Scisoft

Introduction

"The discovery of nuclear chain reactions need not bring about the destruction of mankind any more than did the discovery of matches. We only must do everything in our power to safeguard against its abuse."

Albert Einstein

One of the most mysterious phenomena in the universe is the conversion of mass into energy. The whole universe is "powered" by this process. The energy radiated by stars, including the Sun, arises from nuclear reactions (called fusion) deep in their interiors. The release of nuclear energy occurs through the fusion of two light hydrogen nuclei into a heavier nucleus of helium.

SOHO: The Solar and Heliospheric Observatory

Until about 1800, the principal fuel on our planet was wood, its energy originating from solar energy stored in plants during their lifetimes.
Since the Industrial Revolution, people have depended on fossil fuels—coal, petroleum, and natural gas—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. This amount of energy is typical of chemical reactions resulting 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.
In a nuclear reaction, however, the energy released is often about 10 million times greater than in a chemical reaction, and the change in mass can easily be measured.

History

The history of generating huge amounts of energy in a nuclear reaction, is basically the history of the atom bomb...Here are few important steps that led to the release of nuclear energy on demand.

• In 1896, Antoine Henri Becquerel discovered radioactivity in uranium.
• In 1902, Marie and Pierre Curie isolated a radioactive metal called radium
• In 1905, Albert Einstein formulated his Special Theory of Relativity. According to this theory, mass can be considered to be another form of energy. According to Einstein, if somehow we could transform mass into energy, it would be possible to "liberate" huge amounts of energy.
• During the next decade, a major step was taken in that direction when Ernest Rutherford and Niels Bohr described the structure of an atom more precisely. It was made up, they said, of a positively charged core, the nucleus, and of negatively charged electrons that revolved around the nucleus. It was the nucleus, scientists concluded, that had to be broken or "exploded" if atomic energy was to be released.
• In 1934, Enrico Fermi of Italy disintegrated heavy atoms by spraying them with neutrons. However he didn't realize that he had achieved nuclear fission.
• In December 1938, though, Otto Hahn and Fritz Strassman in Berlin did a similar experiment with uranium and were able to verify a world-shaking achievement. They had produced nuclear fission (they had split an atom)
  - 33 years after Einstein said it could be done mass was transformed into energy.
• On August 2, 1939, Albert Einstein wrote a letter to the American President, Franklin D. Roosevelt. "In the course of the last four months, it has been made probable - through the work of Joliot in France as well as Fermi and Szilard in America - that it may become possible to set up nuclear chain reactions in a large mass of uranium... And this new phenomenon would also lead to the construction of bombs... A single bomb of this type, carried by boat or exploded in a port, might very well destroy the whole port together with some of the surrounding territory." He urged Roosevelt to begin a nuclear program without delay.
  In later years Einstein deplored the role he had played in the development of such a destructive weapon: "I made one great mistake in my life," he told Linus Pauling, another prominent scientist, "when I signed the letter to President Roosevelt recommending that atoms bombs be made."
• 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.
• In August 1942, during World War II, the United States established the Manhattan Project. The purpose of this project was to developed, construct, and test the A-bomb. Many prominent American scientists, including the physicists Enrico Fermi and J. Robert Oppenheimer and the chemist Harold Urey, were associated with the project, which was headed by a U.S. Army engineer, then-Brigadier General Leslie R. Groves.
• On May 31, 1945, sixteen men met in the office of Secretary of War Henry L. Stimson. The sixteen men were there to make decisions about a weapon the average American had never heard of - the atom bomb. They picked future targets for "The Bomb." They were not talking about "just another weapon." What they were discussing was "a new relationship of man to the universe," as said by Stimson. Humankind, the Secretary seemed to be saying, was at the most critical turning point in its entire recorded history.
  The super-secret group also had many questions about the future including:
  • What were the chances of producing nuclear weapons more powerful that the one being developed?
  • How long would it take other nations, especially the Soviet Union, to catch up with the United States?
  • What hope was there to use atomic energy for peaceful purposes, such generating as electricity?
• On July 16, 1945, the first atomic bomb (or A-bomb), was tested at Alamogordo, New Mexico.
• On August 6, 1945, the Enola Gay, an American airplane, dropped the first atomic bomb ever used in warfare on Hiroshima, Japan, eventually killing over 140,000 people. On August 9, 1945, the United States dropped a second atomic bomb, this time on the Japanese city of Nagasaki. The drop is one mile off target, but it kills 75,000 people.
• On August 29, 1949, the Soviet Union tested its first atomic device at the Semipalatinsk test range. (Up to 20kt yield)
• On November 1, 1952, a full-scale, successful experiment was conducted by the United States with a fusion-type device.
• In 1946, the Atomic Energy Commission (AEC), civilian agency of the United States government, was established by the Atomic Energy Act to administer and regulate the production and use of atomic power.
  Among the major programs of the new commission were production of fissionable materials; accident prevention; research in biology, health, and metallurgy and production of electric power from the atom; studies in the production of nuclear aircraft; and the declassification of data on atomic energy.
  The most important goal of the 1946 act, however, was to put the immense power and possibilities of atomic energy under civilian control, although nuclear materials and facilities remained in government hands.
• A revised Atomic Energy Act in 1954 allowed for licensed private ownership of facilities to produce fissionable materials.
• In 1964 an amendment permitted private ownership of nuclear fuels, which aided the growing nuclear power industry.
• Under the Energy Reorganization Act of October 1974, the AEC was abolished, and two new federal agencies were established to administer and regulate atomic-energy activities: the Energy Research and Development Administration and the Nuclear Regulatory Commission.
• In 1977, the responsibilities of the former were transferred to the newly established Department of Energy.

The Equivalence of Mass and Energy

In 1905, Albert Einstein formulated his Special Theory of Relativity. According to this theory, mass can be considered to be another form of energy.

In our daily life, mass and energy seem to be separate. In a nuclear reaction, however, the energy released is often about a million times greater than in a chemical reaction, and the change in mass can easily be measured.
Mass and energy are related by what is certainly the best-known equation in physics:

E=mc²

in which E is the energy equivalent (called mass energy) of mass m, and c is the speed of light.

A very small amount of matter is equivalent to a vast amount of energy.
For example, 1 kg (2.2 lb) of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNT.

Nuclear Energy

Nuclear Energy is released during the splitting (fission) or fusing of atomic nuclei. The energy of any system, whether physical, chemical, or nuclear, is manifested by the system's 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. According to the Law of Conservation of Energy, if we add up the total amount of energy in the universe (we can describe energy quantitatively with units such as Joules or kilowatt-hours), the total amount never changes. In other words, energy is neither created nor lost, even though it may be converted from one form to another. Thus the law of conservation of energy is really the law of conservation of mass-energy.

The Atom

The atom consists of a small, massive, positively charged core, nucleus, surrounded by electrons.
The nucleus contains most of the mass of the atom. It is itself composed of neutrons and protons.

• The proton is 1,836 times as heavy as the electron.
  For an atom of hydrogen, which contains one electron and one proton, the proton provides 99.95 percent of the mass.
• The neutron weighs a little more than the proton.
  Elements heavier than hydrogen usually contain about the same number of protons and neutrons in their nuclei, so the atomic mass, or the mass of one atom, is usually about twice the atomic number.

Protons are affected by all four of the fundamental forces that govern all interactions between particles and energy in the universe.

• The electromagnetic force arises from matter carrying an electrical charge. It causes positively charged protons to attract negatively charged electrons and holds them in orbit around the nucleus of the atom.
• The strong nuclear force binds the protons and neutrons together into a compact nucleus. This force is 100 million times stronger than the electrical attraction that binds the electrons. It must be strong enough to overcome the repulsive force of the positively charged nuclear protons and to bind both protons and neutrons into the tiny nuclear volume. The nuclear force must also be of short range because its influence does not extend very far beyond the nuclear "surface." The nuclear force is due to a strong force that binds quarks together to form neutrons and protons.
• The other two fundamental forces, gravitation and the weak nuclear force, also affect the proton. Gravitation is a force that attracts anything with mass (such as the proton) to every other thing in the universe that has mass. It is weak when the masses are small, but can become very large when the masses are great.
• The weak nuclear force is a feeble force that occurs between certain types of elementary particles, including the proton, and governs how some elementary particles break up into other particles.

Chemical Elements

The number of protons in the nucleus of an atom determines what kind of chemical element it is. All substances in nature are made up of combinations of the 92 different chemical elements, substances that cannot be broken into simpler substances by chemical processes. The atom is the smallest part of a chemical element that still retains the properties of the element. The number of protons in each atom can range from one in the hydrogen atom to 92 in the uranium atom, the heaviest naturally occurring element.

The Nucleus

The nucleus contains most of the mass of the atom. It 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 nucleus of an atom is described by these characteristics:

• The mass number "A" of a nucleus is the number of nucleons (protons and neutrons) it contains.
• The atomic number "Z" is the number of positively charged protons.
  For example, the expression U, represents uranium with A=235 (number of nucleons) and Z=92 (number of protons).
• The binding energy of a nucleus is a measure of how tightly its protons and neutrons 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 Nuclear Binding Energy

The total energy required to break up a nucleus into its constituent protons and neutrons can be calculated from , called nuclear binding energy.

If we divide the binding energy of a nucleus by the number of protons and neutrons (number of nucleons), we get the binding energy per nucleon. This is the common term used to describe nuclear reactions because atomic numbers vary and total binding energy would be a relative term dependent upon that. The following figure, called the binding energy curve, shows a plot of nuclear binding energy as a function of mass number. The peak is at iron (Fe) with mass number equal to 56.

The binding energy per nucleon is a function of the mass number
The curve of binding energy implies that if two light nuclei near the left end
of the curve coalesce to form a heavier nucleus (the process is called fusion),
or if a heavy nucleus at the far right splits into two lighter ones (the process is
called fission), more tightly bound nuclei result, and energy will be released.

The rising of the binding energy curve at low mass numbers, tells us that energy will be released if two nuclides of small mass number combine to form a single middle-mass nuclide. This process is called nuclear fusion.

The eventual dropping of the binding energy curve at high mass numbers tells us on the other hand, that nucleons are more tightly bound when they are assembled into two middle-mass nuclides rather than into a single high-mass nuclide. In other words, energy can be released by the nuclear fission, or splitting, of a single massive nucleus into two smaller fragments.

Nuclear Fusion

The binding energy curve shows that energy can be released if two light nuclei combine to form a single larger nucleus. This process is called nuclear fusion. The process is hindered by the electrical repulsion that acts to prevent the two particles from getting close enough to each other to be within range and "fusing."

To generate useful amounts of power, nuclear fusion must occur in bulk matter. That is, many atoms need to fuse in order create a significant amount of energy. The best hope for bringing this about is to raise the temperature of the material so that the particles have enough energy - due to their thermal motions alone - to penetrate the electrical repulsion barrier. This process is known as thermonuclear fusion. Calculations show that these temperatures need to be close to the sun's temperature of 1.5 X 10⁷K.

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 atom, a free neutron, and 3.2 MeV (3.2x10⁶eV).

Although the energy release in the fusion process is less per nuclear reaction than in fission, 0.5 kg (1.1 lb) of the lighter material contains many more atoms; thus, the energy liberated from 0.5 kg (1.1 lb) of hydrogen-isotope fuel is equivalent to that of about 29 kilotons of TNT, or almost three times as much as from uranium. This estimate, however, is based on complete fusion of all hydrogen atoms. Fusion reactions occur only at temperatures of several millions of degrees, the rate increasing enormously with increasing temperature; such reactions consequently are known as thermonuclear (heat-induced) reactions. Strictly speaking, the term thermonuclear implies that the nuclei have a range (or distribution) of energies characteristic of the temperature. This plays an important role in making rapid fusion reactions possible by an increase in temperature.
Development of the hydrogen bomb was impossible before the perfection of A-bombs, for only the latter could yield that tremendous heat necessary to achieve fusion of hydrogen atoms. Atomic scientists regarded the A-bomb as the trigger of the projected thermonuclear device.

Thermonuclear Fusion in the Sun and other Stars

The sun radiates energy at the rate of 3.9 X 10²⁶ W (watts) and has been doing so for several billion years. The sun burns hydrogen in a "nuclear furnace." The fusion reaction in the sun is a multi-step process in which hydrogen is burned into helium, hydrogen being the "fuel" and helium the "ashes." Hydrogen burning has been going on in the sun for about 5 billion years and calculations show that there is enough hydrogen left to keep the sun going for about the same length of time into the future. The burning of hydrogen in the sun's core is alchemy on a grand scale in the sense that one element is turned into another.

Fusion takes place when the nuclei of hydrogen atoms with one proton each fuse together to form helium atoms with two protons. A standard hydrogen atom has one proton in its nucleus. There are two isotopes of hydrogen which also contain one proton, but contain neutrons as well. Deuterium contains one neutron while Tritium contains two. Deep within the star, A deuterium atom combines with a tritium atom. This forms a helium atom and an extra neutron. In the process, an incredible amount of energy is released.

When the star's supply of hydrogen is used up, it begins to convert helium into oxygen and carbon. As a star evolves and becomes still hotter, other elements can be formed by other fusion reactions. If the star is massive enough, it will continue until it converts carbon and oxygen into neon, sodium, magnesium, sulfur and silicon. Eventually, these elements are transformed into calcium, iron, nickel, chromium, copper and others until iron is formed. When the core becomes primarily iron, the star's nuclear reaction can no longer continue. Eements more massive than those with atomic number equal to 56 (iron) cannot be manufactured by further fusion processes as atomic number equal to 56 makes the peak of the binding energy curve. If nuclides were to fuse after that, then energy would be consumed as opposed to produced.
The inward pressure of gravity becomes stronger than the outward pressure of the nuclear reaction. The star collapses in on itself. What happens next depends on the star's mass.

Nuclear Fission

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 cesium-140, rubidium-93, three neutrons, and 200 MeV.
A nuclear fission reaction releases 10 million times as much energy as is released in a typical chemical reaction. In practical units, the fission of 1 kg (2.2 lb) of uranium-235 releases 18.7 million kilowatt-hours as heat.

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 two more atoms, thereby releasing four or more additional neutrons and initiating a self-sustaining series of nuclear fissions, or a chain reaction, which results in continuous release of nuclear energy.

Using the binding energy curve, we can estimate the energy released in this fission process. From this curve, we see that for heavy nuclides (mass about 240u), the mean biding energy per nucleon is about 7.6MeV. For middle-mass nuclides (mass about 120), it is about 8.5 MeV. This difference in total binding energy between a single large nucleus and two fragments (assumed to be equal) into which it may be split is then close to 200MeV. This is a relatively large amount of released energy per fission event. When a chain reaction occurs, many atoms and nuclei are involved so lots of energy is released.

Fission in Nuclear Reactors

To make large-scale use of the energy released in fission, one fission event must trigger another, so that the process spreads throughout the nuclear fuel as in a set of dominos. The fact that more neutrons are produced in fission than are consumed raises the possibility of a chain reaction. Such a reaction can be either rapid (as in an atomic bomb) or controlled (as in a reactor).

Core of the US Geological Survey (USGS)
nuclear research reactor, Triga II, while operating.

In a nuclear reactor, control rods made of cadmium or graphite or some other neutron-absorbing material are used to regulate the number of neutrons. The more exposed control rods, the less neutrons and vice versa. This also controls the multiplication factor k which is the ratio of the number of neutrons present at the beginning of a particular generation to the number present at the beginning of the next generation. For k=1, the operation of the reactor is said to be exactly critical, which is what we wish it to be for steady-power operation. Reactors are designed so that they are inherently supercritical (k>1); the multiplication factor is then adjusted to the critical operation by inserting the control rods.

An unavoidable feature of reactor operation is the accumulation of radioactive wastes, including both fission products and heavy "transuranic" nuclides such as plutonium and americium.

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nuclear weapons

Nuclear Weapons

In the 20th century, the rapid advance of industry and modern technology greatly increased both the destructive power of armed forces and the capacity of societies both to resist and to recover from an attack. Nuclear weapons carry the possibilities of destruction to a new level and are able to inflict far greater damage within a few hours than previously resulted from years of warfare. This not only makes the consequences of war worse but also raises new concerns about controlling such a destructive process. Indeed, nuclear weapons have not been used in war since the first two atomic bombs were dropped on Japan in 1945, but many countries, including many Third World countries, now have nuclear weapons.

Nuclear Weapons are explosive devices designed to release nuclear energy on a large scale for military reasons. The first atomic bomb (or A-bomb), was tested on July 16, 1945, at Alamogordo, New Mexico.

All explosives prior to that time derived their power from the rapid burning or decomposition of some chemical compound. Such chemical processes release only the energy of the outermost electrons in the atom. Nuclear explosives, on the other hand, involve energy sources within the nucleus of the atom. The A-bomb gained its power from the splitting (fission) of all the atomic nuclei in several kilograms of plutonium. A sphere about the size of a baseball produced an explosion equal to 20,000 tons of TNT.

Although nuclear bombs were originally developed as strategic weapons to be carried by large bombers, nuclear weapons are now available for a variety of both strategic and tactical applications. Not only can they be delivered by different types of aircraft, but rockets and guided missiles of many sizes can now carry nuclear warheads and can be launched from the ground, the air, or underwater. Large rockets can carry multiple warheads for delivery to separate targets.

The first USSR's ICBM thermonuclear warhead
Up to 3 Mt yield. Up to 8500 km flight range.
In service in 1960-1966

The Atom Bomb

The atomic bomb works by a physical phenomenon known as fission. In this case, particles, specifically nuclei, are split and great amounts of energy are released. This energy is expelled explosively and violently in the atomic bomb. The massive power behind the reaction in an atomic bomb arises from the forces that hold the atom together called the strong nuclear force.

The element used in atomic bombs is Uranium-235. Uranium's atoms are unusually large, and henceforth, it is hard for them to hold together firmly. This makes Uranium-235 an exceptional candidate for nuclear fission. Uranium is a heavy metal and has many more neutrons than protons. This does not enhance their capacity to split, but it does have an important bearing on their capacity to facilitate an explosion.

Uranium is not the only material used for making atomic bombs. Another material is the element Plutonium, in its isotope Pu-239. However, Plutonium will not start a fast chain reaction by itself. The material is not fissionable in and of itself, but may act as a catalyst to the greater reaction. The bomb basically works with a detonating head starting off the explosive chain reaction.

The Chain Reaction

When a uranium or other suitable 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 continuous release of energy.

Fission of uranium 235 nucleus.

When a U-235 atom splits, it gives off energy in the form of heat and Gamma radiation, which is the most powerful form of radioactivity and the most lethal. When this reaction occurs, the split atom will also give off two or three of its "spare" neutrons, which are not needed to make either of the parts after splitting. These spare neutrons fly out with sufficient force to split other atoms they come in contact with. In theory, it is necessary to split only one U-235 atom, and the neutrons from this will split other atoms, which will split more...so on and so forth. This progression does not take place arithmetically, but geometrically. All of this will happen within a millionth of a second.

Critical Mass

A small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, would not sustain a chain reaction. Too many neutrons escape through the surface area, which is relatively large compared with its volume, and thus are lost to the chain reaction. In a mass of uranium-235 about the size of a baseball, however, the number of neutrons lost through the surface is compensated for by the neutrons generated in additional fissions taking place within the sphere. The minimum amount of fissile material (of a given shape) required to maintain the chain reaction is known as the critical mass. Increasing the size of the sphere produces a supercritical assembly, in which the successive generations of fissions increase very rapidly, leading to a possible explosion as a result of the extremely rapid release of a large amount of energy.
In an atomic bomb, therefore, a mass of fissile material greater than the critical mass must be assembled instantaneously and held together for about a millionth of a second to permit the chain reaction to propagate before the bomb explodes. A heavy material, called a tamper, surrounds the fissile mass and prevents its premature disruption. The tamper also reduces the number of neutrons that escape.
If every atom in 0.5 kg (1.1 lb) of uranium were to split, the energy produced would equal the explosive power of 9.9 kilotons of TNT. In this hypothetical case, the efficiency of the process would be 100 percent. In the first A-bomb tests, this kind of efficiency was not approached. Moreover, a 0.5-kg (1.1-lb) mass is too small for a critical assembly.

Detonation of Atomic Bombs

Various systems have been devised to detonate the atomic bomb. The simplest system is the gun-type weapon, in which a projectile made of fissile material is fired at a target of the same material so that the two weld together into a supercritical assembly. The atomic bomb exploded by the United States over Hiroshima, Japan, on August 6, 1945, was a gun-type weapon. It had the energy of anywhere between 12.5 and 15 kilotons of TNT. Three days later the United States dropped a second atomic bomb over Nagasaki, Japan, with the energy equivalent of about 20 kilotons of TNT.

A more complex method, known as implosion, is used in a spherically shaped weapon. The outer part of the sphere consists of a layer of closely fitted and specially shaped devices, called lenses, consisting of high explosive and designed to concentrate the blast toward the center of the bomb. Each segment of the high explosive is equipped with a detonator, which in turn is wired to all other segments. An electrical impulse explodes all the chunks of high explosive simultaneously, resulting in a detonation wave that converges toward the core of the weapon. At the core is a sphere of fissile material, which is compressed by the powerful, inwardly directed pressure, or implosion. The density of the metal is increased, and a supercritical assembly is produced.
The Alamogordo test bomb, as well as the one dropped by the United States on Nagasaki, Japan, on August 9, 1945, were of the implosion type. Each was equivalent to about 20 kilotons of TNT.

Regardless of the method used to attain a supercritical assembly, the chain reaction proceeds for about a millionth of a second, liberating vast amounts of heat energy. The extremely fast release of a very large amount of energy in a relatively small volume causes the temperature to rise to tens of millions of degrees. The resulting rapid expansion and vaporization of the bomb material causes a powerful explosion.

I'll soon post an article with the effects of these reactions........

Data source: world-mysteries.com