05000317/LER-2012-003
| Calvert Cliffs Nuclear Power Plant, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 3172012003R01 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS:
On August 12, 2012, Unit 1 was operating at 100 percent rated thermal power. There were no inoperable systems, structures, or components that would have impacted this event.
B. EVENT:
On August 12, 2012, a condition was identified that resulted in a plant shutdown required by Technical Specifications. At 1234, shutdown group Control Element Assembly (CEA) 9 dropped to the fully inserted position. At the time of the event, no planned CEA motions were in progress. Operators stabilized reactor power at 87 percent and entered the following Technical Specification Action Statements:
3.1.4.B for CEA alignment 3.1.5.B for one shutdown CEA withdrawn less than 121.5 inches 3.2.4.A for azimuthal power tilt 3.4.1.A for Reactor Coolant System pressure, temperature, and flow departure from nucleate boiling.
Operators discovered that the Control Element Drive Mechanism (CEDM) power supply breaker for CEA 9 had tripped open. In an attempt to recover the dropped CEA, one attempt was made to reclose the breaker. Additional troubleshooting revealed low resistance in the upper gripper coil, indicating an internal short circuit. It was determined that the CEA alignment could not be restored within the required completion time and operators entered Action Statement 3.1.4.F, which thereafter, required the unit to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />. Operators began the Technical Specification required unit shutdown at 1425. Unit 1 entered Mode 3 at 1959 on August 12, 2012 and Mode 5 at 1257 on August 13, 2012. Following replacement of the failed coil, the unit was returned to full power at 1630 on August 18, 2012.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT:
The dropped CEA resulted in entering applicable Technical Specification Action Statements.
Operators then entered Action Statements 3.1.4.F and 3.1.5.0 when it was determined the CEA alignment could not be restored. This resulted in a Technical Specification required unit shutdown for repairs.
Following repairs, Operators began unit startup and returned the unit to full power on August 18, 2012.
indicating an internal short circuit in the coil. The average resistance value for all other CEDM upper gripper coils was 7.2 ohms. The internal short circuit was most likely caused by overheating. The most probable cause is degradation in the CEDM cooling system. A contributing cause is potential low margin in the CEDM cooling system.
III. ANALYSIS OF EVENT:
This event is reportable in accordance with the following:
10 CFR 50.73(a)(2)(i)(A) - The completion of any nuclear plant shutdown required by the plant's Technical Specifications.
Operators completed the Technical Specification required shutdown on August 12, 2012 which meant it is reportable under 10 CFR 50.73(a)(2)(i)(A).
This event did not result in any actual nuclear safety consequences. The CEA drop event is analyzed in Section 14.11 of the Calvert Cliffs' Updated Final Safety Analysis Report. The most limiting CEA drop event is an uncontrolled CEA insertion at hot full power. The event is bounded by the Section 14.11 safety analysis.
The risk associated with the CEA drop event is considered to have a relatively low impact. While the probabilistic risk assessment does not model the failure likelihood of a single dropped CEA, the failure could be a precursor to a plant trip. A plant trip has a Conditional Core Damage Probability of 6.6E-07 and a Conditional Large Release Probability of 2.8E-08. A dropped CEA is considered to be within the bounds of a plant trip. As a result, the risk associated with this event is considered small and is not significant from a probabilistic risk assessment perspective.
This event has limited impact on the Nuclear Regulatory Commission Reactor Oversight Process Performance Indicators for Unit 1. The performance indicator for Unplanned Power Changes per 7000 Critical Hours is projected to rise by 1 and remain low in the green band.
IV. CORRECTIVE ACTIONS:
A. ACTION TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:
The affected CEDM coil stack was replaced.