British Energy reported an event at Heysham 1 involving a fuelling machine operation at a fuel storage tube. The machine had collected a new fuel assembly from the tube and had replaced this with a short shield plug. Unknown to the operators, and contrary to indications at the fuelling machine, the shield plug had not disconnected from the machine grab. A mechanical interlock designed to prevent the machine moving until safe to do so had failed. This allowed the machine to move sideways, causing the shield plug to collide with the storage tube. The shield plug broke in two, with the lower part falling into the storage tube.