05000454/LER-2002-003
| Docket Number | |
| Event date: | |
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| Report date: | |
| 4542002003R00 - NRC Website | |
LICENSEE EVENT REPORT (LER) estimate to the information and Records Management Branch (t-6 f33), U.S.
Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. It an Information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Byron Station, Unit 1 STN 05000454 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17)
A. Plant Conditions Prior to Event:
First Reactor Trig Event Date/Time: October 15, 2002 / 1217 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.630685e-4 months <br /> Unit 1, Mode 1 — Power Operations, Reactor Power— 100% Reactor Coolant (RC) [AB] System - Normal Opefating Temperature and Pressure No structures, Systems or components were inoperable at the start of the event that contributed to the event.
Second Reactor Trip Event Date/Time: November 7, 2002 / 1247 hours0.0144 days <br />0.346 hours <br />0.00206 weeks <br />4.744835e-4 months <br /> Unit 1, Mode 1 — Power Operations, Reactor Power— 100% Reactor Coolant System - Normal Operating Temperature and Pressure No structures, systems or components were inoperable at the start of the event that contributed to the event.
All times referred to in this report are Central Standard Time.
B. Description of Event:
The Turbine is equipped with a Digital Electrohydraulic Control (DEHC) [TG] System consisting of a solid-state electronic controller and a high pressure fluid supply used for control of the turbine valve operators. The controller compares signals representing turbine speed and first stage pressure with reference values initiated by a load demand signal. The controller then puts out a comparison signal that actuates hydraulic control of the main turbine governor valves to match generator output to load demand.
First Reactor Trip On October 15, 2002, at approximately 1217 hours0.0141 days <br />0.338 hours <br />0.00201 weeks <br />4.630685e-4 months <br />, the Unit 1 reactor automatically tripped due to a RC overtemperature condition. The immediate cause of the RC overtemperature condition was the turbine governor valves closing causing a loss of turbine load. The Operators observed the Main Generator electrical output drop from 1282 to 232 megawatts. Licensed Operators appropriately responded to the reactor trip. The loss of turbine load resulted in an increase in both RC pressure and temperature. The Pressurizer Spray Valves and Power Operated Relief Valves (PORVs) automatically opened, as (KABYR_DOCS \_LERS‘20021ers '1454-2002-001-00 doc)
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LICENSEE EVENT REPORT (LER) estimate to the information and Records Management Branch (t-6 f33), U S Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503 If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Byron Station, Unit 1 STN 05000454 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17) B. D Description of Event (continued):
expected. The temperature increase in the RC and the corresponding RC pressure decrease by the PORVs opening caused the setpoint for the automatic reactor protection function of Overtemperature Delta Temperature (OTDT) to be reached and the reactor tripped as expected. The Main Steam [SB] dumps and steam generator PORVs opened appropriately in response to the secondary pressure increase from the governor valves going close'd. The 1A RC pump was energized via the unit auxiliary transformer due to surveillance testing of its normal electrical feed and consequently lost power when the turbine tripped. A Unit Operator (licensed) observed RC pressure decreasing towards the Safety Injection setpoint and took action to manually close the Pressuizer
- PORVs in advance of the automatic reclosure signal. The 1 A and 1B Auxiliary Feedw'ater (AF) [BA] Pumps were manually started.
A root cause investigation was initiated. All causes for an RC overtemperature condition were ruled out except for DEHC system. A malfunction in the DEHC system was apparent due to an unusual display on the DEHC system panel in the Main Control Room and several distorted parameters. However, thorough diagnostic testing did not identify any problems with the ;DEHC system and extensive testing could not recreate the failure. Areview of the data from the computer that tracks the DEHC system showed that all three speed channels went to zero in the DEHC system at the start of the event. The DEHC system responded by sending a close signal to the governor valves, which responded normally.
Within 10 seconds, the governor valves were closed which caused a loss of load. Approximately 55 seconds into the event the DEHC system computer reset itself and most of the data began to appear correct.
The investigation identified four circuit cards in the system that would be the most likely cause of the event. These four cards were replaced. A monitoring computer was installed to save additional data points from the DEHC system computer. Under this continuous monitoring of the DEHC system, the unit was restarted with no indications of abnormal conditions'on the DEHC system.
Second Reactor Trip On November 7, 2002 at approximately,1247 hours0.0144 days <br />0.346 hours <br />0.00206 weeks <br />4.744835e-4 months <br /> the Unit 1 reactor again automatically tripped due to a RC overtemperature condition. Licensed Operators appropriately responded to the reactor trip. The immediate cause of the RC overtemperature condition was the turbine governor valves closing causing a loss of turbine load. As with the first trip, the plant responded as expected. The Pressurizer Spray valves and Power Operated Relief Valves (PORVs) automatically opened, as expected._ The temperature increase in the RC and the corresponding RC pressure decrease created by the PORVs opening caused the setpoint for the automatic reactor protection function OTDT to be reached and the reactor tripped as expected. The Main Steam dumps and steam generator PORVs opened appropriately in response to the secondary pressure increase from the governor valves going closed.
The 1B AF pump automatically started on expected low steam ,generator level. The 1A AF pump was manually started prior to the low level setpoint being reached. A review of the data from the plant process computer and the special monitoring computer set up after the last reactor trip indicated a very (K BYR_DOCS\_LERS20021ers454-2002-001-00.doc) LICENSEE EVENT REPORT (LER) estimate to the information and Records Management Branch (1-6 133), US.
Paperwork' Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503 If an Information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the Information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Byron Station, Unit 1 STN 05000454 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17) B. Description of Event (continued):
similar scenario between the previous trip and this trip. As in the original event, some of the DEHC system computer data showed some incorrect data as compared with non-DEHC system based plant data. This led investigators to believe that the data again had been corrupted. The same unusual display of the DEHC panel in the MCR was noted. However, the data remained incorrect and the computer did not reset itself. The special monitoring computer was found locked up after the event. A review of the data indicated that it had stopped collecting data prior to the event. However, immediate indications and subsequent diagnostic testing of the DEHC system this time revealed a failure of one of the 8 cards in the Central Processing Unit (CPU). The card is a Bit Logic (BL3) circuit board in the DEHC system computer's CPU.
This BL3 card was then taken to the DEHC system simulator where the failure was repeated. This confirmed a hard failure had occurred that was repeatable.
An Emergency Notification System (ENS) notification for the first reactor trip and manual starts of the 1A and 1B AF pumps at 1432 hours0.0166 days <br />0.398 hours <br />0.00237 weeks <br />5.44876e-4 months <br /> on October 15, 2002 and the ENS notification for the second reactor trip and automatic start of the"1B AF pump and the manual start of the 1A AF pump was made at 1347 hours0.0156 days <br />0.374 hours <br />0.00223 weeks <br />5.125335e-4 months <br />. These events are also require an LER in accordance with 10 CFR 50.73 (a)(2)(iv).
C. Cause of Event:
At the time of the first reactor trip the cause was indeterminate. Circuitry within the CPU was not suspected since it was operating normally and it was believed that a failure in the CPU would lock it up.
The cause of the second reactor trip and, in retrospect, the first reactor trip is attributed a malfunctioning Bit Logic (BL3) circuit board in the DEHC computer CPU. The circuit board malfunction was intermittent in nature and could not be diagnosed for the first reactor trip. The failure of the board was due to a single chip, type SN74H60. The cause of the failure of the chip was not determined at this time. This card is one of an eight-card set that makes up the Arithmetic and Control portion of the Westinghouse DEHC Mod II Mark III Computer.
D. Safety Analysis:
There were no safety consequences impacting plant or public safety as a result of these events. The unit is designed to cope with a loss of turbine load event. In both trips, the reactor trip system functioned as designed and shut down the reactor without incident. The 3rd quarter 2002, Unit 1 NRC performance indicator for unplanned scrams per 7000 critical hours is in the green band at a value of zero. With these two reactor trips in the 4th quarter 2002 it is expected the Performance Indicator will remain in the green band.
(KABYR_DOCSN_LERS120021ers4154-2002-001-00 doc) ■ APPROVED BY OMB NO. 3150-0104 EXPIRES 07/31/2004 collection request. 50 hrs. Reported lessons learned are incorporated into the licensing process and fed back to industry. Forward comments regarding burden estimate to the information and Records Management Branch (t-6 133), U S.
Paperwork Reduction Project (3150-0104), Office Of Management And Budget, Washington, DC 20503. If an Information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the Information collection FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Byron Station, Unit 1 STN 05000454 (If more space Is required, use additional copies of NRC Form 366A)(17)
E. Corrective Actions:
First Reactor Trip Four circuit cards with a high probability of causing this type of malfunction in the DEHC system were replaced. The removed cards were inspected for visible damage and sent to a lab for diagnostic testing. No defects were found on the cards.
A special monitoring computer was installed to monitor performance of the DEHC system.
Second Reactor Trio The DEHC system CPU BL3 card was replaced. Subsequent diagnostic testing of the DEHC system was performed with no failures. Further analysis will be conducted on the failed chip to determine the cause of the failure. Following this analysis, further corrective actions will be determined, as appropriate.
Further evaluation will be conducted to determine the cause of special monitoring computer failure to yield data on second event.
F. Previous Occurrences:
None
G. Component Failure Data:
Manufacturer � Nomenclature � Model Number Westinghouse � Bit Logic Card � N/A (K 1 BYR_DOCS\_LERS20021ers454-2002-001-00 doc)