Information Notice 2008-12, Braidwood Unit 1 Reactor Trip Due to Off-Site Power Fluctuation: Difference between revisions

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{{Adams
#REDIRECT [[Information Notice 2008-12, Reactor Trip Due to Off-Site Power Fluctuation]]
| number = ML081830163
| issue date = 07/07/2008
| title = Braidwood Unit 1 Reactor Trip Due to Off-Site Power Fluctuation
| author name = Case M J
| author affiliation = NRC/NRR/ADRO/DPR
| addressee name =
| addressee affiliation =
| docket = 05000456
| license number =
| contact person = lgs1, dls10
| document report number = IN-08-012
| document type = NRC Information Notice
| page count = 4
}}
{{#Wiki_filter: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 consi
 
der 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, 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 syst
 
em and the resulting Unit 1 tr
 
ip 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 ac
 
tions 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).
 
==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.
 
==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}}
 
{{Information notice-Nav}}

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