The operators of the mothballed Dounreay nuclear complex in Caithness have been fined £15,000 after an employee breathed in plutonium.
The plant's Marshall Lab carried out research for Japan
The UK Atomic Energy Authority (UKAEA) pleaded guilty to the health and safety breach at Wick Sheriff Court.
Speaking outside the court, a UKAEA spokesman said procedures had been tightened since the incident.
In February, the operator was fined £140,000 for illegally dumping waste at the former fast reactor plant.
Simon Middlemas, Dounreay's site director, said: "We accept some mistakes were made in the Marshall Lab at the beginning of last year resulting in a worker receiving a very minimal plutonium intake.
"It should not have happened and we, very quickly, addressed a number of issues."
Dounreay was Britain's centre of fast reactor research from 1954 to 1994
The site is now being decommissioned at a cost of £2.9m
The Marshall Lab was built in 1981 and carried out development and trials for Sellafield and Japan's mixed oxide fuel reprocessing programme
UKAEA's director of safety, Dr John Crofts, added: "Our procedures have been tightened and a number of behavioural safety initiatives have been set in place to ensure this type of incident should never occur again."
The worker breathed in plutonium at the plant in January last year.
The prosecution followed an investigation by inspectors into record keeping and the storage of material at a former fuel reprocessing laboratory which the employee had been sent to help decommission.
The charge, brought under the Health and Safety at Work Act, cited the source of contamination as a number of lead bricks.
Earlier this year the Nuclear Installations Inspectorate, the nuclear industry's on-site regulator, served two formal improvement notices on the UK Atomic Energy Authority.
It was ordered to take action to remedy shortcomings identified in record-keeping and storage of material at the laboratory.