05000317/LER-2007-001
| Calvert Cliffs Nuclear Power Plant | |
| Event date: | 01-17-2007 |
|---|---|
| Report date: | 03-19-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3172007001R00 - NRC Website | |
I. DESCRIPTION OF EVENT
Results from a hydrostatic pressure test performed in January 2007 on the Pump Cover Heat Exchanger (HX), previously installed on the 11B Reactor Coolant Pump (RCP), indicates that a Reactor Coolant System (RCS) pressure boundary leak existed when the component was installed in Unit 1. The HX was removed during the 2006 Unit 1 refueling outage (RFO) as a corrective action to address failures of the lower seal stage and the low level of radioactivity detected in the Unit 1 Component Cooling Water (CCW) system. The low level of radioactivity (indicative of RCS leakage) was documented in September 2004. The RCS leak rate was so small (approximately 0.016 gallons per day) that the exact location could not be identified. The leakage was considered unidentified RCS operational leakage, and was less than the one gallon per minute Technical Specification allowable. The Technical Specifications limit RCS pressure boundary leakage to zero. The RCP pump cover HX was identified as one of the potential sources of the leak. Component cooling water flows through several HXs in and on the pump cover to provide cooling to the mechanical seal. Chemistry tests performed on the CCW system subsequent to startup from the 2006 RFO indicated that the active RCS leak was no longer present. The hydrostatic pressure test results, and the fact that the RCS leakage stopped after the 11B RCP HX was replaced, confirm that an RCS pressure boundary leak existed.
II. CAUSE OF EVENT
Low levels of radioactivity detected in the Unit 1 CCW system slightly increased between June and September 2004. Plant Chemistry tracked the increasing trend and initiated Condition Report Number IR4-025-347 on September 10, 2004. At that point, the activity level was below the target level, but still indicated an increasing trend. A feed and bleed of the CCW system had been initiated on September 3, 2004 to reduce the activity level; however, the level continued to increase and eventually exceeded the station's self-imposed administrative target value on September 22, 2004. Activity levels in the CCW system returned to near target value with additional feeding and bleeding of the system. A complex troubleshooting plan was developed to establish non-routine sampling and testing of the possible leaking systems containing components that interface between the CCW system and radioactive fluids. Assuming a continuous leak, the leak rate was calculated to be approximately 0.016 gallons per day (approximately 0.04 ml/min).
This very small leak rate made identification of the exact leak location extremely difficult using traditional sampling methods.
A detailed assessment identified the Unit 1 RCS sample cooler, the Chemical and Volume Control System letdown HX and the RCP HXs, as the most likely sources of in-leakage into the CCW system. The Unit 1 RCS sample cooler was replaced on line in March 2005 with no resulting change in leak rate. The remaining components could not be isolated at power. The 11B RCP Pump Cover HX was removed during the 2006 RFO and shipped offsite for testing. Following startup from the 2006 RFO, Chemistry testing confirmed that the short-lived fission products were no longer present in the CCW system. This indicates that an active leak of RCS into the CCW NRC FORM 366AU.S. NUCLEAR REGULATORY COMMISSION (1-2001) CALVERT CLIFFS, UNIT 1 05000 317 2007 03-� � 001 -�� 00 p 0 004 system was no longer present. The 11B RCP Pump Cover HX removed during the 2006 RFO failed its as-found hydrostatic pressure test during overhaul activities performed at an offsite facility (Reference Condition Report Number IRE-019-549). During the test, hydrostatic pressure was applied to the CCW side of the HX at 44 psig. After 30 minutes, pressure had dropped to 42 psig. The vendor's specification for a successful test is a pressure drop of less than 0.5 psig over 30 minutes. The failure mechanism of the HX is unknown. The exact location of the leak was not determined. The leak could be in the tube-in-tube HX or in the drilled port HX. Either location would constitute an RCS pressure boundary failure. The hydrostatic pressure test results, and the fact that the RCS leakage stopped after the 11B RCP HX was replaced, confirm that an RCS pressure boundary leak existed.
III. ANALYSIS OF EVENT
The Technical Specification for RCS operational leakage (Technical Specification 3.4.13.a) limits pressure boundary leakage to zero. If any RCS operational pressure boundary leakage exists, the Technical Specifications require the operating Unit to be in Mode 3 within six hours and to be in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. The results from the hydrostatic pressure test confirm that the activity detected in the CCW system in 2004 was due to a pressure boundary leak in the RCS.
Specifically, the leakage was determined to be in the 11B RCP Pump Cover HX. The condition existed from June 2004 until the RCP was removed during the spring 2006 RFO. The RCS pressure boundary leakage condition existed for a time longer than allowed by the Technical Specification. Therefore, this condition is reportable pursuant to 10 CFR 50.73 (a)(2)(i)(B).
An examination of the failed component to determine the exact location of the leak was not performed. Therefore, the actual failure mechanism of the pump cover heat exchanger is unknown. However, the design of Calvert Cliffs' RCP pump covers is such that thermal cracking can occur which may propagate to the drilled port heat exchanger in the thermal barrier region.
Since the failure may have occurred due to a material problem that resulted in abnormal degradation of a principal safety barrier (i.e., it was necessary to take corrective actions to restore the barrier's capability), this event is also reportable pursuant to 10 CFR 50.73(a)(2)(ii)(A).
This event did not result in any actual nuclear safety consequences. The monitored CCW system radioactivity level was maintained near the threshold value for the duration that the condition existed. The CCW system activity level and leak rate were monitored on a weekly basis. The licensee performed an interim corrective action to "feed and bleed" inventory to and from the system, thus the monitored activity level in the CCW system was maintained at approximately 1.2 E-5 micro Curies per milliliter. At that level, the system did not present a safety hazard to plant personnel or to the environment. The monitored leak rate remained relatively constant at approximately 0.016 gallons per day for the known duration of the condition.
From a qualitative perspective, there was no significant increase in core damage frequency or large early release frequency due to the small thermal barrier leak. The leak was extremely small (0.016 gallons per day) for the duration that the condition existed. As such, there is no indication that the rupture likelihood of the leak should be increased. If the leakage rate had increased, then it is likely that a plant shutdown would ensue prior to the leakage reaching the level where the event would be considered an initiating event.
IV. CORRECTIVE ACTIONS
A. A complex troubleshooting plan was developed and implemented to identify possible sources of the activity detected in the CCW system.
B. A feed and bleed of the CCW system was performed to minimize CCW system contamination levels and to maintain the activity level near the target value.
C. The 11B RCP Pump Cover HX was removed and replaced during the 2006 RFO.
D. Chemistry samples performed subsequent to removal of the 11B RCP Pump Cover HX verified that an active leak of RCS into the Unit 1 CCW system no longer exists.
V. ADDITIONAL INFORMATION
A. Component Identification � IEEE 803 IEEE 805 � Component EIIS Function System ID � 11B RCP Pump Cover HX HX� AB
B. Previous Occurrences
No previous Calvert Cliffs occurrences were identified involving an RCS pressure boundary leak due to failure of an RCP pump cover heat exchanger.