The Nuclear Instalations Inspectorate has issued a report into the incident at Dungeness B which forced British Energy (BE) into an unscheduled maintenance outage and resulted in a significant financial loss.
According to the NII, on 19 November 1999 reactor 2l was being prepared for the repair of a leak in a superheated tubeplate. Workers noticed significant defects in a weld joining the tubeplate to the superheated header. BE shut down the other Dungeness B reactor as a precautionary measure.
Using ultrasonic inspection the weld defect has been shown to exist all the way round the weld and extends to a depth of 50mm. The discovery has brought into question the quality of a number of welds manufactured using the submerged arc aoutomatic welding method. BE is inspecting all such steam penetration welds attempting to inspect all accessible gas side welds for rogue ferritic material.
Safety cases are being reviewed to ensure that penetration weld failures are covered by ‘consequence of failure’ arguments.
The NII will have to approve safety cases prior to both the Dungeness B reactors returning to operation.
The incident has been classified as Level 2 on the International Nuclear Events Scale.