Is there a minimum size for an atomic bomb? Would it be possible to make a microscopic one that results in a very small explosion?
Simon McLeish
Lechlade, Gloucestershire, UK
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Nuclear explosions are different from conventional explosions. A normal explosive substance uses an external detonating trigger, and smaller quantities of explosive cause smaller explosions (from bombs akin to those used in the second world war, which destroyed buildings, to the cap in a toy pistol). A nuclear bomb doesn鈥檛 work like that 鈥� at least, not a fission bomb such as the ones set off in Hiroshima and Nagasaki.
A radioactive substance releases radiation all the time, produced by its inherent atomic instability. In radioactive fission, atoms break apart, causing the emission of radiation. This can cause other atoms to fission, with the release of more particles 鈥� like balls on a snooker table when hit by the cue ball. This is a chain reaction, but for small quantities of the radioactive material, it will just fizzle out rather than cause an explosion.
When there is enough of the radioactive material (the 鈥渃ritical mass鈥�), the chain reaction can start to accelerate uncontrollably, and the bomb explodes.
The actual mechanism of an atomic bomb is, in principle, fairly simple: two masses of radioactive material, each less than the critical mass but together larger than it, are forced together very rapidly (using, for instance, a conventional explosive). The chain reaction quickly goes out of control and the bomb explodes. The critical mass is the minimum possible size at which this happens, and how big that is depends on the radioactive material being used.
The uranium used at Hiroshima in 1945 , of which only about 1 kilogram fissioned. However, as with any 鈥渟uccessful鈥� atomic bomb, it required shielding and a delivery mechanism to keep the person deploying it safe, which ended up making it larger. The overall weight of , codenamed Little Boy, was 4400 kilograms, almost 70 times the weight of the uranium.
A smaller atomic bomb requires a less stable radioactive substance, so is more dangerous to build and control. A microscopic one with less than a gram of fissionable material would be impossible to manufacture or control.
Richard Miller
London, UK
Anyone able to give an accurate answer reflecting the state of the art in nuclear weapons design is unlikely to be at liberty to respond, but enough information is in the public domain to put a reasonable upper bound on the size of the smallest possible atomic bomb.
In pursuit of that figure, the W54 warhead developed for the Davy Crockett tactical nuclear weapon was about the size of a suitcase, incorporating 4 kilograms of fissile material and 12 kilograms of conventional explosives.
This is already less than half the quantity of fissile material required to reach the spherical critical mass of pure plutonium (11 kilograms), and only works due to the use of explosive lenses to compress the core and a neutron-reflecting layer of beryllium (or something similar) to amplify the chain reaction once detonated.
Stephen Johnson
Eugene, Oregon, US
All atomic bombs start with a fission reaction, where a neutron causes a nucleus to split, releasing some number of neutrons and an amount of kinetic energy (around 200 megaelectronvolts in the case of uranium-235).
Although this is a lot of energy from a single atom, it doesn鈥檛 produce any sort of explosion. The explosion we see with nuclear weapons is caused by the splitting of around a trillion trillion atoms in a microsecond.
At this scale, the combined kinetic energy heats the core of the weapon to a temperature hotter than the surface of the sun, which causes massive expansion of the core. This expansion and the subsequent shock wave is what we see as an atomic explosion.
In the late 1950s, the US designed the W54 atomic weapon for use in artillery, small, air-launched missiles and even as a portable backpack explosive. This weapon had a yield as small as the equivalent of around 10 tons of high explosives.
An even smaller weapon could be created by changing the geometry of the core and surrounding it with neutron-reflecting materials. However, no atomic weapon will ever be really small, let alone microscopic.
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