Science

Nuclear weapons – largest and most devastating

Nuclear weapons are the most powerful weapons ever invented. The devastating effects from nuclear bombs emanate from the forces that keep atomic nuclei together.

There are two main types of nuclear weapons:  Fission bombs, where energy is released through splitting of heavy nuclei (uranium or plutonium); and thermonuclear bombs, or hydrogen bombs, where joining (fusion) of the lightest nuclei (hydrogen) contributes to the release of explosive power, even greater that of fission bombs.

How a fission bomb works

In a fission bomb, energy is released when heavy atom nucleic are hit by slow, so called “thermal”, neutrons and split as a result. During fission, vast quantities of energy are released along with additional neutrons, which can split more nuclei and release more energy in a chain reaction, if conditions are right.

The fission process

The fission bomb is designed to release as much energy as possible before the bomb disintegrates, stopping the chain reaction. The longer a chain reaction lasts, the more powerful the explosion will be. In order to start such a reaction, a “critical mass” is required. The critical mass depends on the properties of the fissile material, its density and geometry. For uranium-235 it is around 25 kg and for plutonium-239 some 5 kg. The nuclei of these elements can be split by thermal neutrons, setting off a chain reaction. The splitting process produces a few hundred different radioactive isotopes such as krypton, barium, iodine-131, cesium-137, and strontium-90.

Diagram: A nuclear fission, i e the chain reaction that is basis for a nuclear bomb

Diagram: A nuclear fission, i e the chain reaction that is basis for a nuclear bomb

1. A neutron with a suitable energy level (thermal neutron) hits a nucleaus of U235 (a corresponding sequence is valid for Pu239).
2. The uranium atom is split, which results in release of energy and a number of additional thermal neutrons. These, in turn, may either escape from the material or hit a U238 nucleus so that nothing further happens. But if the neutron hits another U235 nucleus, that one will be split and emit additional neutrons.
3. The number of free thermal neutrons will grow exponentially for the duration of the chain reaction, resulting in release of huge amounts of energy and radioactive fission products as fallout.

The bombs over Hiroshima and Nagasaki

The bombs that were dropped on Hiroshima and Nagasaki were of the fission type. The Hiroshima bomb had a blasting power corresponding to 13 kilotons (kt) of TNT, while the Nagasaki bomb was 21 kt. The Hiroshima bomb had a core of U235; the Nagasaki bomb Pu239.

Advanced level: HIROSHIMA AND NAGASAKI

How a thermonuclear bomb works

A hydrogen bomb is detonated in three steps: first a plutonium bomb, then a fusion bomb, and finally a uranium bomb (U238). The plutonium bomb goes off first and ignites a fusion process in hydrogen gas (tritium), setting off a large number of fast neutrons. These, in turn, start a fission in a shielding – tamper – of U238, which encloses the bomb. The primary yield (explosive energy) of a hydrogen bomb is generated by fission of the tamper. The diagram below shows schematically the design of such a bomb.

The explosion will continue over 600 nanoseconds, with the initial fission taking 550 ns. (one nanosecond relates to a second as a second relates to 30 years).

The explosive power of a hydrogen bomb is, in theory, unlimited, which in not the case for a fission bomb. The largest hydrogen bomb that has been tested had a power corresponding to 58 megatons (Mt), which equals about 4 600 Hiroshima bombs. This test was done in 1961 over Novaja Zemlja, in what was then the Soviet Union. A strategic bomb today has a blasting power of some 200 – 500 kilotons, which is frightful, considering the devastating effects caused by the bombs over Japan, which were less powerful by  an order of magnitude.

The fusion process

Fusion occurs when two isotopes of hydrogen combine to form a single nucleus of helium, which releases enormous amounts of energy. In order for this to happen, extreme levels of temperature and pressure are required. Fusion naturally occurs in the interior of stars, which is the source of their energy. On Earth, fusion occurs only in hydrogen bombs or in experiments with nuclear physics (e. g. fusion energy research).

In a hydrogen bomb, two isotopes of hydrogen – deuterium and tritium – are joined to form a nucleus of helium and a neutron is emitted. Fusion of the two isotopes releases large quantities of energy and a shower of fast neutrons.

Material for atomic bombs

The common materials for nuclear weapons are uranium (U) and plutonium (Pu). While uranium occurs as a natural element on Earth, plutonium does not exist in natural form and is therefore created synthetically from U238 through neutron irradiation in nuclear reactors. Natural uranium consists mainly of two different isotopes: U235 and U238. Both of these isotopes have a very long half life, 700 and 4500 million years, respectively. The uranium that occurs in nature has a low content of U235 (0.7%). Fuel for nuclear reactors usually contains 3-4% of U235. For weapons grade uranium, the proportion of U235 needs to be over 90%. Therefore the uranium needs to be enriched, a process performed in plants that separate the two uranium isotopes from each other so that U235 can be concentrated.

Plutonium from nuclear reactors used for energy production can be used to build a nuclear bomb. This process is cumbersome, however, due to dangerous radiation from the plutonium and because there is a mixture of other plutonium isotopes than Pu239, which reduces the power of a bomb and makes it unpredictable. In conventional warfare, depleted uranium (i. el, tailings from enrichment) is used for armor-breaking ammunition and also as reinforcement of the armor shieldind on vehicles. Use of depleted uranium in weapons implies health effects for the population in areas where such weapons are used, since it still is toxic.

This diagram shows different ways for acquisition of material for nuclear weapons.

This diagram shows different ways to acquire material for nuclear weapons.

A state, that aspires to obtain nuclear weapons, might retrieve irradiated fuel from a research reactor and use it for nuclear bombs after reprocessing (1). This is the road that Israel may have gone down, according to the assumption that it possesses nuclear weapons. If a state has capacity for Uranium enrichment for production of nuclear reactor fuel, it is relatively simple to proceed with the enrichment up to weapons grade material (2) Today there are concerns that for instance Iran might take this path for production of nuclear bombs. An alternative way to obtain material for nuclear explosives is separation of plutonium from (partially) burnt-out nuclear fuel (3). It is likely that North Korea produced material for its nuclear tests in this way in the first decade of this century.

Advanced level: URANIUM CHAIN

Delivery Mechanisms

For a nuclear weapon to reach the intended target, some sort of delivery mechanism is required. The most common carriers today are missiles. These may either be ballistic missiles, without control of flight track after launching, or cruise missiles that can adjust the course of flight and even navigate. Depending on range, nuclear weapon systems are divided into three categories: strategic, eurostrategic/regional or tactical.
There is no common definition of these categories and it is not clear where the borders between them should be drawn. The difference between strategic and tactical nuclear weapons may be defined based on size of charge or on range or on planned use. If the type of nuclear weapon is defined based on range, a strategic bomb would be one that is intended for long-range deployment  (>5500 km). Tactical nuclear weapons exist as missiles, mines, torpedoes and nuclear artillery.

Advanced level: DELIVERY MECHANISMS

Hair Trigger Alert

The US and Russian presidents are always accompanied by a case, that is carried by a carefully selected and trained military officer. The case contains a satellite radio along with the codes for launching that country´s nuclear arsenals. This case is often called “the nuclear football”. The “football” was established during the Cold War, when the leaders of the USA and the Soviet Union wanted to be in constant control of the ability to launch a nuclear attack.

The US president has a sovereign last say about deployment of nuclear weapons. After the collapse of the Soviet Union, the president of Russia took over the “football”, but in Russia an order to deploy nuclear weapons may be issued by the president, the minister of defense or the commander in chief of the army.

Nuclear War by Mistake

The large nuclear weapon states have a wide network of satellites and radar stations, which would give early warning about nuclear attacks. The intent is to allow the state under attack to detect incoming missiles early enough for counter-attack before impact of the hostile nuclear missiles.

False alarms are, unfortunately, not uncommon. Such alarms may be caused by fires, research missiles, electronic disturbances, or other spurious events. So even if the nuclear weapon states have no intent to use their nuclear weapons, a risk is always present that nuclear war could break out as a consequence of a system malfunction or through human errors. From the beginning of the nuclear era, there have been concerns about nuclear war by mistake.

If the number of nuclear weapon states increases, the risk for nuclear war will increase significantly, since it can be assumed that new nuclear weapon states would not have the skills and technology to effectively filter out false alarms.

Advanced level: WAR BY MISTAKE

What happens at the bang?

When a nuclear charge goes off, there will be an intense flash of light, like a giant lightning. It blinds and burns everything within a certain radius. There is no chance to take cover, unless there has been prior warning.

Everything within a certain radius from ground zero of the explosion will be smashed and burned. What remains would be smoke, gasses and small particles, which rise up in the sky. A mushroom-shaped cloud will be formed. Simultaneously there will be gamma radiation, which causes radiation sickness, and an electromagnetic pulse that ruins electrical equipment. Both of these effects are invisible. Immediately afterwards the blast wave hits. It is so violent that it smashes buildings and blows people away many kilometers from ground zero. After these immediate effects there will be radioactive fallout, which makes large areas un-inhabitable for a long time into the future.

Advanced level: EFFECTS OF A NUCLEAR EXPLOSION

Page last revised 2012-07-11