Dungeness B reports potential INES-2 incident

8 July 2009

The UK's Dungeness B nuclear power station has reported an incident that it has provisionally rated level 2 ('incident') on the International Nuclear and Radiological Event Scale.

The incident happened on 29 June at British Energy's 660MWe Dungeness B1 AGR.

The station said:

"Whilst lowering a fuel plug unit in order to latch it to a new fuel stringer, it became apparent that the coupling had not latched correctly and that foreign material was trapped between the spring collet assembly and the neutron scatter plug. The debris appears to be a rubber sheet, the source of which is likely to be one of the three covers used to cover the maintenance tubes during earlier cell maintenance activities. The failure to correctly latch was identified during a procedural check as the assembly was being raised. Fuel handling activities were suspended, but the assembly remains suspended by approximately 3 metres.

"As part of the recovery process, polyurethane foam was injected below the suspended stringer to minimise the potential drop height in the event of it de-latching. The foam did not come in contact with the stringer. The subsequent analysis of the foam indicated that it was a material that could act as a moderator, and thus challenge the applicable criticality safety certificates.

A mechanical restraint has now been applied to prevent the stringer falling.

The event is provisionally rated INES Level 2, as degradation of defence in depth has the potential to result in a criticality incident. This rating will be reviewed as part of the follow up investigation."

A British Energy spokesman released this statement:

"There was no impact on the safety of our workforce or the public at large and there was no damage to the plant. Both units continue to operate as normal. Safety is our number one priority at all times.

"The event, which was not connected to the reactor in any way, relates to a new fuel assembly. In the process of connecting the new fuel to the supporting fuel plug unit, a piece of rubber was found to have become trapped in the coupling, threatening the integrity of the connection. Operations ceased immediately and as a precaution, polyurethane foam was injected under the fuel assembly. Subsequent review confirmed that this foam is not permitted under the rules.

"The coupling did not fail, there was no plant damage, no staff were injured and there was no release of radioactivity. The load has now been secured by fitting primary and secondary clamps and we are now working on a plan to safely return the fuel handling equipment to full service.

"In co-ordination with the regulator we are carrying out a full investigation into the cause of this event and have undertaken to put necessary measures in place to prevent this happening again.

"The company takes such incidents extremely seriously and will take any appropriate action which may be necessary.

"This is the seventh INES 2 event at BE since the scale was introduced in 1990. BE has had no INES events rated above level 2."




One of the great myths perpetrated about nuclear power
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par with diamonds, gold and other precious metals.





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