D1211 effluent pits

The effluent disposal system was an essential requirement for operation of the site and was therefore one of the earlier constructions.

It was constructed in the 1950s, the bulk of the work being completed by 1957. It comprised two large effluent receipt mixing and disposal tanks situated at the lowest point of the site.

Waste liquors flowed into the tanks by gravity via the low-active drain that was connected to the chemical plant complex and the reactors. After sampling, the contents of the tanks were discharged to sea through four, 23cm diameter cast iron pipes.

The positioning of two large effluent holding tanks as the prime components of the  effluent control system meant that one tank received liquid arisings while liquid was recirculated in the other and sampled prior to discharge.

Agreement with the Scottish Development Department, the then regulator, for the control and authorisation of liquid effluent discharges from the Dounreay site under the Radioactive Substances Act, stipulated that the pit would be sampled during recirculation for sentencing purposes and that a further drip sample would be taken during discharge.

The latter  sample was to be divided into two, so that as well as an analysis being carried out at Dounreay a sample would be sent to the SDD for an independent analysis. The pit discharge was carried out after high tide, and initially discharges were made down a temporary line because of the delay in excavation of the tunnel that would be used for the permanent lines. The temporary line discharged some 76 metres beyond high water mark, and was used until March 1958 when the permanent lines came into use.

The first authorisation for the disposal of liquid radioactive waste was issued in January 1957 and was valid for one year.

Major modifications and improvements were made over the years, including inlet pipes, drainage ducts, discharge pipes, and improved sampling techniques. A revised authorisation for the discharge of liquid effluent to sea issued in 1967 was changed in 1989. This new authorisation put limits not only on total alpha and beta activity and strontium 90 but additionally on various radionuclides.

A decision was taken in the 1990s to construct a new effluent treatment facility to meet modern requirements for the control of the discharge pH. The new low level liquid effluent treatment plant came into service in 2003, when use of the D1211 effluent pits ceased.

D1211 today is in care and maintenance pending the start of its decommissioning. This will involve the construction of a containment building over the facility and dedicated ventilation to allow it to be cleaned out and dismantled safely. Decommissioning is expected to take three years to complete.

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Image: Excavating part of the low-active drain in 2006

Excavating part of the low-active drain in 2006

Image: The old effluent tanks where low level liquid waste from all parts of the site, including the fuel plants, was collected for sampling prior to sea discharge

The old effluent tanks where low level liquid waste from all parts of the site, including the fuel plants, was collected for sampling prior to sea discharge