ICAN around the world
The science of nuclear weapons
Contents
Types of nuclear weapons
Nuclear weapons are the most powerful weapons ever invented. Their enormous power 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 core of a nuclear weapon are minuscule. 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 5000km 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 — that is, hydrogen atoms with different numbers of neutrons in the nucleus.
Design of atomic bombs
In an 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 is a certain minimum amount of material 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.
A fission bomb is technically simpler to design than a thermonuclear weapon, and produces a lower yield. They are constructed to release as much energy as possible in as short a 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. The most common materials for fission processes in nuclear weapons are uranium-235 and plutonium-239.
The bombs dropped on Hiroshima and Nagasaki were both atomic bombs. The Hiroshima bomb had an explosive power of about 13 kilotons, and the Nagasaki bomb about 21 kilotons. The Hiroshima bomb was based on uranium-235 and the Nagasaki bomb on plutonium-239.
Design of hydrogen bombs
Energy is released from the hydrogen bomb through the process of fusion. During fusion, the energy develops out of 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. For fusion to occur, the nuclei must move towards each other at great speed. For this speed to be achieved, the nuclei must be heated to tens of millions of degrees centigrade.
Thermonuclear weapons don’t have the same kind of limit to their explosive effect as fission weapons. The largest thermonuclear bomb ever tested had a yield of 58 megatons, which is the equivalent of about 4600 Hiroshima-sized bombs. This test took place over Novaya Zemlya in what was then the Soviet Union.
Nuclear weapons material
The most common materials used for nuclear weapons are uranium and plutonium. While uranium occurs in nature, plutonium must be produced synthetically in a nuclear reactor using uranium-238. Natural uranium consists mainly of two isotopes: uranium-235 and uranium-238. Both of these have very long half-lives (the time it takes for the radiation to reduce by half) — 0.7 billion and 4.5 billion years respectively.
Naturally occurring uranium has a low level of uranium-235: only about 0.7 per cent. Fuel for nuclear power reactors usually contains 3 to 4 per cent of uranium-235. To create weapons-grade uranium, around 95 per cent of uranium-235 is required. To obtain these higher levels of uranium-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 uranium-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 uranium-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 criticized, because it contaminates the areas where uranium weapons have been used and has been linked to serious health consequences. A global campaign has been initiated to outlaw them.
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. Strategic weapons are defined by their role in supporting the “strategy” of deterrence, and they are usually thought of as long-range weapons. Intermediate weapons are somewhere between strategic and tactical weapons, while tactical nuclear weapons are intended for battlefield use. The yield of the weapons is irrelevant in determining how they are classified: many strategic weapons have yields far smaller than those of the largest tactical weapons.
Hair-trigger alert status
The US and Russian presidents are constantly followed by a carefully selected military officer carrying a suitcase which contains 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 United States, 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.
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 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 of nuclear war caused by technical problems or human error. From the moment the United States and the former Soviet Union started targeting each other’s cities with nuclear weapons, people have expressed concern about nuclear war by mistake.
On a number of occasions, false alarms have been received and created 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 substantially increase the chances of nuclear war by accident or miscalculation.
Describing 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 pulverized 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, ionizing 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 shockwave 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.