A fire at the Dounreay nuclear plant in the north of Scotland resulted in an “unauthorised”release of radioactivity, an investigation has found.

The nuclear plant has been censured by safety regulators after admitting human error led to the release of radioactivity into the atmosphere.

The criticism comes just weeks after another nuclear plant, Sellafield in Cumbria, was also tackled by the safety regulator over its management of asbestos while pictures emerged of badly corroded storage ponds there.

Environmentalists said the problems highlighted dangers from building a new generation of nuclear plants which however well designed would be vulnerable to human fallibility.

Staff at Dounreay were blamed in a report prepared for the Office for Nuclear Regulation (ONR) of “procedural non-compliances and behavioural practices” that led to a fire on 7 October.

The ONR has served an “improvement notice” on Dounreay Site Restoration Limited (DSRL) requiring a raft of changes to ensure there is no repeat of the accident in which traces of tritium – a radioactive type of hydrogen – escaped into the environment. Refusal to comply with the notice could eventually lead to prosecution, it said.

DSRL, controlled by a consortium involving the engineering group Babcock, is involved in a £1.6bn contract to dismantle Dounreay, a prototype “fast reactor” built to experiment with nuclear fission in the 1950s and 60s.

“Our investigation identified unacceptable behaviours and practices that fell well short of our values and standards,” said Mark Rouse, managing director at DSRL. “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 was satisfied with the changes. The reactor stopped operating in 1994 and is more than halfway through decommissioning.

John Sauven, executive director of Greenpeace UK, said the problems at Dounreay were disturbing. “Until we can clone enough popes to staff all the world’s nuclear reactor control rooms, there will always be a fallible human component in nuclear power stations. This worrying news demonstrates why it’s important to remember that even a reactor design which looks safe on paper can never completely design out the actions of human beings. Its why renewable energy always scores better on safety, because far less can go wrong.”

The Nuclear Free Local Authorities Secretariat, the local government voice on nuclear issues, described the Dounreay incident as “alarming” and called for more information.

“To hear that critical staff were not fully conversant in fire safety procedures and that a release of tritium occurred; putting into danger staff and the wider public; is quite disgraceful at such a sensitive nuclear site like Dounreay,” said councillor Mark Hackett, the NFLA’s chair.

The nuclear industry has had a difficult week after Areva, one of the companies involved in the management of Sellafield, issued a profit warning and said it might need a cash injection.

On Tuesday EDF, Areva’s partner on the Hinkley Point C new building project in Somerset, warned that a similar scheme at Flamanville in Normandy was even more behind schedule than previously thought.

The pictures from Sellafield also caused widespread concern as the ponds of sensitive waste had cracked concrete, seagulls bathing in the water and weeds growing around derelict machinery. Babcock is not involved at Sellafield but runs Dounreay alongside two US companies, CH2M Hill and URS. Babcock also holds contracts in rail, defence and even education.