05000318/LER-2013-001
| Calvert Cliffs Nuclear Power Plant, Unit 2 | |
| Event date: | 02-17-2013 |
|---|---|
| Report date: | 04-18-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| Initial Reporting | |
| ENS 48763 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown, 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 3182013001R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS:
Unit 2 was in Mode 3 (Hot Standby) when the condition was discovered.
B. EVENT:
On February 17, 2013, while Unit 2 was in Mode 3 during a refueling outage, a pinhole leak was identified at the upper packing leakoff line cap seal weld of valve 2CV-100F (pressurizer spray valve), which constituted a Reactor Coolant System (RCS) pressure boundary leak.
The Technical Specifications limit RCS pressure boundary leakage to zero. The leak was identified during the Mode 3 boric acid walkdown. The leakoff line was located below the valve packing. Therefore, per Technical Specification definition, the pinhole leak constituted an RCS pressure boundary leak.
Based on visual inspection performed during the boric acid walkdown, the leak most likely existed during plant operation. Based on a review of the maintenance history and discussions with experienced Calvert Cliffs' personnel, the most likely cause of the pinhole was a latent weld defect created when performing the cap seal weld. The valve bonnet assembly, which includes refueling outage.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT:
There were no inoperable structures, components, or systems that contributed to the event.
D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:
02/17/2013 At 1345, during Mode 3 boric acid walkdown per the boric acid corrosion control program, an active boric acid leak was identified at valve 2CV-100F.
02/17/2013 02/17/2013 03/13/2013 03/23/2013 At 1551, event number 48763 immediate (four and eight hour) notification report submitted pursuant to 10 CFR 50.72(b)(3)(ii)(A) (degraded condition) and 10 CFR 50.72(b)(2)(i) (Plant Shutdown Required by Technical Specifications).
At 2259, Unit 2 entered Mode 5 and exited the Technical Specification action statement.
Replaced valve bonnet assembly.
At normal operating temperature and pressure, verified no leakage existed on the valve upper packing leakoff line.
Visual inspection during the Mode 3 boric acid walkdown identified an active boric acid leak at the valve. The pinhole was located at the packing leakoff line cap seal weld.
G. MAJOR OPERATOR ACTION:
There were no major operator actions. At the time of discovery, the unit was shut down for a refueling outage.
H. SAFETY SYSTEM RESPONSES:
No safety system responses were expected. None occurred.
II. CAUSE OF EVENT:
The event is documented in station condition report number CR-2013-001245. Based on boric acid walkdown visual inspection data, the leak most likely existed during plant operation. Based on discussions with Calvert Cliffs' personnel, the most likely cause of the pinhole was a latent weld defect created when performing the cap seal weld.
Ill. ANALYSIS OF EVENT:
The Technical Specifications for RCS operational leakage (Technical Specification Limiting Condition for Operation 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 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> 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 />. Based on review of boric acid walkdown data the leak most likely existed during operation 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).
As stated earlier, the failure was most likely due to a latent weld defect created during the installation of the cap seal weld. 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).
boundary leakage was contained within the Containment Building and did not interface with any other systems. Unidentified RCS pressure boundary leakage remained within Technical Specification limits during the time the pressure boundary leak may have existed (i.e., during the period of applicability after startup from the spring 2011 Refueling Outage, until the leak was repaired in March 2013, approximately two years maximum).
Due to the leak rate of approximately one drop per minute that was noted when the leak was identified, there was no impact on the Reactor Coolant System Leakage performance indicator.
The performance indicator remained green.
From a qualitative perspective, there was no significant increase in core damage frequency or large early release frequency due to the RCS pressure boundary leak. The leak was extremely small for the duration that the condition existed. The leakoff line hole size is within the capacity of the charging pumps.
If additional information is subsequently developed that would significantly affect the understanding of this event, a supplemental licensee event report will be submitted.
IV. CORRECTIVE ACTIONS:
A. ACTION TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:
1. The valve bonnet assembly, which includes the packing leakoff line, was replaced.
B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:
1. We plan to delete the valve leakoff lines from pressurizer spray valve bonnet assemblies.
V. ADDITIONAL INFORMATION:
A. FAILED COMPONENTS:
2CV-100F pressurizer spray valve.
B. PREVIOUS LERS ON SIMILAR EVENTS:
A review of Calvert Cliffs' events over the past several years was performed. One previous Calvert Cliffs occurrence was identified involving an RCS pressure boundary leak due to failure of a valve leakoff line weld. This was reported to the NRC in 2010 in Licensee Event Report 318/2010-002. The subject valve was a manual valve of a different design than 2CV-100F. Following the 2010 event, all required inspections were performed. The leak on 2CV-100F did not exist during a subsequent inservice inspection walkdown in 2011.