ONE of the most hazardous tasks in shutting down the Dounreay nuclear plant has been completed after a painstaking eight years.
All 1,500 tonnes of liquid metal have been drained from the 250 megawatt Prototype Fast Reactor (PFR), once the hub of Britain's fast breeder research and development programme.
It operated between 1974 and 1994, when it produced electricity for
the national grid. The liquid sodium metal was used as the coolant to transfer heat from the reactor core through three secondary circuits to a steam-generating plant for electricity production.
After the plant closed, the nuclear fuel was removed from the reactor. The last batch of hazardous sodium has now been transferred from the base of the reactor vessel and is being destroyed in a £17 million purpose-built chemical treatment plant that extracts the radioactivity and converts the sodium to common salt that can be discharged into the sea. The radioactivity in the sodium is isolated for specialist disposal.
James McCafferty, the PFR decommissioning manager, said: "The team behind this achievement is drawn from several different companies but all had one thing in common – to deliver the removal of this hazard from the reactor to the highest standards of personal and environmental safety."
Staff were able to see the last remaining few tonnes being removed from the bottom of the reactor by attaching a camera to a purpose-built pump that was lowered into the "heel" of the reactor vessel.
This was the most difficult part of the sodium to reach and required innovative design work and extensive testing by the reactor clean-up team to develop a system that would clear the final pools of metal. It was the first time anyone had seen inside the vessel since its construction 40 years ago.
Attached to a steel hose, the camera assembly manoeuvred through the complex reactor to reach the bottom. Operating in an extreme radiation environment, with temperatures in excess of 200C, it filmed the pump removing the final 20 tonnes of the pool of sodium.
Billy Husband, who was in charge of removing alkali metal residues, said: "Achieving this project is a major step forward in our programme to decommission PFR. We have worked on this particular stage of the project for two years and it has, without doubt, been a success due to the dedicated team of UKAEA (UK Atomic Energy Authority] and contractor staff working together."
Evan Park, the lead operations engineer, who worked at PFR in its infancy and watched the sodium being loaded into the reactor in 1974, said: "When I worked here during the construction of PFR, I never thought I would be standing here today, overseeing such a major phase of its decommissioning."
Decommissioning the entire Dounreay complex, which has been built up over more than 50 years, is due to end in 25 years, at a total cost of £2.9 billion.
