05000373/LER-2009-001
| Lasalle County Station Unit 1 | |
| Event date: | 05-21-2009 |
|---|---|
| Report date: | 07-20-2009 |
| 3732009001R00 - NRC Website | |
A. PLANT AND SYSTEM IDENTIFICATION
General El ectr ic - Boiling Water Reactor, 3489 Megawatts Thermal Rated Core Power
CONDITION PRIOR TO EVENT
�Unit(s): 1 Event Date: 5/21/09�Event Time: 1635 � Reactor Mode(s): 1 Power Level(s): 100 Mode(s) Name: Power Operation
B. DESCRIPTION OF EVENT
On May 21, 2009, at 1635 CDT, LaSalle Unit 1 automatically scrammed from 100% power. The scram was due to a generator lockout signal caused by a failure of the surge arrester on the 'A' phase of the Unit 1 West Main Power Transformer (1W MPT) (MP)[EL]. Deluge protection of the 1W MPT actuated and no fire resulted from the arc caused by the surge arrester failure.
Following the scram, reactor water level was restored using the Motor Driven Reactor Feed Pump (FW)[SJ], and pressure control was established with the Main Condenser (CD)[SG] via the turbine bypass valves.
The surge arrester failure caused a voltage transient on both LaSalle Units. On Unit 2, the 24B and 25B Low Pressure Feedwater Heaters (HD)[SM] and the 24C Low Pressure Heater Normal Drain tripped, which forced a reduction in reactor power to approximately 85%. The voltage transient also resulted in a number of other non-valid isolation signals, including Reactor Water Cleanup (RT) [CE] and Reactor Building Ventilation (VR)[VA]. The VR system isolated on both Units 1 and 2.
Unit 2 returned to full power on May 22, 2009, at 1100 CDT. Following replacement of the 1W MPT 'A' phase surge arrester and verification that the MPT had not been damaged, Unit 1 was restarted on May 23, 2009, and was synchronized to the grid on May 25, 2009. The Unit returned to full power at 1412 CDT on May 27, 2009.
An Emergency Notification System call was made at 1946 CDT on May 21, 2009, in accordance with 10 CFR 50.72(b)(2)(iv)(B) as an event or condition that resulted in actuation of the reactor protection system when the reactor was critical.
C. CAUSE OF EVENT
A forensic analysis was performed on the failed surge arrester. The analysis determined that the root cause was a manufacturing defect. The glass tape used to hold together the metal oxide blocks inside the arrester was folded and wedged between the metal oxide blocks in two � different areas, which prevented face-to-face contact between the blocks. As a consequence, under normal operating conditions an air gap existed between the blocks, leading to uneven current flow through adjacent blocks. Surges from normal switching operation and lightning strikes in the years following the installation caused punctures near the edge of the affected blocks.
Although all arresters are tested prior to shipping, the defect in this arrester was either not detected by the factory testing or the defective arrester was not discarded upon identification of the defect. It is the opinion of the vendor that this defect should have been identified in production testing. Retrieval of the original Joslyn test reports is no longer possible. The arresters were satisfactorily Doble tested on site prior to installation, but this test was not capable of identifying the defect.
As a result of Doble test results in 2004, the original 1W MPT 'A' phase surge arrester was replaced, along with the 1 East (1E) MPT 'A' phase surge arrester. The Joslyn Manufacturing Co. (now MacLean Power System) ZSH series (MOV-Metal Oxide Varistor) station class surge arresters were selected to replace the original Westinghouse CPL series silicon carbide gapped surge arresters because they use metal oxide technology, which represented an improved design over the silicon carbide gap type arresters.
These arresters performed without problems until the day of the event. On April 30, 2009, a temperature scan of the Unit 1 transformers was performed and no differential temperatures were noted on either the MOV or silicon carbide arresters. Due to the manufacturer defect, degradation of the arrester would have started with the installation in 2004, but the degradation did not result in significant heat generation due to the low current in the arrester. Therefore, thermography did not identify the defect.
D. SAFETY ANALYSIS
The safety significance of this event was minimal. All control rods fully inserted and all systems responded as expected to the scram. The Emergency Core Cooling Systems were not challenged. This event did not meet the NEI 99-02 definition of an unplanned scram with complications. There were no safety system functional failures.
E. CORRECTIVE ACTIONS
Corrective Actions:
- The `A' phase arresters were replaced on both the 1 E and 1W MPT.
- The 1 E and 1W MPTs were tested to verify that they had not been degraded as a result of the transient.
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CONTINUATION SHEET
2. DOCKET 6. LER NUMBER 05000373 YEAR NUMBER NO.
2009 -0 01 -00 00 Corrective Action to Prevent Recurrence:
- Procurement requirements will be changed to require that vendor test data be supplied for future station surge arrester purchases. (AT# 923160-15)
F. PREVIOUS OCCURENCES
LaSalle Station has not previously experienced a failure of a surge arrester.
G. COMPONENT FAILURE DATA
Joslyn ZSH Metal Oxide Arrester with 264kV Rating