may be overcome by chilling the apparatus to almost absolute zero (–273° C, or –460° F). The metal in the coils and circuits then offers almost no resistance to the flow of current, and the power requirement might be reduced enough to make the method practical.
Another method being explored is inertial confinement, or, more specifically, laser fusion. In this method small pellets of fusion material (deuterium or tritium) are compressed to extremely high density for a short period of time by very powerful laser (focused light) beams. Other inertial confinement methods involve beams of electrons or heavy ions instead of beams of light. At present scientists do not expect that a workable reactor that uses these fusion methods to generate electricity will be realized imminently.
In 1989 two research chemists announced that they had triggered a nuclear fusion reaction at room temperature using relatively common materials—heavy water (in which the hydrogen atoms have been replaced with deuterium), a platinum electrode, and a palladium electrode. However, attempts by other scientists to duplicate the reaction, which was dubbed “cold fusion,” produced conflicting results.
In the United States control of nuclear-energy activities is the responsibility of the Nuclear Regulatory Commission. The commission grants licenses for the building and operation of nuclear reactors and for the ownership and use of nuclear materials. Among its other duties is the establishment of procedures to protect the health and safety of the public. The construction and operation of nuclear reactors have also come under increased scrutiny by state and local governments in the locales of their operation.
An accident involving the Unit 2 reactor at Three Mile Island, near Harrisburg, Pa., on March 28, 1979, in which the reactor core was severely damaged, was caused by a combination of equipment malfunctions and human error. Although the direct health effects from the resulting release of radioactivity into the environment are still being investigated, the psychological effects of the event, which was given wide coverage by the news media, contributed to the concerns about reactor safety.
The Nuclear Regulatory Commission and the industry that it regulates have given special attention to the lessons learned from the Three Mile Island accident, and this has led to an increased understanding of the safe operation of nuclear power plants. However, the construction of new nuclear power plants has slowed dramatically in the wake of the events at Three Mile Island.
International concern over the issue of reactor safety was renewed following an accident at a facility in the Soviet Union in April 1986. The Chernobyl nuclear power plant, which is located about 80 miles (128 kilometers) northwest of Kiev in Ukraine, suffered a catastrophic meltdown of its nuclear fuel. A radioactive cloud spread from the plant over most of Europe, contaminating crops and livestock. Lesser amounts of radiation even spread so far as to appear in Asia and in North America.
Another issue of concern is the question of where to put nuclear waste. This waste is largely the spent fuel of reactors. It is radioactive, and some of its components remain so almost indefinitely. The waste is currently being held at temporary sites until a permanent solution to the problem can be found. In the 1980s it was reported that radioactive wastes from these sites had begun to leak into the environment. At present the most promising solution to the problem of waste storage involves forming waste material into a glassy substance in a process termed vitrification.
In 1970 the Treaty on the Nonproliferation of Nuclear Weapons went into effect. The non-weapons nations that signed the treaty agreed not to develop weapons in exchange for materials and technology for peaceful purposes to be supplied by the nations that already had nuclear weapons. In a major effort to limit the nuclear arms race between the United States and the Soviet Union, negotiations such as the Strategic Arms Limitation Talks (SALT) were pursued during the 1970s and '80s, followed by the Strategic Arms Reduction Talks (START), which continued beyond the 1990s. The International Atomic Energy Agency (IAEA) attempts to provide assurance that weapons proliferation does not occur.
Mass and Atomic Numbers
In many experiments in physics the weight of the entire nucleus is important. The atom is identified by the total number of protons and neutrons. This total is called a mass number. Neutrons, however, are not important in chemistry. The only part of the nucleus that is important to chemists is the proton that holds the electron. For this reason, atoms are commonly identified in chemistry by the number of protons in the nucleus. This is the atomic number of the element.Another reason for identifying atoms by atomic number is the existence of isotopes. Every kind of atom (including hydrogen) can have different numbers of neutrons in its nucleus, making different isotopes of the element. The isotopes are distinguished by using the mass number, but the element is identified by the atomic number.
Atomic Weights in Chemistry
In the days before the structure of the atom was known, chemists and other scientists developed a system of atomic weights for identifying atoms of the different elements. For the chemist it was enough to know how the various kinds of atoms compare in weight. First the lightest element, hydrogen, was assigned the arbitrary weight of one. Then equal amounts of other elements were measured against the hydrogen to obtain a table of relative atomic weightsBecause of isotopes these weights are not the actual relative weights of single atoms. In nature the various isotopes of each element are found in a certain proportional abundance. They differ only in their mass numbers. The atomic weight of the element is the average weight of the mixture of the element's isotopes. Hydrogen is a good example.
The natural abundance of normal hydrogen atoms without neutrons is about 99.985 per cent; the abundance of hydrogen atoms containing one neutron (atoms of the isotope deuterium) is about 0.015 per cent. The isotope tritium, containing two neutrons, occurs too rarely to consider. The weights of the two main isotopes in atomic mass units are 1.0078252 and 2.0141022, respectively. The two weights, considering relative abundance, give the average atomic weight of hydrogen, 1.00797.
How Protons and Neutrons Change
When the protons and neutrons in a nucleus of an atom are in a suitable ratio to each other, the nucleus shows no tendency to change, and the atom retains its chemical character. It is said to be stable.Atoms of elements heavier than lead or bismuth (atomic numbers 82 and 83) are radioactive, because the nucleus is unstable. Atoms of light elements such as carbon can also be made unstable by bombardment with nuclear particles. Carbon, for example, has two stable isotopes, each with six protons and six or seven neutrons (6C12 and 6C13). Atoms of unstable carbon 14 (6C14) can be produced by bombardment, either in nature with cosmic rays or with high-powered laboratory devices.
The nucleus cannot, however, hold the second excess neutron indefinitely. It gains a more normal neutron-proton balance by expelling an electron. The electron's charge apparently comes from changing a neutron to a proton. This changes carbon 14 to an isotope of nitrogen (6C14 becomes 7N14). The atomic number of the new element changes from 6 to 7. The mass number (14) is the same, because the loss of a neutron is balanced by the gain of a proton.
An opposite change occurs if bombardment drives out one or two neutrons from normal carbon atoms, making carbon with mass number 11 or 10. The remaining neutrons can hold all six protons for only a few minutes or seconds. The nucleus, however, does not expel protons. It expels a particle called a positron. This has the mass of an electron but a positive charge. The charge seems to come from changing a proton to a neutron. The expulsion changes carbon 11 and 10 to isotopes of boron (5B11 and 5B10).
Electron Shells and Chemical Activity
Electrons draw atoms into chemical reactions by forming pairs with electrons from the outer edges of other atoms. In heavier atoms most of the electrons are arranged in a way that binds them firmly to the atom. They cannot enter into chemical reactions. Electrons arrange themselves in a series of shells around the nucleus. The electrons in each shell tend to repel each other, but they overcome this repulsion in part by forming pairs within the shells.The simplest example of this is the helium atom. Around its nucleus of two protons and two neutrons (2He4) it has a pair of electrons. All the atomic forces are completely satisfied, making the helium atom the most strongly bound of any in nature. It will not form two-atom molecules, and it shows no chemical activityAll heavier atoms have a helium pair around the nucleus, and additional electrons circle outside this pair. Some of these can pair with electrons from other atoms until this shell has four pairs. This again satisfies all atomic forces, and the atom (of argon) is chemically inert. Additional shells have more complex arrangements.The structure and chemical activity of electrons were revealed by studies of light with spectroscopes and of crystals of solid matter with X rays. Scientists who used X rays named the shells with letters beginning with K, to avoid confusion with other systems of labeling. Since chemical properties arise from an atom's outer electrons, they vary as the different shells are built up. The periodic table of the elements used in chemistry corresponds roughly to the shells. The principal periods end exactly where the X-ray shells do, with an atom that has four paired electrons in its outer shell
The Energy Within the Atom
The orbits made by electrons rotating in their shells are shown in the diagrams as circles. Actually their paths form far more complicated patterns. Most of the knowledge of the electron structure of atoms has been obtained by the study of the light and X rays given out by atoms. Light is emitted by the atom when it is excited by high temperatures. This emission consists of lines of definite wave lengths. This unique combination of lines is called the line spectrum of the element. It can be recorded and measured.
Careful
analysis of line spectra of elements began about 1880. The quantum
theory was proposed by Max Planck in 1900 and modified by Albert
Einstein in 1905. Niels Bohr successfully applied this theory to
explain the hydrogen spectrum in terms of the electronic structure of
the hydrogen atom.
The Bohr Model of the Hydrogen Atom
The hydrogen atom is selected for study because it has the simplest atomic structure. As the single electron whirls around the nucleus, it is attracted to the nucleus by the electrostatic force of unlike charges. It is also acted upon by a centrifugal force that tends to pull it away from the nucleus. When these two forces balance, the electron moves in its most stable orbit. The most stable state is called the ground, or normal, state of the atom. It represents the condition of minimum possible energy for the atom. It is assumed that, in this state, the electron may revolve without radiating energy.There are only certain energy states in which the hydrogen atom can exist. If the atom absorbs more energy from an outside source, it is then considered to be in an excited state. An excited state, compared to the ground state, is less stable but possesses more energy. The reverse action also takes place. When an atom in an excited state falls to the ground state, energy is lost in the form of radiation. There can be only certain excited states because of the quantities in which energy is exchanged. Emission or absorption of energy occurs in specific amounts called quanta. A quantum is a specific amount of energy. Every energy exchange must involve at least one quantum of energy or some whole number multiple of one quantum. If the amount of energy available for exchange is less than one quantum, no exchange can occur.
The Energy Level Diagram
A physical representation of this energy exchange can be shown by an energy level diagram. If the atom absorbs energy, the orbit of the electron increases because the atom is in an excited state. If the atom emits energy, the electron falls into a more stable orbit closer to the nucleus. The diagram can be constructed by representing various energy levels (or states) with horizontal lines. The emission of light accompanying the fall of an electron from one energy level to a lower energy level is shown on the diagram by a line drawn between the corresponding energy levels. When the electron falls from an outer to an inner orbit, the atom emits light. The energy of the emitted light is equal to the energy lost by the electron in its fall. The light may be ultraviolet, visible, or infrared. In the visible range it appears as a series of lines.The theory of wave, or quantum, mechanics modifies the Bohr theory to account for both the particle and wave characteristics of the electron. This theory appears today to explain more completely the motions and energy exchanges of electrons.The Atomic Nucleus
The nucleus of an atom is composed of specific numbers of protons and neutrons, called nucleons. The mass of the nucleus is slightly smaller than the sum of the masses of its protons and neutrons. This difference is known as the mass defect and is related to an equivalent energy, called the binding energy. This relation, based on Einstein's statement for the equivalence of mass and energy, is E = mc2, where E = energy, m = mass, and c = the speed of light. It means that energy can be transformed into mass and mass can be transformed into energy. The atomic mass unit (amu) corresponds to a multiple of an energy unit called the electron volt. Mass-energy calculations are usually expressed in million electron volts (MeV). The equivalence is 1 amu = 931 MeV.The mass defect or binding energy per nucleon is different for various elements. It has the least value for light elements, such as deuterium, or heavy hydrogen. The deuterium mass is less than the mass of its constituents of one proton and one neutron by about 2 MeV. For heavier elements, the mass defect per nucleon first increases to a maximum and then decreases. Thus if a heavy nucleus, such as uranium-235 (U-235) is split into fragments consisting of lighter elements, the sum of the mass defects of the fragments is found to be larger than that of the original U-235 nucleus. The difference is the energy released during the fission process. In the fusion process two light nuclei are joined to form a heavier element, also releasing larger amounts of energy. The final mass of the element is less than the mass of the two original nuclei.
The Study of Nuclear Particles
The oldest method used to study nuclear characteristics involves the tracks made by nuclear particles when cosmic rays collide with atomic nuclei in the atmosphere. Cosmic rays are streams of particles, usually positively charged, traveling at tremendous speeds. A sensitive photographic emulsion is taken to a high point on the Earth's surface or carried into the upper atmosphere by a balloon. Cosmic rays penetrate the emulsion and split the nuclei of the emulsion. During their passage the particles produce silver granules at points along the ionizing path. The track is photographed and viewed through a microscope.The tracks of nuclear particles are also studied with the Wilson cloud chamber or Geiger counters. A new instrument designed especially for nuclear track recording is called the liquid-hydrogen bubble chamber. Its successful operation depends upon this fact: if a liquid is heated under pressure to a temperature that is above its atmospheric boiling point, it will not boil when pressure is reduced unless nuclei are present on which bubbles can form. Charged particles act as such nuclei. Thus, if a fast particle passes through the superheated liquid, boiling begins along the track. The track is photographed and studied.
Particle Accelerators (Atom Smashers)
Particle accelerators are machines that accelerate charged particles in a high vacuum by electromagnetic interactions. When the particles reach a high velocity, they are made to strike either on a fixed metal target or, in machines called colliders, on a beam of particles traveling in the opposite direction.The main purpose of an accelerator is to produce particle collisions. When these take place at high speeds, new particles are often formed and new phenomena are detected. This helps to explain the nature of elementary particles and their interactions.
The first accelerators were cyclotrons, in which a magnetic field is used to move charged particles, such as protons, in a spiral path as they are accelerated by an alternating electric field. A cyclotron is limited in its acceleration to a few hundred MeV. As particle velocities approach the speed of light, their relativistic masses increase and the alternating electric fields fall out of synchronization with the particles. In synchrocyclotrons, specialized cyclotrons, the frequency of the alternating field is changed to synchronize it with the particle motion. These machines can produce energy levels just below one gigaelectron volt (GeV) (1 GeV = 1,000 MeV).
The next advance in accelerators came in the late 1940s with the development of synchrotrons. These maintain charged proton particles in a nearly circular trajectory by increasing the magnetic field when the frequency of the electric field is increased. Particles are preaccelerated and then injected from the outside. The operation of a synchrotron can be compared to a person repeatedly using a bat to strike a ball that is tied with a string to a post so that it revolves around the post. The string represents the magnetic field that holds the particle motion in a nearly circular orbit. The bat corresponds to the electric accelerating field. As the speed and the energy of the particles increase, the magnetic field and the frequency of the electric field are also increased, similar to an increased frequency in striking the ball as it speeds up.
The latest particle accelerators are the alternating-gradient synchrotrons (AGS). Here a varying magnetic field is applied to the particle beam alternately in the horizontal and vertical directions to prevent the particles from drifting apart. This keeps the beam in focus, thus overcoming a serious problem encountered in the earlier machines. The location and energy levels of some powerful AGS devices are:
Brookhaven National Laboratory, Upton, N.Y., 33 GeV
Serpukhov Institute for High Energy Physics, near Moscow, Russia, 76 GeV
European Organization for Nuclear Research, or CERN, near Geneva, Switzerland, 500 GeV
Fermi National Accelerator Laboratory, or Fermilab, near Chicago, Ill., 1,000 GeV
In
a linear accelerator a rapidly alternating electric field is set up in
a resonant cavity by radio-frequency oscillators. Inside the cavity is
a series of pipes called drift tubes. As the particles pass between
tubes, they receive accelerating pulses from the electrical field. By
the time a proton passes through the entire cavity, it has acquired
energy of many gigaelectron volts. It is then aimed at the stationary
target.
The 2-mile- (3.2-kilometer-) linear accelerator at
Stanford University, California, is called SLAC, an acronym for
Stanford Linear Accelerator Center. It accelerates electrons to about
50 GeV.Colliders accelerate electrons and positrons, protons and antiprotons, heavier ionized atoms, or combinations of other particles. The world's most powerful accelerator is the Tevatron Collider (proton-antiproton collider) of the Fermi National Accelerator Laboratory. CERN's Large Hadron Collider (LHC), scheduled to begin operation in 2005, is expected to accelerate beams of protons to 7,000 GeV and beams of heavy ions such as lead to even greater energies.
Mysteries of the Nuclear Structure
The particles that make up a nucleus are too small to be seen by microscopes. Their effects, however, can be measured through accelerator testing, dislodging large particles with high-energy particles, by the production of new particles, or by measuring the radiation that results from colliding matter. The high frequencies (or short wavelengths), used in accelerators, which lead to high energy particles, make these machines short distance probes capable of exploring sub-nuclear size effects. These in turn provide physicists with insight into the nuclear structure.The particles that make up a nucleus were once considered the most elementary of matter. They are now thought to be made up of even smaller components known as quarks. Quarks, leptons, and bosons are believed to be some of the fundamental particles of matter. Leptons are electrons and similar particles that, unlike quarks, do not experience nuclear interactions. Another class of particles, called bosons, is believed to be responsible for transmitting the forces of nature by mediating the interaction between all other kinds of particles. Gluons, for example, are bosons that exhibit a strong interactive force and bind quark particles together
DEVELOPMENT OF TECHNOLOGY
Elements of atomic number greater than 83 are naturally radioactive. Their nuclei emit particles spontaneously to become different nuclei. Elements of atomic number greater than 92 are so unstable they are not found in nature. Scientists assumed that if the nucleus of a heavy element were split into two or more fragments it would release great energy.The Release of Nuclear Energy
Such a splitting, or fission, reaction was achieved in 1939. The nucleus of a uranium isotope (U-235), when bombarded by neutrons, split into two smaller nuclei. The reaction also produced new neutrons and released about 200 mev of energy per fission. One example of the nuclear fission reaction is as follows:92U235 + 0n1 → 36Kr90 + 56Ba142 + neutronsThe significance of this reaction is that if one pound of uranium undergoes fission, it will release energy equal to that released by the burning of 3,000 tons of coal or 9,000 tons of TNT. Later experiments proved that plutonium and thorium could also undergo nuclear fission. Plutonium is an element made artificially by bombarding uranium 238 with neutrons.
The Chain Reaction
The production of new neutrons by nuclear fission suggested the possibility of a chain reaction. Take a block of pure U-235 and assume fission occurs with the release of two neutrons for each nuclear fission. Further assume that no neutrons escape and that all neutrons that hit U-235 nuclei cause fission. The first fission produces two neutrons. Each of these neutrons causes a fission and four neutrons are produced. The chain grows, and after 20 such cycles a million neutrons have been produced. If the process continues, an astronomical number of fissions occurs in a fraction of a second. The corresponding energy release would create a pressure large enough to blow the system apart. In 1942 it was experimentally proved that a self-sustaining chain reaction could be produced in U-235 and plutonium.The A-Bomb—Uncontrolled Nuclear Energy
Because these discoveries were made during World War II, it was almost inevitable that nuclear energy would be used first for destructive purposes. The Allied governments made an all-out effort to produce weapons that would use the explosive energy provided by a chain reaction. It was necessary to overcome three main problems.First, huge plants were built to separate the fissionable uranium isotope U-235 from the isotope U-238, which is more abundant. Second, plutonium had to be manufactured by neutron bombardment of U-238. Production of these materials was a very slow process. Continuous production for many months was necessary to produce the quantities required for an atomic bomb.
The third task was the design and construction of the bomb. In a small bomb neutrons would fly out of the material without splitting enough nuclei to sustain the chain reaction. After much calculation and experiment, a sufficient, or “critical,” size was decided upon and construction was begun. On July 16, 1945, a test bomb was exploded near Alamogordo, N.M. The energy released by the blast proved to be as great as had been predicted theoretically. In August 1945 two atomic bombs were dropped on the cities of Hiroshima and Nagasaki in Japan. The explosive power of each bomb was that of approximately 20,000 tons of TNT.
The Fusion Reaction
At extremely high temperatures nuclei of light elements fuse into heavier nuclei. This fusion reaction is accompanied by a much greater conversion of mass to energy than occurs in a fission reaction. One example of the fusion reaction follows:1H2 + 1H3 → 2He4 + 0n1
It is the basic mechanism by which the sun and other stars generate energy as light and heat.
Thermonuclear Weapons
Two light elements, such as deuterium and tritium (actually isotopes of hydrogen), can be mixed to form a layer surrounding an atomic bomb. When the atomic bomb explodes, the high temperature reached sets off the fusion, or thermonuclear reaction. Hydrogen bombs have been exploded. A thermonuclear explosion releases thousands of times as much energy as is released by an atomic bomb.To make maximum application of nuclear energy, its release must be controlled exactly. In an atomic bomb explosion only a small percentage of the maximum possible number of fissions occurs. By using a moderator material to convert fast neutrons to slow neutrons, the number of nuclear fissions is greatly increased. A nuclear reactor controls the release of nuclear energy for practical use.
The Nuclear Reactor
A nuclear reactor is a unit constructed to enclose all the equipment and material necessary to produce and control the process of nuclear fission. There are different types of reactors. The power reactor is used to generate heat for conversion into steam. A research or experimental reactor is operated as a source of neutrons for producing radioactive isotopes or for performing neutron diffraction studies of materials. Both must contain certain basic components.The nuclear fuel may be either uranium or plutonium. In the case of uranium it may be in the form of natural uranium containing 0.7 percent U-235 and 99.3 percent U-238. It may be natural uranium metal that is enriched with fissionable U-235. It also may be pure U-235 that has been completely separated from its more abundant isotope U-238.Compounds of uranium are also used as fuel. These include uranium oxide, uranyl sulfate, and uranyl nitrate. Depending on the construction and purpose of the reactor, the uranium fuel is made into a number of different elements. Solid uranium fuels are used in the shape of rods, slugs, cylinders, flat plates, curved plates, and pellets. Plutonium, separated by special chemical means, is sometimes used in the form of a pure metal.
The second essential part of a nuclear reactor is the moderator material. It must be relatively light in mass and must not absorb neutrons. Fast neutrons, produced by fission, are slowed by a series of collisions with the nuclei of the moderator atoms. A light nucleus is most effective because its weight is relatively close to that of a neutron. Energy lost by the neutrons in collision is absorbed by the nuclei of the moderator. Among the moderators that have been used in nuclear reactors are carbon in the form of graphite, the light metal beryllium, heavy water (having a deuterium nucleus), and ordinary water.One other essential part of the reactor is the coolant. The main coolant is a liquid or gas that is pumped or blown through the reactor core to remove heat given off mainly by the fuel. If this circulation is not maintained, the temperature of the fuel assembly can become high enough to melt the fuel elements.
The commonest method of controlling fission is by the insertion and withdrawal of a neutron-absorbing material such as a cadmium rod. As the rod is moved into the reactor, more and more neutrons are absorbed, and the fission reaction is slowed down. As it is withdrawn, and less and less of its surface is exposed for neutron absorption, the reaction rate increases. Safety, or shut-off, rods may also be provided in a reactor. These rods are made from boron or other good neutron absorbers. Any number of them can be inserted into the reactor simultaneously to bring the fission reaction to a complete stop.A layer of material called the reflector surrounds the core containing the fuel and moderator. The reflector may be ordinary water, heavy water, graphite, or beryllium. The purpose of the reflector is to reflect escaping neutrons back into the core. In turn the reflector is surrounded by a thermal shield. The shield reflects neutrons and absorbs some radiation that is produced by the fission reaction. The thermal shield, as its name indicates, also absorbs heat created by radiation.
Finally, all internal parts of the reactor that have been described are surrounded by a biological shield. The biological shield stops radiation and neutrons that pass through the thermal shield. It is made of concrete, which may contain a heavy material such as iron and steel punchings. The biological shield is usually many feet in thickness. Its main purpose is to protect personnel working near the reactor.
Research and test reactors.
There is a large variety of reactors with different design features for special uses. One type is the tank reactor. This reactor system has a grid of fuel elements enclosed in a reactor tank. The tank is usually installed in a concrete radiation shield but may be installed in a pool. The core of the tank reactor is an assembly of aluminum-clad fuel plates usually made of an enriched uranium-aluminum alloy. The plates are placed in a fixed position in the closed reactor tank. Beryllium is often used as the primary reflector. The reactor is cooled and moderated by either ordinary water or heavy water. The heavy water usually permits lower fuel consumption and provides more uniform neutron beams from the reactor. The power ratings of most research tank-type reactors range up to about 40,000 kilowatts. These reactors are used to study many nuclear problems. They are also employed in the manufacture of radioisotopes for medical and industrial purposes. New reactor designs, materials, and operating procedures are constantly being tested. This is done with test reactors. Those designed to check the irradiation of materials and components are usually called general-purpose test reactors. Those developed for a specific function are generally termed special test reactors.Power reactors.
A power reactor generates heat that is converted into steam. The steam can be used directly for power, as in a nuclear submarine. It can also be used to generate electric power—for example, in a commercial nuclear power plant.In 1954 the nuclear-powered submarine Nautilus was successfully launched. It became fully operational a year later, and more submarines with atomic reactors followed. The United States Navy then began construction of nuclear-powered aircraft carriers and guided-missile cruisers. By the late 1970s the Navy had more than 75 nuclear-powered craft. A nuclear-powered merchant vessel, the Savannah, was launched in 1959. The success of the United States Navy nuclear submarine program was largely due to the efforts of Adm. H. G. Rickover.
The first commercial nuclear power plant that generated electricity in the United States was actually an outgrowth of the United States Navy reactor program. The pressurized water reactor (PWR) began production (capacity, 60,000 kilowatts) at Shippingport, Pa., in 1957. In this type of reactor, ordinary water functions both as moderator and coolant. The reactor vessel has steel walls several inches thick, and the reactor core is 3.7 meters (12 feet) high. The water enters the reactor vessel, rises through the core, and absorbs heat released by nuclear fission.
This water, under pressure of 140 kilograms per square centimeter (2,000 pounds per square inch) and at a temperature in excess of 260° C (500° F) travels through a closed coil system to a heat exchanger that also contains water. Heat is transferred from the hot reactor water through the coils to the water in the heat exchanger. The temperature and pressure of the reactor water fall. As relatively cool water, it passes out of the heat exchanger to pumps that return it to the nuclear reactor to be reheated and begin the cycle again. The reactor vessel, coolant pumps, heat exchangers, and piping are all enclosed in a concrete containment building.
The water originally in the exchanger absorbs the heat given up by the reactor water and is changed to steam. The steam is piped to a turbineSteam pressure turns the turbine, which drives a generator . The steam then goes to a condenser, where it is changed back to water. Another cycle begins when the water returns to the heat exchanger. The electricity produced goes to an electric power station for distribution.The pressurized water reactor is one of two basic light (or ordinary) water reactor (LWR) designs that have been approved and are in use or under construction in the United States. The other basic design is the boiling water reactor (BWR). The major difference between the PWR and the BWR is that the latter converts water to steam directly in the reactor core. The steam then turns the turbine, which drives the electricity generator. Thus the steam generator circuit of the PWR can be eliminated in the BWR.
Canada's commercial heavy water reactor (HWR), the CANDU (Canadian-Deuterium-Uranium) reactor, replaces ordinary water (H2O) with heavy water (D2O) in the pressurized loop to remove heat from the core. The deuterium in D2O is twice as heavy as the hydrogen in H2O. Since heavy water absorbs fewer neutrons than ordinary water, more thermal neutrons survive in the reactor, increasing the chance that they will hit the fissionable U-235. This permits the use of natural uranium fuel, which consists of (99.3 percent) U-238 and (0.7 percent) fissionable U-235. Because of this advantage, an HWR does not require expensive U-235 fuel enrichment.
Gas-cooled reactors (GCR) employ either carbon dioxide or helium as the coolant instead of water. Carbon dioxide is used in commercial nuclear plants in the United Kingdom and France. The use of helium is under development in the United States, where nuclear power plants supply about 18 percent of the electricity generated. More than 70 percent of the electricity in France comes from nuclear plants.
Breeder reactors.
A breeder, or fast, reactor is designed to produce both power and new fuel at the same time. Breeder reactors do away with the moderator so that the neutrons retain higher velocity and kinetic energy. When these neutrons are captured by U-238, which is nonfissionable, they can convert it into a transuranic element, known as plutonium-239 (Pu-239), which is fissionable. This new fuel can be separated out after generation in a reactor for use as fuel in other reactors. Since U-238 is much more plentiful than naturally occurring U-235, the development of breeder reactors may bring a long-lasting nuclear fuel supply.The small EBR-1 breeder reactor first produced electric power in the United States in 1952. Other breeder reactors have since been developed in the United States, France, Germany, Italy, the United Kingdom, Japan, Russia, and India. The world's only operating commercial plant is the Super-Phoenix, a fast-breeder reactor in France.
Radiation Hazards
When nuclear fission of U-235 occurs, the atom may split in any of 30 or more ways and produce a total of about 200 fission products. These products are radioactive. They decay and release beta and gamma radiation. Beta rays can penetrate a short distance into the human body. Gamma rays have great penetrating powers and can pass through the human body with relatively little absorption. Exposure to such radiation can be injurious to the human body, and repeated exposures have a cumulative effect. In excessive amounts it can produce cataracts or forms of cancer. It can also damage reproductive cells and cause mutation of genes, producing possible physical defects in future generations.In the operation of nuclear reactors, great care is exercised to protect personnel in the area and to safeguard instruments that are sensitive. The thermal shield and biological shield are examples of this built-in protection. Precise tools have been designed to handle radioactive material by remote control. When objects or areas have been irradiated, they are decontaminated by special methods. Radiation-resistant clothing has been designed for those who work in dangerous areas.
The Nuclear Fuel Cycle
The nuclear fuel cycle refers to the overall scheme in which nuclear fuel is mined, enriched, fabricated into fuel assemblies, used in a reactor, and then reprocessed. Reprocessed fuel material may follow one of three routes: return of material to the reactor, return of the material to the reactor after it undergoes enrichment, or temporary storage as waste material.One form of reactor fuel is natural uranium. It is mined as an ore in the usual manner. To produce fissionable material the ore is first crushed and ground to a powder. The powder is mixed with water to form a slurry, which is dissolved in acid. Addition of barium carbonate precipitates radium and other metallic impurities. The uranium solution remaining is mixed with hydrogen peroxide, and uranium oxide is precipitated. The oxide is separated by filtration. It is dissolved in nitric acid, purified with ether, and precipitated as ammonium diuranate (NH4U2O7), a bright yellow solid. The ammonium diuranate is heated in an electric furnace and passed over hydrogen fluoride to produce solid uranium tetrafluoride. This compound is reduced at a high temperature to produce pure uranium metal. The metal is machined, enclosed in aluminum containers, and is then ready for use in a reactor. In a second method of uranium ore treatment, natural uranium is changed to a compound known as uranium hexafluoride. This compound, though corrosive and very active, is the only compound of uranium that is gaseous at moderate temperatures (around 60° C, or 140° F). Uranium hexafluoride is treated in several ways to separate the U-235 isotope from the U-238 isotope. It can be separated by gaseous diffusion through porous barriers. The compound uranium hexafluoride is most suitable because fluorine has only one isotope. Thus the course of diffusion is determined by the uranium and is not influenced by the fluorine. The process consists of passing the gas through barriers that contain billions of holes smaller than two millionths of an inch. The U-235 passes through the barriers more rapidly and goes on to the next higher stage for further concentration. The weaker portion is recycled through a lower stage. The U-235 isotope is gradually separated and concentrated. This method is utilized by the light-water reactor industry in the United States. The enriched material is converted chemically into uranium dioxide and fabricated into pellets for use in the reactor.
After plutonium is produced in a reactor, it must be separated from all other fission products that are present. This is a hazardous procedure because of radiation. The irradiated uranium slugs containing plutonium go to a primary separating plant. From this separation come four liquids, including one of impure plutonium. The plutonium solution is separated chemically and converted to pure plutonium metal. This plutonium may be used as reactor fuel and thus renew the fuel cycle. In every ton of slugs there are only a few ounces of plutonium.
Radioactive Isotopes
Radioactive isotopes are radioactive atoms of ordinary elements such as carbon, cobalt, sodium, or phosphorus. Some radioisotopes are found in the atomic ash that remains after uranium atoms are split in a nuclear pile. Others are created by exposing normal elements to intense radiation inside a nuclear reactor while fission is taking place.Radioactive isotopes emit radiation in the form of beta and gamma rays. The intensity of the radiation is proportional to the rate at which the radioactive material decays. Thus the different radioisotopes can be used for special purposes and processes.
Tools of research and industry.
Tracers, as radioactive isotopes are sometimes called, have been described as the most useful research tool since the invention of the microscope in the 17th century. Physiologists using tracers, for instance, are learning where and at what speed physical and chemical processes occur in the human body.An example of this technique is an experiment with radioactive sodium 24. A very small amount of the isotope is added to a salt solution that is injected into the body. Instruments that are sensitive to radioactivity follow the tracer in the salt as it travels through the body. Scientists have learned that it passes through the walls of the veins, is carried to the sweat glands, changes into sweat, and appears on the surface of the body in less than a minute. Tracers are also being used to study the flying habits and travel patterns of insects. Agricultural and botanical research has benefited from the use of radioactive isotopes. Scientists have determined how plants absorb chemicals as they grow. With radioactive cobalt, botanists can produce new types of plants. Structural variations that normally take years of selective breeding to develop can be made to occur in a few months. Industrial operations often use radioactive tracers instead of X rays or radium in the detection of flaws in cast or welded metal. A few dollars' worth of cobalt can replace thousands of dollars' worth of radium in such work. The petroleum industry employs radioelements in checking almost every kind of operation, from the drilling of wells to the distribution and use of finished products.
Aid to medicine.
The field of medicine has benefited greatly from nuclear energy in the form of radioisotopes. Physicians use radioisotopes to locate tumors, to diagnose and treat patients suffering from thyroid irregularities, and to study and treat cancer. The element cobalt has been adapted for many medical needs. A small quantity of the natural element becomes strongly radioactive after it has undergone prolonged exposure to radiation in a nuclear pile. It is placed in a thick lead case with a tiny opening that is covered by a shutter, and it is then shipped to hospitals. Patients suffering from cancer can then be exposed to the healing effects of the radiation under controlled conditions. Radioactive material in this form is much less costly than radium, and it is far simpler to use than X-ray radiation.The radioisotope of phosphorus is another important diagnostic aid. If a solution containing radiophosphorus is injected into the veins of a patient, it concentrates in the cells of certain brain tumors. A specially designed Geiger counter is then passed over the surface of the head. It accurately locates the tumor by recording the radiation that is emitted from the radiophosphorus lodged in the tumor. The thyroid gland strongly attracts iodine. Hence radioactive iodine is used both in diagnosing and in treating diseases of the thyroid.
Harnessing Fusion for Peaceful Use
A beginning has been made toward harnessing the most powerful release of nuclear energy, that of the thermonuclear reaction (nuclear fusion, using light elements). The heat needed to start the reaction is at least 50 million degrees Centigrade (122 million degrees Fahrenheit). The fission bomb yields such heat; for peaceful application, however, the heat must be produced without destroying the apparatus.In present experiments a magnetic field instead of a container is used to hold the material. Ionized deuterium (heavy water) is placed inside the coil of a powerful electromagnet. The magnetic field of the coil confines the material, called plasma in this state, to the axis of the coil. This arrangement is often called a “magnetic bottle.” The plasma is heated by shooting a tremendous electric charge through it.
Experiments with laboratory-size “bottles” have generated heats of from 20 to 30 million degrees Centigrade (70 to 85 million degrees Fahrenheit) and perhaps some fusion. Further development is retarded by the enormous amount of power needed for the coils and the charge. This obstacle may be overcome by chilling the apparatus to almost absolute zero (–273° C, or –460° F). The metal in the coils and circuits then offers almost no resistance to the flow of current, and the power requirement might be reduced enough to make the method practical.
Another method being explored is inertial confinement, or, more specifically, laser fusion. In this method small pellets of fusion material (deuterium or tritium) are compressed to extremely high density for a short period of time by very powerful laser (focused light) beams. Other inertial confinement methods involve beams of electrons or heavy ions instead of beams of light. At present scientists do not expect that a workable reactor that uses these fusion methods to generate electricity will be realized imminently.
In 1989 two research chemists announced that they had triggered a nuclear fusion reaction at room temperature using relatively common materials—heavy water (in which the hydrogen atoms have been replaced with deuterium), a platinum electrode, and a palladium electrode. However, attempts by other scientists to duplicate the reaction, which was dubbed “cold fusion,” produced conflicting results.
CONTROL OF NUCLEAR ENERGY
After World War II a major effort was made to apply nuclear energy to peacetime uses. However, the potential hazards of radiation demand a relentless vigilance with regard to the safe use and disposal of nuclear materials. The large increase in nuclear-weapons arsenals throughout the world has also been of grave concern.In the United States control of nuclear-energy activities is the responsibility of the Nuclear Regulatory Commission. The commission grants licenses for the building and operation of nuclear reactors and for the ownership and use of nuclear materials. Among its other duties is the establishment of procedures to protect the health and safety of the public. The construction and operation of nuclear reactors have also come under increased scrutiny by state and local governments in the locales of their operation.
Nuclear Reactor Safety
Nuclear power plants are designed and operated in a manner that emphasizes the prevention and mitigation of accidental releases of radioactivity into the environment. No deaths have thus far been conclusively attributed to the operation of any commercial nuclear power plants that are located in the United States. Nevertheless, the potential for cancer and genetic damage as the result of the accidental release of radioactivity has led to an increased public concern about the safe operation of reactors.An accident involving the Unit 2 reactor at Three Mile Island, near Harrisburg, Pa., on March 28, 1979, in which the reactor core was severely damaged, was caused by a combination of equipment malfunctions and human error. Although the direct health effects from the resulting release of radioactivity into the environment are still being investigated, the psychological effects of the event, which was given wide coverage by the news media, contributed to the concerns about reactor safety.
The Nuclear Regulatory Commission and the industry that it regulates have given special attention to the lessons learned from the Three Mile Island accident, and this has led to an increased understanding of the safe operation of nuclear power plants. However, the construction of new nuclear power plants has slowed dramatically in the wake of the events at Three Mile Island.
International concern over the issue of reactor safety was renewed following an accident at a facility in the Soviet Union in April 1986. The Chernobyl nuclear power plant, which is located about 80 miles (128 kilometers) northwest of Kiev in Ukraine, suffered a catastrophic meltdown of its nuclear fuel. A radioactive cloud spread from the plant over most of Europe, contaminating crops and livestock. Lesser amounts of radiation even spread so far as to appear in Asia and in North America.
Another issue of concern is the question of where to put nuclear waste. This waste is largely the spent fuel of reactors. It is radioactive, and some of its components remain so almost indefinitely. The waste is currently being held at temporary sites until a permanent solution to the problem can be found. In the 1980s it was reported that radioactive wastes from these sites had begun to leak into the environment. At present the most promising solution to the problem of waste storage involves forming waste material into a glassy substance in a process termed vitrification.
Nonproliferation and Arms Control
After World War II five additional nations—the Soviet Union, the United Kingdom, France, China, and India—demonstrated the capability to explode nuclear devices. Several other nations had the potential capability to assemble nuclear weapons or were near this capability.In 1970 the Treaty on the Nonproliferation of Nuclear Weapons went into effect. The non-weapons nations that signed the treaty agreed not to develop weapons in exchange for materials and technology for peaceful purposes to be supplied by the nations that already had nuclear weapons. In a major effort to limit the nuclear arms race between the United States and the Soviet Union, negotiations such as the Strategic Arms Limitation Talks (SALT) were pursued during the 1970s and '80s, followed by the Strategic Arms Reduction Talks (START), which continued beyond the 1990s. The International Atomic Energy Agency (IAEA) attempts to provide assurance that weapons proliferation does not occur.
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