Dounreay Site Restoration Ltd

Subsea pipeline and diffuser

The construction of Dounreay in the 1950s included a system for discharging low-level radioactive effluent to the sea.

A tunnel was excavated 600 metres out to sea and 25 metres below the seabed. Four cast iron pipes, each of about 230 mm diameter, were laid along the wall of the tunnel and encased in a concrete surround. The pipes were connected at one end to the D1211 effluent pits and at the other to a diffuser - now known as the old diffuser - at the seaward end of the tunnel.

The diffuser was a concrete-lined chamber some 3.5 metres in diameter and 10m long. Inside the diffuser, each pipe branched via valves and reducers into "risers" that rose vertically through the rock to the seabed discharge point above.

Upstands were fitted to each riser outlet at the seabed to prevent waves of sand and debris which may have resulted in blockages. The upstand pipes protruded a few feet above the seabed, causing fishing nets to snag and break the upstands.

The pipework beneath the seabed probably started to corrode as soon as the tunnel system was flooded and used for the first time in 1956. A section of pipework within the old diffuser chamber is believed to have been made from mild steel, so may have had poor resistance to corrosion.

In 1979 and again in 1983, substantial pumping pressures - reportedly of the order of 3-10 bar - were applied to the discharge pipeline in an effort to improve flow rates. This action may have led to failure of already corroded pipework within the diffuser.

In 1989, during an inspection by remotely-operated vehicle at the tunnel side of the bulkhead, a flow was seen coming from the diffuser through a small pipe in the bulkhead when pumping operations were underway. It wasn’t possible to see inside the chamber as the bulkhead door had been closed prior to flooding of the tunnel after construction in the 1950s.

The system remained in routine operational use until 1992 when it was replaced by a new multi-bore polythene pipe. The pipe runs through the same tunnel but stops short of the old diffuser, rising upwards at 45 degrees and connecting to a new diffuser on the seabed near where the old upstands emerged.

In the late 1990s, divers located the old risers on the seabed and installed plugs as a precaution against the escape of any particles that may be lodged in the disused chamber.

The condition of the old diffuser and old effluent pipes are being investigated to enable decisions to be made about how they should be decommissioned.

In April 2008, a diving contractor working under the supervision of DSRL project staff surveyed the area of the old diffuser and outfall and probed one of the risers with radiation detection and camera equipment. See here for the news story.

In February 2009, a pipeline crawler carrying a camera and radiation monitor inspected the inside of three of the old effluent pipes. It was inserted into the pipes near its connection to the D1211 tanks. An umbilical cable gave the crawler a potential reach of 450 metres through each pipe.

Data gained from the surveys will be used to form judgements about how to leave the old system in a safe condition as part of the site closure programme. A proposal is to be presented to the Scottish Environment Protection Agency in December 2009.