05000277/LER-2016-001

LER-2016-001, Leak in High Pressure Service Water Pipe Results in Condition Prohibited by Technical Specifications

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
2772016001R00 - NRC Website

LER 16-001-00 for Peach Bottom, Unit 2, Regarding Leak in High Pressure Service Water Pipe Results in Condition Prohibited by Technical Specifications

ML16316A114
Person / Time
Site:	Peach Bottom
Issue date:	11/11/2016
From:	Navin P D Exelon Generation Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
CCN: 16-102 LER 16-001-00
Download: ML16316A114 (5)

comments regarding burden estimate to the FOIA, Pnvacy and Information Col ections Branch

• T 5 F53), U.S Nuclear Regulatory Commission, Washington, DC 20555-0001 or by e ma to 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.

05000-277

3. LER NUMBER

001 0 2016 Unit Conditions Prior to the Event Unit 2 and Unit 3 were both operating in Mode 1 at approximately 100% rated thermal power. There were no structures, systems or components out of service that contributed to this event.

Description of Event

The High Pressure Service Water (HPSW) system (EIIS: BI) is a safety-related system that provides cooling water to four heat exchangers in the Residual Heat Removal (RHR) system (EIIS: BO) during post-accident conditions. The HPSW system consists of four 4,500 gpm pumps that normally draw water from the Conowingo Pond. The system can also be lined up to draw water from the Emergency Cooling Tower (ECT) (EIIS: BS) in the event that suction from the pond becomes unavailable. Water is pumped to the RHR heat exchangers, each of which is located in a watertight room. Water exits the heat exchangers through 14" diameter pipes. Those pipes from the 'A' and 'C' heat exchangers combine into an 18" diameter line, which later combines with an 18" diameter line from the 'B' and 'D' heat exchanger into a 24" diameter line that goes to the plant discharge canal. On the 18" diameter return line from the 'A' and 'C' heat exchangers, there is a pipe connection for a line to the HPSW radiation monitor sample pump.

The line is a 1" diameter Schedule 40S (0.133" wall thickness) Type 304 stainless steel pipe that was installed as part of a 1994 modification. The line is connected to the 18" diameter HPSW line with a socket weld connection to a half-coupling. The 1" pipe has a short span from the half-coupling to a rigid support on the wall, with two small valves in the span.

On 8/16/16, at approximately 0830 hours0.00961 days <br />0.231 hours <br />0.00137 weeks <br />3.15815e-4 months <br />, in PBAPS Unit 2, a 120 drop per minute leak was identified on the 1" pipe to the '2C' HPSW radiation monitor sample pump. Inspection determined that the leak was from a crack in the pipe located at the toe of the weld to the half-coupling on the 18" diameter HPSW return line from the 'A' and 'C' RHR heat exchangers. The crack was approximately 1" long and located between the 11:00 o'clock and 2:00 o'clock positions on the pipe.

Due to the length of the crack, the structural integrity of the pipe was questionable. If the pipe were to shear at the crack location, it was estimated that 77 gpm would flow through the opening into the 'C' RHR room. As a result of the identified condition, the following equipment was declared inoperable at 1500 hours0.0174 days <br />0.417 hours <br />0.00248 weeks <br />5.7075e-4 months <br /> on 8/16/16:

• One low pressure ECCS injection subsystem per TS 3.5.1.A due to flooding potential in the 'C' RHR room

• One RHR suppression pool cooling subsystem per TS 3.6.2.3.A due to flooding potential in the 'C' RHR room

• One RHR suppression pool spray subsystem per TS 3.6.2.4.A due to flooding potential in the 'C' RHR room

• One RHR drywell spray subsystem per TS 3.6.2.5.A due to flooding potential in the 'C' RHR room

• One HPSW subsystem per TS 3.7.1.A due to through-wall leak in ASME Class 3 piping 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 e-ma 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 X 20503. If a means used to impose an 'nformation co lection does not display a currently valid OMB control number, the NRC may nut conduct or sponsor, and a person is not requ'red to respond to, the information collection.

05000-277

3. LER NUMBER

2016 0 001

• The emergency heat sink per TS 3.7.3.6 due to potential for loss of inventory (applies to both Units 2 and 3) The pipe was replaced and the affected equipment was restored to an operable status on 8/18/16 at 0610 hours0.00706 days <br />0.169 hours <br />0.00101 weeks <br />2.32105e-4 months <br />. The elapsed time from the discovery of the failure to when the equipment was returned to service was approximately 45 hours5.208333e-4 days <br />0.0125 hours <br />7.440476e-5 weeks <br />1.71225e-5 months <br /> and 40 minutes.

Analysis of the Event

Initial evaluation of the flaw could not determine if the pipe would have failed during a seismic event.

More detailed modeling of the pipe from its connection on the 18" diameter HPSW line to the first anchor point on the wall determined that seismic stresses could have caused the crack to propagate and result in a complete fracture of the pipe. This conclusion was reached on 9/12/16 and is considered to be the time at which a reportable event was identified. It is unknown at what time the crack developed into the as- found condition, however it is likely that it existed for a period of time greater than the TS required completion time for the systems to be restored to an operable status (7 days for each condition). Since the condition existed for a period of time longer than the TS required completion time, the condition resulted in a condition prohibited by TS and is being reported pursuant to 10CFR 50.73(a)(2)(i)(B).

Cause of the Event

To determine the cause of the crack, the section of the pipe and the socket weld were sent to an off-site lab for failure analysis. The cracking initiated at the toe of the weld on the outer diameter of the pipe and propagated in one general direction through the pipe wall. The fracture surface was examined by scanning electron microscopy and had transgranular features which are typical of fatigue. There were no material anomalies at the fracture origins and the weld appeared to be of generally good quality. The analysis concluded that the leak was caused by fatigue cracking caused by cyclic loading conditions, such as vibration.

The pipe was anchored to the wall a short distance from its connection to the 18" diameter HPSW line.

This design did not adequately account for vibration in the HPSW line, which resulted in bending stresses in the 1" pipe at the toe of the fillet weld connection to the half coupling. In addition, a 2:1 weld was not used with the socket weld. The current standard for fillet welds on small bore piping subject to vibration is to use a weld with a length to depth ratio of 2:1 in order to mitigate stress from vibration. A contributing cause is that the pipe appeared to have been slightly distorted in the downward direction.

Corrective Actions

The pipe was replaced between the 18" HPSW pipe and the first valve. A 2:1 weld was utilized at the socket to mitigate vibration impacts. The configuration of the 1" pipe is used at a total of 16 locations at the plant for the supply and return lines for radiation monitors for the HPSW system. All were visually inspected to determine if similar flaws existed on the other lines. No flaws were identified. Non- comments regarding burden estimate to the FOIA, Pnvacy and Information Collecfions Branch (T-5 F53), U S Nuc'ear Regulatory Commission Washington. DC 20555-0001, or by e-mail to Infocollects Resource@nrc gov and to the Desk Officer, Office of Information and Regulatory Affairs, 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.

05000-277

3. LER NUMBER

001 0 2016 destructive examinations have been scheduled to obtain additional information on the condition of the piping.

Previous Similar Occurrences Since the modification in 1994 that installed the radiation monitors and associated piping, there have been no other documented cases of leakage from this piping.