UNITED STATES 

 NUCLEAR REGULATORY COMMISSION 

 OFFICE OF NUCLEAR REACTOR REGULATION 

 WASHINGTON, DC 20555-0001 

 

July 7, 2008 

NRC INFORMATION NOTICE 2008-12: BRAIDWOOD UNIT 1 REACTOR TRIP DUE TO 

OFF-SITE POWER FLUCTUATION 

ADDRESSEES

All holders of operating licenses for nuclear power reactors, except those who have permanently 

ceased operations and have certified that fuel has been permanently removed from the reactor 

vessel. 

PURPOSE

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

addressees of the results of a staff evaluation of a recent automatic trip of the Braidwood Unit 1 

Nuclear Power Plant due to an offsite power voltage fluctuation coupled with a failed protective 

relaying circuit. 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

Braidwood Unit 1 experienced an automatic reactor trip from full power on June 27, 2007. The 

reactor trip was triggered by low flow in the 1D Reactor Coolant Loop after the 1D Reactor 

Coolant Pump (RCP) tripped (appropriate reactor protection system actuation given loss of flow 

due to the RCP trip). The event was initiated by a lighting strike 4.3 miles from the plant that 

created a single-phase-to-ground fault on transmission line number 2001, which is a 345kV 

transmission line. The ground fault suppressed the B-phase voltage to 48 percent of nominal 

until 345kV protective breakers at Braidwood and the East Frankfort Transmission Substation 

isolated the line about 3 cycles after fault initiation. The suppressed B-phase voltage created a 

momentary phase imbalance on the Braidwood RCPs resulting in elevated current on the 

unaffected A- and C-phases. The C-phase current on the 1D RCP exceeded the trip set point of 

an instantaneous overcurrent protective relay. Normally (by design), the RCP trip on momentary 

overcurrent would have been blocked by an in-series impedance relay; however, at the time of 

the event, the impedance relay was in a tripped state due to a failed Bussman type KTN-R fuse 

in an impedance monitoring circuit. 

The licensee has been working on a generic reliability issue with Bussman KTN and KTN-R style 

fuses over the last two years (based on Title 10, Part 21, “Reporting of Defects and 

Noncompliance,” of the Code of Federal Regulations reporting for these fuses). There have 

been several intermittent or complete fuse failures caused by fuse element cracking 

characterized as fatigue failure. After a fuse failure on 6.9kV Bus 157 in January 2007, 

ML081830163 
IN 2008-12 

Page 2 of 3 

the licensee made the decision to replace all Bussman fuses installed on the 6.9kV bus 

protective relay circuits. However, due to the risk of tripping an RCP during fuse replacement, 

these fuses were to be replaced during outages. The fuse that failed in the 1D RCP protective 

relay circuit was scheduled for replacement in the next refueling outage. The licensee 

considered this a small risk, since fuse failures had been self-revealing and promptly replaced to 

restore the designed relay scheme. However, the only fuse in the RCP protective relaying circuit 

without a loss-of-bus-voltage indication or alarm (not self-revealing), failed in this event. Only 

the disturbance in the transmission system and the resulting Unit 1 trip indicated the vulnerability 

within the RCP protective relaying circuitry. 

The corrective action planned by the licensee was to either implement a design change to 

eliminate the identified vulnerability from the RCP protective relaying system, or, if design 

changes were not appropriate, to develop alternate actions to address the vulnerability. 

Additional information is available in Braidwood Station, Unit 1, Licensee Event Report 2007-

001-00, dated August 27, 2007 (ADAMS Accession No. ML072400074). 

DISCUSSION

The event was initiated by a lightning strike to a 345kV transmission line which caused a 

line-to-ground fault. The fault caused the voltage on B-phase to drop to 48 percent of the 

nominal voltage until the transmission line breakers opened, appropriately isolating the fault. The 

duration of the event from fault initiation until the breakers opened was about 0.051 seconds 

(about 3 cycles). With the B-phase voltage low, the plant’s RCPs responded by drawing more 

current on A- and C-phases to maintain RCP power output. The 1D RCP’s supply breaker 

tripped on overcurrent in the C-phase. The instantaneous over-current relay set point was 

exceeded and it actuated appropriately. Based on the circuit’s design, a series impedance relay 

should normally block the momentary overcurrent condition; however, at the time of this event, 

the impedance relay was in a tripped state due to a failed fuse in the impedance relay’s voltage 

sensing circuit. 

The root cause of this event was a design flaw which allowed the RCP protective relay circuit to 

be in a degraded state without proper indication to the operators. This degraded relay circuit 

state placed the plant in a condition less tolerant to grid voltage excursions without the requisite 

knowledge of the operators. A contributing factor was the failure of the fuse used in the relay 

voltage sensing circuit. Consequently, as a result of the design flaw coupled with the grid 

voltage excursion, a reactor trip occurred from full power. Unplanned reactor trips are identified 

in the NRC’s Reactor Oversight Process as a performance indicator associated with the 

“Initiating Events” cornerstone. 

Licensees rely on plant controls, indications, and alarms to inform operators of plant, system, 

and sub-system conditions such that proper operation of the plant can be conducted during 

normal, abnormal, and emergency conditions. More specifically, these controls, indications, and 

alarms allow the operation of the plant while avoiding unnecessary transients and automatic 

trips. Relaying circuits should provide indication to the operators upon failure or partial failure of 

such circuits. In particular, circuits should indicate protective relay actuation within relaying 

circuits (such as the fuse opening and the impedance relay operation, in this case). 
IN 2008-12 

Page 3 of 3 

CONTACT

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

matter to the technical contact listed below. 

/RA by TQuay for/ 

Michael J. Case, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contact: Kenneth A Miller, NRR 

 (301) 415-3152 

 e-mail: Kenneth.miller2@nrc.gov 

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

http://www.nrc.gov. Select Electronic Reading Room and then Document Collections. 
IN 2008-12 

Page 3 of 3 

CONTACT

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

matter to the technical contact listed below. 

/RA by TQuay for/ 

Michael J. Case, Director 

Division of Policy and Rulemaking 

Office of Nuclear Reactor Regulation 

Technical Contact: Kenneth A Miller, NRR 

 301-415-3152 

 e-mail: Kenneth.miller2@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. 

Distribution: IN Reading File 

ADAMS Accession Number: ML081830163

OFFICE TECH EDITOR EEEB:DE BC:EEEB:DE D:EEEB 

NAME KAzariah-Kribbs KMIller GWilson PHiland 

DATE 06/26/2008 7/2/08 7/2/08 7/3/08 

OFFICE LA:PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR 

NAME CHawes DBeaulieu MMurphy TQuay for MCase 

DATE 7/3/08 7/03/08 7/07/08 7/07/08 

OFFICIAL RECORD COPY