The Dounreay shaft and wet silo

During the construction of the tunnel for the LLLE pipework in the 1950s, a vertical access shaft was sunk near the shoreline. After construction of the tunnel, the access shaft was sealed off from the tunnel with an in-situ cast concrete plug. In 1958 this 4.6 m diameter, 65 m deep shaft was author­ised by the UK government agency of the time to allow its use as a disposal facility for ILW. A wide variety of ILW, ranging from soft waste in bags to complete steel machinery, was tumble tipped into the shaft until 1977 when an explosion in the shaft led to the cessation of disposals.

The shaft is unlined and so there is groundwater ingress with the result that the water residing in the shaft becomes contaminated. The water level in the shaft is kept below that of the surrounding groundwater level by pumping. The extracted water is discharged through the LLLE authorised route.

During the late 1960s UKAEA recognised that the tumble tipping of ILW into a near-surface facility that was not designed as a disposal facility and had no engineered barriers between the ILW and the host environment would become an unacceptable practice. A replacement vault, the wet silo, was constructed as a single skin tanked concrete walled vault excavated into the host rock. It came into operation as a store in 1971. Its waste entry ports are at ground level and ILW was dropped into the silo from bottom opening vertical flasks through shielded gate valves. This facility is water filled to aid cooling and to provide shielding of the ILW.

Around 1980 UKAEA recognised that the historical disposals to the shaft, and emplacements in the wet silo had been so divorced from current practices that the ILW should be retrieved from these facilities. There had been no thought of ever retrieving ILW from the shaft but it had always been planned to empty the wet silo as it was licensed as a store. However, neither facility has any features to enable emptying and decommissioning and both present major challenges. Engineering studies and practical exper­iments for waste treatment have been carried out to inform the future retrieval projects.

As there are no in-built monitoring features to detect possible leakage in either facility, a number of boreholes have been sunk over the years in the shaft and wet silo areas. Monitoring of the groundwater in these bore­holes has not shown increases in the levels of radioactivity to be of signifi­cance to the wellbeing of the environment or operators working on the site (Environment Agency et al, 2010).

In preparation for retrieval of ILW from the shaft, a major project costing £27 million was completed in 2008 to encircle the shaft with a series of boreholes through which grout was pumped to form a water barrier around and under the shaft. This barrier is not completely impervious but has reduced the groundwater flow into the shaft by a factor of 10-15. Although not a design parameter, the grout curtain provides additional retention for any radioactivity migration from the shaft. The reduced ingress of water into the shaft will allow practicable contaminated water management to be undertaken when retrieval of the ILW is underway.

Добавить комментарий

Ваш e-mail не будет опубликован. Обязательные поля помечены *