Origins of the shaft
A vertical shaft was sunk at Dounreay in the 1950s to build a tunnel for the discharge of radioactive effluent from the various research and industrial facilities then under construction.
The shaft was used to transfer tunnelling equipment in and out of the low-active effluent discharge tunnel. It was the main route for removing spoil and the main collection and pumping point for groundwater to keep the tunnel dry during excavation.
The shaft and tunnel were constructed using drilling and blasting methods. The shaft has an average diameter of 4.6 metres and a total height of 65.4 metres. The upper 7.9 metres of the shaft were completed with concrete lining. Beneath this is a 5.6 metre wire meshed and rock bolted section. The remainder of the shaft is unlined, with occasional rock bolting, wire meshing and gunnite grouting at intervals down the shaft as well as along the tunnel, although the extent and location of these were not recorded.
A stub tunnel leaves the shaft approximately five metres above the shaft base level and extends for approximately 29 metres where it intersects the tunnel, some 50 metres below the seabed. The tunnel slopes at a gradient of 1:3 down to the end of the stub tunnel. From this point the tunnel extends with an upward gradient of 1:200 in a north-westerly direction approximately 600 metres out beneath the sea where it ends at a diffuser chamber.
Following completion of the tunnel, a concrete plug was installed in the stub tunnel to provide hydraulic and mechanical isolation between the shaft and the tunnel. This plug is approximately 2.4 metres thick and was keyed and grouted into the surrounding rock.
Initially, it was intended to keep the tunnel dry to allow access to the diffuser chamber, while the shaft would be allowed to fill naturally with groundwater. Consequently, the plug was designed to withstand a hydraulic load equivalent to a 60-metre head of water.
Higher than expected volumes of groundwater flowed into the tunnel, however, and the concept of keeping it dry for personnel access was abandoned. Both the shaft and tunnel were allowed to fill naturally with groundwater once commissioning of the diffuser chamber had been completed.