The energy of any system, whether physical, chemical, or nuclear, is manifested by its ability to do work or to release heat or radiation. The total energy in a system is always conserved, but it can be transferred to another system or changed in form.
Until about 1800 the principal fuel was wood, its energy derived from solar energy stored in plants during their lifetimes. Since the Industrial Revolution, people have depended on fossil fuels-coal and petroleum-also derived from stored solar energy. When a fossil fuel such as coal is burned, atoms of hydrogen and carbon in the coal combine with oxygen atoms in air; water and carbon dioxide are produced and heat is released, equivalent to about 1.6 kilowatt-hours per kilogram or about 10 electron volts (eV) per atom of carbon. This amount of energy is typical of chemical reactions, which result from changes in the electronic structure of the atoms. A part of the energy released as heat keeps the adjacent fuel hot enough to keep the reaction going.

The Atom
   The atom consists of a small, massive, positively charged core (nucleus) surrounded by electrons. The nucleus, containing most of the mass of the atom, is itself composed of neutrons and protons bound together by very strong nuclear forces, much greater than the electrical forces that bind the electrons to the nucleus. The mass number  of a nucleus is the number of nucleons, or neutrons and protons, it contains; the atomic number  is the number of positively charged protons.

   The binding energy of a nucleus is a measure of how tightly its neutrons and protons are held together by the nuclear forces. The binding energy per nucleon, the energy required to remove one neutron or proton from a nucleus, is a function of the mass number A. The curve of binding energy implies that if two light nuclei coalesce to form a heavier nucleus, or if a heavy nucleus splits into two lighter ones, more tightly bound nuclei result, and energy will be released.

   Nuclear energy, measured in millions of electron volts (MeV), is released by the fusion of two light nuclei, as when two heavy hydrogen nuclei, deuterons, combine in the reaction producing a helium-3 nucleus, a free neutron, and 3.2 MeV, or 5.1 × 10^-13 J. Nuclear energy is also released when the fission of a heavy nucleus such as U is induced by the absorption of a neutron, as in producing caesium-140, rubidium-93, three neutrons, and 200 MeV, or 3.2 × 10^-11 J. A nuclear fission reaction releases 10 million times as much energy as is released in a typical chemical reaction.

²₁H + ²₁H ------> ³₂He + ¹₀n +3.2 Mev     

Nuclear Energy from Fission

¹₀n + ²³⁵₉₂U -----> ¹⁴⁰₅₅Cs + ⁹³₃₇Rb +3 ¹₀n +200 Mev    

   The two key characteristics of nuclear fission important for the practical release of nuclear energy are both evident in equation above. First, the energy per fission is very large. In practical units, the fission of 1 kg (2.2 lb) of uranium-235 releases 18.7 million kilowatt-hours as heat. Second, the fission process initiated by the absorption of one neutron in uranium-235 releases about 2.5 neutrons, on the average, from the split nuclei. The neutrons released in this manner quickly cause the fission of several more atoms, thereby releasing four or more additional neutrons and initiating a self-sustaining series of nuclear fissions, a chain reaction, which results in continuous release of nuclear energy.

   Naturally occurring uranium contains only 0.71 per cent uranium-235; the remainder is the non-fissile isotope uranium-238. A mass of natural uranium by itself, no matter how large, cannot sustain a chain reaction because only the uranium-235 is easily fissionable. The probability is rather low that a neutron produced by fission, having an initial energy of about 1 MeV, will induce fission, but can be increased by a factor of hundreds when the neutron is slowed down through a series of elastic collisions with light nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for the design of practical energy-producing fission reactors.

   In December 1942, at the University of Chicago, the Italian physicist Enrico Fermi succeeded in producing the first nuclear chain reaction. This was done with an arrangement of natural uranium lumps distributed within a large stack of pure graphite, a form of carbon. In Fermi's "pile", or nuclear reactor, the graphite moderator served to slow the neutrons and make a chain reaction possible.

Nuclear Fusion
   The release of nuclear energy can occur at the low end of the binding energy curve through the coalescence of two light nuclei into a heavier one. The energy radiated by the Sun arises from such fusion reactions deep in its interior. At the enormous pressures and temperatures existing there, hydrogen nuclei combine in a series of reactions equivalent to equation:

²₁H + ²₁H ------> ³₂He + ¹₀n +3.2 Mev     

and give rise to most of the energy released by the Sun. Other reactions lead to the same result in stars more massive than the Sun.

   Artificial nuclear fusion was first achieved in the early 1930s by bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with high-energy deuterons (deuterium nuclei) in a cyclotron. To accelerate the deuteron beam a great deal of energy was required, most of which appeared as heat in the target. As a result, no net useful energy was produced. In the 1950s the first large-scale but uncontrolled release of fusion energy was demonstrated in the tests of thermonuclear weapons by the United States, the USSR, Great Britain, and France. Such a brief and uncontrolled release cannot be used for the production of electric power.

   In the fission reactions discussed earlier, the neutron, which has no electric charge, can easily approach and react with a fissionable nucleus-for example, uranium-235. In the typical fusion reaction, however, the reacting nuclei both have a positive electric charge, and the natural repulsion between them, called Coulomb repulsion, must be overcome before they can join. This occurs when the temperature of the reacting gas is sufficiently high-50 to 100 million ° C (90 to 180 million ° F). In a gas of the heavy hydrogen isotopes deuterium and tritium at such a temperature, the fusion reaction occurs, releasing about 17.6 MeV per fusion event. The energy appears first as kinetic energy of the helium-4 nucleus and the neutron, but is soon transformed into heat in the gas and surrounding materials.

   If the density of the gas is sufficient-and at these temperatures the density need be only 10^-5 atmospheres, or almost a vacuum-the energetic helium-4 nucleus can transfer its energy to the surrounding hydrogen gas, thereby maintaining the high temperature and allowing a fusion chain reaction to take place. Under these conditions, "nuclear ignition" is said to have occurred.

   The basic problems in attaining useful nuclear fusion conditions are (1) to heat the gas to these very high temperatures, and (2) to confine a sufficient quantity of the reacting nuclei for a long enough time to permit the release of more energy than is needed to heat and confine the gas. A subsequent major problem is the capture of this energy and its conversion to electricity.

   At temperatures above 100,000° C (180,000° F), all the hydrogen atoms are fully ionized. The gas consists of an electrically neutral assemblage of positively charged nuclei and negatively charged free electrons. This state of matter is called a plasma.

   A plasma hot enough for fusion cannot be contained by ordinary materials. The plasma would cool very rapidly, and the vessel walls would be destroyed by the temperatures present. However, since the plasma consists of charged nuclei and electrons, which move in tight spirals around strong magnetic field lines, the plasma can be contained in a properly shaped magnetic field region.

   In any useful fusion device, the energy output must exceed the energy required to confine and heat the plasma. This condition can be met when the product of confinement time t and plasma density n exceeds about 10¹⁴. The relationship t n >_-  10¹⁴ is called the Lawson criterion.

   Numerous schemes for the magnetic confinement of plasma have been tried since 1950 in the United States, the former USSR, Great Britain, Japan, and elsewhere. Thermonuclear reactions have been observed, but the Lawson number rarely exceeded 10¹². One device, however-the tokamak, originally suggested in the USSR by Igor Tamm and Andrey Sakharov-began to give encouraging results in the early 1960s.

   The confinement chamber of a tokamak has the shape of a torus, with a minor diameter of about 1 m (about 3.3 ft) and a major diameter of about 3 m (about 9.8 ft). A toroidal magnetic field of about 5 tesla is established inside this chamber by large electromagnets. This is about 100,000 times the Earth's magnetic field at the planet's surface. A longitudinal current of several million amperes is induced in the plasma by the transformer coils that link the torus. The resulting magnetic field lines are spirals in the torus, and confine the plasma.

   Following the successful operation of small tokamaks at several laboratories, two large devices were built in the early 1980s, one at Princeton University in the United States and one in the USSR. In the tokamak, high plasma temperature naturally results from resistive heating by the very large toroidal current, and additional heating by neutral beam injection in the new large machines should result in ignition conditions.

   Another possible route to fusion energy is that of inertial confinement. In this technique, the fuel-tritium or deuterium-is contained within a tiny pellet that is bombarded on several sides by a pulsed laser beam. This causes an implosion of the pellet, setting off a thermonuclear reaction that ignites the fuel. Several laboratories in the United States and elsewhere are currently pursuing this possibility. Progress in fusion research has been promising, but the development of practical systems that produce more power than they consume will probably take decades to realize. The research is expensive, as well.

   However, some progress has been made in the early 1990s. In 1991, for the first time ever, a significant amount of energy-about 1.7 million watts-was produced from controlled nuclear fusion at the Joint European Torus (JET) Laboratory in England. In December 1993, researchers at Princeton University used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction that output 5.6 million watts. However, both JET and the Tokamak Fusion Test Reactor consumed more energy than they produced during their operation.

   If fusion energy does become practicable, it offers the following advantages: (1) a limitless source of fuel, deuterium from the ocean; (2) no possibility of a reactor accident, as the amount of fuel in the system is very small; and (3) waste products much less radioactive and simpler to handle than those from fission systems.

The Chain Reaction
   When the uranium nucleus fissions, it breaks up into a pair of nuclear fragments and releases energy. At the same time, the nucleus emits very quickly a number of fast neutrons, the same type of particle that initiated the fission of the uranium nucleus. This makes it possible to achieve a self-sustaining series of nuclear fissions; the neutrons that are emitted in fission produce a chain reaction, with a continuous release of energy.

   The light isotope of uranium, uranium-235, is easily split by the fission neutrons and, upon fission, emits an average of about 2.5 neutrons. One neutron per generation of nuclear fissions is necessary to sustain the chain reactions. Others may be lost by escape from the mass of chain-reacting material, or they may be absorbed in impurities or in the heavy uranium isotope, uranium-238, if it is present. Any substance capable of sustaining a fission chain reaction is known as a fissile material.

Atomic Bomb
   Extremely powerful explosive weapon whose force is fuelled by the splitting, or fission, of the nuclei of specific isotopes of uranium or plutonium (uranium-235, uranium-238, and plutonium-239) in a chain reaction.

   The process of fission releases enormous energy in the form of extreme heat and a massive shock wave. A slow, carefully controlled fission reaction generates power for electricity companies worldwide, but in an atomic bomb the release of energy continues unabated until all fissile material is exhausted. In addition to its virtually limitless destructive effects-flash burns, and widespread destruction through pressure waves, and high winds-a nuclear explosion also produces deadly radiation in the form of gamma rays and neutrons, which destroy living matter and contaminate soil and water.

Fission and Fusion
   Atomic bombs are nowadays called nuclear weapons, which are of two general types: fission or fusion. Fission weapons were the first atomic bombs to be developed, tested, and used in war, when the United States dropped two atomic bombs on Hiroshima and Nagasaki in Japan in 1945, at the end of World War II.

   Fusion bombs, also called hydrogen or thermonuclear bombs, are vastly more powerful than fission bombs. They were developed and tested in the early 1950s, but these have never been used in warfare. A thermonuclear device depends on a fission reaction to produce extreme heat that causes hydrogen isotopes of deuterium and tritium to come together, or fuse. This process yields energy many times greater than that of fission-type devices. Most nuclear weapons in present-day stockpiles are thermonuclear devices.

Development of the First Atomic Bombs
   In the late 1930s, physicists in Europe and the United States realized that, in theory, the fission of uranium could be used to create an extremely powerful explosive weapon. In August 1939, the physicist Albert Einstein sent a letter to US President Franklin D. Roosevelt that described this possibility and warned of its potential development by other nations.

   The US government explored this possibility for several years before establishing in 1942 the top-secret Manhattan Project, under the directorship of US Army Brigadier-General Leslie Groves. This team, working in several locations but in large part at Los Alamos, New Mexico, under the scientific leadership of physicist J. Robert Oppenheimer, designed and built the first atomic bombs, based on uranium-235 and on the more experimental plutonium-239.

   The first atomic explosion was conducted, as a test code-named Trinity, of the plutonium bomb. It was carried out near Alamogordo, New Mexico, at dawn on July 16, 1945. The energy released from this explosion was equivalent to that released by the detonation of 20,000 tons of trinitrotoluene (TNT). The United States dropped the first atomic bomb on the Japanese city of Hiroshima on August 6. It followed with a second against Nagasaki on August 9. As many as 100,000 people were killed by the Hiroshima uranium device, called Little Boy, and some 40,000 by the Nagasaki plutonium bomb, called Fat Man. Japan agreed to US terms of surrender on August 14.

   These are the only times that a nuclear weapon has ever been used in a conflict between nations. Since then, several nations have exploded nuclear devices in tests, in the atmosphere, under the earth, and under the sea. Only the United States, Russia, Britain, France, and China admit to possessing nuclear weapons. Other nations, such as Israel and India among others, are also thought to have them, or to be able to assemble them quickly.

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