Science

TABLE OF CONTENTS

1. Nuclear weapons – the biggest and the most destructive
   
   
   
2. The design and function of the hydrogen bomb
   
   
   
3. Weapon launchers
   
   
   
4. With the finger on the trigger
   
   
   
5. What happens at the bang?
   

Nuclear weapons – the biggest and the most destructive
Nuclear weapons are the most powerful weapons ever invented. The enormous power of nuclear weapons derives from the release of energy holding atomic nuclei together. The amount of energy a nuclear device can produce is measured in the equivalent number of tonnes of Trinitrotoluene (TNT) needed to obtain a corresponding yield of energy from a conventional explosive.

The atoms at the foundation of a nuclear weapon are minuscule – so tiny it is hard to imagine. To get an idea of the size of an atom, think about a baseball about seven centimetres in diameter. If an atom were the size of a baseball, the ordinary baseball, by comparison, would be almost 5,000 kilometres across. If atoms are so tiny, how can they create the enormous amounts of energy released in a nuclear explosion?

There are two main types of nuclear weapons: the atomic bomb, in which the energy is released by the division (fission) of the large atomic nuclei of uranium and plutonium; and the hydrogen bomb, also called a thermonuclear bomb, in which the energy release is caused by the merging (fusion) of the small atomic nuclei of hydrogen isotopes (i.e., hydrogen atoms with different numbers of neutrons in the nucleus).

The design and function of the atomic bomb
In the atomic bomb, energy is released through the process of fission. Heavy nuclei – uranium or plutonium – are divided when hit by neutrons. During the fission process, enormous amounts of energy are released. More neutrons shoot out from the divided nuclei and cause other nuclei to divide, starting a chain reaction that releases energy at an explosive rate. One condition for such a reaction to start is a certain minimum amount of material: what is called a critical mass. The amount of material needed to make up a critical mass depends on the physical properties of the nuclear isotope, and the density and shape of the material in the device's core.

Fission process
The fission bomb, also called the atomic bomb, is technically simpler to design than a thermonuclear weapon; fission bombs also produce lower yields. The atomic bomb is constructed to release as much energy as possible in as short time as possible through a chain reaction, before the released energy creates an explosion and the chain reaction is terminated. The longer the chain reaction, the larger the explosion. To start such a process, a certain minimum amount of material, what is called a critical mass, is required. The amount of material needed for a critical mass depends on the physical properties of the nuclear isotope, and the density and shape of the material in the device's core.

The most common materials for fission processes in nuclear weapons are uranium-235 and plutonium-239. These substances can maintain a chain reaction long enough to produce a very powerful reaction. The products of the fission process are some hundred radioactive isotopes of krypton, barium, iodine-131, cesium-137, and strontium-90.

The picture shows a nuclear fission, that is, the chain reaction causing a nuclear explosion. In this example, uranium-235 is used, but it could also be the isotope plutonium-239.

1. A uranium-235 atom absorbs a neutron and divides into two new atoms (fission fragments), releasing three new neutrons and some binding energy.

2. One of those neutrons is absorbed by an atom of uranium-238 and does not continue the reaction. Another neutron is simply lost and does not collide with anything, also not continuing the reaction. One neutron, however, does collide with an atom of uranium-235, which then divides and releases two neutrons and some binding energy.

3. Both of those neutrons collide with uranium-235 atoms, each of which divides and releases between one and three neutrons, that can then continue the fission reaction.

Nuclear fusion is the process by which multiple light atomic particles join together to form a heavier nucleus. It is accompanied by the release of massive amounts of energy. For fusion to take place, very high temperatures and heavy pressures are necessary. Fusion occurs naturally in the nuclei of stars, creating the energy that causes stars to shine. On Earth, nuclear fusion must be artificially induced in order to create a nuclear explosion or other release of energy.

During fusion, energy is released from the merging of two different atomic nuclei into a third. The most commonly used nuclei are the hydrogen isotopes deuterium and tritium, which fuse into helium-4. Since both nuclei are positively charged, they repel each other; so for fusion to happen, the nuclei have to move towards each other at a very great speed. For this speed to be achieved, the nuclei must be heated to tens of millions of degrees centigrade. This is the reason that the hydrogen bomb is also called a thermonuclear weapon (thermo = heat). The release of energy in the fusion process creates the massive nuclear explosion.

Nuclear weapons material
The most common materials used for nuclear weapons are uranium (U) and plutonium (P). While uranium occurs in nature, plutonium must be produced synthetically in a nuclear reactor using uranium-238. Natural uranium consists mainly of two isotopes: U-235 and U-238. Both of these have very long half-lives (that is, the time it takes for the radiation to reduce by half) – 0.7 and 4.5 billion years respectively. Naturally occurring uranium has a low level of U-235: only about 0.7 percent. Fuel for nuclear power reactors usually contains 3-4 percent of U-235. To create weapons grade uranium, around 95 percent of U-235 is usually required. To obtain these higher levels of U-235, uranium needs to be enriched, which is done in uranium enrichment facilities – large industries that separate the material and produce a high concentration of the necessary U-235 isotope.

Plutonium is a by-product of nuclear power reactors that can be used to produce nuclear weapons. The process is difficult, however, because plutonium is highly radioactive and, when produced in this way, is mixed with other isotopes not wanted in a nuclear weapon.

Depleted U-238 is used by the military as armour-penetrating ammunition and also to reinforce the armour shielding on tanks. The use of depleted uranium has been heavily criticised, because it contaminates the areas where uranium weapons have been used and has been linked to serious health consequences.

Weapon launchers
For a nuclear weapon to reach its target, some kind of launching system is needed. Missiles are the preferred method today for delivering weapons to their targets. A missile is basically an oblong weapon with long-range flight capabilities, carrying a warhead that explodes upon reaching the target. These can be ballistic missiles without steering capabilities, or cruise missiles that can correct their trajectories or even navigate. Depending on the range of the delivery vehicle and the purpose of the weapon, nuclear weapons are classified as strategic, intermediate and tactical.

There is no common definition of these terms, and the distinctions among the different weapon types are somewhat fuzzy. Strategic weapons are defined by their role in supporting the "strategy" of deterrence (also known as mutually assured destruction), and they are usually thought of as long-range weapons -- 5,500 or more is a standard definition. Intermediate weapons, as the name suggests, are somewhere between strategic and tactical, while tactical nuclear weapons are intended for battlefield use. The yield of the weapons is irrelevant: many strategic weapons have yields far smaller than those of the largest tactical weapons.

With the finger on the trigger
The American and Russian presidents have a constant follower wherever they go: a suitcase carried by a carefully selected military officer who has undertaken the most rigorous security training. The suitcase includes a satellite radio and the codes for launching the state's nuclear arsenal. It is called the nuclear football. The ”football” was established during the Cold War, when the US and Soviet leaders wanted to have constant control over the possibility of launching a nuclear attack.

In the US, the President alone has the mandate to order a nuclear launch. After the dissolution of the Soviet Union, Russia inherited the Soviet ”football”. In Russia, however, the President, the Chief of the General Staff, and the Defence Minister, can order a nuclear launch. (1)

Nuclear war by mistake
The big nuclear weapon states have an extensive network of satellites and radar stations to warn them of an enemy nuclear attack. The purpose of the system is to provide information about an incoming attack early enough to launch a counter attack before enemy nuclear warheads strike. False alarms, unfortunately, are not that unusual. Fires, test launches of weapons, electronic disturbances, or computer malfunctions may all cause alarms. Hence, even if the nuclear weapon states do not intend to use their nuclear weapons, there is always a risk for nuclear war – caused by technical problems or the human factor. From the moment the US and the former Soviet Union started targeting each other's cities with nuclear weapons, people have been concerned about nuclear war by mistake.

During the Cold War, the US and the USSR developed so-called launch-on-warning systems to identify and counter an enemy nuclear attack before it reaches its target. High alert status of nuclear weapons also increases the risk of use by mistake.

On a number of occasions, false alarms have been received, creating an imminent risk of nuclear war. If more states acquire nuclear weapons, the risk of use by mistake will also increase. As we already see in South Asia, the very short warning times -- as little as six minutes -- of a possible nuclear attack by either India or Pakistan, place intolerable strains on decision makers and increase the chances of nuclear war by accident or miscalculation enormously.

What happens at the bang?
During a nuclear explosion, enormous amounts of energy from the nuclei of the atom are released; an intense flash of light dazzles, blinds, and burns everyone within a certain radius. There is no time to seek protection from this heat wave unless one is warned beforehand.

Everything at the place where the bomb detonates will be pulverised and burned up. Smoke, gases, and radioactive particles rise from the explosion, creating a giant mushroom-shaped cloud. As soon as the cloud is shaped, ionising radiation is released, causing radiation sickness and related injuries. At the same time, an electromagnetic pulse will destroy all electronic equipment. The explosion also creates a massive shock wave that destroys buildings and other infrastructure, and kills and injures people and living organisms at many kilometres' distance. These immediate effects are followed by radioactive fallout that is spread by wind and rain over large areas that remain contaminated for a very long time.

1. http://findarticles.com/p/articles/mi_m1571/
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