05000373/LER-2015-001
| Lasalle County Station Units 1 And 2 | |
| Event date: | 12-12-2014 |
|---|---|
| Report date: | 02-10-2015 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| 3732015001R00 - NRC Website | |
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LaSalle County Station Units 1 and 2 are General Electric Company Boiling Water Reactors with 3546 Megawatts Rated Core Thermal Power.
A. CONDITION PRIOR TO EVENT:
Unit(s): 1 / 2 Reactor Mode(s): 1 Event Date: December 12, 2014 Event Time: 1324 CST Mode(s) Name: Power Operation Power Level: 100%
B. DESCRIPTION OF EVENT:
On December 12, 2014, both Units 1 and 2 were in Mode 1 at 100% power with no fuel movements in progress.
At 1324 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.03782e-4 months <br /> CST, it was reported that both air-lock doors of the Unit 2 Reactor Building 710' elevation between the Unit 2 diesel generator corridor and the Unit 2 Reactor Building were open at the same time for approximately 10 seconds.
While both interlock doors were open, Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.2 ("Verify one secondary containment access door in each access opening is closed") was not met. Secondary containment was declared inoperable for the time that both interlock doors were open. TS 3.6.4.1 Required Action (RA) A.1 to restore secondary containment to OPERABLE status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> was entered and exited at 1324 hours0.0153 days <br />0.368 hours <br />0.00219 weeks <br />5.03782e-4 months <br /> CST on December 12, 2014.
This occurrence is reportable under 10 CFR 50.73(a)(2)(v)(C) and (D) as an event or condition that could have prevented the fulfillment of the safety function of the structures or systems that are needed to control the release of radioactive material and to mitigate the consequences of an accident. An ENS report was made to the NRC at 1820 hours0.0211 days <br />0.506 hours <br />0.00301 weeks <br />6.9251e-4 months <br /> EST (EN 50674) on December 12, 2014, pursuant to 10 CFR 50.72(b)(3)(v)(C) and (D).
An Engineering Evaluation has determined this event did not meet the NEI 99-02 definition of a Safety System Functional Failure.
C. CAUSE OF EVENT:
The cause of the event was that the door closure mechanism was degraded for door #507 (Reactor Building side of the interlock), which prevented the electro-mechanical solenoid operated bolt from properly aligning with the door-mounted catch. This malfunction resulted in the capability to open both interlock doors #507 and #402 at the same time, and was similar to a previous occurrence on October 22, 2013, and February 18, 2014.
The degradation of the door closure mechanism and door seal was determined to be due to repeated challenges of the door during personnel ingress/egress. The design of the interlock mechanism was considered a contributing cause, as it was considered less than robust for the application.
Corrective actions from the previous occurrences to identify, procure and install a more robust design were in progress at the time of the event. Interim corrective actions were in place to perform quarterly inspections of the assemblies and to tighten the fasteners as required, but did not prevent this event.
D. SAFETY ANALYSIS:
The safety significance of this event was minimal. The Reactor Building-to-outside differential pressure remained negative throughout the period that the secondary containment was inoperable. The secondary containment was inoperable for approximately 10 seconds, which was significantly less than the four-hour Completion Time to restore the secondary containment to operable status allowed by TS 3.6.4.1 Required Action A.1.
The function of the secondary containment is to contain, dilute, and hold up fission products that may leak from the primary containment following a Design basis Accident (DBA). Engineering Evaluation (EC 396711) was performed to show that this event had no impact on the safety function associated with secondary containment.
The time that both doors were simultaneously opened was approximately 10 seconds. This event did not result in the reactor enclosure differential pressure dropping below the design bases set point of -0.25 inches w.g.
Both the inner and outer doors were promptly closed by station personnel which ended the event. This event did not involve any kind of door or airlock material condition preventing door closure. Additionally, both the inner and outer doors were closed by normal expected means and were capable of remaining closed as designed.
The computed dose for EC 396711 was based on the door opening during the 780 second time period prior to Standby Gas Treatment (SBGT) system drawdown and filtration. This discounts the initial 120 seconds of an event where no release takes place, per calculation L-003068, "Re-Analysis of Loss of Coolant Accident (LOCA) Using Alternative Source Terms".
The approximate 10 second opening of the secondary containment doors is bounded by calculation L-003068, "Re-Analysis of Loss of Coolant Accident (LOCA) Using Alternative Source Terms". Should an event occur, with both secondary containment doors open simultaneously for 30 seconds or less, this would result in a potential dose increase of approximately 3.85%. The 3.85% decrease in margin is inconsequential in comparison to the 10 CFR 100 regulatory limits.
EC 396711 also evaluated the pressure impact on the secondary containment and the ability of the SBGT system to achieve the TS required negative pressure. The results of the evaluation show SBGT would restore secondary containment pressure within 3 minutes which is well below the 15 minute maximum drawdown time required by TS.
Based on the short duration of door opening (approximately 10 seconds), no material condition preventing door closure or maintaining the doors closed and attendance by knowledgeable personnel who closed the doors immediately, the secondary containment safety function was maintained.
E. CORRECTIVE ACTIONS:
- The door closure mechanism for door #507 was upgraded to the new, more robust design.
- Quarterly preventative maintenance to inspect the assemblies and fasteners and tighten or replace as necessary remain in progress.
F. PREVIOUS OCCURRENCES:
On February 18, 2014, Unit 1 was in Mode 5 with fuel moves in progress during refueling outage L1R15, and Reactor Building 710' elevation between the Unit 2 diesel generator corridor and the Unit 2 Reactor Building were open at the same time for approximately 3 seconds. While both interlock doors were open, Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.2 ("Verify one secondary containment access door in each access opening is closed") was not met. Secondary containment was declared inoperable for the time and RA A.1 to restore secondary containment to OPERABLE status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> were entered and exited at 1820 CST on February 18, 2014.
The cause of the event was degradation of the door closure mechanism and door frame seal. A contributing cause was a less than robust design of the door interlock assembly. Corrective actions from the previous occurrences to identify, procure and install a more robust interlock assembly design were still in progress at the time of the event. Additional corrective actions included creating a periodic preventative maintenance task to inspect, tighten, and replace fasteners as necessary.
On October 22, 2013, both Units 1 and 2 were in Mode 1 at 100% power. At 1129 hours0.0131 days <br />0.314 hours <br />0.00187 weeks <br />4.295845e-4 months <br /> CDT, it was reported that both air lock doors on the Unit 1 Reactor Building 710' elevation between the Chemistry Hot Lab and the Reactor Building were open at the same time for approximately 10 seconds. While both interlock doors were open, Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.2 ("Verify one secondary containment access door in each access opening is closed") was not met. Secondary containment was declared INOPERABLE for the time that both interlock doors were open. TS 3.6.4.1 Required Action A.1 for both Units 1 and 2 to restore secondary containment to OPERABLE status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> was entered and exited at 1129 CDT on October 22, 2013.
The cause of the event was a less than robust design of the door interlock assembly. Troubleshooting found that the mounting fasteners that secure the entire locking assembly to the frame of door # 226 (Reactor Building side of the interlock) were loose, which prevented the electro-mechanical solenoid operated bolt from properly aligning with the door-mounted catch. This misalignment prevented the bolt from entering the catch on door # 226 when door # 225 (Chemistry Hot Lab side) was opened. This malfunction resulted in the capability to open both interlock doors at the same time, and was similar to a previous occurrence on February 28, 2013.
On February 28, 2013, Unit 1 was in Mode 1 at 100% power and Unit 2 was in Mode 5 for refueling outage L2R14. At 0400 hours0.00463 days <br />0.111 hours <br />6.613757e-4 weeks <br />1.522e-4 months <br /> CST, it was reported that both air lock doors on the Unit 1 Reactor Building 710' elevation between the Chemistry Hot Lab and the Reactor Building were open at the same time for approximately 10 seconds. While both interlock doors were open, Technical Specification (TS) Surveillance Requirement (SR) 3.6.4.1.2 ("Verify one secondary containment access door in each access opening is closed") was not met for Unit 1. Secondary containment was declared INOPERABLE for the time that both interlock doors were open. TS 3.6.4.1 Required Action A.1 to restore secondary containment to OPERABLE status within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> was entered and exited for Unit 1 at 0400 CST on February 28, 2013.
The cause of the event was determined to be a less than robust design of the door interlock assembly.
Troubleshooting found that the mounting fasteners that secure the entire locking assembly to the frame of door # 226 (Reactor Building side of the interlock) were loose, which prevented the electro-mechanical solenoid operated bolt from properly aligning with the door-mounted catch. This misalignment prevented the bolt from entering the catch on door # 226 when door # 225 (Chemistry Hot Lab side) was opened. This malfunction resulted in the capability to open both interlock doors at the same time.
This occurrence was similar to the October 2013 event. In addition to repairing the interlock assembly by tightening the fasteners, actions were initiated to periodically inspect the assemblies and to identify and install a more robust design. The new design had been identified but not installed when the October 2013 event occurred.
G. COMPONENT FAILURE DATA:
Locknetics/SDC style lock