On October 24, 2010,
control rod 10-35 failed to insert upon actuation of an
automatic reactor scram caused by
loss of condenser vacuum (CARD10-29509). An emergent issue team was formed to investigate this event. The apparent cause was determined to be a hydraulic lock caused by blockage in the flow path between the
control rod drive mechanism and the
scram discharge volume. The investigation never found any foreign material, but postulated that the likely foreign material was discharged into the
scram discharge volume, ultimately ending up in the torus room
sump. As a corrective action, for cycle 15 the licensee increased the frequency of performing TS surveillance
SR 3.1.4.2 scram time testing to every 100 days, adjusted the representative sample size to assure all rods would be tested during cycle 15, and included
control rod 10-35 in each quarterly
scram time testing sample. On November 18, 2011, the
control rod failed to fully insert during
scram time testing. The rod was fully inserted and remained there for the rest of the cycle. The inspectors are waiting for the licensees evaluation of this event, specifically their conclusions regarding the foreign material found, and their evaluation of how the foreign material could have been present, causing the first event, but migrated to allow successful
scram time testing on November 11, 2010, and the first three quarters of 2011 before finally causing the failure identified on November 18, 2011. Because the licensee had not completed their evaluation, this issue is being treated as an unresolved (
URI) item.
