Plutonium was left lying in a puddle on the floor for nine months

'Complacency' led to the spillage of 83,000 litres of highly radioactive nuclear liquor at the Thorp reprocessing plant in Cumbria. Francis Elliott on a scandal that could destroy government plans for a second nuclear age

On Friday 19 April, the nation's attention was fixed on the contents of Leo Blair's lunchbox. Journalists, desperate to enliven a dull election campaign, were debating the frequency with which the Prime Minister's son was served chips at his Westminster primary school.

On Friday 19 April, the nation's attention was fixed on the contents of Leo Blair's lunchbox. Journalists, desperate to enliven a dull election campaign, were debating the frequency with which the Prime Minister's son was served chips at his Westminster primary school.

Hundreds of miles away something was happening on the windswept Cumbrian coast that, had it been known at the time, would have blown the campaign wide open.

Managers at the troubled thermal oxide reprocessing plant - Thorp - in Sellafield became aware that they could not account for all the spent fuel, believed to have come from German nuclear power stations, it was supposed to be reprocessing.

Earlier that day they had decided to send a remote-controlled camera into the section of the plant, far too dangerous for human exposure, where the spent fuel is weighed in giant suspended tanks. The images it relayed horrified them. There on the stainless steel floor of the concrete cell housing the tanks lay a huge pool of highly radioactive nuclear liquor.

Altogether 83,000 litres of spent fuel dissolved in concentrated nitric acid shimmered beneath the camera lights. It contained enough plutonium to make 20 nuclear weapons.

The nuclear liquor had been leaking from a badly designed pipe since at least January and possibly from as long ago as last August. The plant is now closed.

How could such a major leak have occurred and why wasn't it detected for up to nine months? These are the subjects of an on-going official inspection that could yet lead to criminal prosecutions.

At the time Barry Sneldon, managing director of the British Nuclear Group, moved quickly to downplay the incident.

"Let me reassure people that the plant is in a safe and stable state," he said in a press release initially reported only in the regional press.

Although the nuclear reprocessing plant's closure was eventually reported in the national press more than two weeks later it failed to achieve widespread coverage as Tony Blair's re-election continued to dominate the news.

In Downing Street and the rest of Whitehall there was near panic, however, as the scale of the incident began to emerge.

An IoS investigation has found that the Nuclear Installations Inspectorate (NII) almost immediately informed Patricia Hewitt, the then Trade Secretary, and Margaret Beckett, the Environment Secretary, that they believed the leak was a significant malfunction.

The regulator also promptly informed the International Atomic Energy Authority which earlier this month classified it as Level 3 - a "serious incident" on the International Nuclear Event Scale. The scale ranges from 0 to 7, with 7 reserved for catastrophes on the scale of Chernobyl and Three Mile Island.

The last Level 3 incident in Britain (also at Sellafield) was in September 1992. There has only been one other incident as serious in the world in the past year. The most recent Level 4 incident led to the deaths from radiation sickness of three workers in a Japanese nuclear plant in 1999. They had been mixing nuclear fuel in a bucket.

Nerves were hardly calmed when the preliminary findings of a Board of Inquiry convened by the British Nuclear Group, formerly BNFL, which runs the plant, began to circulate among a small group of senior ministers and officials.

The company released a copy of that report late on Friday afternoon. It makes devastating reading.

The immediate cause of the leak is blamed on "metal fatigue" arising from a design fault in one of the pipes leading to a suspended tank, known as an accountancy tank. Engineers appear to have overlooked the fact that the tank would rise and fall placing "greater stresses to be exerted on associated pipework than had been anticipated".

Worrying though such a fault is, it is the report's next findings that are the most shocking. "There is some evidence that the pipe may have started to fail in August 2004," it admits, adding that by January of this year "significant amounts of liquor started to be released".

"In the period between January 2005 and 19 April 2005 opportunities... were missed which would have shown that material was escaping. Had these opportunities been taken the quantity of liquid released could have been significantly reduced."

The report stresses that the liquor pooled in a "secondary containment area", which prevented any release to the environment. No personnel were harmed.

In an accompanying statement the company blamed "complacency" for the fact that warning signs were missed.

"I shall be taking action to ensure that any complacency is addressed," said Barry Snelson, who has been at Sellafield since 1 August 2004. He added that the plant was "safe and stable".

Privately, the company knows that Thorp's days may be numbered. Since 1 April, the plant has been owned by a new government quango, the Nuclear Decommissioning Agency (NDA).

The NDA has the unenviable and extremely expensive job of cleaning up after Britain's ageing nuclear installations. Income from Thorp - it was projected to earn around £560m over the next 12 months - is supposed to partially offset the cost, £2.2bn this year, of the clean-up.

Late on Friday night the NDA released a carefully worded statement on its website which gave rise to speculation that Thorp's future was in jeopardy.

The issue of whether to build a new generation of nuclear power stations is one of the most sensitive of Tony Blair's third term.

It had been expected that ministers would aggressively begin to make the case for the carbon-free energy source immediately following the election. That they knew the full scale of the Thorp leak explains why no such exercise was launched.

David Willetts, the shadow Trade Secretary, said he would be calling for ministers to answer an urgent question on the incident when the Commons meets next week.

"This seems like a basic failure of procedure worthy of Homer Simpson. We do need to rationally consider the nuclear case but every incident like this undermines public confidence."

Norman Lamb, the Liberal Democrat trade spokesman, said: "This is staggering, and a timely reminder of why we moved away from nuclear in the first place. The truth is that human error can never be eliminated from this industry."

Mr Lamb urged ministers to give serious consideration to shutting the plant for good.

A DTI spokeswoman said: "It is essential that BNG acts urgently to implement the recommendations of the investigation to improve operating practice and retrieve the escaped liquid.

"Most of Thorp remains closed and the NDA and the regulators are still looking at how best to proceed. We are going to wait for advice before taking a decision on the way forward."

Company sources say there would be huge financial implications in closing the plant. It has £5bn worth of outstanding contracts and hefty penalty payments for non-delivery would have to be met from the public purse. The Government would also have to foot the bill for returning unprocessed spent fuel to customers in countries like Germany, Canada and Japan.

Work began on Monday to pump the highly radioactive liquor back into the system. But the damage to the future of British nuclear energy will take far longer to repair.

* About 3,000 litres of radioactive water leaked on Friday at a Czech nuclear power plant near the border with Austria, it emerged yesterday. An official told AP the water was contained in a tank and did not contaminate the environment.

PAST DISASTERS

Windscale (later renamed Sellafield), UK

1957: Human error during maintenance resulted in a reactor fire, releasing a cloud of radio-isotopes. Thirty years later National Radiological Protection Board estimated 33 premature deaths.

Three Mile Island, USA

1979: Malfunction at Three Mile Island nuclear reactor in Pennsylvania caused near meltdown. Concentrated plume of radioactive smoke exposed local area to radiation.

Chernobyl, Belarus

1986: An early morning explosion at the Chernobyl nuclear plant caused an atomic fire. Ukrainian health authorities estimate the final death toll will be 125,000.

Tokinawa, Japan

1999: Three workers in a nuclear fuel plant caused "criticality accident" by bypassing safety procedure. A chain reaction lasting 17 hours produced large amounts of radiation.

Paks, Hungary

2003: A "design fault" at the Paks nuclear power plant flooded one reactor room with radioactive water 10cm deep, and fuel rods were broken.

Tom Anderson