Bomb disposal experts were called to the Sellafield nuclear reprocessing plant after a routine audit of chemicals stored on the site.
Sellafield Ltd would not confirm the identity of the “range of substances” but said they were non-nuclear and had been on the site since 1992.
However, there were concerns they could become hazardous if exposed to oxygen.
A spokesman said no explosion had taken place and there was no reason for people living locally to be concerned.
Mark Neate, the site’s security and resilience director, said experts will now be looking to extract those materials and dispose of them.
He said: “We have all the appropriate experts, it’s a routine process, I see no reason for any concern.
“As a matter of precaution, what we did was put a cordon around the material and moved people out of that cordon.
“There was no evacuation of the site.”
Environment Agency said it was aware of the situation and was working with partners to monitor it.
Sellafield reprocesses and stores nearly all of Britain’s nuclear waste.
Last year, BBC’s Panorama exposed safety concerns at the plant after a tip-off from a whistleblower, including allegations of inadequate staffing levels and poor maintenance.