05000302/LER-2005-004
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/Anumber No. 05000 | |
| Event date: | 10-28-2005 |
|---|---|
| Report date: | 12-14-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3022005004R00 - NRC Website | |
EVENT DESCRIPTION
At 19:10, on October 28, 2005, Progress Energy Florida, Inc. (PEF), Crystal River Unit 3 (CR-3) was operating in MODE 1 (POWER OPERATION) at 51 percent RATED THERMAL POWER when the breaker for B Train motor-operated Main Feedwater Isolation Valve (MFIV) [SJ, ISV] FWV-29 motor [SJ, MO] tripped soon after the valve started cycling closed. This occurred during power reduction in preparation for a refueling outage. At the time, CR-3 was being shutdown in accordance with Operating Procedure OP-209A, "Refueling Outage Plant Shutdown and Cooldown.
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-29 is the B Train main block valve.
Improved Technical Specification (ITS) 3.7.3, "Main Feedwater Isolation Valves," 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-29 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 21:03, on October 28, 2005, FWV-29 was manually closed. This action isolated the affected MFW flow path and satisfied Required Action A.1 of ITS 3.7.3. At 15:59, on October 29, 2005, MODE 4 (HOT SHUTDOWN) was entered. This action made Required Action A.2 of ITS 3.7.3 no longer applicable.
The cause for failure of FWV-29 to automatically close was initially unknown. Therefore, FWV-29 was considered to have become inoperable when it failed. No 10CFR50.72 or 10CFR50.73 reportability criteria were determined to be applicable. Potential reportability was re-visited upon receipt of a failure report dated November 11, 2005, that was provided by an electric motor repair facility. That failure report supported a conclusion that FWV-29 became inoperable and would not have been able to perform its intended function for some unspecified period of time prior to the actual failure.
In order for this event to be determined not reportable, FWV-29 would have had to become inoperable on or after 19:10 on October 25, 2005. Based on engineering judgment, FWV-29 became inoperable at some point prior to that time. Therefore, FWV-29 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 s 600 pounds per square inch gauge (psig), then the Emergency Feedwater Initiation and Control System [JE] will initiate a Main Feedwater Isolation (MFWI) [JB] signal. The purpose of the MFWI is to limit the overcooling and depressurization of the Reactor Coolant System [AB], which would result in 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 [SJ, P] is tripped to eliminate FW System flow to an OTSG which has lost the ability to maintain its pressure.
Although FWV-29 did not close when required on October 28, 2005, the redundant B Train MFIVs and the A Train MFIVs operated as required during the manual reactor trip, initiated on October 29, 2005, upon loss of both Main FW pumps. Additionally, damage to the motor-operated FWV-29 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-29 for an unspecified period of time prior to October 28, 2005, 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-29 motor was oxidation/corrosion of the motor rotor magnesium fans and shorting ring. After disassembly, the motor rotor shorting ring and fan blades were found discolored, distorted, with pieces broken off the shorting ring and fan blades.
The degradation was noted by the appearance of a grayish-black powdery residue on the destroyed pieces. Corrosion of the magnesium alloy is believed to be the result of exposure to high temperatures and high humidity.
During the course of the failure, portions of the shorting ring and fan degraded and distorted. As the magnesium rotor swelled, the rotor fans came into contact with the motor winding endturns, progressively contacting and nicking the windings. This condition resulted in a locked rotor condition. As a result of the locked rotor condition, motor current increased and produced excessive heating in the stator windings. The increase in current resulted in a breakdown of the insulation system. Once the insulation broke down, failure of the windings occurred. The initial failure was most likely a turn-to-turn short in one of the areas where the rotor fans came into contact with the stator windings. The turn-to-turn short resulted in excessive currents and localized heating that began to break down the coil insulation until a phase-to-phase or phase-to-ground short developed in the slot.
A contributing factor for failure of the motor-operated FWV-29 motor was inadequate preventive maintenance due to applicable Operating Experience not being incorporated into the CR-3 Motor Operated Valve Program Manual. Failure of motor-operated valve motors associated with magnesium rotors has been documented in the following: (1) NRC Information Notice 86-02, "Failure of Valve Operator Motor During Environmental Qualification Testing," dated January 6, 1986; (2) IEEE Transactions on Energy Conversion, Volume 3, Number 1, "An Investigation of Magnesium Rotors in Motor Operated Valve Actuators," dated .March 1988; and, (3) NUREG/CR 6205, 'Valve Actuator Motor Degradation," dated December 1994.
The motor-operated FWV-29 motor is manufactured by the Reliance Electric Company (Serial # SYZ00762-A1-PT) and is used in a Limitorque Corporation Model SMB-4T-150 actuator.
CORRECTIVE ACTIONS
1.� A review of applicable motor-operated valve motors with magnesium rotors has been performed. There are nine safety-related motor-operated valves with magnesium rotors that meet this criterion at CR-3. Five of these motor-operated valve motors have been inspected (visual inspection with a boroscope and an electrical Polarization Index (PI) inspection). Based on the results of these five inspections and corresponding installation history, the three remaining motor-operated valve motors (Decay Heat Removal System (DH) valve DHV-3, FWV-15, FWV-30) will not be inspected at this time due to the comparatively short time these motors have been in service (see next page).
Tag# Frame Start Installed Inspection Results Action Size Torque Date DHV-3 215TY 80ftlb 05/2001 Not inspected Inspect later DHV-4 215TY 80ftlb 06/1992 PI-sat/Boroscope-unsat Replaced DHV-5 256TY 100ftlb 04/1984 PI-sat/Boroscope-sat None required DHV-6 256TY 100ftlb 04/1984 PI-sat/Boroscope-unsat Replaced FWV-14 215R2 80ftlb 12/1997 PI-sat/Boroscope-sat None required FWV-15 210TY 80ftlb 01/1998 Not inspected Inspect later FWV-28 256TY 200ftIb 04/1996 PI-unsat/Boroscope-unsat Replaced FWV-29 256TY 150ftlb 1990 Motor failed Replaced FWV-30 256TY 150ftlb 10/2003 Not inspected Inspect later 2.� Other actions associated with this event are being addressed in CR-3 Corrective Action Program Nuclear Condition Report NCR 174428.
PREVIOUS SIMILAR EVENTS
Based on a keyword search of the CR-3 electronic Licensee Event Report database, no previous similar events involving failure of a motor-operated valve motor due to rotor oxidation/corrosion have been reported to the NRC by CR-3.
ATTACHMENTS
Attachment 1 - Abbreviations, Definitions, and Acronyms Attachment 2 - List of Commitments ATTACHMENT 1
ABBREVIATIONS, DEFINITIONS AND ACRONYMS
CFRMCode of Federal Regulations DHMDecay Heat Removal System DHV Decay Heat Removal Valve FWMMain Feedwater System FWV Main Feedwater Valve ITSMImproved Technical Specifications MFIV Main Feedwater Isolation Valve MFWIMMain Feedwater Isolation MS Main Steam System NCRMNuclear Condition Report NEI Nuclear Energy. Institute OPMOperating Procedure OTSG Once Through Steam Generator PEFMProgress Energy Florida, Inc.
PI Polarization Index RBMReactor Building SLB Main Steam Line Break NOTES:MImproved Technical Specifications defined terms appear capitalized in LER text {e.g., MODE 1} Defined terms/acronyms/abbreviations appear in parenthesis when first used {e.g., Reactor Building (RB)}.
ATTACHMENT 2
LIST OF COMMITMENTS
The following table identifies those actions committed to by PEF in this document. Any other actions discussed in the submittal represent intended or planned actions by PEF. They are described for the NRC's information and are not regulatory commitments. Please notify the Supervisor, Licensing & Regulatory Programs, of any questions regarding this document or any associated regulatory commitments.
RESPONSE COMMITMENT DUE DATE
SECTION
No regulatory commitments are being made in this submittal.