05000361/LER-2002-001
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| 3612002001R00 - NRC Website | |
DOCKET (2) LER NUMBER (e) PAGE (3) FACILITY NAME(1) San Onofre Nuclear Generating Station (SONGS) Unit 2 05000-361 2002 — 001 — 00 San Onofre Nuclear Generating Station, Units 2 and 3 September 11, 2001 January 11, 2002 Plant:
Event Date:
Discovery Date:
Reactor Vendor Mode Power (percent)* * on 9/11/2001
Background:
Combustion Engineering 1 - power operation 98.5 Combustion Engineering 1 - power operation 100 At San Onofre Nuclear Generating Station (SONGS) Units 2 and 3, the Engineered Safety Features Actuation System (ESFAS) use Potter and Brumfield (P&B) series 7032 Motor Driven Relays (MDR) to actuate Engineered Safety Features equipment. The ESFAS subgroup relays are the last relays in the control circuit before the ESF actuated equipment. As required by Technical Specification 3.3.6., "Engineered Safety Features Actuation System (ESFAS) Logic and Manual Actuation," Southern California Edison (SCE) surveils these relays once every six months.
Series 7032 relays are non-latching MDR relays (Figure 1). These relays have two positions: "energized" and "de-energized." When energized, two coils, connected in series and located inside the relay, rotate the relay rotor shaft and operate the relay contacts by a shaft extension. When de-energized, the rotor shaft is returned to its normal position by two connected springs (Figure 2). Relay shaft rotation is limited to a 30-degree arc by the stator faces and a stop ring. At SONGS, the Series 7032 relays are normally energized so that only spring force is used to rotate the relay on a loss of power.
The Series 7032 relays have three decks of main form "Y" contacts (high current applications) and three decks of four (4) auxiliary form "C" contacts (low current applications) mounted in isolated rings.
Units 2 and 3 each have 20 Series 7032 relays (total of 40) installed as follows:
1. 10 are spare relays and not currently being utilized 2. 18 form Y contacts are not being used 3. 12 form Y contacts are being used
Description of the Event:
On September 11, 2001, while performing a subgroup relay test on Emergency Feedwater Actuation Signal (EFAS) 1 Train B, Steam Generator (SG) E-089 Auxiliary Feedwater (AFW) {BA} Isolation Valve 2HV4715 {FCV} failed to close. AFW pump 2P140 started and AFW control valve 2HV4706 to SG E-089 closed as expected. 2HV4715 did not close as expected and was verified locally to be open.
Plant operators manually closed 2HV4715 from the control room. Upon investigation, SCE discovered that the P&B relay (2L035K402) {RLY} used for 2HV4715 had failed. The relay was replaced and the EFAS train returned to operable status within the time allowed by Tech Spec 3.7.5, "Auxiliary Feedwater (AFW) System.
FAC1JTY HAW) � DOCKET (2) � LER NUMBER � PAGE (3) San Onofre Nuclear Generating Station (SONGS) Unit 2 � 05000-361 YEAR � On September 14, 2001, SCE determined the shaft of relay 2L035K402 would not fully counter-rotate to its zero-degrees stop position when de-energized. SCE continued to perform a detailed cause evaluation, which included the use of a Scanning Electron Microscope to examine various components within the relay. During this time, SCE had ongoing discussions with P&B. During November 2001, SCE met with P&B at their facility in Greensboro, North Carolina and analyzed a second sample of the same type and vintage relay. In November 2001, SCE conducted additional laboratory analyses to quantify the degradation of the relay components. P&B confirmed that the springs were within their design specifications on December 28, 2001 and were not a factor in the relay failure. On January 11, 2002 (discovery date), SCE completed the cause evaluation, concluding that the self-lubricating nylon hub in deck 1 of the form "Y" contacts (i.e., closest to the relay coils and the heat they generate) had deteriorated such that reinforcing glass fibers in the nylon were creating an abrasive interface. The increased friction prevented the shaft from rotating to its zero position (Figure 2). Consequently, relay 2L035K402 failed to close contacts 4L-4M, which prevented 2HV4715 from closing.
Cause of the Event:
The primary factor, which increased the frictional force (drag) associated with the main form "Y" contact blocks, was the slow, age-related degradation of the movable nylon hub (Figure 3). Scratching on the "Y" contacts was a secondary factor that increased friction. This cumulative friction eventually overcame the spring return force and prevented the relay from fully counter-rotating to its fail safe "zero degree stop" position.
The 40 relays at SONGS (30 installed and 10 spare) were manufactured from 1989 through 1990 time frame and as such are subject to the same degradation mechanism.
Corrective Actions:
SCE has determined that there is no need to utilize the form "Y", high current contacts at SONGS Units 2 and 3. As such, a proactive approach will be implemented to replace all the SONGS P&B MDR-7032 relays with P&B MDR-7033 or P&B MDR-7034 relays, which do not contain form "Y" contact blocks.
SCE also examined a spare P&B MDR-7032 relay that had been subject to the same conditions (age, number of cycles and normally energized). This spare relay also showed signs of this age related degradation, but its contacts operated correctly.
The testing of 30 installed (i.e., non-spare) MDR-7032 relays are being staggered each month so that at least three relays are tested each month. Thirteen of these relays have already been tested following the initial relay test failure and there have been no additional failures.
The performance of these relays will be trended during the performance of the subgroup relay testing until the new MDR relay is installed.
Safety Significance:
Valve 2HV4715 is located on AFW pump P-141 discharge piping to steam generator (SG) E-089. This valve is closed during plant operation (Mode 1) and fails as-is upon loss of power. Upon an EFAS actuation, the valve opens and closes to provide automatic SG level control. It is also the AFW isolation valve for SG E-089.
— 001 — 00 2002 FACILITY NAAE(1) � DOCKET (2) � LER ROAMER (S) � PAGE (3) There are two actuation signals for this valve generated by the ESFAS: (1) an EFAS, and (2) a Main Steam Isolation Signal (MSIS). These signals come from relays located in the ESFAS auxiliary relay cabinets. The EFAS initiates a signal for the valve to open to supply AFW to the SG for plant cool down in the event of a postulated Chapter 15 accident. MSIS initiates a close signal to isolate SG E-089.
ESFAS actuates separate relays for these actions.
The failed relay is not in the EFAS or MSIS actuation logic for 2HV4715. The P&B relay which opens 2HV4715 on EFAS (2L035K724) and the relay which close 2HV4715 on MSIS (2L035K723) did not experience the failure found in relay 2L035K402. Therefore, 2HV4715 would have been capable of closing on an MSIS. The failure of relay 2L035K402 to close valve 2HV4715 would only prevent the automatic control of SG E-089 water level, but would not (and did not) prevent control room operators from manually operating this valve. The close signal on the failed relay secures excess AFW flow by shutting valve 2HV4715 when the SG water level returns to above the low level setpoint and is not in the actuation logic for MSIS.
Although the EFAS 1 Train B was not able to automatically close 2HV4715 to secure excess AFW flow, it did not impact the ability of plant operators from detecting increasing water level in the SG E-089 and manually closing 2HV4715. Therefore, neither the health nor the safety of plant personnel or the public was affected by this condition.
Additional Information:
On May 5, 1999, SCE reported a defect found in a different type of P&B relays resulting from insufficiently welded contact pads. This condition does not involve the same underlying concern or reason as this event. Therefore, the corrective actions taken would not have prevented this condition.
Potter and Brumfield Series 7032 Motor Driven Relay Form "C" Contacts:
(3 separate decks) Form "Y" Contacts:
(3 separate decks) U.S. NUCLEAR REGULATORY COMMISSION DOCKET (2) PAGE (3) LER NUMBER 6) FACILITY NAME(1) -- 001 -- I 0 00 05000-361 2002 Figure 1 U.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME(1) DOCKET (2) LER NUMBER (6) PAGE (3) 05000-361 2002 -- 001 — I � 00 Figure 2 U.S. NUCLEAR REGULATORY COMMISSION FACILITY NAME(1)
LER NUMBER (S)
DOCKET (2) 05000-361 Figure 3 Pho o he movable showing con ac s the age heat damage the Nylon