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Dounreay death rattle Engineers at the Dounreay nuclear reprocessing plant in Scotland are preparing to carry out one of the most complex decommissioning programmes ever attempted. David Hayward expla

Dounreay is decaying, doomed and will soon be dead. Such convenient labels, now being applied to the nuclear facility on Scotland's north coast, mask the fact that decommissioning engineers at the site are becoming increasingly active.

Plans for a long-gestated £5bn, 100 year decommissioning marathon - destined to be the most varied and complex ever attempted - are already well in hand. And, by the year end, one of the most hazardous reactor coolant clean up operations will already be under way.

Further ahead lies the decommissioning of around 40 structures ranging from three nuclear reactors to a vast 65m deep shaft packed solid with discarded radioactive rubbish.

Earlier this month the Government ruled it no longer economic to continue with Dounreay's only remaining commercial operation, the recycling of imported nuclear fuel.

Dounreay has been dogged by claims of radioactive leaks and contaminated seashores. More significantly there have been recent reports that an abandoned construction shaft at the site had been used for decades as a radioactive waste dump.

Whatever the reality, the Government has approved a £350M plan to isolate the shaft and meticulously recover every item in the 700m3 pile of rubbish (see box).

Meanwhile, concern over the plant's management and safety regimes culminated in a damming report published last week by the Health & Safety Executive's Nuclear Installations Inspectorate. Although it ruled the plant still safe, it criticised substandard equipment as leading to a 'complacent' attitude to safety.

By coincidence, this report was delivered to Dounreay engineers just as a bus load of 10 more NII inspectors arrived to carry out another separate and even more comprehensive safety audit. This ongoing examination was triggered by a total power failure in fuel reprocessing areas last month when an excavator accidentally severed both electricity cables to the plant.

Pending the results of this further safety analysis, all fuel reprocessing work has been suspended.

Closing down the Dounreay plant and removing the nuclear nasties is nothing new for civil engineer Bob Mathews, who leads the site's decommissioning team - he has been doing it for decades. Now the closure is confirmed, he will embark on a decommissioning programme which will go on for another 100 years or so.

Dounreay is a hotchpotch of structures housing containment vessels, fuel stores and waste treatment facilities plus the three long redundant nuclear reactors. The oldest power plant is the distinctive dome-shaped experimental fast breeder reactor commissioned in 1959 to test plutonium instead of scarcer uranium fuel.

This facility closed in 1977, though a newer prototype reactor remained operational until four years ago. A third materials testing reactor closed its doors after a leak in 1967.

Virtually everything else at Dounreay was built to service these three reactors and to treat and store the nuclear waste they produced. This material was supplemented by other contaminated material after decommissioning got under way in the 1970s.

The need for extensive reprocessing facilities on site to deal with spent fuel prompted Dounreay to set up a commercial reprocessing facility to help offset decommissioning costs - around £60M a year.

As a result, Dounreay has treated fuel from Australia, Germany and Belgium, and from the former Soviet Union state of Georgia. But, says UKAEA, some £30M worth of plant modernisation would now be needed to deal with orders over the next decade of just £25M. So it obtained government permission to cease commercial fuel reprocessing. However, recycling its own reactor fuel will - safety audit permitting - take until at least 2006, so little will physically close down for some time.

Dismantling of structures will be even slower. With most of the spent fuel now out of the reactors, Mathews claims it is often more economic to leave the buildings standing for at least 50 years until radiation levels have decayed sufficiently to allow conventional demolition.

The reactor buildings have three containment levels - a steel reactor vessel, thick concrete biological shielding and an outer steel pressure vessel shell. 'We have limited storage on site for anything we remove from the buildings and, as they are themselves the most secure stores we have, it is often more satisfactory to leave equipment insitu,' Mathews explains.

Intermediate waste is ultimately destined for a new national underground store planned by government agency Nirex. But finding a suitable location for this controversial repository has long been delayed and the Dounreay waste may have to remain on its own site for around 50 years.

Mathews reckons that over the next 10 years he will only demolish three buildings. Most will go over the following decade leaving the three reactors, plus a clutch of waste stores, to be dealt with by engineers two generations down the line.

The distinctive reactor dome may be the last to go, if at all. Plans are now well advanced to have it, the first of its kind in the UK, protected as a listed structure.

Mathews' immediate concern however is not steel or concrete but the 1,500t of volatile liquid mix of sodium and potassium coolant still in two of the reactors. Decontaminating this for safe disposal to the sea means adding water which generates hydrogen and heat.

This operation must take place in an inert environment, otherwise, explains Mathews, 'hydrogen and heat together can create a large bang and trigger a sodium fire'. But he emphasises that any explosion would be the result of a chemical rather than nuclear reaction, and would be well away from radioactive material.

Only after removing the coolant can the final batch of fully depleted low grade uranium fuel be removed from the prototype reactor. Many of the 977 remaining fuel rods have been deformed by heat into a banana shape and wedged into place in the reactor tubes. As a result they must be cut remotely in the centre to allow removal.

The equipment for both these delicate decommissioning tasks is now being developed with a hoped for start date later this year. But Mathews stresses that with safety paramount, the whole decommissioning programme must remain flexible. He hopes to have the coolant treated and remaining fuel out by 2006, but accepts it could take until 2014.

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