A fire at the Dounreay nuclear plant resulted in an "unauthorised" release of radioactivity, an investigation has found.

Dounreay Site Restoration Limited (DSRL), the company responsible for decommissioning the site on the north coast of Scotland, said it had identified "unacceptable behaviours and practices" by staff.

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.

A statement from 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 relating to the fire on November 11.

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."

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 reactor stopped operating in 1994 and is more than halfway through decommissioning.

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."

Environment Secretary Richard Lochhead said the investigation and subsequent enforcement notice were "extremely concerning".

He said: "It has been identified that procedural non-compliances and behavioural practices were factors in the fire incident last month. This is simply unacceptable and must end immediately.

"Scottish Ministers expect DSRL to work closely with regulators to ensure that improvements are put in place to prevent incidents like this happening in the future.

"We also expect the ONR to ensure continuing tight regulation going forward so that DSRL are left in no doubt about the need to ensure they have complete compliance and a behavioural culture which is 100% safety focused, 100% of the time.

"Scottish Government officials will be meeting with Dounreay senior management shortly to discuss the issue and find out what measures they are putting in place to address regulators' concerns."