Officials at Dounreay yesterday admitted they do not know what is in the Caithness plant's notorious waste shaft.
With decommissioning work on the shaft now under way, one of the problems facing those charged with isolating and retrieving 20 years' worth of nuclear and chemical waste there is a number of small drums at the bottom.
The drums, containing sodium, were dumped there in 1959 but nobody knows whether they are intact or not.
There was unsupervised fly-tipping into the shaft and workers firing rifles into it to sink polythene bags floating on water, with no regard to the shaft's hazardous contents.
Now it will take those at Dounreay at least another 20 years and more than £200m before the story of the shaft can be brought to an end, and only if the financially troubled Nuclear Decommissioning Agency (NDA) can provide enough financial support.
As it is, the NDA is facing a shortfall of £160m in its £2.2bn budget and there are fears that as many as 500 jobs could be lost from Dounreay's decommissioning programme.
But Simon Middlemass, Dounreay's acting director, said yesterday that it was highly unlikely there would be any delay in the first phase of the shaft project as a result of the NDA's search for savings.
Nothing more was deposited in the shaft after there was an explosion in 1977, which was thought to have been caused by sodium and potassium wastes reacting to water.
The first task is to isolate the 65-metre shaft and work has now started drilling up to 400 boreholes round it. A specially developed grout will then be injected through the boreholes into any fracture in the surrounding rock, creating a giant containment barrier in the shape of a boot around the shaft.
Some 12,000 tonnes of concrete have already been poured to create a working platform at the top of the shaft.
In total the isolation phase should cost around £27m but could be completed by the end of next year.
Warren Jones, Dounreay's shaft isolation project manager, said yesterday: "Decommissioning the waste shaft is one of the biggest clean-up challenges in the world today, so I am delighted that we have now commenced this phase of work to clean it out."
But the next stage, the actual retrieval and treatment of the shaft's contents, will be even more complex and costly (£180m before anything is actually removed from shaft) and may yet be subject to delay if the NDA's funding problems persist. Dounreay would only say that the timescale and funding of the retrieval was the subject of discussion with regulators and the NDA.
Concepts for removing the waste are still being developed, but it is known that remotely operated equipment will perform the actual withdrawal of the shaft's contents, which will be treated in new purpose-built plants.
Steve Efemey, who is in charge of the retrieval and treatment phase, said yesterday: "We don't know exactly what's down there or what condition it is in as there are different degrees of degradation. That's the challenge.
"The worst-case scenario is that the sodium drums are still intact. But that is unlikely after 50 years and 1500 tonnes of waste on top of them."
But systems would be in place to absorb any explosion.
The retrieved waste would be put through a shredder which is capable of cutting a car engine block. The liquid and solid wastes would be separated, encased in cement and stored in drums.
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