Information Notice 2008-12, Reactor Trip Due to Off-Site Power Fluctuation
| ML081830163 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 07/07/2008 |
| From: | Michael Case NRC/NRR/ADRO/DPR |
| To: | |
| lgs1, dls10 | |
| References | |
| IN-08-012 | |
| Download: ML081830163 (4) | |
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,
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 plants RCPs responded by drawing more
current on A- and C-phases to maintain RCP power output. The 1D RCPs 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 circuits 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 relays 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 NRCs 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:
(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:
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