Workers were evacuated from a high-security plant at Dounreay after radioactive contamination was detected there, it has emerged.
The incident occurred on June 7 but site bosses only publicly released details at a meeting of Dounreay Stakeholder Group on Wednesday evening.
Site managing director Martin Moore said human error was to blame for the episode which is the subject of an in-house probe.
He said no harm had come from what turned out to be an “insignificant” level of low-level contamination.
The alarm was sounded as the worker sought to leave a former reprocessing plant within Dounreay’s fuel cycle area.
After follow-up surveys identified further ‘hotspots’ in the plant, all personnel were moved out.
Mr 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.”
Dounreay, an experimental nuclear power site, is being demolished and cleaned up at a cost of £2.32bn in a job expected to last up until 2033.
DSG chairman Roger Saxon questioned why site licence company DSRL had not issued a press release on the day of the incident.
He said that would have let the public know what had happened and help prevent the circulation of rumours and conjecture about what had happened.
A DSRL spokesman yesterday defended the delay and provided some additional information.
He said: “Earlier this month, an employee at Dounreay detected low levels of radioactive contamination during routine personal monitoring while exiting a controlled area.
“As a precaution, work in the area stopped immediately and an investigation was launched. The individual was wearing the correct protective clothing for the area. They have since returned to work and operations have returned to normal.
“There was no risk to members of the public, no increased risk to the workforce and no release to the environment.”
The incident in the D1204 plant has been rated as 0, the lowest level on the International Nuclear Event Scale.
The spokesman added: “The site’s regulator has been informed in accordance with agreed procedures and full details of the event were provided at the next meeting of the site’s stakeholder group.”
The Scottish Environment Protection Authority is meanwhile continuing its investigation into a contamination incident which occurred in the D1203 uranium recovery plant in February.
A spokesman said: “This investigation is ongoing and we have served notice on DSRL to provide some additional information.
The incident did not breach the site’s authorised discharge limits.