05000281/LER-2016-001

LER-2016-001, Unit 2 Reactor Trip due to Generator Differential Lockout

Surry Power Station, Unit 2
Event date:	10-09-2016
Report date:	12-08-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
2812016001R00 - NRC Website

LER 16-001-00 for Surry Power Station, Unit 2, Regarding Reactor Trip Due to Generator Differential Lockout

ML16350A101
Person / Time
Site:	Surry
Issue date:	12/02/2016
From:	Lane N L Virginia Electric & Power Co (VEPCO)
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
16-419 LER 16-001-00
Download: ML16350A101 (4)

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, 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.

1.0 DESCRIPTION OF THE EVENT On October 9, 2016 at 0254 hours0.00294 days <br />0.0706 hours <br />4.199735e-4 weeks <br />9.6647e-5 months <br />, with Unit 1 and Unit 2 at 100 percent power, Unit 2 experienced an automatic reactor trip initiated by a turbine trip due to generator differential lockout relay actuation. At the time of the trip, high wind and heavy rain conditions existed due to the effects of Hurricane Matthew. All three auxiliary feedwater (AFW) pumps [EIIS-BA-P] automatically started on low-low steam generator (SG) [EIIS-AB-SG] water level providing flow to the SGs as expected. Operation of the main steam power operated relief valves (PORVs) [EIIS-SB-RV] and the main steam dump valves [EIIS-SB-TCV], coincident with the initiation of AFW, resulted in the Unit 2 reactor coolant system (RCS) temporarily cooling to 542 degrees F. RCS temperature was restored to nominal average temperature of 547 degrees,,Fat 0320 hours0.0037 days <br />0.0889 hours <br />5.291005e-4 weeks <br />1.2176e-4 months <br />. Main feedwater flow was re-established to control SG level, and AFW was secured at 0336 hours0.00389 days <br />0.0933 hours <br />5.555556e-4 weeks <br />1.27848e-4 months <br />.

At 0609 hours0.00705 days <br />0.169 hours <br />0.00101 weeks <br />2.317245e-4 months <br />, a four-hour report to the NRC was made pursuant to 10CFR50.72(b)(2)(iv)(B) due to valid automatic actuation of Reactor Protection Systems and an eight-hour report was made pursuant to 10CFR50.72(b)(3)(iv)(A) due to automatic actuation of the AFW system.

2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event resulted in no safety consequences or implications. There were no testing or surveillance procedures in progress when the reactor trip occurred. Appropriate operator actions were taken in accordance with station procedures, and the unit was brought to a stable condition. Station equipment relied upon to mitigate the event responded as designed. Normal offsite and emergency power supplies were available during the event. The health and safety of the public were not affected.

3.0 CAUSE The 22 KV electric output of the main generator is sent to the Main Transformers and the Station Service Transformers (SST) [EIIS-EA-XFMR] through the Isolated Phase Bus Duct (IPBD) [EIIS-EA-IPBU] System.

The current Unit 2 IPBD system and SST leads termination enclosures were installed by design changes in 2011 and 2015, respectively. As part of the October 2016 post-trip inspection, water was found in two leads termination enclosures and in some IPBD horizontal runs. Water appears to have entered the system during ongoing heavy rain and wind conditions through gaps at mechanical joints associated with the SST leads termination enclosure and the IPBD.

The direct cause of the reactor trip was a ground of the 'A' phase on the high side (22 KV) of the 'A' SST due to water accumulation in the 'A' SST leads box, resulting in the actuation of the generator differential lockout relay. Without a drain path, the water level in the enclosure increased up the insulator creating a path to ground.

The root cause for the water intrusion and the resulting trip was the lack of testing and inspection of mechanical joints to ensure a weather tight system was accomplished after modifications. The design change specifications included provisions to ensure weather tightness, including detailed instructions for testing of welds and an array of electrical tests. However, no guidance was provided for inspection or testing of mechanical connections for the bus duct insulator hardware and gaskets, or for the termination enclosures.

4.0 IMMEDIATE CORRECTIVE ACTIONS Following the reactor trip, control room operators acted promptly to stabilize the unit in a safe, hot shutdown condition in accordance with station procedures.

5.0 ADDITIONAL CORRECTIVE ACTIONS A root cause evaluation (RCE) team was assembled to determine the cause of this event and to recommend corrective actions. While Unit 2 was shut down, water was removed from the IPBD and leads termination enclosures, and breaches that allowed water intrusion were sealed. Additionally, the 'A' phase SST and IPBD systems were dried using forced dehumidified air. Weep holes were installed at low points in the SST leads termination enclosures and Generator Step-Up transformer (GSU) [El IS-EL-XFMR] leads termination enclosures to allow drainage of potential moisture intrusion. All three phases of the IPBD system were meggered to verify no further grounds existed.

6.0 ACTIONS TO PREVENT RECURRENCE Weep holes installed in the transformer termination enclosures as described in section 5.0 above will prevent the possibility of creating a ground path by water accumulation. Design change testing procedures will be revised to include more rigorous instructions for non-safety design changes and common tests for electrical enclosures. Engineering qualification requirements will be updated to ensureAhat engineers are trained on lessons learned from this event. Additionally, upcoming design changes associatedmith Reserve Station Service Transformers (RSST) [EIIS-EA-XFMR] will be reviewed and revised as neededAo capture lessons learned from this event.

7.0 SIMILAR EVENTS None 8.0 MANUFACTURER/MODEL NUMBER AZZ — Calvert Isolated Phase Bus / 7085 9.0 ADDITIONAL INFORMATION Unit 1 was at 100 percent power at the time of this event and was unaffected by the Unit 2 reactor trip. Unit 1 entered a refueling outage on October 23, 2016. The corresponding Unit 1 IPBD and SST leads termination enclosures were inspected for the accumulation of water. Water was identified on the low side (4 KV) of the 'B' SST leads termination enclosure on Unit 1. As in Unit 2, weep holes were installed on the Unit 1 SST and GSU, leads termination enclosures, and connections were inspected and sealed as necessary. Other transformer terminations (including those associated with emergency power supplies) are not enclosed by bus ducts, and therefore are not affected by this event.