So, how does a nuclear bomb work?

North Korea's ongoing threats about nuclear warfare have put 'the bomb' back on the public's radar.

They were the hot technology of the Cold War, but how do nuclear bombs work?

An atomic bomb is based on releasing energy from breaking apart very heavy atomic nuclei (other nuclear weapons, like hydrogen bombs, use atomic bombs as their triggers).

The nucleus of an atom is a cluster of protons and neutrons, and the heavier the element, the bigger the cluster.

A nucleus of uranium-235, for instance, has 235 protons and neutrons. At this size, the nuclei are unstable and occasionally break apart in a process called fission, leaving smaller nuclei of lighter elements and releasing a lot of energy as radiation.

On its own, fission isn't enough to make a bomb work. Ordinarily fission is rare and produces only a little heat and radiation. But there is something special about uranium-235, along with a few other fissile substances, such as plutonium-239.

When a nucleus of uranium-235 fissions, it doesn't just leave smaller nuclei behind, but also extra neutrons which zoom off on their own. Those extra neutrons are key, because if one hits another nucleus, it can cause it to fission too.


The neutrons move quickly and nuclei are very small, so it's possible that they will zoom right out of the uranium before being absorbed by another nucleus.

But if there is enough uranium - what is called a critical mass - then enough neutrons are absorbed by other nuclei to keep the fission going in a chain reaction by spawning further fissions, which release even more neutrons and so on.

In a bomb, the chain reaction grows and grows as more and more neutrons and fissions take place, quickly spiralling out of control and generating so much energy so rapidly that the nuclear material explodes. It is only the explosion that stops the reaction.

The critical mass is not a constant. Criticality depends on many factors, including the quantity, quality and shape of the material and its surroundings.

Detonating a fission bomb is a matter of altering one or more of these factors to push a subcritical mass over the edge of criticality, setting off the chain reaction.

For example, in the bomb dropped on Hiroshima on August 6, 1945, there were two separate bits of subcritical uranium-235 and, on detonation, an explosive charge smashed them together to create the critical mass. A more common design compresses a single piece of subcritical material with explosives just enough to create a critical mass. Once critical mass is reached, the explosion follows.

The technical difficulties are in arranging things carefully before to make this happen.

As well as the obvious hazards of designing and constructing a bomb, the fissile material required is not easy to come by. Uranium ore is mostly unsuitable uranium-238 and has to be heavily processed to enrich it with uranium-235, while plutonium-239 doesn't occur naturally and must be made in a nuclear reactor.

The difficulties may not dissuade those intent on nuclear armament, but at least they slow them down.

Fairfax New Zealand