05000317/LER-2010-003
| Calvert Cliffs Nuclear Power Plant, Unit 1 | |
| Event date: | 05-12-2010 |
|---|---|
| Report date: | 07-08-2010 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 45920 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3172010003R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS
Unit 1 was operating at 100 percent of rated thermal power on May 12, 2010, prior to the subject event.
B. EVENT
On May 12, 2010, at 13:51 eastern daylight time (EDT), Calvert Cliffs Nuclear Power Plant (CCNPP), Unit 1 experienced an automatic reactor trip from 100 percent power. The reactor trip occurred due to a complete load rejection when main turbine generator output breaker (500 kV Breaker 552-22) opened while breakers (552-21 and 552-23) were opened to support planned maintenance. The load rejection caused Reactor Coolant System (RCS) pressure to rise to the Reactor Protective System high pressure setpoint resulting in the reactor trip. All control rods fully inserted, there was no loss of normal heat removal, and there was no impact on Unit 2.
Both power operated relief valves (PORVs) opened as expected during the event since the PORV setpoint is equal to the high pressure reactor trip setpoint. As RCS pressure decreased, both PORVs shut as expected. However, during the subsequent pressurization of the RCS back to normal operating pressure, one of the pressurizer safety valves (1RV-201) leaked. The Unit was taken to Mode 5 to repair the leaking pressurizer safety valve. The Unit was,paralleled to the grid on May 19, 2010.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT
There were no inoperable structures, systems, or components at the time of the trip that contributed to the event.
D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:
The Unit 1 reactor trip occurred on May 12, 2010 at 13:51 EDT due to a complete load reject event.
Unit 1 was cooled down and taken to Mode 5 on May 14, 2010 to repair pressurizer safety valve 1RV-201.
Following repair d1RV-201, Unit 1 heat up commenced on May 17, 2010.
Unit 1 was paralleled to the grid on May 19, 2010 at 04:27 EDT.
O (500 kV) transfer trip circuit. The maintenance required two 500 kV breakers to be open with a third breaker (552-22) remaining closed so as to provide Unit 1 main turbine generator output to the grid. This configuration placed the station in a single breaker trip vulnerable condition for a Unit trip. During performance of the maintenance, 500 kV output breaker (552-22) opened due to DC electrical noise (voltage spiking) that was introduced in its protective relay circuitry due to a loose electrical connection. Specifically, a loose electrical connection on the 125 VDC distribution panel board caused breaker 552-22 to open unexpectedly while closing the disconnect switch to restore 125 VDC power to one of the switchyard breakers, breaker 552-23.
The disconnect switch for breaker 552-23 is physically located adjacent to the disconnect switch for breaker 552-22 in a common switch bucket. Mechanical agitation of the disconnect switch for the breaker caused voltage spikes of sufficient magnitude and duration to provide a trip signal to the digital relay for 552-22 breaker. The loose electrical connection most likely existed since original plant installation of the 125 VDC distribution panel board. Therefore, cause code B "Design, Manufacturing, Construction/Installation" most clearly describes the root cause of the failure since the failure was traced to a defective component.
III. ANALYSIS OF THE EVENT:
This event resulted in a valid actuation of the Reactor Protective System. The actuation was not part of a pre-planned sequence during testing or reactor operation. Therefore, this event is reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A). Immediate notification of this event (Event Number 45920) was made on May 12, 2010 at 15:35 EDT in accordance with 10 CFR 50.72(b)(2)(iv)(B) and 10 CFR 50.72(b)(3)(iv)(A).
The Nuclear Regulatory Commission Performance Indicator for Unplanned Scrams per 7,000 Critical Hours is projected to rise to approximately 1.807 and remains green. No other NRC performance indicators were impacted.
0 The event occurred due to a loose connection in the 125 VDC distribution panelboard located in Calvert Cliffs' Switchyard. The panelboard is a Vacu-Break (V-B) Fusible Panelboard manufactured by I-T-E Imperial Corporation (now Siemens).
Unit 1 was cooled down to Mode 5 to repair a leaking pressurizer safety valve (1RV-201). The subject valve is an American Society of Mechanical Engineers Boiler and Pressure Vessel Code Approved PSV designed to limit RCS pressure to a maximum of 110 percent of design
B. PREVIOUS LERs ON SIMILAR EVENTS
A review of Calvert Cliffs' events over the past several years was performed. No previous occurrences were identified involving a reactor trip due to complete load reject as the result of a loose electrical connection.
C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EllS) COMPONENT
FUNCTION IDENTIFIER AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM
REFERRED TO IN THIS LER:
IEEE 803 IEEE 805 Component EMS Function System ID 125 VDC Disconnect Switch DISC FK 500 kV Breakers (552-21, 552-22 and 552-23) BKR FK Pressurizer Safety Valve (1RV-201) RV AB