A nuclear flask containing plutonium-contaminated material fell off the side of a wagon at Sellafield.
The cargo was being transported to Sellafield in Cumbria
The container from the waste depository at nearby Drigg was taken to the Cumbria site by rail and fell as it was transferred to a trailer on Thursday.
The Sellafield Site Emergency Control Centre was set up as a precaution.
British Nuclear Group (BNG) said there was no release of radioactivity during the incident and no-one was hurt. The container was later recovered.
BNG said in a statement that an investigation had been launched into how it happened.