Effluent discharge system
An essential part of the original infrastructure of Dounreay was a system to discharge radioactive effluent from the chemical plants, waste facilities and reactors to the sea.
Over time, the system has been overhauled and modern controls installed. The original discharge system was phased out between 1992 and 2003 and is now the focus of a project to decide how it can be decommissioned.
The original system consisted a low-active drain from the various facilities to carry effluent under gravity to the D1211 pits which were built at the lowest point on the site.
The tanks discharged to the sea via a subsea pipeline and diffuser. A bundle of four pipes was laid in a sloping adit and tunnel beneath the seabed. At the end of the tunnel the pipes were connected to a diffuser with a series of vertical risers to the seabed.
One of the first experiments carried out at Dounreay was a series of trial discharges known as Operation Seanuts. The results influenced the design of the effluent discharge system and the early limits on the radiological content of the discharges.
In 2007, divers operating on the seabed penetrated the old diffuser with a camera and radiation monitor. In 2008, a pipeline crawler travelled inside the pipeline from the shore to assess its condition.
Proposals for decommissioning the old system are to be submitted to the Scottish Environment Protection Agency by December 2009.
Effluent from decommissioning activities today is controlled in a modern facility, the low level liquid effluent treatment plant, prior to discharge to sea. Discharges are recorded, their impact on the environment monitored and the results published routinely.
Cutaway of the vertical shaft and the effluent discharge tunnel, showing the old diffuser chamber and its replacement
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
A diving team from Fathoms probed the old diffuser chamber during a survey in April 2008
The new low level liquid effluent treatment plant
The tunnel under construction
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