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Engineers fail to find solution to Windscale pile decommission

News :

A CRISIS exists over stalled attempts to clear up radioactive material left in the Windscale nuclear reactor since it was shut down by fire in 1957, a nuclear expert has told NCE this week.

Plans to decommission Windscale Pile 1 under a £60M contract awarded in 1999 have been abandoned by UK Atomic Energy Authority (UKAEA), the agency responsible for clearing up the UK's nuclear waste. A search for new solutions is continuing.

Windscale was built to fuel Britain's nuclear weapons race.

Fire in the Pile 1 reactor was the worst ever nuclear accident and left one of the largest masses of unstable nuclear material in the world.

There is also no plan at present to decommission the sister reactor Pile 2. Although this was not damaged by fire, it also contains radioactive material abandoned in a 'high-energy' state since 1957.

UK nuclear consultant Dr John Large told NCE: 'UKAEA has reached an impasse and does not seem to know where to go. Yet the cores contain highly radioactive and unstable materials in conditions which could trigger another release of highly radioactive materials.'

Large once worked at Windscale and, now a consultant, recently advised the Russian Federation on recovering the Kursk nuclear submarine. He said that deadly substances in the core include uranium hydride which ignites at room temperature, carbonaceous dust or explosive 'lampblack', and plutonium isotopes which stay active for 250,000 years.

The material sits in a concrete 'bioshield' box up to 3m thick which contained the graphite core where uranium rods were irradiated to form weaponsgrade plutonium. Unlike most later reactors, the core was vented with fans to a filtered chimney, open to the atmosphere. The box is currently kept at negative pressure.

The structure was built in the 1940s. Large said that age combined with high temperatures will have deteriorated concrete and steelwork in the core. And regardless of the fire-related problems, decommissioning was never considered during design.

Where possible, fuel rods were removed from both piles between 1958 and 1961. However, each was abandoned with graphite cores still retaining stored 'Wigner' energy (see box). This makes the material vulnerable to fire or explosion if disturbed.

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

A statement by UKAEA insisted that the piles are in a 'safe, stable condition, and pose no threat to the public or environment. They can be held in this condition for a number of years.'

A spokesman added that this meant that a solution had to be found in the medium term.

The statement said that a 'comprehensive technical review' was ongoing. No start date for decommissioning is possible while the 'review' continues.

Intense heat and instability hamper inspection

AT WINDSCALE, a mammoth 2,000t graphite brick core containing uranium, plutonium and a cocktail of other radioactive materials remains encased within the bioshield box of Pile 1, badly damaged on the inside by the fire.

More than a quarter of the core is severely fire damaged, according to independent nuclear consultant John Large, and contains around 15t of nuclear fuel. However, intense heat and instability make inspections impossible and the exact composition is unknown.

Loose radioactive debris outside the core was removed by 1999, the year that UKAEA awarded a £60M contract to a consortium to start dismantling the core.

Under proposals by the British Nuclear Fuels Limited/Rolls Royce/Nukem consortium, four vertical steel masts were to be installed through 1.5m 2holes in the roof of the box to support remotely operated arms. Sliding on the masts, they were to gnaw into the core removing it in 200,000 sections and using an inert argon gas surrounding to prevent fire.

But further studies revealed new problems. Space constraints within the box made access to some part of the core impossible. The use of heavier tools was examined, but would have required larger masts and with additional support placing loads which the concrete box roof could not withstand.

In addition, the graphite brick core is restrained by a steel 'garter' or belt. Dismantling could have caused this to spring, risking partial collapse of the graphite core, and even sparking another uncontrolled radioactivity release.

Gas could also leak from the unsealed core but dismantling in air could release uranium hydride - believed to have been formed inside the core after water was used to put out the original fire.

This ignites at room temperature and could set the core on fire triggering another nuclear reaction.

Another plan to use 'remotely operated vehicles' was investigated but abandoned as implementation would have taken decades.

The Wigner effect

THE WINDSCALE piles were built in the 1940s. Uranium, held in a graphite core, was bombarded with neutrons to form plutonium.

During the process, graphite also absorbed energy, a phenomenon called the Wigner effect.

This Wigner energy had to be released periodically by 'annealing' in which the core was heated by starting up the reactor with the cooling fans switched off.

However, not all energy was dissipated, so that each anneal required higher temperatures. In October 1957 operators raised the core temperature to 1,200degreesC, causing the fuel rods to ignite and prompting a catastrophic fire.

Large amounts of Wigner energy remain in the core of Pile 2.

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