05000318/LER-2014-001

LER-2014-001, Reactor Trip Due to Inadequate Protection Against Weather Related Water Intrusion

Calvert Cliffs Nuclear Power Plant, Unit 2
Event date:	01-21-2014
Report date:	03-20-2014
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
Initial Reporting
ENS 49754	10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation
3182014001R00 - NRC Website

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.

- Calvert Cliffs Nuclear Power Plant, Unit 2 05000 318 I NUMBER I NO.

A. PRE-EVENT PLANT CONDITIONS:

Unit 2 was operating at 99.5 percent of rated thermal power on January 21, 2014, prior to the subject event.

B. EVENT:

On January 21, 2014, at 9:25 p.m., Calvert Cliffs Nuclear Power Plant Unit 2 experienced an automatic reactor trip from 99.5 percent power. A ground fault on 13 kV Service Bus 21 feeder breaker 252-2104 resulted in a loss of power to 13 kV Service Bus 21.

At the time of the event, 13 kV Service Bus was providing power to Unit 2 non-safety-related 4 kV Buses 22, 23, 25, and 26, and safety-related 4 kV Bus 24. The loss of power to 13 kV Service Bus 21 resulted in a loss of power to the control element drive mechanism motor generator sets, which caused an automatic reactor trip. The loss of 13 kV Service Bus 21 also resulted in a loss of power to the circulating water pumps, which caused a loss of main condenser vacuum.

The loss of 4 kV Bus 24 resulted in the automatic start of the 2B Emergency Diesel Generator (EDG), which re-powered 4 kV Bus 24. The loss of 4 kV Bus 24 also resulted in 120 volt instrument bus 2Y10 being deenergized, which caused the loss of letdown flow through the Chemical and Volume Control System.

The 13 kV Service Bus 21 deenergized when an air filter on the back of breaker 252-2104 became dislodged from blowing wind and snow and formed a pathway for water to drip on the insulation between the "A" phase bus and a current transformer causing a ground fault.

Following the reactor trip, the unit transitioned into an unscheduled outage.

Unit 1 experienced a reactor trip when 13 kV Service Bus 21 deenergized and caused a turbine control system reboot. Details of that event will be documented in a separate Licensee Event Report (LER 317/2014-001).

TO THE EVENT:

There were no inoperable structures, components, or systems that contributed to the event.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

The reactor trip occurred on January 21, 2014, at 9:25 p.m.

Operators implemented Emergency Operating Procedure (EOP)-0, Post-Trip Immediate Actions, and performed post-trip immediate actions.

Operators entered Technical Specification 3.8.1 condition A for loss of one of two offsite power sources.

At 9:57 p.m., operators implemented EOP-1, Reactor Trip, to perform post-trip recovery actions.

At 11:23 p.m., operators paralleled the 2B EDG to off-site power. Power to 4 kV Bus 24 is now being provided by an alternate feeder.

On January 22 at 0:32 a.m., power was restored to 4 kV Buses 22, 23, 25, and 26.

At 0:33 a.m., the Nuclear Regulatory Commission (NRC) was notified.

At 1:07 a.m., operators implemented Operating Procedure-4, Plant Shutdown from Power Operation to Hot Standby.

On January 24 at 8:47 p.m., 13 kV Service Bus 21 was reenergized.

At 9:25 p.m., operators entered Technical Specification 3.8.1 condition J for failure to meet the 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> requirement to exit Technical Specification 3.8.1 condition A.

At 9:58 p.m., operators exited Technical Specification 3.8.1 conditions A and J for loss of one offsite power source. Total elapsed time of inoperability was 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and 33 minutes.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

Letdown flow in the Chemical and Volume Control System was lost due to the loss of power. It was later restored using EOP-1.

The loss of power to the circulating water pumps resulted in a loss of main condenser vacuum.

Due to the duration of the loss of power, a low pressure turbine rupture disc ruptured as designed due to the elevated main condenser pressure. Heat removal was initially achieved using the Auxiliary Feedwater System and atmospheric dump valves.

F. METHOD OF DISCOVERY:

The event was self-revealing.

G. MAJOR OPERATOR ACTION:

The loss of main condenser vacuum resulted in a loss of normal heat removal for the Reactor Coolant System (RCS). Heat removal was initially achieved using the Auxiliary Feedwater System and atmospheric dump valves.

H. SAFETY SYSTEM RESPONSES:

The Reactor Protective System, Emergency AC Power System, and Auxiliary Feedwater Systems operated as designed. There were no Safety System Functional Failures.

II. CAUSE OF EVENT:

The event is documented in station condition report number CR-2014-000569. The event occurred when a winter storm was in progress that consisted of blowing snow and wind speed of approximately 20 miles per hour. The 13 kV Service Bus 21 is located in an outdoor weatherproof metal structure. Two air filters located on the back of 13 kV Service Bus 21 feeder breaker 252-2104 became dislodged such that a corner of one of the filters bent over, allowing snow to enter through the vent and accumulate. A heater housed within the feeder breaker 252-2104 cubicle melted the snow, allowing water to contact the "A" phase bus and cause a ground fault. This resulted in a resulted in a loss of power to 13 kV Service Bus 21. The root cause was determined to be the outdoor 13 kV metal clad switchgear louver and filter configuration did not provide adequate protection against weather related water intrusion because the weather condition was not anticipated. The contributing cause was a latent deficiency of the switchgear rear door filter housing assembly that went unrecognized.

III. ANALYSIS OF EVENT:

The event resulted in valid actuations of the Reactor Protective System and the 2B EDG. The actuations were not part of a pre-planned sequence during testing or reactor operation.

Therefore, the event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). Immediate notification of the event (Event Number 49754) was made on January 22, 2014 at 00:33 a.m. in accordance with 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A).

Inoperability of one off-site power source occurred at the time the ground fault developed on 13 kV Service Bus 21. The power source remained inoperable for 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and 33 minutes.

During this time, off-site power continued to be provided by 13 kV Service Bus 11.

The NRC Performance Indicator for Unplanned Scrams per 7,000 critical hours is projected to increase to approximately 2.8 and remain green. The NRC Performance Indicator for Unplanned Scrams with Complications is projected to increase to 1 and remain green.

This event did not result in any actual nuclear safety consequences. The loss of non- emergency AC power event is analyzed in Section 14.10 of the Calvert Cliffs' Updated Final Safety Analysis Report. The most limiting LOAC event is a loss of turbine load at hot full power with offsite AC power unavailable. In the event described in Chapter 14.10, there is a loss of RCS flow and atmospheric dump valves. In the January 21, 2014 event, reactor coolant pumps continued to operate and atmospheric dump valves remained available for RCS heat removal.

The event is bounded by the Section 14.10 safety analysis. An estimated conditional core damage probability of 5.2E-06 was calculated for this event.

IV. CORRECTIVE ACTIONS:

A. ACTION TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:

1. The 13 kV Service Bus 21 insulation that was damaged by the ground fault was repaired.

2. The new filter housing with additional bracing support was fabricated and installed on feeder breaker 252-2104.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

1. Temporary covers were installed over the middle (if installed) and upper louvers on the rear of both Unit 1 and Unit 2 13kV outdoor metal clad switchgear doors to minimize the probability for snow to enter into the louvers.

2. Design and install a permanent solution for an exterior housing over metal clad louvers to provide a more robust barrier against the environment.

A. FAILED COMPONENTS:

Outdoor 13 kV metal clad switchgear louver and filter assembly was manufactured by Westinghouse Electric Corporation (EPIX Identification Number W120).

B. PREVIOUS LERs ON SIMILAR EVENTS:

Calvert Cliffs Unit 1 experienced an automatic reactor trip on January 18, 2010, due to water intrusion through a safety-related roof into switchgear protective circuitry. This event was documented in LER 317/2010-001. The associated root cause analysis determined that the station lacked sensitivity to the consequences associated with degraded roof conditions. Corrective actions to prevent recurrence focused on preventive measures and preventive maintenance for roof issues.

Calvert Cliffs Unit 1 experienced an inoperability of the 1A EDG on October 21, 2011, due to water intrusion through a floor drain. This event was documented in LER 317/2011-003. The extent of condition considered safety-related critical equipment and equipment that was exposed to the elements. The event is different than the previous two events in that the 13kV Bus 21 is not safety-related and is located in a weatherproof metal structure that was expected to protect the bus from the elements per its original design.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT

COMPONENT

IEEE 803 IEEE 805

SYSTEM ID

Outdoor 13 kV metal clad switchgear FLT EA louver and filter assembly 13 kV Service Bus 21 BU EA Circuit Breaker 252-2104 BKR EA EDG 2B DG EK

D. SPECIAL COMMENTS:

None