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Significant hazard reduction at Dounreay is ‘made in Caithness’

By Alan Hendry

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The heel pool team at Dounreay.
The heel pool team at Dounreay.

One of the most significant hazards at Dounreay has been removed in a 40-hour operation, using cutting-edge tools designed and made in Caithness.

Around 1810 litres of radioactive sodium coolant (some 1.7 tonnes) remaining in a "heel" pool at the base of the reactor vessel in the Prototype Fast Reactor (PFR) has been pumped out. It paves the way for the next step in the decommissioning of PFR to take place.

The project has been described as "Dounreay at its best", and Dounreay Site Restoration Ltd (DSRL) says it showcases the high-quality design and engineering skills available in Caithness-based companies.

The DSRL project team worked closely with the supply chain to develop an innovative technical solution to the removal of the sodium. Since the heel was difficult to access, a flexible 65m long pumping and camera system that could operate in temperatures of several hundred degrees had to be designed, manufactured and tested.

The pump was designed in-house and fabricated locally by Gow's Lybster, which also manufactured the shielded sodium receipt vessel with Forsyths and Arch Henderson.

The sodium, which had solidified at the bottom of the reactor, first had to be melted. Bespoke heaters similar in shape to the petals of a flower were designed by the project team, fabricated locally by JGC Engineering and Technical Services, and deployed through holes drilled through the base of the reactor leak jacket and attached to the underside of the reactor vessel to melt the sodium from the outside.

The heel pool project team has been working for four years from the initial design to get the project to this stage. They spent months rehearsing the moves and skills required to position the pump in the heel pool, using a mocked-up version of the reactor internals and making changes to the equipment where necessary.

The team believe that the process is a world first for the site and that the innovation and learning can be applied to other decommissioning projects UK-wide where hazards are difficult to access.

Project manager James Robertson said: “This has been a significant challenge. The team has delivered a project that is really at the cutting edge of decommissioning, something that’s truly world class.

"This was Dounreay at its best, delivering a highly complex decommissioning task in a way that demonstrated a really innovative and collaborative working approach.”

Senior project manager Graeme Dunnett said: "The removal of the sodium heel from the reactor vessel is a significant achievement and important step forward in the decommissioning programme of the PFR complex by a project team made up of local contractors and DSRL workers.

"The motto from PFR’s operational days was 'Out of Caithness to the world' and, as this project is world-leading, it very much captures the ethos of this sentiment.”

DSRL managing director Mark Rouse said: “This was not an easy task – it was technically and practically difficult and required a lot of resilience in the face of disappointments and setbacks as the team explored the limits of what was known and what has been done before.”

Dounreay Stakeholder Group chairman Struan Mackie said: “The removal of liquid sodium from the heel pool of the Prototype Fast Reactor represents a significant milestone in removing legacy hazards from the Dounreay site. Throughout the project the Dounreay Stakeholder Group was kept informed of progress and the innovative methods being deployed to complete this complex piece of decommissioning work.

“We believe it to be a world's first – made possible due to combination of expert design, fabrication and execution of the Dounreay workforce and supply chain partners from across Caithness and north Sutherland.”

The next phase will be to decommission and remove the pump from the reactor and install the water vapour nitrogen system which will be used to treat the sodium residues from the reactor vessel and pipework. The project is on course to complete the clean-out work by the end of 2023.

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