The revelation about Torness in East Lothian comes as the nuclear industry is proposing to transport significant amounts of potentially dangerous waste across Scotland, and against the background of a steep rise in nuclear transport accidents.
The French operator of Torness, EDF Energy, has been reprimanded by the Scottish Environment Protection Agency (Sepa) for breaking rules concerning the safe handling of radioactive waste. It wrongly sent 28 oil drums containing 26 litres of radioactive residue to be recycled at a waste plant in Cumbria.
When this was discovered in April the waste had to be sent back to Torness, where it is still awaiting disposal. According to Sepa, EDF Energy was not authorised to dispose of such radioactive "sludges" at a low-level waste repository.
The incident was investigated by Sepa, which last month issued EDF Energy with a formal warning that further breaches could result in prosecution. The error, which has remained undisclosed until now, was contained in a report to Sepa's main board on September 19.
"It is deeply alarming that one of the world's largest nuclear operators can make such an elementary mistake with radioactive material," said Dr Richard Dixon, the director of environmental campaign group WWF Scotland.
"This is very embarrassing for EDF at a time when they are desperately trying to get the UK Government to offer them enough subsidies and other sweeteners to build new nuclear reactors in England."
Dixon argued that the incident cast further doubt on the wisdom of EDF's plan to extend the working life of Torness, and its other nuclear power station at Hunterston in North Ayrshire.
The UK Government's Nuclear Decommissioning Authority (NDA) is also consulting on whether 42 drums of intermediate-level radioactive waste from Torness can be transported across the country to a store that has already been built at Hunterston.
But Pete Roche, the policy advisor to the nuclear-free group of local authorities, called on the NDA to drop its "misguided" plan in light of Torness's latest bungle.
"This illustrates the sorts of problems which can occur, and are likely to occur with much greater frequency, if we start moving more dangerous waste from one site to another," he said.
According to Sepa, the radioactive sludges left in the oil drums had gone unnoticed by EDF.
An agency spokesman said: "EDF has taken steps to prevent a recurrence of the incident, including raising awareness of the issue across the EDF fleet. Sepa will review the implementation of these steps during future routine inspections."
EDF Energy accepted that some oil drums had not been sufficiently checked. "We have examined our waste consignment practices and put in place additional checks to avoid similar issues arising in the future," said a company spokesman.
"We returned the oil residue to Torness, where it is currently stored awaiting disposal through the well-established and approved processes used throughout our industry."
The company stressed that the levels of radioactivity involved were very low.
The spokesman added: "EDF Energy takes its responsibilities for safe and clean operation of our facilities very seriously and strives through open reporting practices to continue to engender the trust of the nation."
The number of accidents during the transportation of radioactive materials has risen sharply, prompting fears for public safety.
According to the UK Government's Health Protection Agency (HPA), 38 incidents were reported in 2011, up from 30 in 2010. The last six years have seen a total of 195 mishaps.
Eleven of the incidents last year involved fuel flasks from nuclear power stations, compared with eight the previous year, often due to loose bolts, faulty valves or other defects. Other problems befell consignments of nuclear industry waste, medical isotopes and other radioactive shipments.
In one instance in December, a train carrying nuclear fuel flasks hit a tree on the line. In November, a courier van carrying radioactive packages to a nuclear site was stopped by police who discovered the driver had been disqualified from driving.
In October, the surfaces of three high-level radioactive waste containers shipped abroad were found to be contaminated in breach of permitted limits. In August 2011, 46 waste oil drums triggered radiation alarms when they left a nuclear site.
According to the HPA one incident in May 2011, when a vial containing medical radioactivity broke, gave a member of staff a "potentially significant" radiation dose. A spillage of uranium ore in June last year may also have exposed a worker to radiation.
The locations of the incidents are not disclosed in the report, so it is not known how many of them took place in Scotland.
Peter Burt, of the Nuclear Information Service, argued that transporting radioactive materials was one of the nuclear industry's riskiest activities, and urged safety regulators to ensure companies comply with the rules before authorising large-scale movements.
He added: "It's a concern that in the months after the Fukushima accident, the number of nuclear transport incidents recorded in the UK actually went up instead of down."