Another 21 boreholes have been drilled around the underground waste shaft at Dounreay.
They are being used to measure how much less groundwater is reaching the shaft following its hydraulic isolation.
Swedish company SolExperts is taking measurements from each borehole and their report is expected to be complete in a few weeks.
The results will provide important information about the effectiveness of a “grout curtain” installed around the unlined shaft.
About 400 boreholes were drilled over the last two years and grout injected into the rock. This sealed up fissures in a 10-metre wide band around and below the shaft.
Diverting groundwater around the shaft is the first phase of decommissioning the 65-metre deep facility.It greatly reduces the amount of contaminated liquid that will need to be treated when the water level inside the shaft is lowered and the waste retrieved, saving an estimated £200 million in waste management costs.
“The results of these boreholes will tell us if the grout curtain is meeting its performance targets or if we need to do further work to seal up more of the fissures,” explained project manager Warren Jones.
A further four boreholes have been drilled to provide data about groundwater to the Scottish Environment Protection Agency.
Once hydraulic isolation has been confirmed, attention will turn to the enabling works for construction of the major plant needed to retrieve waste from the shaft and nearby silo.
This involves levelling a large area of ground and installing underground services in preparation for construction beginning in 2012.
Local members of Highland Council were updated on progress to decommission the shaft when they visited on Monday, June 2.