05000315/LER-2013-001
| Donald C. Cook Nuclear Plant Unit 1 | |
| Event date: | 03-31-2013 |
|---|---|
| Report date: | 05-28-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3152013001R00 - NRC Website | |
Conditions Prior to Event Mode 5 Cold Shutdown
Description of Event
On March 30, 2013, at 0124 EDST, Donald C. Cook Nuclear Plant (CNP) Unit 1 reactor was in Mode 5, Cold Shutdown, after shutdown for a refueling outage.
A small deposit of dry boric acid was identified on the weld face of a 3/4 inch socket welded elbow-pipe connection upstream of 1-NFP-222-V2 (1-NFP-222 REACTOR COOLANT LOOP 2 CHANNEL 3 FLOW TRANSMITTER LOW PRESSURE SIDE ROOT SHUTOFF VALVE)[V]. A liquid dye penetrant examination was performed on March 31, 2013, at 0414 EDST. A 10 mm long linear indication was found on the face of the weld not acceptable per ASME Section Xl. The weld is located in a non-isolable section of the Reactor Coolant System (RCS) piping. The indication is considered a through wall leak of the RCS[AB] pressure boundary that was active during the past operating cycle during modes of applicability. Technical Specification Limiting Condition for Operation (LCO) 3.4.13 limits the RCS to no pressure boundary leakage and is applicable in Modes 1, 2, 3, and 4.
The weld defect was reported in accordance with 10 CFR 50.72(b)(3)(ii)(A), Any event or condition that results in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded.
The weld defect and the subsequent through wall leak of the RCS pressure boundary are reportable as a Licensee Event Report (LER) in accordance with 10 CFR 50.73(a)(2)(ii)(A), Any event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers, being seriously degraded; and 10 CFR 50.73(a)(2)(i)(B), Operation or Condition Prohibited by Technical Specifications.
Cause of Event
Vibration fatigue of a cantilever tap connection is the cause of the identified weld defect and subsequent RCS through wall leak, based on engineering evaluation of previous similar events.
Material failure analysis of the socket weld and associated piping is pending at the time of this report. A revision to this LER will be provided if the failure analysis results in substantial changes in the cause or planned corrective actions.
Analysis of Event
Extent of condition to the weld failure upstream of 1-NFP-222-V2 was completed. Performance Verification performed VT-2, visual examinations on all similar locations in RCS loops in Unit 1 and found no evidence of boric acid or leaks.
No leak or other indications of a leak were noted by personnel performing the containment walkdown at the start of the current Unit 1 Cycle 25 (March-April 2013) outage. Additionally, the RCS unidentified leak rate over the last cycle was typically less than 0.01 gallon per minute.
This condition did not affect any other safety related, or otherwise important to safety, equipment. Based on the observations indicating that there was no active leak, backed up by the low RCS leak rates noted during the last cycle, and that this leak was not in a location to impact the function of any equipment necessary to mitigate RCS leaks, this condition did not represent any significant risk to the unit and did not pose a risk to the health and safety of the public.
Corrective Actions
Completed Corrective Actions
ASME Section XI Repair and Replacement activities have been completed that include the modification of the piping to shift the natural frequency, revised, fillet welds, and replacement of the elbow.
Vibration analysis was completed on the cantilever piping where the defect was identified and other similar components on the RCS to address the extent of condition. All readings were found acceptable without the need for additional analysis.
Planned Corrective Actions
Material failure analysis of the socket weld and associated piping is pending at the time of this report and a full causal evaluation will follow.
A revision to this LER will be provided if the material failure analysis or causal evaluation results in substantial changes in the cause or planned corrective actions.
Previous Similar Events
LERs for CNP Unit 1 and Unit 2 for the past five years were reviewed for similar events. The following was identified:
05000315-2008-004-00, Non-Isolable Reactor Coolant System Pressure Boundary Leak On April 25, 2008, personnel at CNP Unit 1 identified a non-isolable RCS pressure boundary leak on a 3/4- inch instrument line connected to the RCS. Technical Specification LCO 3.4.13 limits the RCS to no pressure boundary leakage and is applicable in Modes 1, 2, 3, and 4.
The apparent cause was determined to be vibratory fatigue based on the piping configuration making it susceptible to high cycle fatigue, with the source of energy being the Reactor Coolant Pump. High cycle fatigue caused flaw propagation and the subsequent leak.
The apparent cause documented in the LER in 2008 is apparently the same as the current weld failure in this LER, as described as cyclic fatigue. The weld repairs and corrective actions are different for each event. The repair in 2008 was completed using a seal weld and weld overlay under the requirements of Code Case N-666. The current event repair included removing the elbow, modifying the piping, and use of a different fillet weld profile. It is noted that the 2008 and current events involve the same elbow component with the defects located at opposite socket welds.
Corrective actions taken in the 2008 event would not have prevented the current reportable event, based on the different location of the previous weld defect and the nature of the repairs completed.