Information Notice 2010-04, Diesel Generator Voltage Regulation System Component Due to Latent Manufacturing Defect

Diesel Generator Voltage Regulation System Component Due to Latent Manufacturing Defect

ML093340392
Person / Time
Issue date:	02/26/2010
From:	Dan Dorman, Mcginty T, Tracy G NRC/NMSS/FCSS, Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking
To:
Beaulieu, D P, NRR/DPR, 415-3243
References
IN-10-004
Download: ML093340392 (5)

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

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 transformers 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 hours0.0347 days <br />0.833 hours <br />0.00496 weeks <br />0.00114 months <br />) 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 licensees corrective action program.

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

laminated windings of the transformers 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.

DISCUSSION

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

plant where the licensees 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.

CONTACT

S

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.

CONTACT

S

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 TMcGinty

(MTschitlz for)

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

OFFICIAL RECORD COPY