DEEP under the Nevada desert last week, plutonium from a nuclear warhead was blasted with chemical explosives. The purpose was to study "the effects of ageing on the behaviour of nuclear materials", according to the UK's Ministry of Defence (MoD).

Code-named Krakatau after the explosive Indonesian volcano, the detonation did not trigger a nuclear chain reaction, but it did raise questions about the commitment of the US and UK to nuclear disarmament and to banning nuclear tests.

The blast took place about 140 kilometres north-west of Las Vegas on 23 February, and is only the second "sub-critical" nuclear test to be jointly conducted by the US Los Alamos National Laboratory and the UK Atomic Weapons Establishment. Both the US National Nuclear Security Administration and the MoD insist that the test was to ensure the safety of existing warheads.

However, critics argue that the US and UK could use the test data to help design new nuclear weapons, which both countries are contemplating building. "The likelihood is that this is part of a larger plan to deploy new nuclear weapons," claims Matt Martin from the Stanley Foundation, a think tank in Muscatine, Iowa.

The test could also weaken the 176-nation Comprehensive Test Ban Treaty, which has been left in the lurch because of opposition from the US, says Dan Plesch, a nuclear policy specialist at Birkbeck, University of London.