BNF probes nuclear leak.
It was the second safety scare this month at the plant.
The company said workers at the Chapelcross plant, near Annan, Dumfries and Galloway had not been exposed to the "small quantity" of powder and the incident posed no risk to the public.
They said the incident was rated a Level 1 - the lowest level on the International Nuclear Event Scale.
It follows an incident when 24 fuel rods fell from a machine during a routine refuelling operation on a reactor on July 5.
An operation is underway to retrieve the rods, which were all accounted for by officials yesterday.
A BNFL spokeswoman said: "We have notified the health and safety executive and we are carrying out a full investigation into the event.
"We can confirm that there was no contamination of any members of the workforce and the area affected was very localised."
She added that the area was now being cleaned up.