05000275/LER-2004-002
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 07-21-2004 |
|---|---|
| Report date: | 09-20-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| ENS 40890 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 2752004002R00 - NRC Website | |
I. Plant Conditions
Unit 1 was in Mode 1 (Power Operation) at 100 percent reactor power.
II. Description of Problem
A. Background
The component cooling water (CCW) System [BI] provides a heat sink for the removal of process and operating heat from safety-related and nonsafety-related components during a design basis accident (DBA) or transient. During normal operation, the CCW System provides this function for safety-related components, various nonessential components, and the spent fuel storage pool [DA]. The CCW System serves as a barrier to the release of radioactive byproducts between potentially radioactive systems and the Auxiliary Saltwater (ASW) System [BS], and thus to the environment.
The CCW System consists of three CCW pumps powered from separate vital buses, two CCW heat exchangers, and a shared CCW surge tank with a divider plate. The piping system consists of three normally cross tied headers. The headers extend from the outlet of the heat exchangers, through the header heat loads (components), to the suction of the CCW pumps.
The two vital headers serve redundant engineered safety feature (ESF) loads. A third, nonvital header serves nonvital equipment. Only one ASW pump and one CCW heat exchanger are required, as assumed in the safety analysis, to provide sufficient heat removal from containment to mitigate a design basis accident. However, to ensure maximum heat removal capability, operators are instructed to place the second CCW heat exchanger in service early in the emergency operating procedures.
Each of the vital headers is separable from the others to mitigate a passive single failure during post loss-of-coolant-accident (LOCA) long term cooling. The divided surge tank is connected to the vital header return piping and is sized to meet system leakage requirements and maintain adequate net positive suction head (NPSH) on system pumps.
In the event that CCW system leakage occurs and system makeup is not available, the surge tank volume provides a minimum of 20 minutes, based on a nonmechanistic leakage rate of 200 gallons per minute (gpm), for operators to locate and isolate the leak or separate the CCW system into separate loops before the system becomes impaired due to water loss LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) as discussed in Supplemental Safety Evaluation Report (SSER) 16, which did not credit automatic or manual CCW makeup to mitigate a CCW system leak.
For a graphical representation of the CCW System, refer to Figure 1.
Additional information on the design and operation of the system, along with a list of the components served, is presented in the Final Safety Analysis Report, Section 9.2.2, "Component Cooling Water System.
Technical Specification (TS) 3.7.7, "Vital CCW System," Limiting Condition for Operation (LCO) requires; "Two vital CCW loops shall be OPERABLE.
With "One vital CCW loop inoperable," and the "Required Action and associated Completion Time of Condition A not met," the unit will "Be in MODE 3 (in) 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> AND Be in Mode 5 (in) 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />." TS 3.7.7 does not specify an LCO for two vital CCW loops inoperable.
TS 3.0.3 requires that; 'When an LCO is not met and the associated ACTIONS are not met, an associated ACTION is not provided, or if directed by the associated ACTIONS, the unit shall be placed in a MODE or other specified condition in which the LCO is not applicable. Action shall be initiated within 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> to place the unit, as applicable, in:
a. MODE 3 within 7 hours8.101852e-5 days <br />0.00194 hours <br />1.157407e-5 weeks <br />2.6635e-6 months <br />; b. MODE 4 within 13 hours1.50463e-4 days <br />0.00361 hours <br />2.149471e-5 weeks <br />4.9465e-6 months <br />; and c. MODE 5 within 37 hours4.282407e-4 days <br />0.0103 hours <br />6.117725e-5 weeks <br />1.40785e-5 months <br />.
Previously on December 16, 1998, the reactor coolant pump (RCP) 1-3 upper bearing lube oil cooler channel head failed due to a crack in the channel head at the toe of the weld connecting the nozzle to the shell resulting in an orderly manual shutdown of Unit 1 to repair. The failure was due to high cycle fatigue cracking of the channel head. The immediate solution was to replace the channel head and change the modal response of the piping so as to detune it from the pump frequency of approximately 20 Hz. Analysis calculation 4A-125 supports the addition of a 45-pound, plus or minus 5 percent, mass firmly affixed to line 1-K-136-4. The longer-term solution was to replace the channel head with new, reinforced channel heads as part of the 10-year RCP maintenance program.
In May of 2002, during Unit 1 eleventh refueling outage (1R11), the upper bearing lube oil cooler channel head for RCP 1-3 was replaced as part of the 10-year RCP motor preventive maintenance program.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Vital
HDR
Non-Vital t 1 HDR`C" j�Vital
HDR
Vent To ATM Head Tank 'se Ne, Aux Bldg Sump B A Derain Water LCV-70 Make Up ) � LCV-69 Figure 1 B.�Event Description On July 21, 2004, leakage was detected by increasing containment sump level. The leak rate was estimated to be approximately 0.13 gpm. Utility personnel entered containment and discovered a cracked joint on the 3-inch CCW piping. The maximum leak, if the CCW pipe completely sheared off, was conservatively assumed to be greater than 200 gpm. The maximum analyzed leak rate for the CCW system is approximately 200-gpm leak, as stated in the Final Safety Analysis Report Update.
- LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) On July 21, 2004, at 1900 PDT, based on the potential for the condition to rapidly degrade under loading such as a seismic event, PG&E licensed plant operators conservatively declared TS 3.7.7 not met, entered TS 3.0.3, and initiated a Unit 1 shutdown at 2020 PDT.
On July 21, 2004, at 2157 PDT, plant operators made a nonemergency event notification (EN #40890) in accordance with 10 CFR 50.72(b)(2)(i), for the required shutdown due to a nonvital CCW leak inside containment.
On July 22, 2004, at 0033 PDT, Unit 1 entered Mode 3, the lube oil cooler isolated, and TS 3.0.3 was exited.
On July 23, 2004, at 1432 PDT, following RCP 1-3 lube oil cooler replacement TS 3.7.7 was exited.
On July 23, 2004, at 1546 PDT Unit 1 entered Mode 2.
On July 24, 2004, at 0104 PDT Unit 1 entered Mode 1.
On July 24, 2004, at 0257 PDT plant operators paralleled Unit 1 main generator to the grid.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected None.
E. Method of Discovery Utility licensed plant operators in the control room during routine observations responded to rising sump level indications and initiated an investigation that identified the leakage location.
F. Operator Actions Utility licensed plant operators evaluated the identified leakage location and questioned the capability of compliance with the Final Safety Analysis Report (FSAR) Update maximum 200-gpm leak rate evaluation in the LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) event of an earthquake. Based upon questionable compliance with design basis leak rate capability, plant operators conservatively entered TS 3.7.7 and TS 3.0.3 and performed an orderly manual shutdown.
G.�Safety System Responses All safety systems, including the emergency diesel generators were fully operable and the offsite electrical grid was stable during the shutdown.
III.�Cause of the Problem
A. Immediate Cause
High cycle fatigue failure at the nozzle connection of the CCW supply piping to the RCP 1-3 upper bearing lube oil cooler channel head.
B. Root Cause
Component replacement of the RCP upper bearing lube oil cooler channel head with a reinforced (stiffer) connection resulted in the piping system being closer to the resonant frequency (20 Hz) resulting in fatigue failure.
C. Contributory Cause 1. Previous nonconformance report (NCR) corrective action was not maintained, i.e., action(s) for a fatigue tolerant weld profile was not maintained for the new heads. The weld quality was a contributor in increasing stress levels (locally).
2. Previous NCR corrective action was not adequately maintained, i.e., action(s) to maintain a margin between the RCP forcing frequency and the resonant frequency of the piping system was not maintained.
IV.�Assessment of Safety Consequences The CCW System is required to provide cooling to safety-related components to assure that they are capable of performing their required functions following an accident. As previously described, the CCW System consists of normally cross tied vital and nonvital headers. The headers can be isolated from each other within 20 minutes following an assumed CCW leak of 200 gpm, as discussed in SSER 16.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Had a 200 gpm or greater leak occurred in header C, and makeup was not available, the leakage would have been terminated when the header C isolation valves were closed. Left uncorrected, the ongoing leakage could result in a loss of the CCW function.
However, a subsequent calculation using seismic stresses and the as-found condition shows the pipe would not have catastrophically failed.
Therefore, the event is not considered risk significant and it did not adversely affect the health and safety of the public.
Also, the condition is not considered a Safety System Functional Failure.
V.�Corrective Actions A.�Immediate Corrective Actions
- Licensed plant operators entered TS 3.0.3 and initiated shutdown
- RCPs were shut down
- Nondestructive examination inspection of similar Unit 1 RCP lube oil cooler piping was performed with no flaw indications identified
- RCP vibration monitoring was performed that found vibration levels on other RCPs were lower than RCP 1-3.
- Visual inspection of the Unit 2 connections and lines was performed B.�Corrective Actions to Prevent Recurrence Move the piping natural frequency off-resonance (i.e., away from a 20hz natural frequency).
VI.�Additional Information A.�Failed Components Component:�RCP upper bearing lube oil cooler Supplier: Westinghouse Electric Co.
Manufacturer: Senior Engineering Company LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) B.�Previous Similar Events Properly due to Misadjusted Travel Stops — Personnel Error," reported outside design basis due to misadjusted travel stops — personnel error.
Corrective action to prevent recurrence includes requiring maintenance verification testing for similar valves to ensure the travel stops are left properly adjusted after maintenance. Corrective actions to prevent recurrence would not have prevented this event as it involved actions regarding valve maintenance only.
NCR N0002080, "Unit One Plant Shutdown due to CCW Leak on RCP 1-3," documents "a [previous] through-wall leak, adjacent to the channel head nozzle weld for the CCW supply to the upper bearing cooler for RCP 1-3, [that] resulted in the curtailment of DCPP Unit 1 and replacement of the cracked channel head." The corrective actions taken as a result of the previous event were ineffective in preventing this event.