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| Start date | Reporting criterion | Title | Event description | System | LER | |
|---|---|---|---|---|---|---|
| ENS 43657 | 21 September 2007 04:00:00 | 10 CFR 21.21 | Part 21 - Defective Analog Electronic Circuit Card (Sbsr Avr) | The SBSR AVR card is manufactured by Basler Electric Company in Highland, Illinois. Basler is a supplier of generator control equipment, including generator excitation systems and protective relays. The company maintained a 10 CFR 50, Appendix B Quality Assurance program until the mid 1990s but is presently a commercial supplier with an ISO 9001:2000 certified quality program. The basic component is an analog electronic circuit card in Basler voltage regulators used on emergency diesel generators (EDGs) in US nuclear power plants. The Basler SBSR excitation system provides closed-loop control of EDG stator voltage via an automatic voltage regulator (AVR) and an exciter. The AVR is the controller, while the exciter converts the control signal from the AVR to a field voltage that is applied to the EDG field winding. The AVR is an analog assembly. The main component of this assembly is an electronic circuit card that is referred to as the SBSR AVR card. The SBSR AVR card contains a variety of mounted components. The defect identified within this report deals specifically with the soldered electrical connections between the L1 magnetic amplifier module (magamp) and the card. While there are many Basler SBSR AVR card part numbers, all cards perform the same function and have similar components. Further, all cards utilize the same L1 magamp and all cards mount and electrically connect the L1 magamp in a similar manner. The nature of the defect is that over a period of many years, cracks can form in the solder joint connections between the L1 magamp and the circuit board. There are nine such solder connections at the L1 magamp, and all nine are susceptible to cracking. Cracks have been observed in five SBSR AVR cards that have been in service at two separate nuclear power plants. Also, another case (at one of the same two plants) exhibited symptoms similar to those exhibited by cards with cracked solder joints, but the card was not inspected for the presence of cracks nor retained. Accordingly, cracks are suspected but not confirmed in this other case. It has been observed that full circumferential cracks can form. When a full circumferential crack forms, the electrical resistance at the connection can increase significantly and electrical continuity at the connection could be lost. In four of the six cases, the observed symptom associated with the cracks is that during surveillance testing, the EDG starts and takes longer than specified to reach its rated voltage. The condition is observed to be intermittent, i.e., when the test is repeated the EDG typically starts and meets its time requirement to establish rated voltage. This intermittent nature has made troubleshooting more difficult as the symptom is typically not reproducible. This observation is consistent with the fact that a cracked solder joint may not fully and permanently lose electrical connectivity. In one of the six cases, the observed symptom was that during surveillance testing, the voltage regulator failed to properly control output voltage of the EDG. The EDG had to be shut down, and it may not have been capable of powering its design basis loads. In one of the six cases there were no symptoms or deficiencies during surveillance testing. Rather, the cracks were seen during a visual observation. For SBSR AVR cards that have been supplied by MPR, MPR has not specified maintenance requirements related to inspection or replacement. Accordingly, there are no supplier-specified actions that would ensure that degraded cards are detected or avoided. The safety hazard is that when electrical continuity is lost at L1 magamp soldered connections, the voltage regulator does not perform as intended, and the EDG could fail to deliver emergency AC power as intended. Analyses and testing by MPR in conjunction with Basler Electric confirmed that, although various symptoms can be observed, loss of electrical connectivity at some combinations of L1 magamp soldered connections could render the voltage regulator unable to control voltage. In such a case, the associated EDG would not be able to provide emergency AC power as intended. Because EDGs are important safety-related components at nuclear power plants that are relied upon for safety-related functions, the cracked solder joints could be a substantial safety hazard. MPR has provided SBSR systems, including AVR circuit cards and spare AVR cards to the following nuclear plants: Beaver Valley, Millstone, Davis-Besse, Pilgrim, Cooper Nuclear, Ginna. US Nuclear plants believed by MPR to utilize the SBSR excitation system are: ANO, Beaver Valley, Calvert Cliffs, Cooper, Crystal River, Davis-Besse, Diablo Canyon, Duane Arnold, Farley, Fermi, FitzPatrick, Ginna, Hatch, Indian Point, Kewaunee, McGuire, Millstone, North Anna, Palisades, Peach Bottom, Pilgrim, Prairie Island, Robinson, Salem, Sequoyah, and Vermont Yankee. For SBSR AVR cards that MPR has supplied, MPR will issue a maintenance bulletin to advise SBSR AVR owners that an inspection and repair program should be established. The inspection program should periodically inspect for L1 magamp solder joint cracks. The inspections should occur on a fuel cycle periodicity after 15 years of service. If cracks are found, the card should be replaced, or the joints repaired by remaking the solder connections. | Emergency Diesel Generator |