A security alert was sparked at the Nuclear Decommissioning Authority (NDA) headquarters in Cumbria with the discovery of a suspect package.
Three mailroom workers at the West Lakes Business Park site in Whitehaven were isolated when unidentified grains were found in a package on Wednesday.
A cordon was set up by the emergency services while police officers took the substance away for laboratory tests.
Cumbria Police declared the area safe after a five-hour operation.
The NDA said it was reviewing security procedures as a result of the alert.
The site's 124 workforce have been told they can now work normally.
At least 20 firefighters, along with police and two ambulance crews were on standby for more than five hours as the situation was assessed.
Cumbria Police declared the situation "low risk" at about 1500 BST.
An NDA spokesman said: "Three staff were isolated as a precaution and were reassured by ambulance staff. They are fit and well, but have been told to seek advice should their health change in the near future.
"The alert sparked when a suspicious substance was discovered in an envelope early on Wednesday.
"We are waiting for a helicopter to bring in a specialist team to take the substance away for testing.
"We are happy our security measures worked well, but we will review these in case any lessons can be learned."
A Cumbria Police spokeswoman said an investigation into the incident was ongoing and the substance would be examined in due course.
The NDA was set up two years ago to oversee the decommissioning of the UK's ageing nuclear facilities.