05000277/LER-2014-002
| Peach Bottom Atomic Power Station Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(vii)(A), Common Cause Inoperability |
| Initial Reporting | |
| ENS 50395 | 10 CFR 50.72(b)(3)(v)(A), Loss of Safety Function - Shutdown the Reactor, 10 CFR 50.72(b)(3)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 2772014002R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose 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.
Peach Bottom Atomic Power Station Units 2 and 3 14 - 002 00 Unit Conditions Prior to the Event Units 2 and 3 were both at approximately 100% power when this condition was discovered. There were no structures, systems or components out of service that contributed to this event.
Description of the Event
On August 23, 2014, a pin-hole leak was identified in a 6" diameter, carbon steel pipe elbow in the Emergency Service Water (ESW) system (EIIS: CC). The ESW system is a standby system that provides cooling water to the emergency diesel generators, Emergency Core Cooling Systems (EGGS) room coolers and other safety-related equipment during design basis accident conditions. During normal operating conditions, this equipment is cooled by the non-safety related Service Water system. The ESW system consists of two redundant subsystems, and is common to Unit 2 and Unit 3. Normal water supply for the ESW system is from the Conowingo Pond. The ESW piping is classified as safety related, ASME Code Class 3, and of moderate energy (piping design rating of 150 psig © 100 degrees F).
The pin-hole leak was located on an elbow between a check valve and a normally open manually operated isolation valve on a line that provides cooling to Unit 2 (which can be supplied by either ESW subsystem). The leak was located on a section of ESW piping (EIIS: P) that is connected to the Service Water (SW) system at a point just downstream of the check valve that prevents SW flow into the remainder of the ESW system. As a result, the elbow is normally pressurized at SW system pressure, but only experiences flow when the ESW system is being operated (e.g., periodic testing).
The leak rate was measured with a graduated cylinder and determined to be approximately 3 ml/min.
Since operability of the degraded piping could not be immediately established, both ESW subsystems were declared inoperable at 1300 hours0.015 days <br />0.361 hours <br />0.00215 weeks <br />4.9465e-4 months <br /> on August 23, 2014. Technical Specification 3.7.2, Condition B was entered for both units, which requires the units to be in Mode 3 in 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in Mode 4 in 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
Ultrasonic examination of the pipe elbow was performed to determine the size and nature of the flaw. An area around the leak with a diameter of approximately 0.6 inches was identified as being below the minimum wall thickness of 0.100 inches. To obtain additional time to pursue ASME code relief, a verbal request was made to the NRC for a Notice of Enforcement Discretion (NOED) to extend the required completion time for TS 3.7.2, Condition B, by 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br />. The NRC verbally granted the NOED at 1922 hours0.0222 days <br />0.534 hours <br />0.00318 weeks <br />7.31321e-4 months <br /> on August 23, 2014 with a follow-up written approval on August 28, 2014.
The ASME code relief request was made in a telephone call to the NRC on August 24, 2014 at 1157 hours0.0134 days <br />0.321 hours <br />0.00191 weeks <br />4.402385e-4 months <br />. Verbal approval was provided by the NRC with a follow-up written approval on August 26, 2014.
Based on the code relief, an operability evaluation determined that the through wall leak and surrounding wall thickness is acceptable for continued operation. The ESW system was declared operable and TS 3.7.2 Condition B was exited at 0348 hours0.00403 days <br />0.0967 hours <br />5.753968e-4 weeks <br />1.32414e-4 months <br /> on August 25, 2014.
Initial notification of this event was made to the NRC (EN# 50395) in accordance with 10CFR 50.72(b)(3)(v)(A) and (B), as a condition that could have prevented the fulfillment of the safety function of structures or systems needed to shutdown the reactor and maintain it in a safe shutdown condition, and to remove residual heat. With receipt of the relief request, this follow-up report is being made in accordance with 10CFR 50.73(a)(2)(i)(B), as a condition prohibited by the plant's technical specifications, and in accordance with 10CFR 50.73(a)(2)(vii)(A) and (B), as an event where a single cause or condition caused two independent trains to become inoperable in a single system designed to shut down the reactor and maintain it in a safe shutdown condition, and to remove residual heat.
Analysis of the Event
There were no actual safety consequences associated with this event. The ESW system was available to perform its design function at all times during this event.
The ultrasonic examination (UT) of the elbow identified an area of interior corrosion around the location of the through-wall leak. The corrosion is on the outer radius of the elbow, immediately downstream of the check valve. The corrosion consists of localized pitting, likely caused by corrosion under deposits and microbiologically influenced corrosion. The corrosion was likely accelerated by flow directed towards the pipe wall as it leaves the check valve. The area found to be less than the minimum wall thickness value of 0.100 inch is approximately 0.6 inch in both the axial and circumferential direction.
The surrounding area was examined to identify the extent of the area with less than 87.5% of nominal wall thickness (i.e., 0.245 inch) and was determined to have a diameter of approximately 2.9 inches in the axial direction and 3.6 inches in the circumferential direction.
The piping minimum wall thickness is specified in ANSI B31.1 and ensures that the piping stresses will remain within code allowable limits. Failure of the piping to meet minimum required wall thickness due to a flaw requires an evaluation to determine if stresses are within code allowable values.
10 CFR50.55a(g)(4) and NRC Regulatory Gide (RG) 1.147 allow the use of ASME Code Case N-513-3, "Evaluation Criteria for Temporary Acceptance of Flaws in Moderate Energy Class 2 or 3 Piping," to evaluate flaws located in straight pipe. Evaluation criteria for fittings such as elbows has been added to a revision to N-513-3 (designated as N-513-4), which has been approved by ASME but not yet approved by the NRC. The ASME Code relief requested by PBAPS and approved by the NRC on August 24, 2014, allowed for evaluation of the flaw in the ESW elbow to be performed using N-513-3 provided the stresses used in the evaluation are adjusted to account for geometric differences, as required by N-513-4.
The flaw evaluation determined that the calculated stress intensity factors for normal and emergency conditions were well below the allowable fracture toughness. This demonstrates the integrity of the elbow and the code case allows for temporary acceptance of the flaw. The NRC requires the repair or replacement activity to be performed during the next scheduled outage.
Cause of the Event
The cause of the leak has been determined to be localized pitting caused by corrosion under deposits and microbiologically influenced corrosion. In addition, the upstream check valve disc tends to direct flow towards the corrosion area, which may have accelerated the corrosion rate.
Corrective Actions
The elbow is scheduled to be repaired or replaced during the Unit 2 refueling outage, which is scheduled to begin in October 2014. Until then, inspection during daily operator rounds (once per shift) is being performed to identify any increase in leakage. Additional evaluation in accordance with the code case will be performed if additional leakage is identified. An additional UT exam was performed on September 23, 2014, and no change to the component's condition was identified.
Adjustments are being made to the site's risk-based inspection program to be able to better identify low- margin piping in raw water systems and perform repairs or replacement to meet code requirements.
Previous Similar Occurrences On July 6, 2008, a 6" diameter ESW pipe was found to have a leak on the supply header to the El Emergency Diesel Generator (EDG) coolers. The leak was determined to be caused by under deposit corrosion. Corrective actions were focused on ESW piping to the EDGs and, as a result, did not prevent this event.