A chemical incident at the Sellfield nuclear decommissioning site in Cumbria sparked an evacuation on Friday night (21 October).
Bomb disposal experts were called to the scene during a routine audit in one of the facility's laboratories involving non-nuclear chemicals that have been stored there since 1992.
It is understood the materials involved can become unstable if exposed to oxygen and an evacuation of the area was undertaken. A decision is being made on how to dispose of the chemicals.
Sellafield is a nuclear fuel reprocessing and nuclear decommissioning site that, according to its recent annual report, is "the only organisation in the UK with the expertise, experience and facilities to safely manage all three forms of nuclear waste: high level, intermediate level and low level waste."
The Windscale nuclear reactor, scene of Britain's worst ever nuclear accident in 1957 when a fire ripped through the site, is being decommissioned there.
Sellafield reprocesses and stores nearly all of Britain's nuclear waste.
In a statement, Sellafield Ltd said: "Following a chemical inventory audit in a laboratory, we took the decision to dispose of a number of chemicals which are no longer used in our operations and have been stored since 1992.
"In line with best practice and established procedures, we alerted the relevant partner agencies and sought advice on managing this material in accordance with the Control of Substances Hazardous to Health (COSHH) regulations.
"As is usual in these scenarios, a specialised unit was invited to attend the Sellafield site to assess the material and advise on its safe disposal. A operational decision will be taken in due course on how best to dispose of the material."
Environment Agency said it was aware of the situation and was working with partners to monitor it.