Effects of nuclear explosions

The energy released from a nuclear weapon detonated in the troposphere can be divided into four basic categories[1]:

* Blast—40-50% of total energy
* Thermal radiation—30-50% of total energy
* Ionizing radiation—5% of total energy
* Residual radiation—5-10% of total energy

However, depending on the design of the weapon and the environment in which it is detonated the energy distributed to these categories can be increased or decreased to the point of nullification. The blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. Locations such as submarine, surface, airburst, or exo-atmospheric determine how much energy is produced as blast and how much as radiation. In general, denser mediums around the bomb, like water, absorb more energy, and create more powerful shockwaves while at the same time limiting the area of its effect.

The dominant effects of a nuclear weapon where people are likely to be affected (blast and thermal radiation) are identical physical damage mechanisms to conventional explosives. However the energy produced by a nuclear explosive is millions of times more powerful per gram and the temperatures reached are briefly in the tens of millions of degrees.

Energy from a nuclear explosive is initially released in several forms of penetrating radiation. When there is a surrounding material such as air, rock, or water, this radiation interacts with and rapidly heats it to an equilibrium temperature. This causes vaporization of surrounding material resulting in its rapid expansion. Kinetic energy created by this expansion contributes to the formation of a shockwave. When a nuclear detonation occurs in air near sea level, much of the released energy interacts with the atmosphere and creates a shockwave which expands spherically from the hypocenter. Intense thermal radiation at the hypocenter forms a fireball and if the burst is low enough, its often associated mushroom cloud. In a burst at high altitudes, where the air density is low, more energy is released as ionizing gamma radiation and x-rays than an atmosphere displacing shockwave.

In 1945 there was some initial speculation among the scientists developing the first nuclear weapons that there might be a possibility of igniting the Earth's atmosphere with a large enough nuclear explosion. This would concern a nuclear reaction of two nitrogen atoms forming a carbon and an oxygen atom, with release of energy. This energy would heat up the remaining nitrogen enough to keep the reaction going until all nitrogen atoms were consumed. This was, however, quickly shown to be unlikely enough to be considered impossible [2]. Nevertheless, the notion has persisted as a rumour for many years.

THE PRINCIPLES

FISSION/CHAIN REACTION

The atomic bomb gets it's energy from fission (splitting) of the nulcei (core) of uranium or plutonium atoms. Albert Einstein explained how the fission of heavy atoms can produce energy released as dangerously high levels of heat and radiation. He published his theory in 1905 which is the well-known equation E = m c-squared.
This states that a given mass (m),
is associated with an amount of energy (E),
equal to this mass multiplied by the square of the speed of light (c).

A very small amount of matter is equivalent to a vast amount of energy. For example, 1 kg of matter converted completely into energy would be equivalent to the energy released by exploding 22 megatons of TNThe neutron is the most effective particle to cause uranium fission. Only one nuetron is needed to split an atom. When the atom fissions (splits), it splits into two smaller atoms which are most always radioactive and releases an enormous amount of energy and two or three nuetrons. The nuetrons released could then possibly hit other nuclei of uranium which causes them to split in the same fashion. This is a chain reaction (a series of fissions). A baseball made of plutonium produced an explosion equal to 20,000 tons of TNT.

CRITICAL MASS

If you had a small sphere of pure fissile material, such as uranium-235, about the size of a golf ball, it would not sustain a chain reaction. Too many neutrons escape through the surface area, and in turn are lost to the chain reaction. This is called a subcritical amount.
In a mass of uranium-235 about the size of a baseball, there are more neutrons hitting the atoms of the fissile material than are escaping through the surface area, thus sustaining the chain reaction.

The minimum amount of fissile material 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.

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.

PRODUCING AN EXPLOSION

When the scientist assemble the bomb, they cannot just create a supercritical mass of fissile material because it would explode.
We get around this problem by creating two subcritical amounts of fissile material then assembling them in the bomb apart from each other.
They do not become critical until an explosion is set off to fire one of the subcritical masses at the other one. The force of the impact welds the two pieces together. Together, these create a critical mass.
It takes about 1 millionth of a second for the nuclear explosion to occur.

THE MANHATTON PROJECT

Research on atomic bombs was begun around the same time in several countries, including Germany, but in the United States, the actual building of an atomic bomb was already underway by 1942 under the code name "Manhattan Project."
The project was carried out in extreme secrecy using a large amount of the national budget. 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, Major General Leslie Groves.
In September 1944 it was determined that an A-bomb would be used against Japan.
On July 16, 1945 in the desert near Alamogordo, New Mexico, the United States successfully conducted the world 's first nuclear test, the "Fat Man" test, codename Trinity. he bomb used in the Trinity test was called the "Fat Man".

When the bomb exploded and the fireball continued to consume the desert, General Thomas F. Farrell, Groves' assistant cried out, "the longhaors have let it get away from them!"

The next day he described the blast a bit more accurately.

"For the first time in history there was a nuclear explosion; and what an explosion! The lighting effects beggarded description. The whole country was lighted by a searing light with the intensity many times that of the midday sun. It was golden, purple, violet, grey and blue. It lighted every peak.....of the nearby mountain range with a clarity and beauty that......the great poets dream about but describe most poorly and inadequately. Thirty seconds after the explosion came, first, the air blast pressing hard against people and things, to be followed almost immediately by the strong, sustained, awesome roar which warned of doomsday and made us feel that we puny things were blasphemous to dare tamper with the forces heretofore reserved to the Almighty."

History of nuclear weapons

The history of nuclear weapons chronicles the development of nuclear weapons. Nuclear weapons are devices that possess enormous destructive potential derived from nuclear fission or nuclear fusion reactions. Starting with the scientific breakthroughs of the 1930s which made their development possible, continuing through the nuclear arms race and nuclear testing of the Cold War, and finally with the questions of proliferation and possible use for terrorism in the early 21st century.

The first fission weapons, also known as "atomic bombs," were developed jointly by the United States, Britain and Canada during World War II in what was called the Manhattan Project. In August 1945 two were dropped on Japan ending the pacific war. An international team was dispatched to help work on the project. The Soviet Union started development shortly thereafter with their own atomic bomb project, and not long after that both countries developed even more powerful fusion weapons also called "hydrogen bombs."

During the Cold War, these two countries each acquired nuclear weapons arsenals numbering in the thousands, placing many of them onto rockets which could hit targets anywhere in the world. Currently there are at least nine countries with functional nuclear weapons. A considerable amount of international negotiating has focused on the threat of nuclear warfare and the proliferation of nuclear weapons to new nations or groups.
There have been (at least) four major false alarms, the most recent in 1995, that resulted in the activation of either the US' or Russia's nuclear attack early warning protocols.

The Difference Between Nuclear Fission & Nuclear Fusion

There are two types of atomic explosions that can be facilitated by Uranium-235: fission and fusion. Fission, simply put, is a nuclear reaction in which an atomic nucleus splits into fragments, usually two fragments of comparable mass, emitting 100 million to several hundred million volts of energy. This energy is expelled explosively and violently in the atomic bomb. A fusion reaction is usually started with a fission reaction, but unlike the fission (atomic) bomb, the fusion (hydrogen) bomb derives its power from the fusing of nuclei of various hydrogen isotopes into helium nuclei. This article discusses the A-bomb or atomic bomb.

The massive power behind the reaction in an atomic bomb arises from the forces that hold the atom together. These forces are akin to, but not quite the same as, magnetism.

About Atoms

Atoms are comprised of various numbers and combinations of the three sub-atomic particles: protons, neutrons and electrons. Protons and neutrons cluster together to form the nucleus (central mass) of the atom while the electrons orbit the nucleus much like planets around a sun. It is the balance and arrangement of these particles that determine the stability of the atom.

Splitability

Most elements have very stable atoms which are impossible to split except by bombardment in particle accelerators. For all practical purposes, the only natural element whose atoms can be split easily is uranium, a heavy metal with the largest atom of all natural elements and an unusually high neutron-to-proton ratio. This higher ratio does not enhance its "splitability," but it does have an important bearing on its ability to facilitate an explosion, making uranium-235 an exceptional candidate for nuclear fission.

Uranium Isotopes

There are two naturally-occurring isotopes of uranium. Natural uranium consists mostly of isotope U-238, with 92 protons and 146 neutrons (92+146=238) per atom. Mixed with this is a 0.6% accumulation of U-235, with only 143 neutrons per atom. The atoms of this lighter isotope can be split, thus it is "fissionable" and useful in making atomic bombs. Neutron-heavy U-238 has a role to play in the atomic bomb as well since its neutron-heavy atoms can deflect stray neutrons, preventing an accidental chain reaction in a uranium bomb and keeping neutrons contained in a plutonium bomb. U-238 can also be "saturated" to produce plutonium (Pu-239), a man-made, radioactive element also used in atomic bombs.

Both isotopes of uranium are naturally radioactive; their bulky atoms disintegrating over time. Given enough time (hundreds of thousands of years) uranium will eventually lose so many particles that it will turn into lead. This process of decay can be greatly accelerated in what is known as a chain reaction. Instead of disintegrating naturally and slowly, the atoms are forcibly split by bombardment with neutrons.

Chain Reactions

A blow from a single neutron is enough to split the less-stable U-235 atom, creating atoms of smaller elements (often barium and krypton) and releasing heat and gamma radiation (the most powerful and lethal form of radioactivity). The chain reaction occurs when "spare" neutrons from this atom fly out with sufficient force to split other U-235 atoms they come in contact with. In theory, it is necessary to split only one U-235 atom, which will release neutrons which will split other atoms, which will release neutrons ... and so on. This progression is not arithmetic; it is geometric and takes place within a millionth of a second.

The minimum amount to start a chain reaction as described above is known as super critical mass. For pure U-235, it is 110 pounds (50 kilograms). No uranium is ever quite pure, however, so in reality more will be needed. U-235, U-238 and Plutonium.

About Plutonium

Uranium is not the only material used for making atomic bombs. Another material is the Pu-239 isotope of the man-made element plutonium. Plutonium is only found naturally in minute traces, so useable amounts must be produced from uranium. In a nuclear reactor, uranium's heavier U-238 isotope can be forced to acquire extra particles, eventually becoming the plutonium.

Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the Plutonium itself. In certain types of bombs, a mixture of the elements Beryllium and Polonium is used to bring about this reaction. Only a small piece is needed (super critical mass is about 32 pounds, though as little as 22 can be used). The material is not fissionable in and of itself, but merely acts as a catalyst to the greater reaction.