Plans to clear up one of the world's most unstable collections of nuclear material held at the Windscale nuclear reactor have run into a crisis due to serious technical difficulties.

The material is contained in the Windscale Pile 1, a giant nuclear reactor built in the late 1940s to produce weapons-grade plutonium for Britain's first atomic bomb. It went on fire in October 1957 in what was at the time the world's worst ever nuclear disaster.

Proposals made as part of a contract awarded in 1999 to clear up Windscale's waste have been abandoned. A search by the British Atomic Energy Authority for ideas is continuing as part of a `review'. A world-leading nuclear expert has described the situation as a crisis, posing a risk of a release of radioactive material from the plant at Sellafield in Cumbria.

"UKAEA has reached an impasse and does not seem to know where to go from here," said nuclear engineer and consultant Dr John Large. "Yet the reactors at Windscale contain highly radioactive and unstable materials in conditions which could trigger another release of highly radioactive materials.

"They do not know what precisely is contained within the core of the plant which went on fire, or what state the material is in."

Large, who worked at Windscale, recently advised the Russian Federation on raising the sunken Kursk nuclear submarine and has advised the Irish government and Tony Blair when he was British shadow energy spokesman.

He said that deadly substances in the core include uranium hydride, which ignites at room temperature, carbonaceous dust or `lampblack', which explodes, and plutonium isotopes with a half-life of 24,400 years, which stay active for 250,000 years. "There could also be pockets of hydrogen, which could cause an explosion if disturbed," said Large.

The material sits in a giant concrete bioshield box up to three metres thick which contains the graphite core where uranium rods were converted in the nuclear reactor to irradiated to form weapons-grade plutonium.

The core was vented with fans to a filtered chimney, unusually leaving it open to the atmosphere, unlike most other reactors. American technology at the time favoured water-cooled facilities, but British engineers opted for an air-cooled system because of the huge volumes of water required to avert meltdown.

Currently, the box is kept at negative pressure to avoid release of radioactive material to the atmosphere. Any explosion or uncontrolled reaction could result in radioactive material being released into the air, as the plant is neither airtight nor watertight.

The core graphite contains large amounts of Wigner energy (see panel: How the Windscale fire happened) created during nuclear reactions, which if released uncontrolled could trigger another fire leading to release of radioactivity. This could occur through disturbance of the material, chemical reaction, or changes in temperature.

"The graphite core is basically a pile of graphite bricks, held together with a restraining garter which has deteriorated. Any attempt to remove this would naturally risk instability," said Large.

"The core is very heavily contaminated with radioactive products produced during the nuclear reactions when the plant was operating. As soon as you start moving these, or introduce thermal mechanical processes such as cutting, you could get thermo-chemical reactions which could convert currently inert material such as caesium, into very volatile substances."

Apart from the risk of allowing materials to escape, the open ventilated design of the plant beside the sea meant sea air would have brought salts into the core which could cause other problems as well as assisting corrosion of steel in the plant. "Salt will lower the temperature at which the graphite can go on fire," Large said.

The age of the structure, built in the late 1940s, and the high temperatures reached in the fire, will have severely damaged the concrete and steel girders inside the core, some of which are likely to be corroding, he said. Concrete disintegrates at high temperatures; the temperatures within the core reached over 1,200 degrees centigrade during the fire. No information on the state of the concrete or steel structures inside the plant has been made public.

"There is danger of a collapse within the bioshield box, which could cause a release of the stored Wigner energy, releasing radioactive materials into the atmosphere," he said.

"Because the box is not sealed, the presence of large quantities of radioactive dust and fuel particles left in the car increases the risk of release of radioactive material."

No decommissioning strategy exists for the sister reactor, Pile 2, which did not go on fire but was abandoned with its graphite core still retaining stored Wigner energy. Fuel rods were removed between 1958 and 1961. Similar dangers exist of a structural collapse leading to an uncontrollable release of energy which could release radioactivity, Large said.

"I would be just as concerned about Pile 2, because it stores lots of Wigner energy, which is available for release in the right conditions. When the accident happened in Pile 1 most of the energy stored by it was released. Pile 2 was closed down without any of this stored energy being annealed, or released in a controlled way, and it remains there. Also, this limits the kind of machinery you could use in dismantling, because of the risk of instability," Large said.

"The biggest danger is of collapse from within the core. This could lead to an uncontrollable release of energy which could lead to the discharge of radioactive material, having serious effects on whatever community it landed on.

"While the Irish government recently launched an action against the British government over the operation of the mixed oxide fuel fabrication plant, a potentially far more serious situation exists with the failure of the plans to clean up Windscale."

A UKAEA spokesman said the piles were in a "safe, stable condition, and pose no threat to the public or environment". Emissions from the reactors were "minimal and well below statutory limits".

"They can be held in this condition for a number of years, but we are not talking lots of lots of years, however. In the long term, a solution must be found."

He said a "comprehensive technical review" was ongoing, and no start date for physical decommissioning was possible while this continued. "As the technical review is still to be completed, UKAEA is not presently committed to any single engineering solution," he said.

Diarmaid Fleming is a civil engineer and freelance journalist with New Civil Engineer magazine in London.

Windscale timeline

November 1947: Construction begins on two nuclear reactors Pile 1 and Pile 2 to produce weapons grade plutonium for Britain's first atomic bomb.

1949: Hungarian-American scientist Eugene Wigner warnings of fire dangers in Windscale-type reactors ignored.

1950: Plutonium production begins at Pile 1.

1951: Pile 2 operational.

1952: Incidents of unexpected high temperature rises noticed in both Piles 1 and 2.

October 1952: Britain carries out first atomic bomb test off the north west coast of Australia using Windscale plutonium.

October 1957: Windscale Pile 1 goes on fire. Radioactivity spread across Ireland, Britain and Europe. Plant closed down.

1958-61: Fuel rods removed from undamaged sections of Pile 1 and from Pile 2.

1990-99: Radioactive debris removed from outside reactor core.

1999: Contract awarded to decommission reactor core.

December 2001: Plans remain stalled because of technical difficulties.

How the Windscale fire happened

The two reactors at Windscale, Pile 1 and Pile 2, were built as part of Britain's headlong rush after the Second World War to become the third nuclear power after the United States and the Soviet Union.

Operating from 1950 until the fire in 1957, they converted uranium to weapons-grade plutonium for nuclear bombs.

The nuclear reaction took place within a concrete box with walls up to three metres thick. Uranium rods were placed inside round slots in a core made from graphite blocks. These were bombarded with neutrons which converted the uranium to plutonium.

But the technology was not fully understood -- and led to disaster. What was not realised was that after each reaction involving bombardment of neutrons, the graphite itself which was thought to be inert would grow and retain energy. Gradually increasing amounts of energy were retained in the graphite after each reaction. This was stored in the form of heat energy, and was known as the Wigner effect after the Hungarian-American scientist Eugene Wigner, who discovered it.

Wigner discovered that unless the energy gradually stored in the core was released in a carefully controlled way, there was a risk of very high temperatures, causing fire or explosions.

He visited Britain and pointing this out to the British in 1949, but his warnings were ignored by the Windscale designers.

US reactors were water-cooled, but required a huge quantity of 25,000 gallons a minute to prevent meltdown and were built in very remote sites. Windscale's designers opted for an air-cooled system instead, partly because of fears about guaranteeing water supply.

The pressure to build the plant is evident in contemporary technical accounts of the construction, which reveal major problems on site. Holes or voids which extended right through the 3m thicknesses of the walls were discovered. The walls were later patched up with grout.

To release the Wigner energy a process known as annealing was done -- heating up the graphite in a controlled manner by starting up the reactor and turning off the cooling fans. Between 1951 and 1957 eight anneals were undertaken.

Heat inside the core was measured by using a tree-like device which inserted a series of probes into the graphite. Because the core had become so distorted after many nuclear reactions, the probes eventually did not fit -- yet the operators continued working.

"They continued operating. This was total arrogance on their part, because they couldn't tell what the temperature was. They were operating completely in the blind," said Dr Large.

Because the energy stored in the graphite increased with each nuclear reaction, the temperature needed to release the energy also rose. Uncertainty about the correct temperature or what heat was needed led to disaster.

On the ninth anneal, in October 1957, the confusion prompted the operators to continue raising the temperature, thinking the core was cooler than it was.

The core temperature soared to over 1,200 degrees centigrade, reaching the ignition temperature of the uranium fuel rods and starting the fire.

Initial frantic attempts to put out the fire failed. Injection of carbon dioxide to smother the fire actually added to it. Flooding with water -- which posed the risk of explosion from the creation of hydride gases -- along with a decision to shut off the cooling fans to starve the fire of oxygen, eventually brought the temperature down.

The fire belched radioactive smoke and gases which spread across Ireland, Britain, the Isle of Man and northern Europe.

Attempts to put out the fire were marked with panic. Burning fuel and debris were shoved out of the core before it was deluged with five million litres of water. Water hoses attached to scaffold pipes initially provided the only alternative to a facility designed without a proper extinguishing system to cope with a fire in the core.

Problem of decommissioning

Forty-four years after it went on fire, the core of Windscale's Pile 1 reactor is in much the same state as it was after the fire. It forms one of the largest masses of unstable nuclear material in the world. Radioactivity within the core is so high that no human access is possible.

More than a quarter of the core is severely fire-damaged and contains around 15 tonnes of nuclear fuel, some of which melted inside the core. Because of the radioactivity, intense heat and instability of the material, inspections have been impossible, meaning that the exact composition or state of what is in the core is unknown.

A limited clearup of debris which ended up outside the core during the fire and in ensuing panic to put it out. It took nine years to complete; work ended in in 1999. In the same year, UKAEA awarded a stg£60 million contract to begin clearing up the waste within the plant.

The consortium of British Nuclear Fuels Ltd, Rolls Royce and German company Nukem was chosen after a competition which attracted the world's leading firms in nuclear decommissioning.

The concrete box has been turned into a Pandora's Box, with further studies revealing further problems. Decommissioning features were not included in the original design.

BNFL, with over 20 successful decommissionings of nuclear plants behind it, teamed up with German specialists Nukem and Rolls Royce with their expertise in robotic equipment.

Each bidding team came up with different proposals on how to remove the mammoth 2,000-tonne graphite core and its cocktail of radioactive materials.

The consortium won out with a plan to install four massive vertical steel masts through 1.5-metre-square holes cut into the concrete roof of the box. These would be supported by an arrangement of steelwork built on the roof.

Remotely operated arms inside the box could slide up and down the masts and gnaw into the core. The proposal aimed at removing the core in 200,000 sections, within argon gas as an inert surrounding to prevent fire.

But the concrete box was like a Pandora's Box, with further studies revealing further problems.

Decommissioning features were not included in the original design. Space constraints within the box make access to some parts of the core impossible. The use of heavier tools and arms was examined, but would have required larger masts, needing additional support.

Then engineers found that the the steelwork needed to support bigger masts and arms became so heavy it would cause the roof to collapse. They were forced to abandon the idea.

The graphite brick core is restrained by a steel garter, or belt. Dismantling could have caused this to spring like an elastic band, risking partial collapse of the graphite core containing radioactive materials, then risking another uncontrolled energy and radioactivity release.

Gas could leak from the unsealed core, while working in water could also result in leakage. Dismantling in air could release uranium hydride -- believed to be inside the core because of the water used to put out the original fire -- which ignites at room temperature. This could set the core on fire, releasing more Wigner energy and setting off another nuclear reaction.

Another plan involving remotely operated vehicles was investigated, but shelved because it would take decades.

Consultant Dr John Large said that decommissioning Windscale posed particular difficulties. "At Windscale, you are starting off with a bad egg. The original designers did not design for decommissioning, and they did not design for the accident. Then bad decisions were made during the fire, because the plant operators were in a state of panic," he said.

"But these are not passive structures -- you cannot just leave them," said Large. "These are old structures, built using old technology, which were hurriedly thrown up as part of a military programme. The longer you leave them, the more difficult it becomes to deal with them."

One solution to seal the plants could be to place a massive concrete shield around the plants, a mammoth civil engineering task likely to be highly costly. This would run against the British government's policy of sustainable development, which prevents work which would have a negative influence on decisions to be made by future generations. A huge concrete shroud could mean bigger problems in the event of future instability within the core.

The haste to build Britain's first nuclear bomb has now presented British scientific and political leaders with a new race to find a solution to clear up the mess left behind.