Plutonium offers several advantages over uranium as a component in a nuclear weapon. Only about 4kg of plutonium is needed to make a bomb. Such a device would explode with the power of 20 kilotons.
To produce 12kg of plutonium per year, only a relatively small reprocessing facility would be needed.
A warhead consists of a sphere of plutonium surrounded by a shell of material such as beryllium, which reflects neutrons back into the fission process.
This means that less plutonium is needed to achieve critical mass, and produce a self sustaining fission reaction.
A terrorist group or country may find it easier to acquire plutonium from civil nuclear reactors, rather than enriched uranium, to produce a nuclear explosive.
Experts believe a crude plutonium bomb could be designed and assembled by terrorists possessing no greater level of skill than needed by the AUM cult to attack the Tokyo underground with nerve gas in 1995.
A nuclear explosive of this nature could explode with the power of 100 tonnes of TNT - 20 times more powerful than the largest terrorist bomb attack to date.