Leaked report details incidents at three plants during February that were serious enough to be reported to ministers

There have been two spillages of radioactive waste and a breakdown in an emergency cooling system at Britain's nuclear plants in the last three months, according to a report to ministers leaked to the Guardian.

A brown puddle containing plutonium five times the legal threshold at which the site must notify the Health and Safety Executive leaked from an old ventilation duct at the Sellafield nuclear complex in Cumbria. This exposed "a number of shortfalls in the design", says the report.

Groundwater at the Torness nuclear power station near Edinburgh was contaminated with radioactive tritium (an isotope of hydrogen) leaking from two pipelines. At Hartlepool nuclear station on the north-east coast of England, the backup cooling system was put out of action by a faulty valve.

All three incidents occurred in February and are still under investigation by the Office for Nuclear Regulation (ONR), the government's new nuclear safety watchdog. They were sufficiently serious to be reported to ministers under safety guidelines agreed after the Chernobyl accident in Ukraine 25 years ago.

Disclosure of the incidents could further delay the government's plans for a programme of nuclear power station building, already being held up by a safety review prompted by the Fukushima accident in Japan. Critics will press for the incidents to be included in the review, being led by the executive head of the ONR, Mike Weightman.

The Guardian has been passed a copy of a report on the incidents sent to ministers on 18 April by Weightman. It was circulated to energy secretary Chris Huhne; business secretary Vince Cable; environment secretary Caroline Spelman; employment minister Chris Grayling; Scottish secretary Michael Moore; and Scotland's first minister, Alex Salmond.

In a covering letter, Weightman says that a fourth incident involving contaminated ground at Sellafield is still under investigation. The criteria for reporting nuclear incidents, which are compiled every three months, include leakages, breaches of safety limits and events where "safe operation may be significantly affected".

According to the ONR, the response to the incidents from the companies that run the three plants was "appropriate". The plutonium spillage at Sellafield has been cleaned up, use of the leaking pipelines at Torness was suspended and the faulty valve at Hartlepool was fixed.

But the ONR is continuing to question operators and monitor plants, to try to ensure similar mishaps do not recur. A spokesman for the ONR told the Guardian that the report to ministers on the incidents is due to be posted online.

Sellafield, which is run by a group of US, French and British companies, said only a very small amount of radioactive water had leaked and that it had been discovered by its own inspectors in a redundant 60-year-old facility on the site. It said that a small amount — "half an eggcup" — of radioactive water was discovered but radiation limits for workers were not breached.

"Safety is our priority at Sellafield and as soon as the water was discovered, access to the area was restricted in order to protect our workforce," said a site spokesman. "There was no release to the environment and no threat to anyone outside the building."

He said: "This is not a legal safety limit in the sense that most people would understand it (ie, a 'target' figure that if exceeded means we've broken the law, like a speed limit for example). The statutory reference made by the ONR refers to our statutory responsibility to notify the HSE if a certain level is exceeded."

The French company, EDF Energy, which operates Britain's nuclear power stations, said that there had never been any danger to staff or the public and it always reported "low-scale events" to the regulator. The levels of radioactivity that leaked at Torness were "extremely low", according to a company spokeswoman. "One would have to drink one tonne of the fluid found to receive a dose equal to thirty minutes flying time in an aeroplane," she said.

The control room at Hartlepool had not shown the true position of a valve for some cooling pumps, she said. "It is believed the valve misalignment occurred following earlier maintenance. This plant configuration is outside of our station operating procedures, however it was corrected as soon as it was identified."

George Regan, chair of the Nuclear Free Local Authorities group, said that it was unusual for three incidents to be reported to ministers in one three-month period. There was only one incident in each quarter of 2010, three at Sellafield and one at Dungeness nuclear power station in Kent.

"With Fukushima very much in the public's mind, we are particularly concerned to hear of a coolant incident at Hartlepool," Regan said. "We will be urgently seeking clarification from the ONR on why these incidents occurred and whether they are of any relevance to the current safety review."

• This article was amended on 20-21 April. On the day of publication, it was updated to include excerpts of responses from Sellafield and EDF. On 21 April a further clarification was inserted in the story text from Sellafield, explaining the legal aspect referred to in the article, whose opening paragraph originally said leaked plutonium at the site was "five times the legal limit". Corresponding with that point, the photo caption was also amended: it originally referred to "a puddle containing plutonium five times the legal safety limit".