Nuclear Energy

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.

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"And Jesus said unto them, Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you."

Jesus

Matthew, 17:20, The King James Bible

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.

For a very different and controversial theory of the atom 
visit The Living Atom Theory 

Another controversial theory: Metaparticles

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.

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.

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.

On August 9, 1945, the American B-29 bomber, Bock's Car dropped Fat Man, a plutonium implosion-type bomb on Nagasaki.  Of the 286,00 people living in Nagasaki at the time of the blast, 74,000 were killed and another 75,000 sustained severe injuries.

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.

The Hydrogen Bomb

The Hydrogen bomb works on a different physical principle known as nuclear fusion. In nuclear fusion, the nuclei of atoms join together, or fuse to form a heavier nucleus (specifically, nuclei of the isotopes of hydrogen combine to form a heavier helium nucleus). This happens only under very hot conditions. The explosion of an atomic bomb attached to a hydrogen bomb provides the heat to start fusion. 

Fusion releases energy due to the overall loss in mass. The total of the masses of the particles which go into a fusion reaction is bigger than the total of  the masses of the particles which come out.  The "mass difference" takes the form of energy. 

The hydrogen bomb is thousands of times more powerful than an atomic bomb. There have not been any hydrogen bombs used in warfare, however there have been hydrogen bomb tests. Most of these tests are done underwater due to risk of destruction. 

The H-bomb is extremely powerful.  A common hydrogen bomb has the power of up to 10 megatons. This atomic bomb dropped on Hiroshima, Japan which killed over 140,000 people had the power of 13 kilotons. All the explosions in World War II totaled "only" 2 megatons - 20%  of the power of ONE common hydrogen bomb.

Thermonuclear Tests

On November 1, 1952, a full-scale, successful experiment was conducted by the United States with a fusion-type device. This test, called Mike, which was part of Operation Ivy, produced an explosion with power equivalent to several million tons of TNT (that is, several megatons). 

	The first U.S. hydrogen bomb test, Mike, is shown in the Pacific at Eniwetok Atoll, Marshall Islands, November 1, 1952.
	The Soviet Union detonated a thermonuclear weapon on August 12, 1953 at the Semipalatinsk test range. Up to 400 kt yield.

On March 1, 1954, the United States exploded a fusion bomb with a power of 15 megatons. It created a glowing fireball, more than 4.8 km (more than 3 mi) in diameter, and a huge mushroom cloud, which quickly rose into the stratosphere.
The March 1954 explosion led to worldwide recognition of the nature of radioactive fallout. The fallout of radioactive debris from the huge bomb cloud also revealed much about the nature of the thermonuclear bomb. Had the bomb been a weapon consisting of an A-bomb trigger and a core of hydrogen isotopes, the only persistent radioactivity from the explosion would have been the result of the fission debris from the trigger and from the radioactivity induced by neutrons in coral and seawater. Some of the radioactive debris, however, fell on the Lucky Dragon, a Japanese vessel engaged in tuna fishing about 160 km (about 100 mi) from the test site. This radioactive dust was later analyzed by Japanese scientists. The results demonstrated that the bomb that dusted the Lucky Dragon with fallout was more than just an H-bomb.

The world's most powerful experimental bomb.  Tested on November 30, 1961 at the Novaya Zemlya test range.  More that 100 Mt rated yield.  Test-detonated at half the yield.

Fission-Fusion-Fission Bomb 

The thermonuclear bomb exploded in 1954 was a three-stage weapon. The first stage consisted of a big A-bomb, which acted as a trigger. The second stage was the H-bomb phase resulting from the fusion of deuterium and tritium within the bomb. In the process helium and high-energy neutrons were formed. The third stage resulted from the impact of these high-speed neutrons on the outer jacket of the bomb, which consisted of natural uranium, or uranium-238. No chain reaction was produced, but the fusion neutrons had sufficient energy to cause fission of the uranium nuclei and thus added to the explosive yield and also to the radioactivity of the bomb residues.

The Neutron Bomb - Clean H-Bomb

On the average, about 50 percent of the power of an H-bomb results from thermonuclear-fusion reactions and the other 50 percent from fission that occurs in the A-bomb trigger and in the uranium jacket. A clean H-bomb is defined as one in which a significantly smaller proportion than 50 percent of the energy arises from fission. Because fusion does not produce any radioactive products directly, the fallout from a clean weapon is less than that from a normal or average H-bomb of the same total power. If an H-bomb were made with no uranium jacket but with a fission trigger, it would be relatively clean. Perhaps as little as 5 percent of the total explosive force might result from fission; the weapon would thus be 95 percent clean. The enhanced-radiation fusion bomb, also called the neutron bomb, which has been tested by the United States and other nuclear powers, does not release long-lasting radioactive fission products. However, the large number of neutrons released in thermonuclear reactions is known to induce radioactivity in materials, especially earth and water, within a relatively small area around the explosion. 
Thus the neutron bomb is considered a tactical weapon because it can do serious damage on the battlefield, penetrating tanks and other armored vehicles and causing death or serious injury to exposed individuals, without producing the radioactive fallout that endangers people or structures miles away.

Effects of Nuclear Weapons

Nuclear Explosions produce both immediate and delayed destructive effects. Blast, thermal radiation, prompt ionizing radiation are produced and cause significant destruction within seconds or minutes of a nuclear detonation. The delayed effects, such as radioactive fallout and other possible environmental effects, inflict damage over an extended period ranging from hours to years.

The effects of nuclear weapons were carefully observed, both after the bombings of Hiroshima and Nagasaki and after many test explosions in the 1950s and early 1960s. The basic effects of nuclear explosion are:

• Blast Effects
• Thermal Effects
• Radiation
• Electromagnetic pulse (EMP)
• Fallout
• Climatic Effects

Blast Effects

As is the case with explosions caused by conventional weapons, most of the damage to buildings and other structures from a nuclear explosion results, directly or indirectly, from the effects of blast.

The very rapid expansion of the bomb materials produces a high-pressure pulse, or shock wave, that moves rapidly outward from the exploding bomb. In air, this shock wave is called a blast wave because it is equivalent to and is accompanied by powerful winds of much greater than hurricane force. Damage is caused both by the high excess (or overpressure) of air at the front of the blast wave and by the extremely strong winds that persist after the wave front has passed. 
In general, large buildings are destroyed by the change in air pressure, while people and objects such as trees and utility poles are destroyed by the wind.

The degree of blast damage suffered on the ground depends on:

• TNT equivalent of the explosion; 

• the altitude at which the bomb is exploded (the height of burst);

• and the distance of the structure from ground zero (the point directly under the bomb). 

Assuming a height of burst that will maximize the damage area, a 10-kiloton bomb will cause severe damage to wood-frame houses, such as are common in the United States, to a distance of more than 1.6 km (more than 1 mi) from ground zero and moderate damage as far as 2.4 km (1.5 mi). (A severely damaged house probably would be beyond repair.) The damage radius increases with the power of the bomb, approximately in proportion to its cube root. If exploded at the optimum height, therefore, a 10-megaton weapon, which is 1,000 times as powerful as a 10-kiloton weapon, will increase the distance tenfold, that is, out to 17.7 km (11 mi) for severe damage and 24 km (15 mi) for moderate damage of a frame house.

When a nuclear weapon is detonated on or near Earth's surface, the blast digs out a large crater. Some of the material that used in be in the crater is deposited on the rim of the crater; the rest is carried up into the air and returns to Earth as radioactive fallout. An explosion that is farther above the Earth's surface than the radius of the fireball does not dig a crater and produces negligible immediate fallout. For the most part, a nuclear blast kills people by indirect means rather than by direct pressure.

Thermal Effects

Approximately 35 percent of the energy from a nuclear explosion is an intense burst of thermal radiation, i.e., heat. The effects are similar to the effect of a two-second flash from an enormous sunlamp. The very high temperatures attained in a nuclear explosion result in the formation of an extremely hot incandescent mass of gas called a fireball. 
For a 10-kiloton explosion in the air, the fireball will attain a maximum diameter of about 300 m (about 1,000 ft); for a 10-megaton weapon the fireball may be 4.8 km (3 mi) across. A flash of thermal (or heat) radiation is emitted from the fireball and spreads out over a large area, but with steadily decreasing intensity. The amount of heat energy received a certain distance from the nuclear explosion depends on the power of the weapon and the state of the atmosphere. If the visibility is poor or the explosion takes place above clouds, the effectiveness of the heat flash is decreased. 
The thermal radiation falling on exposed skin can cause what are called flash burns. A 10-kiloton explosion in the air can produce moderate (second-degree) flash burns, which require some medical attention, as far as 2.4 km (1.5 mi) from ground zero; for a 10-megaton bomb, the corresponding distance would be more than 32 km (more than 20 mi). Milder burns of bare skin would be experienced even farther out. Most ordinary clothing provides protection from the heat radiation, as does almost any opaque object. Flash burns occur only when the bare skin is directly exposed, or if the clothing is too thin to absorb the thermal radiation.
The heat radiation can initiate fires in dry, flammable materials, for example, paper and some fabrics, and such fires may spread if conditions are suitable. 
The evidence from the A-bomb explosions over Japan indicates that many fires, especially in the area near ground zero, originated from secondary causes, such as electrical short circuits, broken gas lines, and upset furnaces and boilers in industrial plants. The blast damage produced debris that helped to maintain the fires and denied access to fire-fighting equipment. Thus, much of the fire damage in Japan was a secondary effect of the blast wave.
Under some conditions, such as existed at Hiroshima but not at Nagasaki, many individual fires can combine to produce a fire storm similar to those that accompany some large forest fires. The heat of the fire causes a strong updraft, which produces strong winds drawn in toward the center of the burning area. These winds fan the flame and convert the area into a holocaust in which everything flammable is destroyed. Inasmuch as the flames are drawn inward, however, the area over which such a fire spreads may be limited.

Since the thermal radiation travels at roughly the speed of light, the flash of light and heat precedes the blast wave by several seconds, just as lightning is seen before thunder is heard.
The visible light will produce "flashblindness" in people who are looking in the direction of the explosion. Flashblindness can last for several minutes, after which recovery is total. If the flash is focused through the lens of the eye, a permanent retinal burn will result. At Hiroshima and Nagasaki, there were many cases of flashblindness, but only one case of retinal burn, among the survivors. On the other hand, anyone flashblinded while driving a car could easiIy cause permanent injury to himself and to others.

Penetrating Radiation

Besides heat and blast, an exploding nuclear bomb has a unique effect—it releases penetrating nuclear radiation, which is quite different from thermal (or heat) radiation. Direct radiation occurs at the time of the explosion. It can be very intense, but its range is limited. For large nuclear weapons, the range of intense direct radiation is less than the range of lethal blast and thermal radiation effects. However, in the case of smaller weapons, direct radiation may be the lethal effect with the greatest range. Direct radiation did substantial damage to the residents of Hiroshima and Nagasaki. Human response to ionizing radiation is subject to great scientific uncertainty and intense controversy. It seems likely that even small doses of radiation do some harm.
When absorbed by the body, nuclear radiation can cause serious injury. For an explosion high in the air, the injury range for these radiations is less than for blast and fire damage or flash burns. In Japan, however, many individuals who were protected from blast and burns succumbed later to radiation injury.
Nuclear radiation from an explosion may be divided into two categories, namely, prompt radiation and residual radiation. The prompt radiation consists of an instantaneous burst of neutrons and gamma rays, which travel over an area of several square miles. Gamma rays are identical in effect to X rays (see X Ray). Both neutrons and gamma rays have the ability to penetrate solid matter, so that substantial thicknesse of shielding materials are required.
The residual nuclear radiation, generally known as fallout, can be a hazard over very large areas that are completely free from other effects of a nuclear explosion. In bombs that gain their energy from fission of uranium-235 or plutonium-239, two radioactive nuclei are produced for every fissile nucleus split. These fission products account for the persistent radioactivity in bomb debris, because many of the atoms have half-lives measured in days, months, or years.
Two distinct categories of fallout, namely, early and delayed, are known. If a nuclear explosion occurs near the surface, earth or water is taken up into a mushroom-shaped cloud and becomes contaminated with the radioactive weapon residues. The contaminated material begins to descend within a few minutes and may continue to fall for about 24 hours, covering an area of thousands of square miles downwind from the explosion. This constitutes the early fallout, which is an immediate hazard to human beings. No early fallout is associated with high-altitude explosions. If a nuclear bomb is exploded well above the ground, the radioactive residues rise to a great height in the mushroom cloud and descend gradually over a large area.
Human experience with radioactive fallout has been minimal. The principal known case histories have been derived from the accidental exposure of fishermen and local residents to the fallout from the 15-megaton explosion that occurred on March 1, 1954. The nature of radioactivity, however, and the immense areas contaminable by a single bomb undoubtedly make radioactive fallout potentially one of the most lethal effects of nuclear weapons.

Spectacular photo album of Nuclear Explosions
 

Electromagnetic Pulse

Electromagnetic pulse (EMP) is an electromagnetic wave similar to radio waves, which results from secondary reactions occurring when the nuclear gamma radiation is absorbed in the air or ground. It differs from the usual radio waves in two important ways. First, it creates much higher electric field strengths. Whereas a radio signal might produce a thousandth of a volt or less in a receiving antenna, an EMP pulse might produce thousands of volts. Secondly, it is a single pulse of energy that disappears completely in a small fraction of a second. In this sense, it is rather similar to the electrical signal from lightning, but the rise in voltage is typically a hundred times faster. This means that most equipment designed to protect electrical facilities from lightning works too slowly to be effective against EMP.

An attacker might detonate a few weapons at high altitudes in an effort to destroy or damage the communications and electric power systems of the victim. There is no evidence that EMP is a physical threat to humans. However, electrical or electronic systems, particularly those connected to long wires such as power lines or antennas, can undergo damage. There could be actual physical damage to an electrical component or a temporary disruption of operation.

Fallout Radiation

Fallout radiation is received from particles that are made radioactive by the effects of the explosion, and subsequently distributed at varying distances from the site of the blast. While any nuclear explosion in the atmosphere produces some fallout, the fallout is far greater if the burst is on the surface, or at least low enough for the firebal to touch the ground. The significant hazards come from particles scooped up from the ground and irradiated by the nuclear explosion. The radioactive particles that rise only a short distance (those in the "stem" of the familiar mushroom cloud) will fall back to earth within a matter of minutes, landing close to the center of the explosion. Such particles are unlikely to cause many deaths, because they will fall in areas where most people have already been killed. However, the radioactivity will complicate efforts at rescue or eventual reconstruction. The radioactive particles that rise higher will be carried some distance by the wind before returning to Earth, and hence the area and intensity of the fallout is strongly influenced by local weather conditions. Much of the material is simply blown downwind in a long plume. Rainfall also can have a significant influence on the ways in which radiation from smaller weapons is deposited, since rain will carry contaminated particles to the ground. The areas receiving such contaminated rainfall would become "hot spots," with greater radiation intensity than their surroundings.

Climatic Effects

Besides the blast and radiation damage from individual bombs, a large-scale nuclear exchange between nations could conceivably have a catastrophic global effect on climate. This possibility, proposed in a paper published by an international group of scientists in December 1983, has come to be known as the "nuclear winter" theory. According to these scientists, the explosion of not even one-half of the combined number of warheads in the United States and Russia would throw enormous quantities of dust and smoke into the atmosphere. The amount could be sufficient to block off sunlight for several months, particularly in the northern hemisphere, destroying plant life and creating a subfreezing climate until the dust dispersed. The ozone layer might also be affected, permitting further damage as a result of the sun's ultraviolet radiation. Were the results sufficiently prolonged, they could spell the virtual end of human civilization. The nuclear winter theory has since become the subject of enormous controversy. It found support in a study released in December 1984 by the U.S. National Research Council, and other groups have undertaken similar research. In 1985, however, the U.S. Department of Defense released a report acknowledging the validity of the concept but saying that it would not affect defense policies.

Alıntı: http://www.world-mysteries.com/sci_9.htm 

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