Previously unseen pictures of two storage ponds containing hundreds of highly radioactive fuel rods at the Sellafield nuclear plant show cracked concrete, seagulls bathing in the water and weeds growing around derelict machinery. But a spokesman for owners Sellafield Ltd said the 60-year-old ponds will not be cleaned up for decades, despite concern that they are in a dangerous state and could cause a large release of radioactive material if they are allowed to deteriorate further.

“The concrete is in dreadful condition, degraded and fractured, and if the ponds drain, the Magnox fuel will ignite and that would lead to a massive release of radioactive material,” nuclear safety expert John Large told the Ecologist magazine. “I am very disturbed at the run-down condition of the structures and support services. In my opinion there is a significant risk that the system could fail.

The storage ponds at Sellafield, as shown in this photograph sent to The Ecologist, have been called ‘disgracefully degraded’ by the executive director of the Institute for Resource and Security Studies. Photograph: The Ecologist

“It’s like an concrete dock full of water. If you got a breach of the wall by accident or by terrorist attack, the Magnox fuel would burn. I would say there’s many hundreds of tonnes in there. It could give rise to a very big radioactive release. It’s not for me to make comparisons with Chernobyl or Fukushima, but it could certainly cause serious contamination over a wide area and for a very long time.”

Gordon Thompson, executive director of the Institute for Resource and Security Studies in Cambridge, Massachusetts, who is an expert at assessing radiological risk, said: “[Sellafield] contains large inventories of radioactive material that could be released to the environment in a variety of ways. The site’s overall radiological risk has never been properly assessed by the responsible authorities. [The] photos, showing disgracefully degraded open-air ponds at Sellafield, indicate that a thorough assessment of risk is overdue.”

The images, taken over a period seven years and leaked via a local nuclear watchdog group to the Ecologist, are said to show two ponds that were commissioned in 1952 and used until the mid-1970s as short-term storage for spent fuel until it could be reprocessed, producing plutonium for military use. One is open to the elements.

This photograph sent to The Ecologist shows a seagull bathing in the water of a storage pond used to hold radioactive material. Photograph: The Ecologist

The government is paying private companies £1.7bn a year to decommission ageing buildings at Sellafield.

“It is urgent that we clean up these ponds [but] it will be decades before they are cleaned up. Before that, a lot of other work has to be done. We are aiming to get them to the point where they are in a condition where the waste can be taken out of them,” said a Sellafield spokesman.

In a statement, Sellafield Ltd said: “These dated pictures do not present an accurate reflection of work across the Sellafield site today, but they are an indication of the scale of the challenge inherited by the NDA, Sellafield Ltd and Nuclear Management Partners to clean up the UK’s nuclear legacy.”

According to Large, who gave evidence to the House of Commons environment committee investigation into nuclear safety in 1986, the ponds were abandoned after they were overwhelmed with spent fuel during the 1974 miners’ strike when Britain was put on a three-day working week by prime minister Edward Heath.

This photograph sent to The Ecologist shows weeds growing around derelict machinery within the Sellafield facility. Photograph: The Ecologist

“In order the ‘keep the lights on’, the UK’s fleet of nuclear power stations were run at full tilt, producing high volumes of spent fuel that the Sellafield reprocessing facilities were unable to keep up with. During the three-day week they powered up the Magnox reactors to maximum, and so much fuel was coming into Sellafield that it overwhelmed the line, and stayed in the pool too long,” Large told the Ecologist.

“The magnesium fuel rod coverings corroded due to the acidity in the ponds, and began to degrade and expose the nuclear fuel itself to the water, so they just lost control of the reprocessing line at a time when the ponds were crammed with intensely radioactive nuclear fuel,” he said

The Office of Nuclear Regulation (ONR), the statutory nuclear safety regulator, said: “The legacy ponds at Sellafield are old and as a result, do not meet the high engineering standards that would be required for modern nuclear facilities. These legacy ponds bring significant challenges, but we must focus our attention on improving the current situation. This does not mean that operations and activities on those facilities are unsafe.”