A FIRE at the Dounreay nuclear plant resulted in an "unauthorised" release of radioactivity into the atmosphere, an investigation has revealed.

Dounreay Site Restoration Limited (DSRL), the company responsible for decommissioning the site on the north coast of Scotland, has been marked down by the Office for Nuclear Regulation (ONR) following the incident.

DSRL blamed the fire on "unacceptable behaviours and practices" by staff, and said that steps had been taken to prevent similar mistakes being made. However, environmentalists have said the accidental release of hazardous material is "extremely worrying" and called for a rethink on the way emergency situations at the site are handled.

The fire broke out on October 7 in the prototype fast reactor sodium tank building, where sodium residues from the operation of the reactor are stored.

The Dounreay on-site fire brigade extinguished the blaze and no-one was hurt. The Nuclear Decommissioning Authority and regulators were informed and an investigation was launched.

In a statement, DSRL said: "The investigation thoroughly checked each aspect of the work and identified procedural non-compliances and behavioural practices that were factors in the incident, and fell short of the values and standards expected of our people.

"It also confirmed the release of radioactivity via an unauthorised route."

The Office for Nuclear Regulation (ONR) served DSRL with an improvement notice on November 11 relating to the fire.

Managing director Mark Rouse said: "We have accepted the improvement notice and will co-operate fully with the regulators.

"Our investigation identified unacceptable behaviours and practices that fell well short of our values and standards. It is important to take the time to ensure as many lessons are learned from this incident as possible.

"We are determined to improve our behaviours and compliance to ensure that we always meet the high standards expected on a nuclear site."

WWF Scotland director Lang Banks said: "Thankfully no one was injured in this incident.

"However, that the fire resulted in the release of radioactive material is deeply worrying and calls into question the processes currently in place to protect people and the environment.

"There needs to be a thorough review of procedures to ensure nothing like this can ever happen again.

"This incident again highlights the hazardous and expensive problem of dealing with the radioactive legacy of the nuclear industry.

"Anyone who thinks that nuclear power has a role to play in our energy future needs to seriously think again."

The company said a safety improvement plan was in place and work would not restart at the tank building until ONR is satisfied with the changes.

The Dounreay reactor stopped operating in 1994. It is currently more than halfway through the decommissioning process.

The incident happened on the same day a blaze broke out on a ship transporting radioactive waste from Dounreay to Belgium.

The blaze started in one of the MV Parida's two funnels on the night of October 7.

It lost power and started drifting towards an oil platform in the Moray Firth.

Fifty-two people on the rig were evacuated as a precaution. Following repairs the ship was able to resume its journey.