05000336/LER-2009-005
| Millstone Power Station - | |
| Event date: | 12-2-2009 |
|---|---|
| Report date: | 01-29-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3362009005R00 - NRC Website | |
Event Description
On November 13, 2009, with Millstone Pcwer Station Unit 2 exiting a refueling outage and preparing to go from Mode 5 to Mode 4, both the inner and outer containment personnel hatch air lock doors [DR] were closed, verified closed and successfully pressure tested.
On December 2, 2009, while in Mode 1 al 100% power, personnel entered the containment for a planned entry.
Subsequently, while in the process of entering the containment, a second entry team opened the outer air lock door and observed the inner door ajar. This condition should have been prevented by a design interlock which prevents opening the outer air lock door if the inner air lock door is not closed. The interlock failed to perform its design function because it was not properly reset during the refueling outage due to inadequate procedures. The outer air lock door was immediately closed. The time both air lock doors were open was less than one minute.
This condition is being reported pursuant to 10 CFR 50.73(a)(2)(v)(C) as any event or condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to control the release of radioactive material.
Cause
The interlock failed to perform its design function because the interlock had not been properly reset in Mode 5 prior to entering Mode 4 on November 13, 2009. Although the interlock had not been properly reset on November 13, 2009, both doors were fully closed and a successful pressure test of the air lock was completed prior to restarting from a refueling outage. (The containment air lock interlock mechanism is defeated in Modes 5 and 6 during refueling outages.) The cause of the interlock not being properly reset was an inadequate procedure that did not provide sufficient details to complete the interlock restoration. A contributing cause is that the surveillance procedure credited with verifying the proper restoration of the interlock was not adequate and incorrectly concluded that the interlock had been restored.
Assessment of Safety Consequences
This condition is judged to be of very low :3afety significance. The requirements on containment penetration closure and operability ensure that a release of radioactive material within containment to the environment will be minimized.
Plant Technical Specification (TS) 3.6.1.3 "Containment Air Locks" and 3.6.1.1 "Containment Integrity' apply.
TS 3.6.1.3a states that the containment a r lock shall be operable with both doors closed except when the air lock is being used for normal transit entry and exit through containment, then at least one air lock door shall be closed.
Upon discovery and recognition the inner air lock door was ajar; the outer door was immediately closed.
Although IS 3.6.1.3 Action b (Action b states in part, "With only the containment air lock interlock mechanism inoperable, verify an OPERABLE air lock door is closed within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> and lock an OPERABLE air lock door closed within 24 (hours.") was met, this condition s being reported as a condition which could have prevented the fulfillment of the safety function of the containment air locks to control the release of radioactive material.
ORM 366A (9-2007) 4C Corrective Action Upon discovery that the inner air lock door was ajar, the outer door was immediately closed.
The maintenance procedure which restores the containment air lock interlock has been revised to provide clearer steps for restoration of the interlock. The surveillance procedure has been revised to ensure that restoration of the interlock has been achieved.
Additional corrective actions are being addressed in accordance with the Millstone Corrective Action Program.
Previous Occurrences
No previous similar events/conditions were identified.
Energy Industry Identification System (ENS) codes are identified in the text as [XXI.
JR F )RM 366A (9-2007)