05000302/LER-2008-004
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | 12-05-2008 |
|---|---|
| Report date: | 01-28-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3022008004R00 - NRC Website | |
At 02:41, on December 5, 2008, Progress Energy Florida, Inc. (PEF), Crystal River Unit 3 (CR-3) was operating in MODE 1 (POWER OPERATION) at approximately 50 percent RATED THERMAL POWER when the breaker [ED, BKR] for A Train motor-operated Main Feedwater Isolation Valve (MFIV) [SJ, ISV] (FWV-30) motor [SJ, MO] tripped soon after receiving a close signal. This occurred during a scheduled power reduction to replace the Main Feedwater Pump FWP-2B [SJ, P] controller [SJ, SC]. At the time, CR-3 power was being reduced in accordance with Operating Procedure OP-204, "Power Operations.
The MFIVs for each Once Through Steam Generator (OTSG) [AB, SG] consist of the Main FW pump suction valve, the main/startup/low load block valves (in parallel) and the Main FW pump discharge cross-connect valve. FWV-30 is the A Train main block valve.
Improved Technical Specification (ITS) 3.7.3, "Main Feedwater Isolation Valves [MFIVs]," requires two MFIVs in each FW System flow path to be operable with at least one MFIV capable of isolating FW within the required isolation time while in MODES 1, 2 and 3. With FWV-30 inoperable, the Required Actions of ITS 3.7.3 were entered for Condition A, "One or more MFW flow paths with one MFIV inoperable." Required Action A.1 requires that the affected flow path(s) be isolated within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. Required Action A.2 requires that the affected flow path(s) be verified as isolated once per 7 days.
At 08:42, on December 5, 2008, FWV-30 was verified to be manually closed. This action isolated the affected MFW flow path and satisfied Required Action A.1 of ITS 3.7.3.
At 16:34, on December 7, 2008, FWV-30 was declared OPERABLE after motor replacement and successful post maintenance testing was performed under Work Order 01459514-02. This action made Required Action A.2 of ITS 3.7.3 no longer applicable.
The cause for failure of FWV-30 to automatically close was initially unknown. A reportability evaluation conservatively concluded that FWV-30 had been unable to complete its intended safety function since the last time it was successfully operated on August 29, 2008.
In order for this event to be determined not reportable, FWV-30 would have had to become inoperable on or after 02:41 on December 2, 2008. Based on engineering judgment, FWV-30 became inoperable at some point prior to that time. Therefore, FWV-30 is considered to have been inoperable for a period of time longer than the 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> allowed by ITS 3.7.3, Condition A, Required Action A.1. This condition is reportable under 10CFR50.73(a)(2)(i)(B).
SAFETY CONSEQUENCES
The safety functions of the FW System are as follows: (1) provide isolation capability for the secondary side of the OTSGs following a Main Steam Line Break (SLB) accident and (2) provide isolation of the FW System lines that penetrate the Reactor Building (RB) [NH] during a high pressure event inside the RB caused by a Loss of Coolant Accident or a SLB accident. If a break in a FW System or Main Steam (MS) [SB] line occurs and causes either or both OTSGs to depressurize to 5 600 pounds per square inch gauge (psig), then the Emergency Feedwater _ Initiation and Control System [JE] will initiate a Main Feedwater Isolation (MFWI) [JE] signal. The purpose of the MFWI is to limit the overcooling and depressurization of the Reactor Coolant System [AB], which would result in a positive reactivity addition to the reactor core and a subsequent increase in reactor power. This is accomplished by isolating the affected (faulted) OTSG(s). The affected train(s) FW block valves, FW System pump suction valve, and the FW System cross-tie valve receive a close signal and the affected trains(s) Main FW pump is tripped to eliminate FW System flow to an OTSG which has lost the ability to maintain its pressure.
Although FWV-30 did not close when required, the remaining A Train MFIVs and the B Train MFIVs operated as required during the scheduled power reduction on December 5, 2008.
Additionally, damage to the motor-operated FWV-30 valve motor did not preclude the capability of closing the valve manually and placing it in the post-accident position.
Based on the above discussion, PEF concludes that inoperability of FWV-30 for an unspecified period of time prior to December 5, 2008, did not represent a reduction in the public health and safety.
Since no loss of safety function occurred, this event does not meet the Nuclear Energy Institute definition of a Safety System Functional Failure (NEI 99-02, Revision 2).
CAUSE
The cause for the failure of the motor-operated FWV-30 motor was selection and use of a magnesium rotor motor that is inappropriate for the application. The failure analysis performed by a metallurgical lab concluded that the root cause was general corrosion, galvanic corrosion, pitting corrosion, and possibly stress corrosion cracking of the magnesium alloy in electrical contact with steel exposed to a high temperature/high humidity environment, with the dominant contributor being galvanic corrosion. General and galvanic corrosion has caused magnesium rotor motor-operated valves (MOVs) to fail in the industry and has led the NRC to issue several information notices relative to these failures. Based upon this event, and previous failures of magnesium rotor motors at CR-3 and the industry, it can be concluded that the magnesium rotor motors are not suited for the operating conditions that lead to the accelerated corrosion and subsequent failure of the magnesium rotors.
Upon disassembly of the magnesium rotor motor, CR-3 Engineering discovered opposite drive end (ODE) end ring separation from the iron core and moderate cracking of the drive end (DE) end ring. The difference in the condition of the ODE and the DE suggests a significant voltage unbalance (i.e., both end rings are not degraded equally). Coating on the balance of the rotor appeared to be the correct color with no indication of temperature induced breakdown. The ODE end ring showed a significant portion of the end ring with minimal coating. The ODE end ring appearance of the cooling fins and the hub displayed a distorted (expanded) diameter difference when compared to the DE end ring. There was evidence of black and white corrosion products on both fractured surfaces. The stator on the ODE showed evidence of a rub between the rotor cooling fins and stator windings. Two blackened areas on the stator indicate contact (shorting) between the rotor and stator approximately 180 degrees apart.
� A contributing cause for this event was corrective actions to prevent recurrence for a previous failure of an identical magnesium rotor motor at CR-3 did not prevent this failure. An alternate design option with a non-magnesium rotor was not available for several of the magnesium rotor motors at CR-3. Therefore, an inspection program was put into place to monitor the degradation mechanisms associated with magnesium rotor motors following the failure of an identical magnesium rotor motor in 2005 (CR-3 Corrective Action Program Nuclear Condition Report (NCR) 174428 for FWV-29). The inspection program implemented during CR-3 Refueling Outage 15 (R15) in the fall of 2007 was not successful in predicting the failure of FWV-30.
The motor-operated FWV-30 motor is manufactured by the Reliance Electric Company (Serial # 1MAF40626-G1-YP) and is used in a Limitorque Corporation Model SMB-4T-150 actuator.
CORRECTIVE ACTIONS
1.AA review of safety-related MOVs with magnesium rotors has been performed. There are eight safety-related MOVs with magnesium rotors that meet this criterion at CR-3 (the Decay Heat Removal System (DH) [BP] valve DHV-3 [BP, V] magnesium rotor motor was replaced in 2001 with an aluminum rotor motor). These eight MOVs were visually inspected with a boroscope during R15 in the fall of 2007 with satisfactory results. Six of these MOVs are not in a harsh (high temperature/high humidity) environment and are not frequently cycled. Therefore, there is no concern that these motors will degrade and fail before their next inspection in R16. MOVs for FWV-29 and FWV-30 were both replaced in 2008 and are not expected to degrade and fail before their next inspection in R16.
Tag# Frame Start Installed Inspection Results Action Size Torque Date DHV-3 215TY 80ftlb 05/2001 Replaced with aluminum None required rotor motor DHV-4 215TY 80ftlb R14 R15 Fall 2007 - Inspect in R16 Fall 2005 boroscope-satisfactory Fall 2009 Not in harsh environment DHV-5 256TY 100ftlb 04/1984 R15 Fall 2007 - Inspect in R16 boroscope-satisfactory Fall 2009 Not in harsh environment DHV-6 256TY 100ftlb R14 R15 Fall 2007 - Inspect in R16 Fall 2005 boroscope-satisfactory Fall 2009 Not in harsh environment FWV-14 215R2 80ftlb 12/1997 R15 Fall 2007 - Inspect in R16 boroscope-satisfactory Fall 2009 Not in harsh environment FWV-15 210TY 80ftlb 01/1998 R15 Fall 2007 - Inspect in R16 boroscope-satisfactory Fall 2009 Not in harsh environment FWV-28 256TY 200ftlb R14 R15 Fall 2007 - Inspect in R16 Fall 2005 boroscope-satisfactory Fall 2009 Not in harsh environment A Although the MOV for FWV-29 was replaced in August 2008, due to a failure to OPEN, the failure was associated with thermal binding, not with the magnesium rotor motor. That event was not reportable to the NRC since FWV-29 failed in its post-accident position of CLOSED.
ATTACHMENTS
Attachment 1 — Abbreviations, Definitions, and Acronyms Attachment 2 — List of Commitments �NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER The following table identifies those actions committed by PEF in this document. Any other actions discussed in the submittal represent intended or planned actions by PEF. They are described to the NRC for the NRC's information and are not regulatory commitments. Please notify the Supervisor, Licensing and Regulatory Programs of any questions regarding this document or any associated regulatory commitments.
COMMITMENT DUE DATE
No new regulatory commitments are contained in this submittal. N/A � NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER