A BBC investigation has uncovered a catalogue of safety concerns at the UK's most hazardous nuclear site.
Panorama found parts of Sellafield regularly have too few staff to operate safely and that radioactive materials have been stored in degrading plastic bottles.
The programme was told that parts of the facility are dangerously rundown.
Sellafield says the site in Cumbria is safe and has been improved with significant investment in recent years.
The Panorama investigation was prompted by a whistle-blower - a former senior manager who was worried by conditions.
He explained that his biggest fear was a fire in one of the nuclear waste silos or one of the processing plants and said: "If there is a fire there it could generate a plume of radiological waste that will go across Western Europe."
The whistle-blower told the BBC that areas of Sellafield - which reprocesses and stores nearly all of the nation's nuclear waste - often didn't have enough staff on duty to meet minimum safety levels. Minimum staff levels are set for both teams of workers and whole plants on the site.
It varies across the site, but in a processing plant of 60 people the safe minimum manning level might be only six workers.
Sellafield's own documents say "any deviation from the safe minimum manning levels is not acceptable".
But figures obtained by Panorama show that between July 2012 and July 2013 there were 97 incidents where parts of the site had too few workers on shift.
Meg Hillier MP, who chairs the Public Accounts Committee, was shocked by the figures: "It is incredible. It defies belief actually that anything could be working at below safe staffing levels. There is no excuse."
Sellafield says there are now less breaches of safe minimum manning levels but the latest figures show they are still being breached on average once a week.
Dr Rex Strong, head of nuclear safety at Sellafield, denied that operating below these levels was dangerous. "You make alternative arrangements, so the things that have to be done get done. Facilities are shut down if we're not able to operate them in the way that we want to."
Panorama has also raised concerns about the way radioactive materials have been stored at Sellafield.
The programme discovered that liquid containing plutonium and uranium has been kept in thousands of plastic bottles for years. The bottles were only intended for temporary storage and some of them are degrading.
Sellafield has been working to remove them, but there are still more than 2,000 bottles containing plutonium and uranium on the site.
Dr Strong told Panorama that Sellafield had been working to get the material into proper storage: "The organisation is now focusing on putting right some underinvestments of the past in order to support the hazard and waste reduction mission that the site has."
Sellafield later said in a statement that plutonium and uranium samples are "kept securely" and that "to imply that such material is inappropriately managed is simply not true".
Panorama has also seen leaked reports that suggest Sellafield had problems with emergency management and with maintaining the site's infrastructure. One report from 2013 says "years of neglect" had led to "intolerable conditions".
Dr Strong rejected the BBC's description of the site. He said: "There's been huge and sustained investment in infrastructure at Sellafield over recent years."
"Safety is our priority and we are managing a very complex site which has got a great deal of hazardous radioactive materials on it."
Panorama - Sellafield's Nuclear Safety Failings was broadcast on BBC One on Monday 5 September at 8.30 BST. Catch up on BBC iPlayer