05000325/LER-2014-003
| Brunswick Steam Electric Plant (Bsep), Unit 1 | |
| Event date: | 03-13-2014 |
|---|---|
| Report date: | 05-09-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| ENS 49910 | 10 CFR 50.72(b)(3)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| 3252014003R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
Background
Initial Conditions At the time of these events, Unit 1 was in Mode 5 at zero percent of rated thermal power (RTP). There were no other structures, systems or components out of service that contributed to this event. Operations with the Potential to Drain the Reactor Vessel (OPDRVs) were in progress, and secondary containment was credited as operable.
Reportability Criteria Two conditions are being reported in accordance with 10 CFR 50.73(a)(2)(v)(C), as conditions that could have prevented fulfillment of a safety function needed to control the release of radioactive material.
During the first, secondary containment [NG] was considered inoperable per Technical Specification (TS) requirements for approximately one minute on March 13, 2014, when both the inner and outer secondary containment airlock doors (i.e., doors 1-RB1-DR-EL050-303 and 1-RB1-DR-EL050-304), on the 50 foot elevation of the reactor building, had been simultaneously opened. The NRC was initially notified of this event on March 13, 2014 (i.e., Event Number 49910), in accordance with 10 CFR 50.72(b)(3)(v)(C).
During the second, secondary containment was considered inoperable per TS requirements for approximately 10 seconds on March 17, 2014, when both the inner and outer secondary containment airlock doors (i.e., 1-RB1-DR-EL020-203 and 1-RB1-DR-EL020-204), on the 20 foot elevation of the reactor building, had been simultaneously opened. The NRC was initially notified of this event on March 17, 2014 (i.e., Event Number 49921), in accordance with 10 CFR 50.72(b)(3)(v)(C).
With both doors open, Surveillance Requirement 3.6.4.1.2 of Technical Specification (TS) 3.6.4.1, Secondary Containment, was not met, rendering secondary containment inoperable. In accordance with the guidance of NUREG-1022, Event Reporting Guidelines 10 CFR 50.72 and 50.73, Revision 3, TS inoperability of a structure, system, or component with no redundant equipment in the same system operable constitutes an event that could have prevented the fulfillment of a safety function.
These conditions are also being reported in accordance with 10 CFR 50.73(a)(2)(i)(B) as operation which was prohibited by the plant's TSs. With secondary containment inoperable, Required Action C.2 of TS 3.6.4.1 requires the initiation of action to suspend OPDRVs. As a result of the short duration of the secondary containment inoperability, this Required Action could not be completed.
Event Description
At 0937 Eastern Daylight Time (EDT) on March 13, 2014, Operations was informed that both the inner and outer secondary containment airlock doors, on the 50 foot elevation of the reactor building, had been simultaneously opened for approximately one minute. This event occurred while an employee was exiting secondary containment at the same time an employee was entering secondary containment. Upon recognition of the condition, the employees took action to secure both doors.
Event Description (continued) Subsequently, on March 17, 2014, at 0750 EDT, Operations was informed that both the inner and outer secondary containment airlock doors, on the 20 foot elevation of the Unit 1 reactor building, had been simultaneously opened for approximately 10 seconds. This event occurred while an employee was exiting secondary containment immediately after another employee had previously entered. Upon recognition of the condition, the employees took action to secure both doors. At the time this condition occurred, OPDRVs were in progress and secondary containment was credited as operable.
Event Cause The root cause of these events is that the design of the secondary containment airlock door interlocks is not robust enough to prevent inoperability of secondary containment.
Statements from personnel involved in the events and maintenance technicians who investigated afterwards clearly demonstrate that the interlock was physically actuating both during and after the events.
The interlock systems have known physical vulnerabilities and require maintenance of moving parts to guarantee proper function. It is possible for the door position switch or plunger to stick in intermittent cases, particularly if the door is experiencing heavy use during an outage. The interlock system, when actuating properly, ultimately relies on the door to be in the fully closed and latched position in order for the plunger to be effective. The door may go out of alignment periodically due to heavy use, may not completely close under a high differential pressure, or may not be challenged by personnel, any of which potentially renders the plunger ineffective.
Safety Assessment The safety significance of these events was minimal. The principal functions of secondary containment are to minimize ground level release of airborne radioactive materials and to provide the means for a filtered and controlled elevated release of the reactor building atmosphere if an accident should occur. The safety objective of secondary containment is to limit the release of radioactivity to the environs after an accident so that the resulting exposures are kept to a practical minimum and are within 10 CFR 50.67 and 10 CFR 100 values. The durations of a potential for release to have occurred during these two events via the open airlock doors was approximately one minute, and approximately 10 seconds, respectively.
Throughout these events, secondary containment negative pressure was maintained and no release occurred.
Corrective Actions
Any changes to the corrective actions noted below will be made in accordance with the site's corrective action program.
The following corrective actions have been completed.
- In each event, the secondary containment airlock doors were closed to re-establish secondary containment.
- Video monitoring equipment was installed for Units 1 and 2 at the 20 foot personnel airlock doors Corrective Actions (continued) were identified as emergency use only. These actions will remain in place until the newly designed interlocks for the airlock doors have been installed.
The following corrective action to prevent recurrence (CAPR) is planned.
- Design and implement a new interlock for the Unit 1 and 2 secondary containment personnel airlock doors.
Previous Similar Events
A review of LERs for the past three years identified the following previous similar occurrences.
- LER 2-2013-005, dated December 19, 2013, reported a loss of secondary containment integrity due to gaps in the Unit 2 secondary containment airlock door seals. Corrective actions included translating design basis requirements for secondary containment into acceptance criteria for the Unit 1 and Unit 2 secondary containment pressure boundary doors, as well as completing procedure revisions to incorporate the acceptance criteria. Since the causes of the events were different, the actions from LER 2-2013-005 could not have reasonably been expected to prevent the condition reported in LER 1-2014-003.
- LER 2-2014-001, dated May 2, 2014, reported that both the inner and outer secondary containment airlock doors, on the 50 foot elevation of the reactor building, had been simultaneously opened for approximately one to two minutes on March 5, 2014. The root cause and corrective actions for the event reported in LER 2-2014-001 are the same as for this LER (i.e., LER 1-2014-003). However, due to the short duration between these events, corrective actions to prevent recurrence had not been implemented. As such, they could not have prevented the condition reported in LER 1-2014-003.
Commitments No regulatory commitments are contained in this report.