ML093340392

UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS 

OFFICE OF NUCLEAR REACTOR REGULATION 

OFFICE OF NEW REACTORS 

WASHINGTON, DC 20555-0001 

February 26, 2010 

NRC INFORMATION NOTICE 2010-04: DIESEL GENERATOR VOLTAGE REGULATION 

SYSTEM COMPONENT DUE TO LATENT 

MANUFACTURING DEFECT 

ADDRESSEES 

All holders of an operating license or construction permit for a nuclear power reactor issued 

under Title 10 of the Code of Federal Regulations (10 CFR) Part 50, “Domestic Licensing of 

Production and Utilization Facilities,” except those who have permanently ceased operations 

and have certified that fuel has been permanently removed from the reactor vessel. 

All holders of or applicants for an early site permit, standard design certification, standard 

design approval, manufacturing license, or combined license issued under 10 CFR Part 52, 

“Licenses, Certifications, and Approvals for Nuclear Power Plants.” 

All holders of or applicants for a license for a fuel cycle facility issued pursuant to 

10 CFR Part 70, “Domestic Licensing of Special Nuclear Material.” 

PURPOSE 

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert 

addressees to possible latent manufacturing defects in emergency diesel generator (EDG) 

voltage regulation components. The NRC expects that recipients will review the information for 

applicability to their facilities and consider actions, as appropriate, to avoid similar problems. 

Suggestions contained in this IN are not NRC requirements; therefore, no specific action or 

written response is required. 

DESCRIPTION OF CIRCUMSTANCES 

On November 12, 2008, during the performance of a monthly surveillance test at Palo Verde 

Nuclear Generating Station Unit 2, the train ‘A’ EDG tripped on a generator differential 

protective relay trip shortly after paralleling it to offsite power. The licensee declared the EDG 

inoperable. 

Licensee troubleshooting revealed damage to the excitation control system for the generator on 

one of the three phase alternating current voltage inputs to the rectifier bridge. The damaged 

electrical component was found to be the ‘C’ phase linear power reactor. A linear power reactor 

is an electrical component consisting of a magnetic coil (inductor). The linear power reactor 

function is to limit the magnitude of the current through the excitation bridge, which supplies the 

generator field during operation. The licensee inspection of the failed magnetic coil found burnt 
IN 2010-04 

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and charred copper conductors and insulation materials. No additional electrical components in 

the cabinet were found damaged or operating out of specifications. Troubleshooting also 

revealed that the failure was isolated to this component and not caused by any other component 

in the voltage regulation system. 

The licensee sent the failed linear power reactor to an external laboratory for an equipment 

failure analysis. The analysis determined the cause of the component failure to be a latent 

manufacturing defect. An iron core lamination was slightly out of alignment with the other 

laminations in the transformer’s ‘E’ core assembly. The slight misalignment created a sharp, 

thin metal edge that, coincident with existing lamination vibration, slowly cut into and degraded 

the internal insulation around the coil wire. The internal insulation was found to be cut/worn 

below the required voltage withstand level resulting in a winding-to-winding fault. The high fault 

current caused very high temperatures and melting of the copper windings. Due to the relatively 

small amount of run time (approximately 3000 hours) on the EDG, the insulation degradation 

developed slowly over a period of approximately 25 years. Manufacturing defects normally 

manifest as an early failure, however the limited actual in-service energized time of the EDG 

delayed the appearance of the defect. However, once a coil winding-to-winding fault develops, 

it is postulated that it grows quickly, resulting in sudden component failure in a relatively short 

period of time. 

Palo Verde licensee corrective actions include the following: 

 • Adding a preventive maintenance task for thermography of EDG excitation system silicon 

controlled bridge rectifiers, power diode bridge rectifiers, current transformers, power 

transformers, and linear reactors. Additionally, performing these new thermography surveys 

may necessitate a plant modification to install new viewing ports for safely performing 

thermography in difficult locations similar to the cabinet housing the linear power reactors. 

 • Using a data recorder to capture various EDG parameters during startup and provide 

trending for troubleshooting can enhance the licensee’s corrective action program. 

 • Inspecting linear power reactors for signs of defects such as observing splits in the 

laminated windings of the transformer’s ‘E’ core and by performing surge/meggar testing to 

detect degrading insulation integrity. 

 • Periodically replacing some power magnetic components based on service time. Availability 

of spares for the excitation system components can increase EDG availability. 

Other actions to be considered by licensees may include: 

 • Performing visual inspections for burn marks on the linear power reactors, conductors, 

cabinet and electrical connections. 

 • Incorporating into plant maintenance procedures the industry's preventive maintenance 

recommendations contained in Technical Report/Maintenance Guide for the individual 

voltage regulator model. 

 
IN 2010-04 

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DISCUSSION 

This IN describes the failure of a linear power reactor in an EDG voltage regulation system at 

plant where the licensee’s preventive maintenance program did not address the EDG excitation 

system magnetic components that can be subject to deterioration with age or time in service. 

The licensee's preventive maintenance strategy included a visual inspection and cleaning at a 

frequency of once every three fuel cycles. The visual inspection was non-intrusive and would 

not reveal latent manufacturing defects. There are no vendor recommendations that specify 

predictive maintenance to identify degrading magnetic components prior to failure. 

Thermography, surge testing, or other maintenance practices may reveal a potential fault 

developing after the insulation sufficiently degrades, but it might not be enough in advance to 

prevent an equipment failure in-service. 

Reviews by the licensee and the NRC revealed past industry experience with degraded voltage 

regulation magnetic components. However, these prior events did not conduct detailed 

laboratory analyses to determine the failure mechanisms. In most cases, the failures were 

attributed to age related degradation. However, the DuPont Nomex insulation material used for 

the linear reactor coils was found to be rated for an extended life while in service up to 

428 degrees Fahrenheit (220 degrees Celsius). In this case, the manufacturing defect was 

attributed to poor workmanship and assembly techniques during original component 

construction. The failed component was originally assembled in the 1970s and installed in the 

1980s. The defect went undetected until its ultimate failure under loaded conditions. The 

relatively small amount of run time on the EDGs, over several years, facilitates characterizing 

these types of defects as age related failures whereas latent component manufacturing defects 

can actually result in failures earlier than what is their expected service life. 
IN 2010-04 

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CONTACTS 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation project manager. 

/RA/ /RA by MTschiltz for/ 

Timothy J. McGinty, Director Daniel H. Dorman, Director 

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards 

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards 

/RA/ 

Glenn Tracy, Director 

Division of Construction Inspection 

 and Operational Programs 

Office of New Reactors 

Technical Contacts: Joseph Bashore Prem Sahay 

 623-393-3737 301-415-8439 

 E-mail: Joseph.Bashore@nrc.gov email: Prem.Sahay@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 
IN 2010-04 

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CONTACTS 

This IN requires no specific action or written response. Please direct any questions about this 

matter to the technical contacts listed below or the appropriate Office of Nuclear Reactor 

Regulation project manager. 

/RA/ /RA by MTschiltz for/ 

Timothy J. McGinty, Director Daniel H. Dorman, Director 

Division of Policy and Rulemaking Division of Fuel Cycle Safety and Safeguards 

Office of Nuclear Reactor Regulation Office of Nuclear Material Safety and Safeguards 

/RA/ 

Glenn Tracy, Director 

Division of Construction Inspection 

 and Operational Programs 

Office of New Reactors 

Technical Contacts: Joseph Bashore Prem Sahay 

 623-393-3737 301-415-8439 

 E-mail: Joseph.Bashore@nrc.gov email: Prem.Sahay@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site, 

http://www.nrc.gov, under Electronic Reading Room/Document Collections. 

ADAMS Accession Number: ML093340392 ME2313 

OFFICE EEEB:DE Tech Editor BC:EEB:DE:NRO BC:EEEB:DE D:DE 

NAME PSahay KAzariah-Kribbs APal GWilson PHiland 

DATE 01/19/2010 12/15/09 email 01/20/2010 email 01/20/2010 01/25/2010 

OFFICE PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DCIP:NRO D:FCSS:NMSS D:DPR 

NAME DBeaulieu CHawes MMurphy GTracy DDorman 

(MTschitlz for) 

TMcGinty 

DATE 01/26/10 01/27/10 01/27/10 02/23/10 02/26/10 02/26/10 

OFFICIAL RECORD COPY