An unprecedented attempt is being to lock up Britain's nuclear past. In the deserts of South Australia, scientists are reaching the critical stage in their attempt to make safe plutonium left littering the ground after a series of nuclear tests in the 1950s.

Deadly fragments of Britain's nuclear legacy are scattered across hundreds of square miles of the Maralinga test site. Now the job is to bury that past and the British and Australian governments are sharing the cost of the operation. Britain is contributing £25m, roughly half the cost of the project.

A huge network of trenches has been dug to accommodate the contaminated topsoil. But the greatest danger comes from fragments of raw plutonium which are like fine particles of dust. Each one is potentially lethal.

Dr Keith Lokan of the Australian Radiation Laboratory said: "Because plutonium is a lot more radioactive per unit mass - you only need to breath in a microgram, one millionth of a gram of plutonium, to produce a significant dose of radiation."

The test site

The explosions contaminated the area with deadly plutonium

The series of test explosions at Maralinga, South Australia in the 1950s thoroughly established Britain's credentials as a nuclear power.

The Ministry of Defence planned to use the site for 100 years, but by the mid 1960s it no longer had any use for it.

Ironically the tests posing the gravest risk of contamination now were a series of so-called minor trials during which the plutonium core never fully exploded.

Trenches have been dug for the contaminated top soil

Peter Burns of the Australian Radiation Laboratory explains: "Of course because they were not nuclear explosions, the plutonium that was within them instead of being thrown up 50,000ft into the air and being diluted by the whole atmosphere of the southern hemisphere, just sprayed the local countryside with plutonium."

Scientists estimate that if the plutonium was left in its current form it would be potentially deadly for a quarter of a million years.

The clean-up operation

Specially adapted vehicles search for plutonium

Special sealed vehicles track the lumps of most radioactive material and once located another vehicle is called out to helps to sweep up the plutonium. To protect the operators the excavator cabs are airtight and checked for contamination before every tea or meal break. Special walkways keep their boots off the potentially contaminated earth.

Those involved in the project are confident that the raw plutonium can be made safe once it is buried and specially treated.

Des Davy, a clean-up co-ordinator, said: "The final stage is to pass large amounts of electricity, or electrical energy, into those pits, with a series of graphite electrodes so as to heat it up and finally melt that soil into a molten rock."

The cabs are checked for radiation

Effectively the plutonium is then locked up inside the newly formed rock, rendering it safe.