Dounreay’s Prototype Fast Reactor has now been drained of all 1500 tonnes of liquid metal that once flowed through its primary and secondary circuits.
The last batch of hazardous sodium has been transferred from the base of the reactor vessel and is now being destroyed in a purpose-built chemical treatment plant that extracts the radioactivity and converts the sodium to common salt that can be discharged safely to the sea.
It means the site has almost completed the destruction of one of the major hazards leftover from its research days.
PFR decommissioning manager James McCafferty said it had been achieved without a lost-time accident for more than three years.
“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,” he said.
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 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 has seen inside the vessel since its construction 40 years ago.
Attached to a steel hose, the camera assembly manoeuvred through the complex internals of the reactor to reach the very bottom. Operating in an extreme radiation environment with temperatures in excess of 200 degrees Celsius, it filmed the pump removing the final 20 tonnes from the mirror-like pool of sodium.
It has taken eight years to drain the reactor in a way that ensured the safety of the workforce and the environment was protected.
The final stage was completed using the innovative camera-pump device designed in-house by UKAEA’s specialist design team. It was built by JGC Technical Services Ltd and operated by Nukem. Initially mock-ups were built and tested at the T3e facility at Janetstown where the whole assembly was also commissioned and the operators trained.
Billy Husband, alkali metal residues removal project manager at PFR, 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 and contractor staff working together.”
Lead designer Robin Herrick helped with the deployment and operations. “Seeing the equipment in use is very exciting. To keep the camera cool way down at the bottom of the reactor vessel, I designed something akin to a 23-metre long flexible Thermos flask. It was a big challenge.”
The lead operations engineer on this project is Evan Park, who also worked at PFR in its infancy and watched the sodium being loaded into the reactor in 1974. He 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.”
The removal of the sodium means the decommissioning team can now “steam-clean” the last traces of sodium from the pipework using a water vapour nitrogen process, making it safe to begin dismantling the entire system.
Notes to Editors:
PFR operated for twenty years from 1974 until 1994. 1,500 tonnes of liquid sodium metal was used as the coolant to transfer the heat from the reactor core through three secondary circuits to a steam-generating plant for electricity production.
All the nuclear fuel was removed from the reactor following its closure and the next phase of decommissioning is to complete the disposal of the remaining sodium residue using the pioneering water vapour nitrogen process.
PFR had the dual role of providing power to the national grid and offering unique research and development facilities.
PFR provided information for the future design and operation of large commercial fast reactor stations and had an electrical output of 250 MW, which was enough electricity to supply a city the size of Aberdeen