 INES 2 |
Introduction of potentially moderating material to fuel route |
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 meters.
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
Event date: Mon, 29-06-2009
