UNITED STATES 

NUCLEAR REGULATORY COMMISSION 

OFFICE OF NUCLEAR REACTOR REGULATION 

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS 

WASHINGTON, DC 20555-0001 

July 7, 2009 

NRC INFORMATION NOTICE 2009-10: TRANSFORMER FAILURES—RECENT 

OPERATING EXPERIENCE 

ADDRESSEES 

All holders of operating licenses for nuclear power reactors under the provisions of Title 10 of 

the Code of Federal Regulations (10 CFR) Part 50, “Domestic Licensing of Production and 

Utilization Facilities,” except those who have ceased operations and have certified that fuel has 

been permanently removed from the reactor vessel. All holders of licenses or certificates of fuel 

cycle facilities. All holders of and applicants for nuclear power plant construction permits under 

the provisions of 10 CFR Part 50. All holders of licenses or certificates for fuel cycle facilities. 

PURPOSE 

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

addressees of recent operating experience involving failures of large transformers. The NRC 

expects that recipients will review the information for applicability to their facilities and consider 

actions, as appropriate, to avoid similar problems. However, suggestions contained in this IN 

are not NRC requirements; therefore, no specific action or written response is required. 

DESCRIPTION OF CIRCUMSTANCES 

For several years, available industry operating experience has indicated an increasing trend in 

transformer failures, and has provided recommendations to reduce the chances of failure. 

Improved preventive maintenance and monitoring practices have helped to identify some 

problems before they developed to the point of failure, but the number of large transformers 

events has not decreased and in fact continues to rise. The following are relatively recent 

examples involving main power transformers: 

• Indian Point, Unit 3—On April 6, 2007, while operating at 92-percent power, a fault 

occurred on the No. 31 main transformer resulting in an automatic reactor trip and 

transformer fire. Licensee Event Report (LER) 50-286/2007-002, which can be found on 

the NRC’s public Web site using Agencywide Documents Access and Management 

System (ADAMS) Accession No. ML071620122, provides additional information. 

• River Bend Station—On May 4, 2007, an unplanned manual reactor scram was initiated 

following the loss of cooling on the No. 2 main transformer (see LER 50-458/2007-002, 

ADAMS Accession No. ML071840161, for more information). 

• Grand Gulf Nuclear Station—On January 12, 2008, a manual reactor scram was initiated 

following a loss of cooling to the main transformers (see LER 50-416/2008-001, ADAMS 

Accession No. ML080700702, for more information). 

ML090540218 
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• Limerick Generating Station, Unit 2—On February 1, 2008, a low voltage bushing 

connection failed on the 2A main transformer resulting in a turbine trip and reactor 

scram. 

• Diablo Canyon, Unit 2—On August 16, 2008, an automatic reactor trip occurred resulting 

from the failure of the main electrical transformer C phase. Plant operators 

subsequently declared a Notification of Unusual Event due to an observed fire at the 

C phase transformer (see LER 50-323/2008-001, ADAMS Accession No. ML082970221, 

for more information). 

• North Anna, Unit 2—On October 29, 2008, while attempting to place the unit on line, the 

turbine tripped on a generator lockout relay actuation. The C main transformer was 

discovered to be spraying oil. 

• Oyster Creek—On November 28, 2008, an electrical fault internal to the M1A main 

transformer led to an automatic reactor scram due to load reject. The transformer was 

replaced with the spare, and on February 1, 2009, this transformer failed due to a 

bushing failure, resulting in a reactor scram, fire, and declaration of a Notification of 

Unusual Event (see LER 50-219/2008-001, ADAMS Accession No. ML090260082, and 

LER 50-219/2009-001, ADAMS Accession No. ML090970735 for more information). 

A review of licensees’ root cause evaluations for the large transformer failures shows that the 

events are often the result of ineffective implementation of the transformer maintenance 

program. 

DISCUSSION 

The events described above illustrate instances in which the loss or tripping of large 

transformers resulted in plant transients, reactor trips, unnecessary starting of the emergency 

diesel generators (EDGs), and declaration of plant events. Transformer failures have resulted 

in eight declared plant events from January, 2007, to February, 2009, making them the second 

leading reason for such declarations. While the large transformers discussed in this IN are 

generally non-safety related, they are within the scope of the Maintenance Rule (Title 10 of the 

Code of Federal Regulations, Section 50.65, “Requirements for Monitoring the Effectiveness of 

Maintenance at Nuclear Power Plants”). 

A relatively high incidence of transformer failures has occurred in the last few years, the majority 

of which could have been avoided had the licensee fully evaluated and effectively implemented 

corrective actions and recommendations identified in industry operating experience. These 

corrective actions included a more effective maintenance program and a more proactive 

approach to addressing abnormal indications. In particular, some utilities have installed online 

automated oil analysis and monitoring system to support decisions regarding preventive and 

corrective maintenance to improve transformer reliability. The Institute of Electrical and 

Electronics Engineers (IEEE) provides industry guidance on this matter in Standard 

C57.140-2006, “IEEE Guide for the Evaluation and Reconditioning of Liquid Immersed Power 

Transformers.” However, it should be noted that the NRC has not endorsed this document, and 

the recommendations it contains do not constitute NRC requirements. 
 IN 2009-10 

 Page 3 of 3 

CONTACT 

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 (NRR) project manager. 

/RA/ /RA/ 

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

Division of Policy and Rulemaking Division of Fuel Cycle Safety 

Office of Nuclear Reactor Regulation and Safeguards 

 Office of Nuclear Materials Safety 

 and Safeguards 

Technical Contacts: Rebecca Sigmon, NRR/DIRS Prem Sahay, NRR/DE 

 (301) 415-4018 (301) 415-8439 

 E-mail: Rebecca.Sigmon@nrc.gov E-mail: Prem.Sahay@nrc.gov

NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov. 

To access this information, select “Electronic Reading Room” and then “Document Collections.” 
 IN 2009-10 

 Page 3 of 3 

CONTACT 

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 (NRR) project manager. 

/RA/ /RA/ 

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

Division of Policy and Rulemaking Division of Fuel Cycle Safety 

Office of Nuclear Reactor Regulation and Safeguards 

 Office of Nuclear Materials Safety 

 and Safeguards 

Technical Contacts: Rebecca Sigmon, NRR/DIRS Prem Sahay, NRR/DE 

 (301) 415-4018 (301) 415-8439 

 E-mail: Rebecca.Sigmon@nrc.gov E-mail: Prem.Sahay@nrc.gov

NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov. 

To access this information, select “Electronic Reading Room” and then “Document Collections.” 

Distribution: IN Reading File 

 Hard Copy to Elizabeth Doolittle (Mail Stop EBB 6003 1-D 2-M) 

ADAMS Accession Number: ML090540218 TAC NO. ME0392 

OFFICE IOEB/DIRS AC/DIRS/IOEB D/NRR/DIRS TECH EDITOR NRR/DE/EEEB BC/EEEB/DE

NAME RSigmon JThorp FBrown KAzariah-Kribbs PSahay GWilson 

DATE 05/08/09 05/13/09 06/02/09 04/03/09 email 05/12/09 05/12/09 

OFFICE A/NRR/DE DPR/PGCB DPR/PGCB BC/DPR/PGCB D/NRR/DPR D:FCSS

NAME ABoland DBeaulieu CHawes MMurphy TMcGinty DDorman 

DATE 05/18/09 06/03/09 06/03/09 07/01/09 07/07/09 06/19/09 

OFFICIAL RECORD COPY