Workers were forced to evacuate the Dounreay nuclear power station in the Scottish Highlands after the discovery of radioactive contamination, it has been revealed.
Site bosses blamed "human error" for the incident, according to the Press and Journal newspaper.
They also said that there was "insignificant" low-level contamination but no harm has come of the leak.
The incident happened on June 7 but has only just been made public following the Dounreay Stakeholder Group on Wednesday evening.
According to reports, workers were forced to leave the plant after follow-up surveys found a number of other contamination 'hotspots'.
An in-house probe into how the contamination occurred is now underway.
Managing director Martin Moore said: “The contamination was very local but it wasn’t in a place it should have been, normally.
“The levels were insignificant but they should not have been there so we cleared the area and then had a controlled re-entry.”
He added: “It came down to a lack of due diligence in monitoring around one of the barriers."
"It was human error. It shouldn’t have happened and we’re very disappointed that it did.
“We took this very seriously and the teams responded well and our investigation is well under way.”
A spokesman for the Scottish Environment Protection Authority said: “This investigation is ongoing and we have served notice on DSRL to provide some additional information.
Dounreay nuclear station was earmarked for early decommissioning after a damning report found a number of issues with the plant.
It is hoped the site will be demolished by 2036 at a cost of £2.9bn to taxpayers.
