The World of Nuclear Science

Warfare - Fusion Weapons

The fusion of two light nuclei to form a heavier nuclei also releases energy. However, fusion is only possible at extremely high temperatures. Thus fusion-based weapons are sometimes also called thermonuclear weapons, thermo- in this context meaning heat-induced.

Fusion is based on joining two light nuclei together. The lightest element is hydrogen, and the two isotopes of hydrogren most readily fused together are hydrogen-2 (deuterium) and hydrogen-3 (tritium). This is why sometimes fusion-based weapons are also called hydrogen-bombs, or H-bombs. A single fusion reaction will generally release less energy than a fission reaction, but since fusion involves light atoms, 1 kg of fusion fuel will release more energy than 1 kg of fission fuel (since more reactions can occur).

1 kg of fusion fuel (light atoms) allows for more reactions than 1 kg of fission fuel (heavy atoms).

It was experimentally impossible to create a thermonuclear device before the invention of the atomic fission bomb, since there was no method of obtaining a high enough temperature for a fusion reaction to occur. (The detonation of a fission bomb releases a large amount of heat energy - several millions of degrees.) Thus all of today's thermonuclear weapons are dependent on fission bombs. They are all based on a fission bomb 'trigger' - used to create the high temperature needed for fusion, and contain a fusion section to add further energy to the overall process.

The first thermonuclear device was detonated by the United States in 1952, after numerous trials in the years preceding. Russia then detonated its first thermonuclear bomb the next year. In 1954 the US exploded another fusion bomb with a power of about 15 megatons. It created a huge fireball about 5 km in diametre and then left a mushroom clound that dispersed into the atmosphere. It was called a 'fission-fusion-fission' bomb - because it contained three stages. The first was a fission trigger, to create the high temperature required for the second stage of fusion. Fusion of hydrogen isotopes created heavier helium nuclei, and high-speed neutrons. These extra high speed neutrons then collided and fissioned the uranium-238 (in the third fission section) which released more energy, although it did not create another chain reaction. Soon after, Russia detonated its first thermonuclear device, leading to the breakout of the Cold War.

Thermonuclear weapons are considered to be highly tactical, because they can destroy armed forces and infrastructure easily, but do not produce as much radioactive fallout compared to fission-based weapons. The fallout from thermonuclear weapons is due primarily to the fission trigger. Thus thermonuclear weapons are sometimes called "clean" bombs. Technically a clean bomb is defined as one where significantly more than half of the destructive power arises from fusion. So-called "radiation fusion bombs", also known as neutron bombs are examples of clean weapons. However, these bombs make nearby objects and structures radioactive because of the number and speed of the released neutrons. Still, the radioactive fallout from a "clean" bomb is significantly less than that from a fission weapon. There are in fact still some fears today about whether the Hiroshima and Nagasaki sites in Japan, bombed by the US in World War II, are safe from radiation. (This was one of the factors that made the death toll hard to accurately quantify.)

Thermonuclear weapons are at least as destructive as fission weapons. In fact, a completely efficient thermonuclear bomb using hydrogen-isotope fuel can release up to three times as much energy compared to a fission bomb, with the same mass of uranium fuel.