Dounreay decommissioning scaffold structure
Client: Dounreay Site Restoration Ltd - Scaffold design for removal of DFR Pond concrete liner. Image Source: Dounreay Site Restoration Ltd
As part of the Dounreay decommissioning project, the concrete liner of the DFR (Dounreay Fast Reactor) Pond is being removed. A Dounreay decommissioning scaffold structure was required.
The DFR Pond, for storing fuel irradiated inside the reactor, was built in 1956 and measures 32.9m long by 9.75m wide and 6m deep internally.
The hot fuel was removed from the reactor and stored underwater in the pond to allow it to cool. From there, it was transported to the reprocessing plants for chemical separation.
The Pond consists of two reinforced concrete tanks, separated by a concrete wall. Each of the tanks contained 500m3 of water, which has been drained before any operation to remove the internal concrete liner is started.
In order to begin removal operations, scaffolding is required to be installed in the Pond area and boarded out to the top of the Pond walls.
Turner Access was commissioned to design and supply the Dounreay decommissioning scaffold structure that could enable fast and safe access to the Pond liner. The structure is currently being trialled at t3|uk trials facility, near Dounreay.
Through continuous collaboration with the project team at Dounreay Site Restoration Ltd (DSRL), a scaffold design and installation methodology was produced. Initial discussions concentrated on the detail of how the concrete liner would be removed. Personnel from Dounreay explained in meticulous detail everything from where the Pond is situated, dimensions, the environment and means of access, to the cutting process of the concrete blocks and how they would be removed.
From these initial discussions, it became apparent that Turner’s OCTO® Scaffolding System was the product for the job; not only because of the company’s design and manufacturing capability, but because standard components meet requirements. This means that, although special components can be produced if required; using predominantly standard components helps control costs and delivery time.
The next step involved preliminary designs being produced, which detailed the build procedure and best practice methodology.
Challenges & Solutions
Due to the ionising radiation hazards of the Pond’s environment, it is a requirement that removal of the concrete lining is done to keep workers’ radiation dose as low as reasonably practicable (ALARP). This was the main reasons that the Turner OCTO® Scaffolding System was chosen.
A requirement of the DFR Pond project was to liaise closely with the RPA (Radiation Protection Adviser). The RPA is appointed, under legislation, to advise on protection from ionising radiation.
Nuclear industry workers’ radiation dose is required by law to be kept ALARP, below a legal limit of 20 mSv effective dose per year. Dounreay’s dose investigation level, agreed with the HSE, is 10 mSv in a calendar year. A Dounreay Managing Director’s review level is in place, at 5 mSv in a calendar year, with doses kept ALARP, within job-specific constraints. Therefore, the speed of scaffold installation was of critical importance.
Time – limit exposure time
One of the unique benefits of the Turner OCTO® System is speed of assembly. Speed is achieved by a combination of less weight and fewer components, resulting in greater productivity.
For the same scaffold area, Turner OCTO® uses fewer components and is less than half the weight of traditional Tube & Fittings and traditional system scaffolding.
Distance – increase distance from exposure source
When the Dounreay decommissioning scaffold structure is re-built in the Pond, it will be assembled in sections and lifted into the Pond by a crane.
The first two lift heights will be installed using this method, with the remaining platform height installed within the Pond.
This methodology ensures workers keep a suitable distance from the Pond for as long as possible and reduces the time they are in the Pond itself. This in turn reduces total and individual radiation dose.
The Pond concrete liner will be cut into blocks for removal; totalling 190 from each Pond structure. In order to accommodate this, the scaffold structure had to be designed around where the cutting positions are. The structure must also accommodate slurry removal equipment, which filters out the concrete particles and re-circulates water back to the blade for cutting.
To facilitate the cutting procedure and provide access to remove the size-reduced sections of the Pond liner, cantilever components were incorporated into the design. The Pond scaffold structure consists of 3 independent towers that are linked together and a singular lift of cantilevers installed to access the pond walls.
The singular lift of cantilevers are adjustable within 0.5m in the height of the structure and can be accessed from each of the 3 independent towers as these are all accessible from the main body of the structure.
Once the cantilevers are in place the cutting saw can be attached to the pond walls and cutting can commence.
When the cut is complete the cantilevers can be removed and reinstalled at the next lower level below the desired cutting line. This allows the cutting saw to be placed at multiple locations on the pond walls.
One of the issues to overcome at the design stage was how the scaffold structure could be installed without workers standing on the floor of the concrete liner. Exposure levels can be minimised through use of two factors; time and distance. By not accessing the floor level, doses can be restricted. If workers do come in contact with the liner floor, doses may not be justified as ALARP. A means of access that keeps workers at a distance showcases best practice. The solution to install the scaffold structure came in the form of a build methodology and pre-fabricated tool.
The first two platform heights will be assembled in sections and then lowered into the Pond using a crane. To ensure the scaffold base-out is level; adjustments have to be made to the jacks.
To prevent workers from making contact with the floor, a special component was designed to enable jack adjustments to be made from above the Pond. Turner Access Scaffold Design Department came up with the initial concept and design of a Jack Key. The key is rotated until the Jack is in contact with the ground surface. On approval of the design, the key was manufactured and tested on-site along with the rest of the scaffold structure installation.
The Turner OCTO® Scaffold System has provided a safe and versatile access platform for a highly sensitive and important project in the nuclear decommissioning sector. The advantage of working with a company like Turner Access is the availability of design and manufacturing innovation. Throughout the project, Turner Access has provided ongoing support in the development of the solution provided to meet the Client’s requirements.
An example of design innovation is the provision of component seals to provide contamination control. A series of ‘plugs’ were manufactured to provide a means of sealing all scaffold components to ensure radioactive dust could not penetrate the equipment.
On completion of the DFR Pond project, the scaffold structure can be surveyed clear from the area, ready for use on another similar application.
The scaffold structure is currently being tested by DSRL. On completion of these tests, the structure will be moved to start removal of the DFR Pond lining.
The importance of testing the scaffold structure before use in the Pond was highlighted when the need for additional component design was identified. The Client requested a device that would help guide and fit the second part of the scaffold structure when being lowered onto the first platform section.
Turner Access designed a spigot-style component that would fit onto the top of the scaffold vertical on the base structure and help guide the platform being lowered into position. Due to Turner’s in-house Design and Manufacturing capability, this request was fulfilled and delivered to the Client within timescale.
When nuclear sites such as Dounreay were being built, there was no priority given to ease of decommissioning. This is proving to be a challenging issue across the country as more and more nuclear plants are being shut down or facilities being scaled back. The decommissioning team at Dounreay have strived to source safe and economical solutions that support a demanding project timetable. The solution provided by Turner OCTO® for the DFR Pond hopes to meet and exceed Client expectations.
“DSRL have a clear Safety, Health, Environment and Quality Policy for all decommissioning activities carried out at Dounreay and the identification of Turner OCTO® system scaffold has assisted the DFR Pond Project to comply with our commitment to achieve ever higher performance in these areas.
The overall service provided by Turner Access was professional and efficient, and their team’s commitment to identify workable solutions for such a unique structure was commendable. The system’s speed and versatility lends itself to the often complex access challenges experienced during nuclear decommissioning.”
Aly Mackay, DFR Decommissioning Project Manager, DSRL.