The Technology of Nuclear Weapons

Abstract

To manufacture a nuclear weapon, the main requirement is to obtain a sufficient quantity of fissionable or fissile material.¹ A fissile element is one whose nucleus is capable of rupture into two lighter fragments, thus forming nuclei of lighter elements, together with free neutrons. These free neutrons may strike other nuclei causing further fission and so on in a chain reaction. The minimum amount of material necessary to sustain a chain reaction is called the critical mass of the substance. Smaller amounts of fissile material will not sustain a chain reaction since too large a fraction of the free neutrons escape through the surface and are unavailable to cause fission in other nuclei. The critical mass can be reduced, however, if the fissile material is compacted to make it more dense, or if it is surrounded by a shell of metallic material such as beryllium (Be) to reflect neutrons which would otherwise escape. If there is enough fissile material, packed closely together for long enough, and if the chain reaction is out of control, the result is a nuclear explosion: an ‘atomic bomb’, the explosive yield of which is equivalent to that of 20 kilotons of TNT. Alternatively, the power from a controlled chain reaction in a nuclear reactor can be used to generate electricity.