A clean-up operation funded by the NDA has destroyed more than half the liquid metal used as coolant in the experimental Dounreay Fast Reactor.
Its annual report published today describes the toxic sodium-potassium alloy it inherited in 2005 as the second biggest hazard in its £3bn-a-year clean-up mission.
Some 57,000 litres remained inside the reactor when its decommissioning stalled in the 1980s.
A purpose-built chemical treatment plant commissioned by the NDA in 2009 has now destroyed more than half of it.
The metal is so radioactive that its removal is bringing down radiation levels inside the reactor at a rate of 20 million becquerels a second.
The two-stage process involves chemical conversion and radiological decontamination to produce a salty water that is safe to discharge to sea.
Designers thought the plant could decontaminate the metal by a factor of up to 1000.
But clean-up rates of up to 4 million have been achieved and the levels of radioactivity in the effluent are now at the limits of detection.
“It has been phenomenally successful at hazard reduction,” said Andy Swan, senior facility manager with contractor DSRL.
The plant currently is undergoing repairs to a valve that will keep it on course to destroy the bulk of the hazard by March next year.
Fast reactors were the only reactors in Britain to use liquid metal instead of gas or water in the cooling circuits. Only two fast reactors were built, both at Dounreay in northern Scotland.