At no time was there any increase in radiation within the area and no personnel were affected

BNFL spokesman

They have also suspended refuelling at their Calder Hall plant in Cumbria, which uses the same refuelling system as Chapelcross, while moving to reassure the public that the incident was a "low level" nuclear incident.

Members of the plant's incident team were called to deal with the situation as carbon dioxide was sprayed over the basket to ensure it did not catch fire.

The government's watchdog the Nuclear Installations Inspectorate (NII) has launched an inquiry alongside that being carried out by BNFL.

A BNFL spokesman said: "In the early hours of Thursday July 5, during a routine refuelling operation on Reactor Three at Chapelcross, some used fuel elements became dislodged inside the refuelling machine.

"At no time was there any increase in radiation within the area and no personnel were affected. There is also no indication that the fuel has been damaged.

"Refuelling has been suspended while the elements are returned to their correct position and the root cause of the event is identified and remedied."

A spokesman for the Health and Safety Executive said it was aware of the incident and was investigating the matter.