Officials at the Dounreay nuclear complex in Caithness are dealing with the second radioactivity alert in less than three weeks.
Eight workers at Dounreay are being monitored
A second part of the site has been closed after tests showed traces of plutonium in eight workers.
They had been working in a laboratory which is being decommissioned.
A treatment plant at the nuclear site was closed last month after an escape of dangerous spent fuel, although no-one was harmed by the spill.
The latest fears were triggered when the employees, who are now being monitored, had routine tests after working in the laboratory.
It is the second problem to affect work on dismantling the one-time test laboratory in less than a year.
It was shut down last November after a similar alert involving 15 workers.
The unit remained closed until a couple of months ago.
Spokesman Colin Punler said of the new scare: "One of the checks the workers go through is a nose-blow into a tissue.
"Overnight last Thursday, eight of the tissue samples which the workers were asked to provide as a routine precaution came back with traces of radioactivity on them."
Monitoring is being carried out after the latest problem
Dounreay, which is run by the United Kingdom Atomic Energy Authority, was used as Britain's centre of fast reactor research and development from 1955 until 1994.
Mr Punler added: "The more you take it apart, the greater the hazard becomes. It is proving to be a difficult job and a dirty job."
Last month a treatment plant was closed after an alert involving a batch of hazardous, dissolved spent fuel.
Officials said nobody had been harmed, or exposed to radioactive waste, as a result of the problem in the cementation area.
No formal disciplinary action was taken against any member of the workforce following the incident.
The authority said the spill was "contained within the cell", but admitted it was a "setback" to the 30-year decommissioning programme.