Clean-up workers at Dounreay celebrated what they called a 'milestone' in the dismantling of the former nuclear research site. A plant built to 'destroy' the radioactive liquid metal coolant of the Prototype Fast Reactor (PFR) completed its work, after neutralising 1533 tonnes of the material.
Dounreay said the sodium had represented the most serious hazard still present at the old reactor. The removal of the most hazardous substance of all - the used nuclear fuel - having already been carried out following the reactor's shutdown in 1994. The PFR started up in 1975 and contributed 254 MWe to the UK grid but was not considered successful enough to be the basis for a series of similar units.
Work to dismantle and cut up the structures of the reactor itself can follow after the "last few tonnes" of sodium residue are removed from its inner circuits.
The last batch of liquid sodium is destroyed by Willie Johnston while Jim McCafferty, Simon Middlemass and Neil Mclean look on
How it was done
The sodium was effectively turned into salt water by a £17 million ($31 million) plant built in the reactor unit's former turbine hall.
Small batches of the sodium were mixed with larger amounts of aqueous sodium hydroxide and then neutralised with hydrochloric acid. The resulting salt water still contained radioactive caesium to the level of 60 Bq per millilitre, and this was removed by passing it through ion exchange columns. This reduced the activity of the water to "below the limit of detection of 0.3 Bq per millilitre," said Neil Mclean, project manager for the task.
For each tonne of sodium that was 'destroyed', some ten tonnes of salt water was returned to the sea.
The 22 ion exchange columns used in the process will be stored as intermediate-level radioactive waste, destined for eventual permanent disposal deep underground.