05000249/LER-2002-003
| Dresden Nuclear Power Station Unit 3 | |
| Event date: | 10-08-2002 |
|---|---|
| Report date: | 12-09-2002 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 2492002003R00 - NRC Website | |
LICENSEE EVENT REPORT (LER) f33), 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)(1 7) A. � Plant Conditions Prior to Event:
Unit: 03 � Event Date: 10-08-2002 Reactor Mode: 1 � Mode Name: Run Reactor Coolant System Pressure: 926 psig Event Time: 0421 CDT Power Level: 017 percent
B. Description of Event:
This event is being reported in accordance with 10 CFR 50.73(a)(2)(i)(A), "The completion of any nuclear plant shutdown required by the plant's Technical Specifications" and 10 CFR 50.73(a)(2)(ii)(A), "The condition of the nuclear power plant, including its principal safety barriers, being seriously degraded.
On October 8, 2002, at approximately 0421 hours0.00487 days <br />0.117 hours <br />6.960979e-4 weeks <br />1.601905e-4 months <br />, with power being reduced for refueling outage D3R17, an inspection of the DNPS Unit 3 drywell was performed. The inspection of the drywell was performed due to an increase in unidentified leakage on September 14, 2002. The leakage had increased from 0.2 gpm to approximately 0.65 gpm. During the inspection a leak was identified on a one inch diameter piping socket weld associated with the Reactor Recirculation (RR) "A" loop low pressure flow venturi differential pressure sensing line. A Unit 3 shutdown was performed as required by Technical Specifications 3.4.4 for primary pressure boundary leakage. The piping was repaired/replaced per the Electrical Power Research Institute (EPRI) guidelines with the installation of 2-1 axial leg socket welds.
C. Cause of Event:
The root cause was high cycle fatigue failure due to an inadequate 1-1 axial leg socket weld application in a system experiencing flow-induced vibration. The 1-1 axial leg socket weld was installed in 1985 as part of a major reactor recirculation piping replacement project. The weld was consistent with industry standards at the time of installation. (NRC Cause Code B).
A contributing cause was a failure to properly address two previous high cycle fatigue weld failures and to build up the RR sensing line socket welds. Although the installation of tie-back supports to reduce vibration-induced stresses was performed during the previous outage (D3R16), a corporate and site engineering knowledge deficiency of a December 1999, EPRI report existed. Specifically, the EPRI Report TR-113890 recommendation to build up 1-1 axial leg socket welds to 2-1 axial leg socket welds was not implemented on the RR differential pressure sensing lines during the D3R16 refueling outage.
D. Safety Analysis:
The reactor pressure boundary leak was detected by the drywell leak monitoring system.
A failure of the instrument sensing line is bounded by the analyzed condition of a small break Loss-of- Coolant Accident (LOCA). This postulated failure would result in drywell high pressure, which would generate a reactor scram signal and initiation of the Emergency Core Cooling Systems (ECCS). The consequence of this accident would be mitigated by the High-Pressure Coolant Injection (HPCI) system or the Automatic Depressurization System (ADS) in conjunction with the Low Pressure Coolant Injection (LPCI) and Core Spray systems.
LPCI loop select logic was not affected by the RR venturi sensing line leakage. The LPCI loop selection logic ensures that LPCI injection flow is directed to an unbroken recirculation pump loop. Four differential pressure ., � .
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) detectors compare the pressure between RR riser pipes in Loop A and the corresponding riser pipes in Loop B. If the Loop A pressure is greater than the loop B pressure, the logic selects Loop A for injection. If the Loop A pressure is not greater than the Loop B pressure, either RR Loop A is considered broken or neither RR loop is considered broken, a 1/2-second timer causes Loop B to be selected for injection. The minimal pressure drop in RR Loop A due to the failed socket weld was not sufficient to adversely affect the LPCI loop select logic.
The RR Loop A increase in flow indication and total reactor core flow was minimal due to the low pressure sensing line weld failure and had a negligible effect on reactor operating characteristics. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. C Corrective Actions:
Replaced the Unit 3 RR Loop A high and low pressure venturi sensing line elbows and piping with a bent pipe configuration with no elbows and replaced sixteen 1-1 axial leg socket welds with 2-1 axial leg socket welds.
Performed an ultrasonic examination of the RR loop B high and low venturi sensing line socket welds. Identified an indication on the tee connection and repaired the RR Loop B high pressure venturi sensing line tee weld indication with 2-1 axial leg socket welds.
Modified an additional twenty-six 1-1 axial leg socket welds on the Unit 3 RR Loop B high and low pressure venturi sensing lines to achieve 2-1 axial socket weld configurations.
Installed vibration monitors on the RR Loop A low pressure and RR Loop B high pressure sensing lines.
Thermal, modal and response spectra analysis was performed for the RR Loop A low pressure and modal and response spectra analysis was performed for the RR Loop B high pressure sensing line configurations. The following conclusions apply to both analyzed configurations and are consistently applicable to the remaining unanalyzed RR Loop A high pressure and RR Loop B low pressure sensing lines:
- The analysis results provide a relative comparison of the response of the sensing lines with and without the tie-back supports.
- Installing 2-1 axial leg socket welds, consistent with the results of the EPRI socket weld testing program is beneficial and necessary to compensate for the lack of operating vibrational data;
- Due to the constraint imposed by the tie-backs supports, additional thermal stresses are induced in the sensing lines. Detailed thermal stresses and fatigue analysis was performed for the RR Loop A low pressure sensing line and thermal stresses were found to be acceptable.
Performed an extent of condition review for DNPS Unit 2. Previous Unit 2 RR system walkdown observations and Dye Penetrant Tests (PT) found no evidence of piping degradation or leakage. Unit 2 has operated at nearly all the potential reactor recirculation pump speeds allowed. Additionally, the Unit 2 piping has not been replaced and thus piping design characteristics for Unit 2 are not the same as Unit 3. Therefore, it is reasonable to conclude that U2 is not susceptible to high-frequency, low-amplitude cyclic vibration. In conclusion, Unit 2 is not considered to be affected by this failure mode.
LICENSEE EVENT REPORT (LER) f33), 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) Developed and implemented a procedure for reviewing and disseminating new industry technical information.
F. Previous Occurrences:
On November 1, 1997, during inspection of the Unit 3 drywell for the source of a previously detected increase in unidentified leakage, a crack was discovered on a socket weld at a one inch tee fitting in the Reactor Recirculation Loop B High Pressure Flow Venturi Differential Pressure Sensing Line. A Unit 3 shutdown was performed as required by Technical Specification 3.6.H for primary pressure boundary leakage. This event was reported in LER 03-97-012.
On March 21, 1999, during inspection of the Unit 3 drywell for the source of a previously detected increase in unidentified leakage, a crack was discovered on the same socket weld at a one inch tee fitting on the Reactor Recirculation Loop B High Pressure Flow Venturi Differential Pressure Sensing Line. A Unit 3 shutdown was performed as required by Technical Specification 3.6.H for primary pressure boundary leakage. This event was reported in LER 99-003-00.
G. Component Failure Data:
N/A