Attention will now turn to cleaning the out the last few tonnes of residue still inside the reactor circuits to allow the whole system to be cut up safely and disposed of.
“Everyone associated with the clean-up of Dounreay can be proud of what this team has achieved,” said Simon Middlemas, managing director of clean-up contractor Dounreay Site Restoration Ltd.
“When the reactor closed in 1994, the major hazards consisted of the nuclear material inside the reactor and the liquid metal that had been used to transfer the heat from the core to the steam-generating plant for electricity. The reactor was de-fueled and the now the liquid metal has gone, too.'
Jim McCafferty, PFR decommissioning project manager, said: 'It is a milestone in hazard reduction at Dounreay for ourselves, the Nuclear Decommissioning Authority and one which the team at PFR are very proud of.
'With this challenge met the focus of the PFR team has now moved onto the next phases which include removing the sodium residue from the reactor vessel and taking the reactor apart.”
Dounreay’s two fast reactors were the only nuclear power plants ever built in Britain to use liquid metal. This meant their clean-up had to be designed from scratch as no-one had ever dealt with such a hazard in the UK before.
A £17 million chemical treatment was constructed in the former turbine hall of the prototype fast reactor to process the bulk of the liquid metal from its cooling circuits.
Behind thick concrete walls designed to protect workers from the toxic hazard, the alkali metal was converted to salt water by mixing small batches with large quantities of aqueous sodium hydroxide and then neutralising it with hydrochloric acid.
Radioactive caesium was extracted by passing the liquid through an ion exchange column, leaving a cleaned up salt water that was discharged safely to the sea.
In total, 1533 tonnes was destroyed this way. The previous record for liquid metal destruction was just over 1000 tonnes set during the decommissioning of a fast reactor in the USA.
“The whole process generated 10 tonnes of clean effluent for every tonne of sodium metal received,” explained project manager Neil McLean.
“The level of caesium in the metal when it came from the reactor was 60 becquerels per millilitre. The level in the salt water that left the plant was below the limit of detection of 0.3 Bq per millilitre. In between, we used 22 ion exchange columns to extract the caesium and these will now be stored as intermediate-level waste.”
The plant was operated by Nuvia and maintained by BNS Services.