Bomb disposal experts were called to the Sellafield nuclear reprocessing plant after a routine audit of chemicals stored in a laboratory.
Sellafield Ltd said it was “not a radiological event” but involved a small number of canisters of solvents which had been on the site since 1992.
However, there were concerns they could become hazardous if exposed to oxygen.
An area of the site was cordoned off for most of the day, and the canisters disposed of by controlled explosion.
Sellafield said in a statement: “These chemicals are used extensively in many industries and are well understood.
“Because this is happening on the Sellafield site we exercise extreme caution and leave nothing to chance.”
‘Dig a trench’
It said a team from the army’s Explosives Ordinance Disposal Team disposed of the chemicals by digging a trench, burying them using sandbags and detonating them in a controlled manner.
The disposal took place in two batches, with the first transferred from the laboratory to another location on the site and successfully and safely detonated at around 14:15 BST.
A second controlled explosion was then carried out at the same location shortly before 16:00 BST.
The statement added: “We have now removed the cordon from around the laboratory, and the site is working as it would be on any other Saturday.”
Environment Agency earlier 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. Source.