The Prototype Fast Reactor building in Caithness
Tonnes of radioactive liquid metal - a legacy of the experimental fast reactor programme at Dounreay in Caithness - have been destroyed.
Dounreay Site Restoration Limited (DSRL) said the material was turned into "harmless" salt water.
The water was put through a further process so it could be discharged into the sea.
A few tonnes left in the circuits of the Prototype Fast Reactor (PFR) will be cleared so they can be cut up.
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."
Dounreay's two fast reactors were the only nuclear power plants built in Britain to use liquid metal.
This meant their clean-up had to be designed from scratch.
A £17m chemical treatment was constructed to process the bulk of the liquid metal from its cooling circuits.
It 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 later to leave what DSRL said was a cleaned up salt water that was discharged safely to the sea.