05000249/LER-2002-002
| Dresden Nuclear Power Station Unit 3 | |
| Event date: | 07-21-2002 |
|---|---|
| Report date: | 09-19-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2492002002R00 - NRC Website | |
COMMISSION
(LER) (7-2001)
LICENSEE EVENT REPORT
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Dresden Nuclear Power Station Unit 3 05000249 (If more space is required, use additional copies of NRC Form 366A)(17)
A. Plant Conditions Prior to Event:
Unit: 03 Event Date: 07-21-2002 Reactor Mode: 1 Mode Name: Run Reactor Coolant System Pressure: 1000 psig Event Time: 2033 CDT Power Level: 100 percent
B. Description of Event:
This event is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A), "Any event or condition that resulted in a manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B)", reactor protection system (RPS) including reactor scram or reactor trip.
On July 15, 2002, at 03:00, Unit 3 turbine was taken off-line following identification of damage to the PMG at the turbine front standard. Following the controlled Unit 3 turbine shutdown, an inspection of the PMG was performed.
With the turbine at rest, General Electric (GE) and Corporate turbine personnel reviewed clearance measurements and concluded that the shaft was centered, which indicates no bearing wear at the MSOP [TD] end of the shaft. Based on the measurements taken, the front standard was not disassembled for further inspection.
The unit was restored with the PMG internal mechanisms removed, turbine speed was verified locally and no abnormalities were noted during acceleration to 1800 rpm. Inspection upon disassembly of the turbine front standard after the July 21, 2002, reactor scram revealed the bearing wear was at the ten and seven o'clock positions, which would not have been revealed when the turbine was at rest.
On July 21, 2002, at 2033 hours0.0235 days <br />0.565 hours <br />0.00336 weeks <br />7.735565e-4 months <br />, the Unit 3 main turbine [TA] tripped due to low discharge pressure from the turbine shaft bearing oil pump, resulting in a Unit 3 reactor scram. An inspection identified extensive damage to the main shaft oil pump rotor and MSOP gears. The PMG shaft was found sheared. Additionally, the MSOP bearings indicated damage and severe wear. Severe wear was also visible on the main shaft rotor thrust bearing face. Further damage was noted on the MSOP wear ring and the shaft seal rings. The cause of this event was determined to be a degradation of the auxiliary control rotor gear coupling insulation, resulting in a current flow through the shaft which caused electrolysis and accelerated wear of the bearings.
A review of design information and GE recommendations was performed. Dresden had previously implemented GE Technical Information Letter (TIL) 973-3, which recommended the installation of an insulated coupling modification to electrically insulate the main shaft rotor gear assembly from the turbine-generator. The insulated coupling protects the gear teeth, journals, and bearings from electrical pitting damage. Also, a copper braided shaft grounding device had been installed to protect the main turbine bearings from electrolysis damage per GE TIL 893-2 and the shaft voltage measurements were taken per station procedures. A review of shaft voltage readings taken per station procedures revealed an increased trend of the shaft voltage levels. The voltages in the months preceding the event had been increasing from 25 VDC in April 2002 to as high as 100VDC in June 2002.
The station procedure for measuring the main shaft voltage did not provide any acceptance criteria for the voltage reading taken, nor was there any trending of shaft voltages performed. Engineering personnel did not communicate the increase in the shaft voltage during the PMG failure and thus no action was taken. It was determined that there was degraded insulation at the coupling between the main shaft rotor and the quill shaft which led to electrolysis in the front standard components. Since the coupling insulation was determined to be degraded, the increase in main shaft voltage accelerated the rate of electrolysis to the front standard components.
Frosting and pitting were found on the front standard bearings and gear teeth as a result of the electrolysis.
, -- � NRC FORM 366A � U.S NUCLEAR REGULATORY 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 f33), U S Nuclear Regulatory Commission, Washington, DC 20555-0001, and to the 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.
COMMISSION
(LER) (7-2001)
LICENSEE EVENT REPORT
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Dresden Nuclear Power Station Unit 3 05000249 (If more space is required, use additional copies of NRC Form 366A)(17) A review of preventative maintenance history was conducted. The review determined that the front standard components were inspected in May 1993 during a forced outage. The review also determined that the six-year preventative maintenance activity for the Unit 3 front standard was deferred from the refueling outage conducted in September 2000 to the next outage, scheduled for October 2002. The documentation justifying the deferral of front standard inspection was determined to be inadequate.
With the degraded insulation, the high shaft voltage accelerated the rate of electrolysis to the front standard components, causing bearing wear, which resulted in the failure of gear mesh, causing the MSOP to stop. This resulted in the loss of MSOP discharge pressure and the subsequent automatic turbine trip and reactor scram.
The main shaft rotor and MSOP rotor were replaced. The MSOP impeller was in good condition, and it was reused. All four bearings were replaced. The main shaft return oil feed nozzle was found mis-positioned and the quill shaft was found with carbonized oil, but otherwise was in good condition. The PMG was left out-of-service.
C. Cause of Event:
The cause of this event was degradation of the auxiliary control rotor gear coupling insulation, resulting in current flow through the gear shafts which caused electrolysis and accelerated wear of the bearings.
(NRC Cause Code X).
D. Safety Analysis:
At no time did this condition compromise the health and safety of the public. The failure of the MSOP resulted in low turbine oil pressure, which initiated a turbine trip and reactor scram as designed. All safety systems functioned as designed following the reactor trip.
E. Corrective Actions:
Immediate Corrective Actions include:
All degraded parts were replaced and the turbine oil reservoir and internals were cleaned and flushed.
Corrective Actions include:
Revise Corporate Turbine Front Standard inspection procedure MA-AB-763-411 to include dismantle and inspection of extension tube. The insulated coupling splines will be disassembled for inspection during regular six- year Preventative Maintenance and the as-found and as-left megger verified. In addition, the front standard bearing clearance acceptance criteria will be specified in the procedure.
Monitor and trend the shaft voltage readings in the system-monitoring plan.
Revise procedure MA-AB-763-411 to include the inspection of front standard insulation kit per GE specification.
Revise corporate procedure WC-AA-112 to include a step that requires documentation of the specific technical basis for Preventive Maintenance deferrals and risk for the deferral.
- ' --- � NRC FORM 366A � U.S. NUCLEAR REGULATORY 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
COMMISSION
(7-2001) LICENSEE EVENT REPORT (LER) 133), U S Nuclear Regulatory Commission, Washington, DC 20555-0001, 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) Dresden Nuclear Power Station Unit 3 05000249 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17) Screen and sort the Preventive Maintenance deferral database by system for critical Preventive Maintenance deferrals and provide a list to system engineering for review.
F. Previous Occurrences:
None
G. Component Failure Data:
N/A