The closure of Scotland's oldest nuclear power plant at Chapelcross in Dumfriesshire will force Britain to buy nuclear explosives for Trident warheads from the US.

Chapelcross is Britain's only producer of tritium, a radio- active gas which is essential for boosting the explosive yield of nuclear bombs. The state-owned company, British Nuclear Fuels, announced last week that it was planning to close the plant between 2008 and 2010.

Nuclear insiders say that this will oblige the Ministry of Defence to make a deal with the US government for the supply of tritium. But the problem with this, according to defence experts, is that it would breach British and American commitments under the nuclear non-proliferation treaty, reaffirmed in New York just a week ago.

"It would encourage the trade in material for nuclear warheads," said Dr Frank Barnaby, a defence analyst who used to work at the Atomic Weapons Research Establishment at Aldermaston in Berkshire. "This would set the world a bad example.

"Because it is such an essential component of modern nuclear weapons, the trade in tritium should be carefully controlled. And we should be setting a good example by not trading in it."

Tritium is produced by irradiating the metal lithium in one of Chapelcross's four electricity- producing nuclear reactors. It is then extracted in a separate plant built on the site in 1980, which is surrounded by high security fences.

The tritium is sent to Aldermaston where it is made into nuclear warheads for the Trident submarines that operate out of Faslane on the Clyde.

The gas is packed into the warheads so that it can be injected into the plutonium core as it explodes, thereby boosting the nuclear yield by up to 10 times.

Barnaby pointed out that tritium is regarded as vital by the military because it means that bombs can be small and light but still powerful. It also enables more warheads to be carried on each missile.

The problem, however, is that because tritium is radioactive it decays and so needs to be regularly replenished in weapons if they are to work properly. It has a half-life - the length of time it takes radioactivity to decrease by 50% - of only 12.3 years and decays at a rate of 5.5% a year.

Before the extraction plant was built at Chapelcross in 1980 Britain acquired its tritium from the US by swapping it for other nuclear materials under a controversial mutual defence agreement. The Ministry of Defence (MoD) revealed for the first time last month that it obtained 6.7kg of tritium from the US under this agreement between 1960 and 1979.

Reopening the tritium trade with the US in the future, though, would be likely to provoke criticisms from non- nuclear weapons states who have become increasingly concerned about nuclear co-operation between Britain and the US.

The only alternative would be for the MoD to contract another nuclear power station, such as Sizewell B in Suffolk, to produce the tritium - but this is unlikely to be politically acceptable in Britain. Shaun Burnie from Greenpeace International pointed out that Britain and the US reaffirmed their promise to eliminate nuclear weapons at a conference of parties to the nuclear non-proliferation treaty in New York earlier this month. "It is now clear that Britain has no intention of meeting that commitment," he said.

"Instead the MoD is looking at options for maintaining Trident after 2010.

The real proliferators here are those plotting in Westminster and Washington to find a way of continuing the work that Chapelcross has been doing for decades.

"The civil and military nuclear industries are really the same and this once again shows that the two are inseparable." A spokesman for the MoD said they were aware of the planned closure of Chapelcross, but insisted that they had no plans for the availability of tritium after 2010. But he added: "There are options under consideration should events require it."