By Steven McKenzie
Highlands and Islands reporter, BBC Scotland news website
Work on sealing The Shaft is well under way
Myths and rumours abound about what was dumped down The Shaft at Dounreay.
Sunk in the 1950s close to the shores of the Pentland Firth, it plunges 65.4m below the surface and has an average diameter of 4.6m.
Local legend tells of a worker putting an urn containing his mother-in-law's ashes into The Shaft. A car was also allegedly rolled in.
What is known is that radioactive waste was disposed there from 1959 to 1977, when an explosion ended the practice.
Work is well under way on the lengthy process of safely sealing The Shaft before removing the hazardous rubbish.
The Shaft Isolation Project (SIP) team has been working on a solution for a unique problem.
What looks like a concrete bunker has been built over the top of The Shaft, but the most challenging operation has been creating a barrier around its entire length to stop water seeping inside through minute fissures in the surrounding rock.
Technology usually used to protect road tunnels in Scandinavia has been applied to the project.
Engineers have sunk about 200 boreholes to a depth of 80m, before injecting a fine grout at high pressure into the holes.
John Whitfield, project engineer on the SIP, said the technique could be used in the construction of a nuclear waste repository in Finland.
Swedish tunnel builders have also been watching the operation with interest.
Mr Whitfield said: "We are in the final stages of the construction and we are getting into testing the isolation barrier.
"We are trying to drastically reduce the amount of water getting through the cracks in rock around the shaft so that when the waste is taken out the amount of water that flows in will be much, much smaller.
"It will leave us with the sort of amount of water we can treat so we don't have to spend a lot of money on water treatment."
He added: "Grouting generally is used throughout the civil engineering industry, but this sort of grouting - because of the very fine cracks and use of very fine cement around a nuclear shaft - is unique."
The SIP team is gearing up to test the barrier, but it will be 25 years before the clean-up operation will see The Shaft finally give up its secrets.