05000318/LER-2005-001
| Calvert Cliffs Nuclear Power Plant, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded |
| 3182005001R00 - NRC Website | |
I.� DESCRIPTION OF EVENT On February 24, 2005, with Unit 2 in Mode 5, ultrasonic (UT) inspections of the Reactor Coolant System (RCS) Alloy 600 piping penetrations were performed in accordance with the Calvert Cliffs In-Service Inspection (ISI) program. As a result of the UT inspections, two RCS nozzles were identified with indications of flaws requiring disposition. No through wall leakage was detected during the inspections. Both nozzles were determined to have a flaw or flaws that could not be found acceptable under American Society of Mechanical Engineers (ASME) Section XI, IWB-3600.
The RCS nozzles requiring repair were the two inch diameter 21 Hot Leg Drain Line and 22A Cold Leg Letdown Line. These are carbon steel nozzles buttered using Alloy 182/82 and a stainless steel safe-end was welded to the buttered nozzle using Alloy 182/82. The 21 Hot Leg Drain Line Nozzle UT identified one circumferential indication and two axial indications in the dissimilar weld regions that required further disposition. An engineering evaluation of this nozzle was completed on February 26, 2005, indicating the circumferential indication was acceptable under ASME Section XI standard, while the axial indications were not. The results of this evaluation indicated repair would be required prior to returning Unit 2 to service. The 22A Cold Leg Letdown Nozzle UT identified one axial indication that required disposition. Evaluation of this nozzle on February 27, 2005, determined that this indication could not be accurately sized in accordance with ASME Section XI requirements and repair was also required prior to returning Unit 2 to service. Although the nozzles required repair prior to restart, the indications did not represent an immediate threat to the structural integrity of the RCS. The condition posed no threat to operation and was not a failure of any structure, system, or component important to safety.
Eight-hour non-emergency Emergency Notification System notifications were made, in accordance with 10 CFR 50.72(b)(3)(ii), for each nozzle affected. A subsequent evaluation determined there is a reasonable expectation for continued operability for the Unit 1 Alloy 600 dissimilar metal welds. This evaluation credited the ongoing inspection program for dissimilar metal welds and the site's ISI Program, which ensures surface and/or volumetric examination of affected welds on a specified basis. By following industry recommendations, the boric acid inspection program, the risk informed ISI program, and the site Alloy 600 program, Calvert Cliffs Nuclear Power Plant addresses the short and long-term susceptibility of all RCS Alloy 600 components.
A weld overlay repair technique (using Alloy 52M which is not susceptible to primary water stress corrosion cracking) was employed to restore each nozzle to ASME Section XI compliance. Code relief was requested from the Nuclear Regulatory Commission (NRC) to allow final disposition of the resulting configuration. This relief request was verbally approved by the NRC via teleconference on March 10, 2005, prior to the restart of Unit 2.
II. CAUSE OF EVENT
The root cause of the 21 Hot Leg Drain Nozzle (circumferential indication) was an original fabrication defect, with no evidence of having grown in service. This indication was determined to be acceptable under ASME Section XI standards. The two axial indications on the 21 Hot Leg Drain Nozzle were caused by primary water stress corrosion cracking.
The 22A Cold Leg Letdown Nozzle axial indication was not found to be a degraded condition, but was instead considered a very local ultrasonic anomaly. Repair was conservatively performed due to the inability to successfully size the anomaly within ASME Section XI Code requirements.
III. ANALYSIS OF EVENT
This event is reportable in accordance with the following:
10 CFR 50.73(a)(2)(ii)(A): "Any event or condition that resulted in the condition of the nuclear power plant, including its principal safety barriers being seriously degraded.
NUREG-1022, Revision 2, Section 3.2.4, includes, as examples of seriously degraded conditions, welding defects in the primary coolant system that cannot be found acceptable under ASME Section XI standards. The subject axial indications were evaluated and found unacceptable under ASME Section XI, IWB-3600, "Analytical Evaluation of Flaws." Based on NUREG-1022, Revision 2, guidance, this represents a seriously degraded condition of the primary coolant system pressure boundary. Reporting under 10 CFR 50.73(a)(2)(ii)(A) is therefore required.
This condition had no actual safety consequences because no failures occurred and the structural integrity of the RCS pressure boundary was not compromised. The repairs to the nozzles were performed to ensure continued integrity through future operating cycles.
IV. CORRECTIVE ACTIONS
A. The affected RCS nozzles were repaired with a weld overlay repair technique that met ASME Section XI requirements, as modified by an NRC approved code relief. Ultrasonic examinations were performed after repairs verifying compliance.
B. Five additional UT examinations were performed, in accordance with ASME Section XI guidelines, upon discovery of flaw indications in the original sample of nozzles. No indications were identified.
V. ADDITIONAL INFORMATION
A. Component Identification IEEE 803 IEEE 805 Component EIIS Function System ID RCS Nozzle NZL AB
B. Previous Occurrences
No other previous similar events have occurred within the past three years.