David Pearson, of Kirkwood School, asks :-

How do nuclear weapons work?

Murray Matthews, an Environmental radioactivity specialist at the National Radiation Laboratory, who is currently advising the New Zealand Government and assisting in disarmament meetings held at the United Nations headquarters in Geneva, responded.

Nuclear weapons work by converting atomic mass into energy.

Einstein's relationship, E = mc$^2$, links energy (E) to mass (m) where c is the speed of light. Atomic nuclei of uranium and plutonium can be made to break into fragments by the process known as "nuclear fission". The combined mass of these fragments is less than that of the original nucleus and the missing mass appears as energy. Only one thousandth of the uranium mass is lost during fission but this represents a lot of energy, with an explosive power equal to 20,000 tonnes of TNT (a chemical explosive) for every kilogram of uranium used. In these "fission bombs" pieces of uranium or plutonium are held together under extremely high pressure (using chemical explosives) while all the atoms fragment during a rapid "chain reaction".

Another type of weapon is the "fusion bomb" where atoms of certain forms of hydrogen combine to produce helium, in reactions similar to those occurring in the Sun. Again the reaction product has a slightly lower mass than the reactants so energy is released. Extremely high temperatures and pressures are required before fusion can occur and a fission explosion is used to trigger the fusion reaction. "Fusion" or "hydrogen" bombs have explosive powers equivalent to more than 1 million tonnes of TNT.

Although the power of these weapons may be frightening, it is encouraging that a lot of work is now being done towards disarmament and the prevention of further weapon production.