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Dounreay - Past Present Future
We are now approaching the Dounreay waste shaft. Shaped like a cylinder, 4.6 metres across and 65.5m deep, it was excavated in the 1950s to allow the removal of rock spoil during construction of an under sea tunnel that would be used for the discharge of low level liquid effluent to the sea. On completion a concrete plug was used to separate the shaft from the tunnel and it was allowed to fill with groundwater.
In 1958 the Scottish Office authorised the UKAEA to use the shaft as a disposal facility for radioactive waste. Today, no-one would authorise such a facility because much better techniques are available for the management of such waste - but it does illustrate how very different the thinking was in the 1950s and how knowledge has increased.
More than 11,000 disposals took place until 1977, when some sodium reacted with the water, ignited and blew off the roof. The radioactive waste is all beneath the water and the blast occurred in the gas space above it, so forcing the roof off. The environmental consequences of this were minimal but it has generated a lot of media interest in the past.
The job now is to recover this waste, which is an engineering challenge that is unique in the world. It is not possible to simply remove the waste straight away. Firstly, because it is radioactive, properly designed waste treatment and storage facilities need to be built to receive the waste as it is removed. That means demolishing many of these buildings around us and relocating the staff who occupy them. Secondly, the shaft is likely to be affected by the pressures of the land and water around it and the engineers need to have a thorough understanding of what is likely to happen once they begin to remove the waste. Pumping on a regular basis keeps the water level in the shaft below the water table and is designed to ensure all groundwater flows inwards, not outwards. When it comes to emptying the shaft, the groundwater will need to be turned off by building a barrier in the rock around it. This could be a curtain of ice, so the contractors need to know a lot more about the groundwater in this area before they can be certain this will work. It is expected an alliance of companies will be commissioned to undertake the waste retrieval.
A number of ongoing surveys will help to build up a picture of the ground characteristics and this information will ultimately help the UKAEA to decide the best way to retrieve the waste. If all goes well, work could begin isolating the shaft from the surrounding rock in 2006.