05000373/LER-2014-003
| Lasalle County Station Unit 1 | |
| Event date: | 04-30-2014 |
|---|---|
| Report date: | 06-27-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material |
| Initial Reporting | |
| 3732014003R00 - 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 Event Date: April 30, 2014 Event Time: 1037 CDT Reactor Mode(s): 1/5 Mode(s) Name: Power Operation/Refueling Power Level: 100% / 0%
B. DESCRIPTION OF EVENT:
On April 30, 2014, Unit 1 was in Mode 1 at 100% power and Unit 2 was in Mode 5 with fuel moves in progress Auxiliary Building 710' elevation between the Chemistry Hot Lab and the Reactor Building were open at the same time for approximately 5 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 both Units 1 and 2. Secondary containment (CS) [NG] was declared inoperable for the time that both interlock doors were open. TS 3.6.4.1 Required Actions (RA) C.1, C.2 and C.3 to immediately suspend irradiated fuel movements, core alterations, and OPDRVs on Unit 2 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 1037 CDT on April 30, 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 (EN 50076) at 1636 CDT on April 30, 2014, pursuant to 10 CFR 50.72(b)(3)(v)(C) and (D).
An Engineering Evaluation was performed that determined that this event did not meet the NEI 99-02 definition of a Safety System Functional Failure (SSFF).
C. CAUSE OF EVENT:
Investigation determined that the electromagnetic lock on air lock door #247 was not functioning. This lock had been installed on February 3, 2014, as a modification to address recurrent problems in maintaining one secondary containment access door closed in accordance with TS SR 3.6.4.1.2. The lock uses a strong electromagnet that attracts a steel armature plate mounted on the door, which prevents the door from opening when the other airlock door is open. The single mounting bolt holding the armature plate to door #247 had become disengaged and the plate had fallen to the floor, causing the locking function to fail.
Further investigation determined that the armature mounting bolt had not been properly tensioned and Loctite had not been used during installation of the armature plate. This allowed the bolt to gradually work its way loose during normal usage of the door.
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 5 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 less than 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 malfunction or failure. Additionally, both the inner and outer doors were closed by normal expected means and were capable of remaining closed as designed.
Total calculated doses throughout the event at the Exclusion Area Boundary (EAB), Low Population Zone (LPZ), and Control Room (CR) are 10.4%, 1.1% and 85.4%, respectively, of the regulatory limits. For this evaluation, the total dose in this calculation is conservatively assumed to be released during the unfiltered 780 seconds time period prior to SBGT drawdown and filtration; this discounts the initial two minutes of the event, as no release has taken place. The dose was then evaluated during this time period as a proportion, based on door opening time, to evaluate an anticipated release through the opened secondary containment doors.
During the DBA LOCA, should an event occur with both secondary containment doors open simultaneously for 30 seconds or less, this would result in a potential dose increase to the three zones (EAB, LPZ, and CR) of approximately 3.85%. The 3.85% decrease in total margin still maintains adequate overall margin to the regulatory limits at the three areas. The dose was conservatively assumed to be unfiltered and released during the entire 30 second event; therefore, the time of equalization will have no effect on the radiological conclusions.
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 5 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 five other doors with the same armature plate installation were inspected for tightness, and Loctite was applied to the mounting bolt threads.
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, TS 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) C.1 to suspend fuel movements on Unit 1 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, TS 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, TS 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.
On September 18, 2012, Units 1 and 2 were in Mode 1 at 100% power. At 0115 hours0.00133 days <br />0.0319 hours <br />1.901455e-4 weeks <br />4.37575e-5 months <br /> CDT, an Equipment Operator (EO) reported the Unit 2 Reactor Building 761'elevation Interlock Doors 424/314 were both open at the same time for approximately 10 seconds. During the time that both interlock doors were open, TS SR 3.6.4.1.2 ("Verify one secondary containment access door in each access opening is closed") was not met. The secondary containment was declared INOPERABLE for the time that both interlock doors were open. LaSalle Station entered and exited 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 />.
The cause of the event was determined to be the solenoid bracket being loose on Unit 2 Reactor Building 761' interlock door 424. Two screws that hold the locking solenoid bracket in place were found loose. The two loose screws on the locking solenoid mounting bracket caused the electric lock assembly to become misaligned. The misalignment of the electric lock assembly prohibited the locking solenoid plunger from being fully engaged with the door catch and damaged the limit switch, which resulted in the capability to open both interlock doors at the same time.
G. COMPONENT FAILURE DATA:
Security Door Controls Model EMLock 1511 D, with Model AR11YD Armature Mounting Plate