05000317/LER-2008-001
| Calvert Cliffs Nuclear Power Plant, Unit 1 | |
| Event date: | 02-25-2008 |
|---|---|
| Report date: | 04-25-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3172008001R00 - NRC Website | |
A. PRE-EVENT PLANT CONDITIONS
On February 25, 2008, prior to discovery of the subject condition, Unit 1 was in Mode 5 during a scheduled refueling outage. Unit 2 was operating at 100 percent rated thermal power and was not affected by the subject condition.
B. EVENT
pressurizer heater sleeve C-2. The leakage was detected during the pressurizer heater sleeve bare metal visual exam which is performed every refueling outage. Subsequent lamb wave ultrasonic examination confirmed the absence of a circumferential flaw. The heater sleeve was repaired by installing an approved mechanical nozzle seal assembly (MNSA) clamp at heater sleeve location C-2 on the Calvert Cliffs Unit 1 pressurizer prior to restart of Unit 1 from the 2008 Refueling Outage.
C. INOPERABLE STRUCTURES, COMPONENTS, OR SYSTEMS THAT CONTRIBUTED
TO THE EVENT
There were no inoperable structures, systems, or components that contributed to the condition.
D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:
February 25,. 2008 at 0140: Dry boric acid was detected on pressurizer heater sleeve C-2 when performing bare metal visual exam.
February 25, 2008 at 0200: Entered Technical Requirements Manual (TRM) Technical Normal Condition (TNC) 15.4.3.A for the structural integrity of an ASME Code Class 1 component not within the limit.
February 25, 2008 Day Shift: Bare metal visual exams were completed on all pressurizer heater sleeves. No other occurrences were discovered.
February 26, 2008 at 1633: Lamb wave ultrasonic examination was completed to determine if flaw was circumferential. The Iamb wave exam confirmed the absence of a circumferential flaw reflector.
March 2, 2008 Night Shift: Installation of an approved MNSA clamp was completed on pressurizer heat sleeve C-2 under Maintenance Order 1200800084.
March 3, 2008 at 2328: Exited TRM TNC 15.4.3.A because the structural integrity of the ASME Code Class 1 component was restored.
On February 25, 2008, during Calvert Cliffs Nuclear Power Plant Unit 1 Refueling Outage, dry boric acid was noted on Unit 1 pressurizer heater sleeve C-2. The leakage was detected during the pressurizer heater sleeve bare metal visual exam which is performed every refueling outage per the Alloy 600 Program Plan and the Boric Acid Corrosion Control Program.
G. MAJOR OPERATOR ACTION
Operations entered TRM TNC Contingency Measure 15.4.3.A "Structural integrity of ASME Class 1 component is not within the limit." Operations limited the Reactor Coolant System temperature per the TRM TNC as required to satisfy nil-ductility temperature considerations.
The TRM TNC was exited following the completion of Maintenance Order 1200800084.
H. SAFETY SYSTEM RESPONSES
There were no safety system responses as the unit was in a refueling outage condition and in Mode 5.
II. CAUSE OF EVENT:
The event is NUREG-1022, Revision 2, Cause Code B The apparent cause of the event as determined by Condition Report IRE-029-507 is Primary Water Stress Corrosion Cracking (PWSCC) of the Alloy 600 type material. The susceptibility of Alloy 600 type material to PWSCC is a well known industry issue. Leaks of this nature have occurred at Calvert Cliffs in the past. The Calvert Cliffs Alloy 600 Program Plan and its references thoroughly document the background, history of issues, susceptibility and safety evaluation of the pressurizer heater sleeves.
Extensive non-destructive examination and destructive failure analysis of heater sleeves that leaked in the past have all concluded the failure mechanism to be PWSCC.
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 (44006) 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 5, indicates that the flaw existed in Mode 1 most likely for a period longer than the 6-hour completion time allowed under Condition 3.4.13.B. The failure to recognize and meet the requirements of Technical Specification Condition 3.4.13.B also should have required entry into Technical Specification LCO 3.0.3. 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.
IV. CORRECTIVE ACTIONS:
A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL
STATUS:
An approved MNSA clamp was installed at heater sleeve location C-2 on the Calvert Cliffs Unit 1 pressurizer.
B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE
Preventive Actions Repair/replace the Calvert Cliffs Unit 1 Pressurizer Heater Sleeves by the end of the 2012 refueling outage to eliminate the susceptibility to PWSCC.
V. ADDITIONAL INFORMATION
A. FAILED COMPONENTS:
Pressurizer Heater Sleeve C-2
B. PREVIOUS LERs ON SIMILAR EVENTS
A review of Calvert Cliffs' events over the past several years was performed. Previous LERs on similar events are as follows:
Penetrations due to lntergranular Stress Corrosion Cracking Caused by Residual Fabrication Stress. (Intergranular Stress Corrosion Cracking has been re-termed to PWSCC.) As stated earlier, corrective and preventive actions established as a result of the events discussed in those previous LERs have been incorporated in the Alloy 600 Program Plan. No new corrective or preventive actions were required to address the condition described in this LER.
C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (EIIS) COMPONENT
FUNCTION IDENTIFIER AND SYSTEM NAME OF EACH COMPONENT OR SYSTEM
REFERRED TO IN THIS LER:
IEEE 803 IEEE 805 Component EIIS Function System ID Pressurizer PZR AB Heater EHTR AB Pressure/Level Penetration PEN AB Heater Sleeve SLV AB
D. SPECIAL COMMENTS
None