US / British subcritical nuclear test

The first joint US/British subcritical nuclear test was carried out at the Nevada Test site on Thursday 14 February.

Since the US halted underground nuclear testing a number of "subcritical" tests have been carried out. These use amounts of plutonium which are less than required for a critical nuclear reaction.

A subcritical test was carried out in 1998 on part of the W76 warhead which is used on US Trident missiles. That test was related to concerns about the effects of ageing on the plutonium in the warhead pit.

The latest test may have been for a similar reason, but involving part of a British Trident warhead. Defence Minister Lewis Mooney descibed it in the following way:

"To ensure that we continue to be confident of the safety and reliability of our nuclear weapons, it is essential to understand the properties of warhead materials such as high explosive and plutonium under a wide range of conditions, and how these properties change with age. Confidence in the safety and performance of Trident is based ultimately on predictions from high fidelity numerical models run on super computers. The Atomic Weapons Establishment has recently announced a major investment in a new super computer that will substantially upgrade its capability. However experimental studies are still essential to validate the computational models and improve understanding of basic theory. As a continuing part of this programme the UK will shortly collaborate with the US in conducting a plutonium hydrodynamic experiment at the U1A facility in Nevade" (Hansard 12/2/02)

- the final phrase refers to the sub-critical nuclear test.

The test may be related to the refurbishment of the British stockpile of Trident warheads which is just beginning. The movement of nuclear weapons convoys indicates that a significant number of warheads have been returned to the Atomic Weapons Establishment at Burghfield during the last 12 months. These are likely to be reassembled with new components and returned to Coulport over the next year. The ageing of the plutonium pit will be one of the crucial areas being considered before this refurbishment goes ahead. Other problem areas are the ageing of the High Explosive, which is different from that used in the US, and the ageing of Highly Enriched Uranium used in the secondary component of the warhead.