NDE and inspection
Down the drain22 May 2009
Dounreay’s corroded undersea drain has been inspected by a specialised robot which investigated radioactivity levels and the integrity of the old pipes. By Martin Howse
An integral part of the original design for Britain’s experimental fast reactor site at Dounreay, in northern Scotland, was a system to discharge radioactive effluent to the sea. A drain was laid beneath the site and connected the reactors, the fuel plants and waste facilities. Effluent flowed under gravity following discharge from the facilities to two collection tanks installed at the lowest point of the site. After sampling, the contents of the tanks were discharged to sea through four, 23cm diameter bitumen-coated cast iron pipes.
The UKAEA laid the discharge pipes and encased them in concrete within an undersea tunnel. The tunnel was excavated 600m out to sea and terminated at a diffuser chamber 25m below the seabed.
A 1:3 sloping adit carried the pipes from the collection tanks to the start of the main tunnel, which was inclined upwards with a slope of 1:200 to the chamber at the end of the tunnel where the pipes terminated in a manifold connected to the seabed.
The concrete-lined diffuser chamber was some 3.5m in diameter and 10m long. Inside the diffuser, each main pipe branched via valves and reducers into four risers that were drilled vertically through the rock to the seabed discharge point above. When work was complete in 1957, the tunnel was allowed to flood naturally.
The pipework beneath the seabed will have started to corrode as soon as the system was used for the first time in 1958. Records show that the final section of pipework within the old diffuser chamber was made from mild steel; so had less resistance to corrosion from the acidic effluent than the cast iron pipes.
In 1979 and again in 1983, following periods of lower than expected discharge flow rates, increased pumping pressures – of the order of 10 bar – were applied in an effort to improve flow rates, but with limited success.
Routine use of the old pipeline ceased in 1992 when a new multi-bore high-density polythene replacement was installed through the tunnel, terminating in a new diffuser system on the seabed.
Today, the site belongs to the Nuclear Decommissioning Authority. Its closure is managed by Dounreay Site Restoration Ltd (DSRL), which is investigating the current condition of the old pipeline and diffuser. Data obtained during these investigations will enable decisions to be made about potential options to safely decommission the system.
In April 2008, local diving contractor Fathoms Ltd, working under the guidance of DSRL project staff, surveyed the area of the old diffuser outfall and accessed the diffuser chamber via one of the risers with radiation detection and camera equipment.
In February 2009, DSRL commissioned MSIS and Hydropulsion to carry out a visual and radiological inspection inside three of the four discharge pipes. At present, one of the four pipes remains connected to the disused effluent tanks and could not be surveyed. The purpose of the survey was to provide information on the physical condition and assess the levels of residual contamination inside the pipes.
The lowest point of the system is at the triple intersection where the sloping adit meets the tunnel at the base of the original vertical excavation shaft, approximately 200m from the access point. From there, the tunnel rises towards the seabed on a 1:200 incline.
The objective of the survey was to reach this lowest point in the effluent pipes, where particulate would be expected to start to settle out.
The equipment used for this project was a customised, long-range modular inspection vehicle capable of operating in water depths of 50m (5atm) with an onboard CZT gamma detector. The vehicle is typically operated within a variety of pipe sizes from a minimum internal diameter of 150mm.
Following a tender exercise, the vehicle was selected due to its combination of available on-site reconfiguration options and ability to travel long distances (up to 457m at speeds of up to 9m/min).
Dependent upon restrictions encountered during the pipeline inspection it was possible to alter the mounting position of the gamma detector from a frontal to a piggy-back position. A selection of skids were also supplied with the vehicle to allow the camera to maintain a central position within the pipe.
Due to the presence of radioactive contamination underwater, the robot apparatus had to be covered on land. Containment of the powered tether reel and radiological monitoring of the tether during recovery were achieved by use of a bespoke PVC glove-bag and construction of an enclosure for the tether reel. The enclosure was located outside the building which contained the access points for the pipelines.
The controller operated the crawler vehicle and powered tether reel from a dedicated CCTV inspection vehicle adjacent to the tether reel enclosure. The system controller’s on-screen display included information on the CZT detector, counts per second (CPS) detected, date and time, and tether reel distance counter. At appropriate points during the inspection, spectrum logs including information on time, distance out, gamma count rate and spectra were recorded on a laptop connected to the system controller.
Early review of the pipeline technical drawings indicated that the twin track in-line configuration with the gamma detector mounted on the front of the vehicle would be the best format for the crawler (which can also be set up as single track unit for short pipe runs, or a steerable parallel configuration).
The front mounting option selected included integration to the gamma detector of a high-resolution, low-lux, virtually indestructible epoxy-encapsulated camera with built in LEDs. This configuration also offered much simpler decontamination than the pan/tilt/zoom camera alternative option supplied with the system.
The crawler vehicle was fitted with deep lug rubber track belts for use underwater and in dirty or muddy conditions. Stainless steel clutched tracks allowed easy retrieval of the vehicle. Due to conditions inside the pipes the tracks were removed as waste on completion of the surveys. The construction materials of the vehicle aided the successful decontamination of the system, on completion of the project.
The live video feed indicated that all three of the surveyed pipelines were clear of accumulations of solid matter. Indeed, when the crawler was halted occasionally to allow the fine material on the pipes to settle, the sediment was observed to move with the sea swell. This provides further evidence that the pipes remain connected to the sea.
The radiation survey was carried out using a gamma detector probe mounted directly behind the camera on the pipe-crawler. The probe was calibrated and function-checked before the crawler entered each pipe.
The survey comprised a live feed in CPS as the crawler travelled the length of each pipe. The survey indicated that there is some radioactive contamination within the pipework, albeit to a relatively low level.
A small number of areas of elevated radioactivity were subjected to gamma spectrometry independently of the full survey.
The survey intent was to provide information on the residual level of radioactivity remaining in the pipes. The data obtained during the survey, including samples of material from within the pipes are under review.
DSRL expects to present its proposals for decommissioning the system to the Scottish Environment Protection Agency later this year.
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