By Steven McKenzie
Highlands and Islands reporter, BBC Scotland news website
The inside of D8550 resembles a Dr Who set
"You could be the last visitors here," said a civil engineer at the end of our tour of D8550.
I was one of the journalists given a chance to see the drab building at a far end of the Dounreay site.
That external appearance gives few clues to the dramatic reasons for its construction.
D8550 owes its existence to a seismic rift in UK and US collaboration on nuclear science.
The wartime allies had been working together on atomic energy and weapons.
However, the passing of the McMahon Act in 1946 prohibited the US from sharing the technology and data with other countries, ending the arrangement.
Because experimental data gathered in the US on uranium and plutonium criticality was denied to UK scientists, a new laboratory was built at Dounreay to carry out the experiments.
However, the days are numbered for what is now a virtually empty concrete shell.
It has been cleared of almost everything it once contained, including some of the most grossly contaminated radioactive material on the nuclear site in Caithness.
Nothing now remains of the steel pressure vessel where tests on plutonium were once carried out and where highly radioactive rubbish was later stored.
Its domed roof was divided up like a huge jigsaw and cut with a gas torch, before the rest of the metal work was stripped away.
What is left of the building resembles a set for Dr Who.
Even the protective clothing used by workers during the clean-up could have featured in the science fiction show.
Contractors wore heavy leather outfits for cutting away the domed roof and bulky plastic ones for other decontamination jobs.
Demolition has been scheduled to be complete by June, when the last of the building - including the 1.5m thick hexagonal concrete "bio shield" that surrounded the vessel - will be pulled down and rubble removed.
Not far from Dounreay lies the Split Stane, a large cleft stone which is said to have been cracked by the Devil.
It seems an appropriate local landmark, given D8550's connections with splitting the atom and the controversy that can sometimes surround the nuclear site.