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Down in the dumps

Test boreholes are being sunk later this year to establish rock and hydrogeological conditions around Dounreay's biggest public relations embarrassment - the 65m deep unlined construction shaft used for decades as a dump and now full to the brim with a 700m3 cocktail of radioactive waste. Some of this is so dangerous that it can only be removed by remote controlled plant.

Amid last year's safety furore, the Government quickly approved a 20- year, £350M plan to isolate the 4.6m diameter cliff top shaft from surrounding rock and then remove its contents (NCE 25 June 1998).

The dozen boreholes, project managed by Gibb, will determine which of two possible containment curtains around the shaft is the most feasible. Needed as a 100% watertight seal 85m deep, a secant piled wall or a ground freeze operation will demand cutting edge technology.

The shaft's senior projects engineer Sandy McWhirter says he needs absolute proof that whichever curtain design is chosen it will withstand five years of disturbance while the debris - including gloves, filters, even paint pots - is painstakingly grabbed out. His favourite at this early stage is ground freezing.

He says the disadvantage of secant piles is that spoil removed by drilling rigs will have to be decontaminated. Ground freezing makes this unnecessary and the technique has the potential to seal fractured rock with ground water frozen at -8degreesC.

Costs for the two methods are thought to be roughly even. But freezing offers a controlled and sequential switch off, allowing ground water to enter the shaft slowly after it is emptied to aid flushing out of ground water.

The main problem with freezing centres on the fact that it involves injecting brine, which has a lower freezing temperature than water, into the ground. This technique could be rendered ineffective if the ground water contains saline sea water, which has a similar sub zero freezing temperature.

Also needed is an inner ring of heating pipes close to the shaft wall to ensure that water and waste in the hole do not freeze and become more difficult to remove. The plan is to start waste removal in about 2015.

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