Dounreay's operators were fined £15,000 yesterday for a series of health and safety failings that led to a worker breathing in plutonium which will remain in his system for life.

It follows a £140,000 penalty that the UK Atomic Energy Authority (UKAEA) received in February for releasing radioactive particles into the sea and illegally dumping radioactive waste over 20 years.

The latest fine came after an incident in January last year when a male employee and a female colleague at the Dounreay site were exposed to the radioactive material as they disposed of contaminated bricks in a laboratory.

The UKAEA admitted breaches of the Health and Safety at Work Act 1974 at Wick Sheriff Court. Tests revealed one of the employees, Brian Grant, received a plutonium intake.

The matter was reported to the Health and Safety Executive, whose nuclear installations inspectorate issued the plant with two improvement notices after an investigation.

UKAEA's solicitor David Stewart said the dose Mr Grant received was less than one-tenth of the annual permitted legal dose. He added that Dounreay now had risk assessment documents in place and general advice directing that respirators should be worn when disposing of low level and intermediate level waste.

The court heard the nuclear plant now required all items which were in transit for storage to be properly labelled and inspected. Mr Stewart said: "There is absolutely no doubt this should not have happened. We put our hands up to that. All we can do is make sure that this does not happen again."

Sheriff David Sutherland told UKAEA he would have fined them the maximum penalty of £20,000 if they had not entered a guilty plea at an early stage.

Lorraine Mann, of Scotland against Nuclear Dumping, who has been Dounreay's most persistent critic, said: "It is absolutely disgraceful that this sort of thing is happening time and time again.

"The UKAEA seems absolutely incapable of getting on top of the profound health and safety problems on the Dounreay site."

In March 2000 the UKAEA was fined £100,000 at Dornoch Sheriff Court for failing to properly protect three workers from exposure to radiation in 1995 when they suffered exposure up to nine times the permitted levels.

It was fined a further £1000 for an incident in 1998 when an 11,000 volt cable was damaged by a digger driver, cutting off power to the fuel cycle area where all nuclear processing and reprocessing was done.