A clean-up of part of the Dounreay nuclear plant in Caithness has been delayed after the discovery of higher than expected levels of radiation.
The total cost of decommissioning Dounreay will run to £2.9bn
An underwater camera survey detected unexplained hot spots in a pond containing spent fuel used in the Dounreay Material Test Reactor (DMTR).
The find has meant plans to drain the pond have been put on hold until a follow-up probe has been carried out.
A spokesman for the 140-acre site said no-one has been put at risk.
Industry regulators have been informed.
A massive clean-up and regeneration project is under way at Dounreay, which is managed by the UK Atomic Energy Authority (UKAEA).
The UKAEA said the priority was to decommission and demolish the site as quickly and safely as possible.
The target of completion has been reduced from 2063 to 2033.
The 25 megawatt DMTR was Scotland's first operational reactor in 1958 and ran for 11 years.