An investigation has been launched after a worker spilled depleted uranium trioxide powder at Sellafield.
The firm has released details of the incident which took place on June 14.
A statement was released by Sellafield Ltd, Nuclear Decommissioning Authority and Office for Nuclear Regulation.
It said: "While carrying out analysis work in a Sellafield site laboratory, a worker spilled approximately 1kg of depleted uranium trioxide powder while moving a container.
"The material is not harmful unless ingested or inhaled."
It said the correct safety procedures were followed and the immediate area was evacuated before returning to normal service on the same day.
"Health checks confirmed the worker suffered no ill effects," it added.
An internal investigation is being carried out.
The event has been rated one (anomaly) on the International Nuclear Events Scale. The scale rates incidents from one which is the least serious to seven which is classed as a major accident.
Sellafield is regulated by the Office for Nuclear Regulation and the Environment Agency.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here