Work has started on the penultimate phase of cleaning out an old plutonium test cell that many people thought could never be decommissioned safely.
The latest phase involves taking apart the control area of the plutonium criticality test cell.
This was where scientists in the 1950s and 1960s controlled a series of experiments that took place inside an adjoining room with solutions of plutonium and uranium.
The experiments contaminated much of the building. For many years afterwards, no-one could enter the area unless they wore breathing apparatus. Radioactive waste accumulated as the building became a store.
Now, eight years after the start of a £7 million clean-up, the bull-dozers are almost ready to move in.
The experimental cell itself has been cleaned and cut up and other rooms that became contaminated have also been gutted.
Now, workers wearing protective suits are beginning to clean out and dismantle what remains of the control area.
Once this is complete, their attention will turn to cleaning out the last of the contaminated rooms – the "active transit area".
Later this year the building itself will be razed to the ground.
The clean-out of a building dubbed one of the dirtiest at Dounreay has been carried out with one of the best safety records of any decommissioning project at the site.
The team – made up of staff from Doosan Babcock, NDSL, NUKEM and DSRL – has made some 15,000 "entries" into contaminated areas wearing fully-protective airline suits.
Three criticality cells were built at Dounreay in the 1950s to provide the developing UK nuclear industry with data about how plutonium and uranium reacted. The other two have already been cleaned out and demolished.