An underground waste shaft at Dounreay which exploded in 1977 is being sealed off in a £16m operation.
Sealing off a waste shaft at Dounreay will take four years
Grout will be poured down 400 boreholes which are being sunk to form a protective ring around the 65m shaft.
The UK Atomic Energy Authority, the site operators, said the shield was designed to eliminate the risk of radioactive particles leaking.
Grouting was selected as the preferred method following a period of public consultation in 2004.
The operation to seal off the Caithness site's waste shaft will take up to four years to complete.
For almost 20 years nuclear and non-radioactive waste was tipped into it.
But this disposal solution was abruptly halted in 1977 when it exploded.
The project, one of the most critical parts of the overall decommissioning of the plant, will allow the contents of the shaft to be isolated.
About 400 boreholes will be drilled to a depth of up to 80m in a protective ring around the shaft and filled with grout - providing a curtain around the waste hole.
Management said that once complete, the curtain would protect against leakage and eliminate any doubts about the shaft being the source of particles found on the seabed and beach nearby.
The development will also provide a stable environment when the waste is ultimately removed.