At 1530 EDT on September 8, 2003, maintenance personnel opened the 'A' train Control Room charcoal filter plenum (XAA-0029A-AH) to replace the charcoal. At 0730 EDT on September 9, 2003, a question was raised about the work being performed on the 'A' train Control Room ventilation Charcoal Filter Plenum. The Operating shift asked Engineering if the Control Room was capable of maintaining the Technical Specification required pressure under the current conditions, should the ventilation system go into the emergency mode of operation.
Prior to getting the answer, the conservative decision was made to cease work on the charcoal filter plenum and restore the integrity of the 'A' train ventilation system. This was accomplished at 0805 EDT on September 9, 2003. At 0915 EDT on September 9, 2003, Engineering determined that with the isolation dampers for the 'A' train charcoal filter plenum open, due to the associated fan being tagged de-energized, the 'A' and 'B' trains of the control room ventilation system were effectively crossed. Breaching the charcoal filter plenum would have prevented the control room ventilation system from maintaining a greater than 1/8th inch positive pressure during the emergency mode of operation.
Replacement of the charcoal has been placed on hold until a methodology can be formulated to allow the ventilation system to maintain the design pressure.
The licensee informed the NRC resident inspector.
