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Crumbling containers of toxic waste abandoned 40 years ago at “rundown” Sellafield are putting Britain at risk of a nuclear disaster, it is claimed.

The fluid – said to be highly radioactive – is being kept in rotting tanks which are open to the elements, according to a worker who leaked images of the 70-year-old plant.

If the containers drain, the spent fuel could allegedly ignite, spreading radiation over a wide area.

It could even call for mass evacuations and a Chernobyl-like permanent exclusion zone in contaminated areas.

Expert John Large told The Ecologist website: “Looking at the photos I’m very disturbed at the rundown condition.

“In my opinion there is a significant risk the system could fail. It could give rise to a very big radioactive release.

"It could certainly cause serious contamination over a wide area for a very long time.”

Around the tanks in Cumbria a mess of old pipes and equipment can also be seen as weeds grow from cracks in the concrete.

Seagulls regularly land in the deadly contaminated water and carry it on their bodies when they fly off.

In one picture a gull has made its nest a metre above the radioactive soup.

The tanks were used until the mid-70s as short-term storage for spent fuel.

But as the photos show the entire area was abandoned in the 1970s and have had little or no attention in 40 years.

Decommissioning work has now started and “de-flocculation” of the water has made it possible to at last to see what is in the tanks. Until recently no one was sure what was stored below the water.

John Large said: “For the first time in decades we can see into the pond and see the contents, and it’s breathtaking.

“It’s all thanks to the whistleblower that I’m looking at them. If the Euratom inspectors could see what we can see now, my there would have been a row.”

The Office of Nuclear Regulation said it was not planning to prosecute anyone for abandoning huge amounts of nuclear waste but was instead “focusing on accelerating the reduction of risk on-site”.

In a statement the ONR said: “The legacy facilities at Sellafield were built in the 1950s and 1960s and therefore don’t meet modern engineering standards.

“ONR is not considering enforcement action in relation to the complex historical chain of events leading to the current situation at Sellafield but instead is focusing, together with other key stakeholders, on accelerating the reduction of hazard and risk on site, and how we can do that quickly and safely.”

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.”