DOUNREAY'S NUCLEAR cleanup has entered a new phase with the start of hydraulic isolation work on a contaminated shaft at the site in north east Scotland.
Last month Highland Council granted planning permission for construction of a raised working platform, drilling of up to 400 grout curtain injection boreholes and an effluent discharge tunnel, all in the vicinity of the Dounreay shaft.
The curtain will restrict water movement to and from the shaft, isolating its contents and, once complete, protecting against leakage from it (GE June 2005).
The £16M contract to isolate the shaft was awarded to Ritchies, the geotechnical division of Edmund Nuttall, as part of a £27M clean-up programme.
Installation of the curtain is intended to meet concerns about the shaft polluting the nearby marine environment. It is also designed to create a stable environment for contaminated waste retrieval from the shaft in the future.
Dounreay deputy director, Simon Middlemas, said: 'Being granted planning permission to isolate the shaft is a major step forward for the decommissioning of the Dounreay site.
'The shaft is one of the major nuclear decommissioning challenges in the world. The decision by Highland Council to grant planning permission allows us to move forward with our programme to decommission the site.'
Drilling the boreholes requires a raised working platform, built by placing layers of rolled compacted concrete from the Dounreay foreshore to the height of the shaft building.
A spokesman for the UK Atomic Energy Authority (UKAEA) said: 'There isn't enough space around the shaft, which is near a cliff edge.
So once the Highland Council gave its consent in March, work began to extend the cliff. This will make a level base all the way around the shaft from which to install the boreholes.'
These will found at up to 80m depths in a ring around the shaft.
Site workers will then inject ultra fine grout into the boreholes under pressure to penetrate any rock fractures.
Once solidified, the grout will form a barrier, reducing the volume of groundwater wing into the shaft which will result in less groundwater having to be dealt with as contaminated waste.
Trials were recently completed on the drilling and grouting of a shaft simulation grout wall.
July should see reinforcement of a plug in a horizontal stub shaft that leads to an effluent outfall tunnel.
Later work will include inlling a section of this liquid discharge outfall tunnel.
The UKAEA spokesman said: 'By 2008 the shaft will be stable, allowing waste removal from 2019.
But we are trying to bring this removal date forward by agreement with the Nuclear Decommissioning Authority and the Nuclear Installations Inspectorate [a branch of the Health and Safety Executive].
Once begun, it should take about six years to empty the shaft.'