Following detailed investigation of the incident which occurred on Reactor 1 on 31 July at Wylfa Power Station on Aglesey, it has now been rated at 2 on the International Nuclear Event Scale - an incident with no significant environmental impact, but which involved some internal plant failure.
The incident involved failure of part of the refuelling equipment. The lower portion of a grab used to access a fuel chute had become detached during refuelling, and was initially thought to be located in a guide tube above the reactor. Subsequent inspection has now confirmed that the component is in fact seated on the top of the fuel channel itself, in a very confined space. As a result, the task of recovering it is more complex. The incident, provisionally rated at 0 on the scale based on information available immediately following the event, has been upgraded to 2.
Work is now underway to recover and lift the grab component. The grab assembly is being modified to prevent a re-occurrence of the incident. Reactor 2 remains in operation, but no further refuelling will be carried out on either reactor until the modification work is complete.
Basis for rating:
Blockage of a single channel is considered a possible event. Sufficient channel flow occurred through gaps between graphite bricks to prevent fuel melting. Therefore, the safety function availability is at least adequate. It is also necessary to consider the potential event that the grab could have fallen onto a higher rated channel where the cooling may not have been adequate. Given that the maximum consequences from this event could not have been higher than level 4, the maximum under defence in depth is level. 2. On the basis of the information currently available, this event has been rated at this level.
Event date: Sat, 31-07-1993