05000318/LER-2013-003
| Calvert Cliffs Nuclear Power Plant, Unit 2 | |
| Event date: | 05-08-2013 |
|---|---|
| Report date: | 07-03-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 49012 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3182013003R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS:
Unit 2 was operating at 99.5 percent of rated thermal power on May 8, 2013.
B. EVENT:
experienced an automatic reactor trip from 99.5 percent power on a high pressurizer pressure signal from the Reactor Protective System. The high pressure condition was caused by a loss of load event and subsequent Reactor Coolant System heat-up when some or all of the main turbine governor valves, intercept valves, and throttle valves closed.
As a result of the loss of load event, Reactor Coolant System pressure rose to approximately 2396 psia causing the two power-operated relief valves to open briefly as designed. All control element assemblies fully inserted as expected. Following the reactor trip, the main turbine automatically tripped. Containment atmosphere parameters were unaffected by the trip.
Radiation levels were not affected by the trip.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT:
There were no inoperable structures, components, or systems at the time of the trip that contributed to the event.
D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:
The reactor trip occurred on May 8, 2013, at 2147. Operators implemented Emergency Operating Procedure (EOP)-0, Post-trip Immediate Actions.
At 2159, exited EOP-0 and implemented EOP-1, Reactor Trip, for an uncomplicated trip. All safety functions were met.
At 2254, exited EOP-1 and implemented Operating Procedure-4, Plant Shutdown from Power Operation to Hot Standby.
Inspections were performed on the affected equipment, including troubleshooting, circuit checks, calibrations of turbine control circuitry, and replacement of selected circuit cards in the Overspeed Protection Control circuit. The reactor was taken critical on May 11, 2013, at 1158.
On May 12, 2013, at 0134, the unit was paralleled to the grid.
On May 13, 2013, at 0330, the unit was returned to 99.5 percent power.
Based on troubleshooting, the overspeed protection circuitry was initially identified as the most likely cause of the event. To verify, four circuit cards from the overspeed protection control circuitry were removed and sent for laboratory analysis and testing. Two of the cards were High Threshold Logic (HTL) Latch 1 boards and two were HTL Gate 2 boards. The laboratory analysis of the removed circuit cards found one failed logic gate, on the HTL Gate 2 boards.
The root cause team reviewed the logic diagrams for the turbine control system and determined that the failed gate was not a factor in the creation of a false close signal to the control valves.
Therefore, the root cause was not identified. However, the most probable cause of the event was determined to be, an intermittent failure of a component or signal path in main turbine control cabinet 2T11 resulted in a close signal to the governor valves only, governor and intercept valves, or throttle valves only, initiating the event.
III. ANALYSIS OF 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 49012) was made on May 9, 2013, at 0009 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 0.9 and remain green. No other performance indicators were impacted.
event is analyzed in Section 14.5 of the Calvert Cliffs' Updated Final Safety Analysis Report.
The most limiting loss of load event is initiated at hot full power with no credit allowed for operation of the steam dump and bypass system and power-operated relief valves. In this event, those systems operated as designed. The event is bounded by the Section 14.5 safety analysis.
An estimated conditional core damage probability of 1.03E-06 and an estimated conditional large early release probability of 9.30E-08 were calculated for this event.
IV. CORRECTIVE ACTIONS:
A. ACTION TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:
1. Four circuit cards of eight in the overspeed protection circuitry were replaced.
2. Installed test equipment in the main control cabinet and connected recording devices to monitor control system signals.
B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:
1. Develop and implement a project plan to replace selected turbine control circuit cards and/or replace the main turbine control system based upon established bridging strategies.
V. ADDITIONAL INFORMATION:
A. FAILED COMPONENTS:
No failed components were identified.
B. PREVIOUS LERS ON SIMILAR EVENTS:
At Calvert Cliffs, there have been no recent similar events involving a reactor trip caused by the failure of the turbine control system.