A section of Sellafield, the defunct nuclear plant, has been evacuated after a routine inspection uncovered hazardous chemicals.
A bomb disposal unit has been called into the site to deal with a "small amount" of organic peroxide, a potentially explosive chemical that must be kept cool to remain safe.
Organic peroxide has a variety of uses across different industries but its storage is subject to strict safety measures.
The plant, which is now used only to store nuclear waste and has not produced energy since 2003, has been made "non-operational", Sellafield said.
A statement by the company that operates the site on behalf of the Government said: "The storage area is safely segregated from the nuclear operations of the plant and the risk has been identified as a conventional safety issue rather than a nuclear safety risk.
"As a precautionary measure, a controlled evacuation of the Magnox Reprocessing Plant was carried out yesterday in order to investigate the chemical and devise the appropriate course of action. The plant was non-operational at the time.
"The plant will remain non-operational while the chemical is disposed of."
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