05000318/LER-2011-001
| Calvert Cliffs Nuclear Power Plant, Unit 2 | |
| Event date: | 02-17-2011 |
|---|---|
| Report date: | 04-15-2011 |
| 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 46623 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 3182011001R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS
Unit 2 was in Mode 6 refueling when the condition was discovered.
B. EVENT
On February 17, 2011 during a refueling outage, dry boric acid was noted on Unit 2 pressurizer heater N3 outer sleeve to weld pad J-Groove weld. The leakage was detected during the pressurizer heater sleeve bare metal visual exam which is performed every refueling outage.
Non-destructive examination (NDE) and destructive examination confirmed weld flaws at the pressurizer heater N3 location. This pressurizer heater penetration was repaired using an American Society of Mechanical Engineers (ASME) Code approved welded plug.
Based on visual examination performed during the boric acid walkdown and chemical analysis of the white crystalline substance, the leak most likely existed for some time during plant operation over the past operating cycle. The NDE at the pressurizer heater N3 location indicated weld flaws with further destructive examination indicating the root cause of leakage to be primary water stress corrosion cracking (PWSCC).
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
02/17/2011 At approximately 2230, during Mode 6 inservice inspection to satisfy ASME Code Case N-722 detected dry boric acid deposits at the pressurizer heater N3 location.
02/18/2011 At 0300, Event number 46623 immediate (eight hour) notification report submitted pursuant to 10 CFR 50.72(b)(3)(ii)(A) (degraded condition).
02/19/2011 Non-destructive and destructive examination performed to characterize the location, orientation, and length of cracks as required by 10 CFR 50.55a(g)(6)(ii)(E)(2) and determine as-found condition, scope of condition, possible failure mechanism, and engineering determination of best repair method.
03/03/2011 Pressurizer heater penetration N3 repair completed.
Measure 15.4.3.A "Structural integrity of ASME Class 1 component(s) is not within the limit.
Operations exited Technical Requirements Manual Technical Normal Conditions Contingency Measure 15.4.3.A at 0438 on March 8, 2011.
H. SAFETY SYSTEM RESPONSES
No safety system responses were expected. None occurred.
II. CAUSE OF EVENT
The event is NUREG-1022, Revision 2, Cause Code B, "Design, Manufacturing, Construction/Installation.
The root cause of the event as determined by Condition Report (CR)-2011-001754 is PWSCC of the Alloy 82 type weld material. The susceptibility of Alloy 82 type weld material to PWSCC is a known industry issue. The Calvert Cliffs Alloy 600 Program Plan and its references document the background, history of issues, susceptibility and evaluation of the pressurizer heater sleeves. This is the first pressurizer heater outer sleeve to weld pad J-Groove weld leak at Calvert Cliffs.
III. ANALYSIS OF THE EVENT
The subject condition satisfies the criteria in NUREG-1022, Revision 2, for principal safety barriers of the nuclear power plant being seriously degraded. Therefore, this event is reportable pursuant to 10 CFR 50.73(a)(2)(ii)(A). An immediate event notification report (46623) was also made pursuant to 10 CFR 50.72(b)(3)(ii)(A). Calvert Cliffs Nuclear Power Plant Technical Specification Limiting Condition for Operation (LCO) 3.4.13, Reactor Coolant System Operational leakage allows no pressure boundary leakage while in Modes 1 through 4. The discovery of pressure boundary leakage, although in Mode 6, indicates that the leak existed in Mode 1 most likely for a period longer than the 6-hour completion time allowed under Specification Condition 3.4.13.B also should have required entry into Technical Specification LCO 3.0.3 if identified in Modes 1 through 4. Therefore, this condition is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B) as a condition prohibited by the plant's Technical Specifications.
This event did not result in any actual nuclear safety consequences. Due to the low probability for significant leakage, the annual risk impact due to this condition is less than 1E-06 in core damage frequency and 1E-07 large early release frequency. Analysis confirmed that ASME Code allowable stress limits were satisfied for all design basis conditions.
IV. CORRECTIVE ACTIONS
A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL
STATUS
An ASME Code approved welded plug was installed at the N3 location on the Calvert Cliffs Unit 2 pressurizer. Visual examination and surface examination (dye penetrant test) were performed satisfactorily. Post-maintenance pressure test during Mode 3 at normal operating pressure and temperature was performed satisfactorily.
B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE
The associated root cause analysis has not been completed at this time. Corrective actions required by the inservice inspection program have been completed. Any additional corrective actions taken as a result of this event will be implemented in accordance with our corrective action program and incorporated in the inservice inspection program as required. If information is subsequently developed, that would significantly affect a reader's understanding or perception of this event, a supplemental Licensee Event Report (LER) will be submitted.
V. ADDITIONAL INFORMATION
A. FAILED COMPONENTS
Pressurizer heater penetration N3.
B. PREVIOUS LERs on SIMILAR EVENTS A review of Calvert Cliffs' events over the past several years was performed. A previous LER on a similar event is provided: