Hydrogen Bomb; Nuclear Weapons
Hydrogen BOMB!For the hydrogen bomb reaction to become self-sustaining, a so-called critical temperature of about 3.5 × 107 K (6.3 × 107°F) must be attained with the aid of the enormous temperature created by a fission explosive. Once this temperature is achieved, the energy released in the initial reaction maintains the temperature, and the chain proceeds either until the supply of fusionable material is exhausted or until sufficient expansion has taken place that the material is cooled below the critical temperature.
There are two ways to use fusion:
boosting of fission explosive yields or generating multistage thermonuclear reactions.
In a fusion-boosted warhead, when the sphere of fissile materials is compressed (imploded) by the chemical explosion, an uncontrolled fission chain reaction begins. If there is fusionable material inside the device, thermonuclear reactions will boost the fission yield. The fusion reactions do not directly contribute very much to the explosive energy, but instead enhance the fission rate, due to the release of a large number of additional neutrons.
If the massive casing is made mostly of uranium-238 (natural or depleted uranium), neutrons from the thermonuclear reactions will cause the uranium nuclei to undergo fission, giving off still more energy. A device of this sort can be regarded as a three-stage fission-fusion-fission bomb.
The yield, or total energy, of a hydrogen bomb is expressed in megatons (1 megaton equals 1015 calories or 4.18 × 1015 joules). Typical fusion-boosted weapons yield hundreds of kilotons (tenths of megatons), and typical multistage weapons yield megatons.
NUCLEAR WEAPONS
Nuclear weapons derive their explosive power from the fission (splitting) and fusion (combining) of atoms. Fusion devices need to be combined with a nuclear fission weapon to generate the intense heat necessary to begin the still more powerful process of fusion. Fusion weapons—the ‘H’ (hydrogen) bomb—can be a thousand times more powerful than fission weapons and these opened the horrific possibility of glob
al destruction through nuclear missile war. Many early ‘fusion’ weapons were in fact ‘boosted fission devices’, gaining most of their power from the fission explosion with a fusion component to enhance its efficiency. Military requirements have also led to enhanced radiation/reduced blast weapons, the so-called ‘neutron bomb’, in which the immediate radiation is multiplied in order to kill troops rather than destroy installations.
The idea of a source of enormous e
nergy for motive power or weapons featured in the work of 19th-century science-fiction writers including Jules Verne, George Earle Bulwer-Lytton, and H. G. Wells who talks of atomic bombs in The World Set Free. By 1914 the Newtonian view that the universe consisted of lumps of indestructible matte
r had given way to the realization that matter could be transformed into energy.
It was not until the eve of WW II that the practical possibility of a nuclear weapon was understood. On 2 August 1939, Albert Einstein signed a letter to Pres Franklin D. Roosevelt, saying that recent work in France and the USA had indicated the possibility of setting up a nuclear chain reaction in a large mass of uranium. This new phenomenon could ‘also lead to the construction of bombs and it is conceivable—though much less certain—that extremely powerful bombs of a new type may thus be constructed. A single bomb of this type, carried by boat and exploded in a port, might very well destroy the whole port, together with some of the surrounding territory. However, such bombs might very well prove too heavy for transportation by air.’ He was wrong on the last point, and six years and four days later the US dropped the first atomic bomb over Hiroshima, the result of the Manhattan Project.
The first test bomb ever to be exploded, at Alamogordo in the New Mexico desert on 16 July 1945, ‘the Gadget’, was an ‘implosion’ device, with a hollow plutonium core weighing about 8.3 lb (3.8 kg), compressed to critical density by about 4, 866 lb (2, 270 kg) of high explosive. The ‘yield’—the size of the explosion—was 22 kilotons. Nuclear weapon yields are measured as kilotons (each 1, 000 tons of TNT) or megaton
s (one million tons of TNT).
The first bomb to be dropped on Japan on 6 August was of a different type—a ‘gun-assembly’ device called ‘Little Boy’. It was cruder than the Gadget—132.3 lb (60 kg) of highly enriched uranium in two pieces, one of which was fired at the other down a gun-type barrel, producing a far less efficient yield of 12 to 15 kilotons. The US B-29 bomber Enola Gay, named after the pilot's mother, carried the bomb to Hiroshima from its base at Guam, escorted by two other planes carrying observers and instruments. The bomb had been brought from the USA by the cruiserIndianapolis, which was to be sunk by a Japanese submarine on its return trip. The city was obliterated.
Although simple in principle, nuclear weapons are difficult to engineer in practice. The 'gun assembly' type is less efficient but more robust and favoured for artillery shells. The implosion type is more difficult to build and more fragile, but is more efficient and is essential to exploit the advantages of fusion
The detonation of a nuclear weapon produces an incandescent ‘fireball’ in which everything is vaporized. Being hot, the fireball climbs upwards, generating the characteristic ‘mushroom cloud’ as it pulls debris from the ground up with it. Nuclear weapons have five main effects. The first is heat, or ‘flash’, causing burns and eye damage far from the immediate source of the explosion. The second is blast, caused by a shock wave moving outwards from the fireball at about the speed of sound. Third is immediate radiation travelling at the speed of light-neutron radiation at shorter distances, and gamma radiation further out. Most of the latter is generated after the explosion, so a dive for cover may still be worth while. Fourth is residual radiation: the neutrons from the explosion make other material radioactive, which falls back to earth as fallout. Fifth is electromagnetic pulse (EMP), which burns out electronic apparatus. A multi-megaton weapon burst high above the earth, outside the atmosphere, would not have perceptible flash-and-blast effects but could knock out power grids and electrical equipment over a wide area.
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