US nuclear plant was close to disaster Â

The safety of ageing nuclear reactors dotted across the United States has been thrown into doubt by the discovery of severe corrosion at a plant in Ohio that could have triggered a massive failure.

The alarm was sounded after engineers discovered acid had eaten a hole almost all the way through the six-inch thick lid of the 25-year-old Davis-Besse reactor outside Toledo. All that was left to hold back cooling water contained at 2,200lbs of pressure was a skin of stainless steel.

All 68 plants in the US that are of similar design have been ordered by the Nuclear Regulatory Commission (NRC), to inspect their lids as soon as possible. Already, electricity and natural gas prices have started to rise as the industry ponders the possibility that many plants may have to shut down.

The commission is especially concerned about six reactors that share the same design as the Ohio plant. Among them is the Three Mile Island reactor in Pennsylvania, which suffered a near-catastrophic failure in 1979. Three Mile Island is owned by Amergen, which is 50 per cent owned by British Energy plc.

The operators of those plants and six others considered particularly high risk have been given until 1 April to convince the commission that regular inspections would have detected corrosion of the kind discovered in Ohio. If they fail to do so, they will be ordered to shut down. The nuclear industry accounts for 20 per cent of electricity generation in the US.

The hole in the lid in Ohio was caused by boric acid, used in the coolant bath surrounding the uranium rods inside the reactor core. The acid had been leaking from base of some of the control rods that enter the reactor vessel head. It had then eaten through the carbon steel lid. Engineers found the corrosion last month during a periodical inspection.

Had the remaining stainless steel skin, which had already been bent by the enormous pressure, given way, very hot and mildly radioactive water would have rushed into the concrete containment building around the reactor. In a worst case scenario, the reactor might have been starved of cooling water and suffered a dangerous failure and possibly even a full-blown meltdown.

The nuclear sector was already in the spotlight this week after a leading US congressman, Edward Markey of Massachusetts, issued a report suggesting the industry had such feeble security in place that terrorists could easily be working under cover at plants across the country.

"Terrorists may now be employed at nuclear reactors in the United States just as terrorists enrolled in flight schools in the US," he suggested in his report. He said the NRC had to work harder to check all employees for possible terror ties, including links to al-Qa'ida.

Meanwhile, the industry is trying to gauge the possible consequences of what has happened in Ohio. Replacing lids on nuclear reactors is tricky and expensive. FirstEnergy, which owns the Davis-Besse plant, has said the cost of fitting a new reactor head is $20m (£14m).