At 0325 on 8/28/05 the `B' Drywell to Suppression Chamber vacuum breaker indication began cycling from closed to intermediate open. Attempts were made to close the vacuum breaker from the control room were not successful. Attempts to establish a differential pressure between the drywall and suppression chamber airspace were also not successful at getting the vacuum breaker to close and stay closed.
Technical Specification 3.6.4.1 was entered at 0325 and a plant shutdown was initiated at 1100 to comply with Technical Specifications. With the vacuum breaker open or partially open, the pressure suppression function of the suppression chamber could be compromised in the event of a postulated loss of coolant accident and therefore this safety function is degraded. The inoperability of this vacuum breaker is reportable in accordance with 50.72(b)(3)(v), The cause of the open vacuum breaker is currently not known. The plant will be taken to cold shutdown to facilitate torus entry to effect repairs.
There was no other equipment that was unavailable that contributed to this event. All other safety related equipment is currently operable.
The licensee is starting a normal shutdown.
The licensee notified the NRC Resident Inspector and will be notifying the
LAC Township.
