Information Notice 2011-12, Reactor Trips Resulting from Water Intrusion Into Electrical Equipment
| ML110450487 | |
| Person / Time | |
|---|---|
| Issue date: | 06/16/2011 |
| From: | Mcginty T Division of Policy and Rulemaking |
| To: | |
| Beaulieu, D P, NRR/DPR, 415-3243 | |
| References | |
| IN-11-012 | |
| Download: ML110450487 (5) | |
ML110450487 UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001
June 16, 2011
NRC INFORMATION NOTICE 2011-12:
REACTOR TRIPS RESULTING FROM WATER
INTRUSION INTO ELECTRICAL EQUIPMENT
ADDRESSEES
All holders of operating licenses or construction permits 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 that 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 about recent events involving water intrusion into electrical equipment that resulted
in reactor trips. The NRC expects recipients to review the information for applicability to their
facilities and to consider actions, as appropriate, to avoid similar problems. The suggestions
contained within this IN are not NRC requirements; therefore, no specific action or written
response is required.
DESCRIPTION OF CIRCUMSTANCES
Calvert Cliffs Nuclear Power Plant, Units 1 and 2
On February 18, 2010, Calvert Cliffs Nuclear Power Plant experienced a dual-unit reactor trip
with complications in Unit 2. The event started in Unit 1 when water leakage through the roof of
the auxiliary building caused an electrical ground on a 13-kilovolt (kV) bus that tripped one of
the four reactor coolant pumps (RCPs), resulting in an automatic reactor trip of Unit 1 on reactor
coolant system low-flow. This electrical ground was not isolated close to the source because of
a failed ground protection relay in the feeder breaker from a 500-kV/13-kV service transformer
in Unit 2 that provides an alternate source of power to the RCP buses in Unit 1. This incident
resulted in the actuation of upstream protective relays and the subsequent deenergizing of a
500-kV/13-kV service transformer in Unit 2, resulting in the loss of power to all Unit 2 RCPs and
an automatic reactor trip of Unit 2 on reactor coolant low-flow.
A subsequent review found that the licensee had previously recognized the degraded condition
of the auxiliary building roof but left it unresolved for approximately 7 years before the dual-unit
trip. The degraded condition of the roof led to a failure of the auxiliary building to protect safety- related switchgear from environmental conditions. After the trip, the licensees corrective
actions to prevent recurrence included (1) improving processes for the categorization, prioritization, and management of roofing issues, (2) revising the relay calibration procedures to perform a final as-left pickup verification to ensure that a relay was not damaged during
maintenance, and (3) revising the procedure to keep both breakers normally open in the
alternate source of power to the Unit 1 RCP buses from a 500-kV/13-kV service transformer in
Unit 2 and to the Unit 2 RCP buses from a 500-kV/13-kV service transformer in Unit 1.
More information is available in Calvert Cliffs Nuclear Power PlantNRC Special Inspection
Report 05000317/2010006 and 05000318/2010006; Preliminary White Finding, dated
June 14, 2010 (Agencywide Documents Access and Management System (ADAMS) Accession
No. ML101650723); Licensee Event Report (LER) 317-2010-001, Reactor Trip due to Water
Intrusion into Switchgear Protective Circuitry, dated May 27, 2010 (ADAMS Accession
No. ML101530461); and LER 318-2010-001, Reactor Trip due to Failure of Protective Relay
Circuitry, dated May 27, 2010 (ADAMS Accession No. ML101530459). In addition to the
previously discussed issues, these documents provide details on (1) the failure of the
emergency diesel generator in Unit 2 to power a safety-related 4-kV bus because an Agastat
relay in the starting circuit of the emergency diesel generator timed out early, thus introducing
the trip signal for the low lube oil pressure before the lube oil pressure increased sufficiently to
clear this signal, (2) the licensees failure to adequately evaluate and correct a history of
degraded CO-8-type ground fault and overcurrent relays, and (3) the licensees failure to set the
actual relay settings for phase overcurrent protection to the values specified in design-basis
calculations.
Braidwood Station, Units 1 and 2
On August 16, 2010, Braidwood Station experienced a dual-unit reactor trip. Unit 2 tripped first, and Unit 1 tripped approximately 13 minutes later. In the main generator isophase bus
framework of Unit 2, a deionizer fin detached from a section of the crossover damper assembly, which caused a ground fault that resulted in a generator lockout and reactor trip. Following the
reactor trip, the Unit 2 condenser hotwell level began to rise, thus requiring the redirection of
condensate from the hotwell to the condensate storage tank. A design deficiency caused the
redirection of the condensate to the auxiliary feedwater (AFW) suction header, resulting in the
discharge of approximately 12,000 gallons of water from the AFW suction header standpipes in
Unit 2 to the turbine deck floor of the turbine building. This volume of water flowed down
through floor openings to the lower elevations of the turbine building that is shared between
both units at Braidwood Station.
The water leaked into the 4-kV switchgear in Unit 1 and caused a ground fault that tripped a
4-kV breaker, deenergizing downstream 480-V switchgear and motor control centers.
Circulating water pumps A and C in Unit 1 tripped because of a loss of power, resulting in a low
condenser vacuum that caused a turbine trip and a reactor trip in Unit 1. In addition, a loss of
power to the motor control centers resulted in a loss of power to the condenser steam dump
valves, which then could not be opened for the normal removal of decay heat. Operators used
the power-operated relief valves of the steam generator to remove decay heat and to maintain
temperature and pressure in Unit 1.
The licensee installed AFW system suction header standpipes in 1986 to provide a standing
head of water to address a low net positive suction head problem when the motor-driven AFW
pumps were started. However, this design change introduced the potential for water spills from
the standpipes because the AFW suction header is also used as the flowpath of condenser hotwell reject water from the hotwell to the condensate storage tank. At certain flow rates, large
amounts of water can be drawn up into the standpipes and could spill onto the turbine deck
because the standpipes are open to the atmosphere on the common turbine building. The
licensee observed water overflowing from the AFW standpipes onto the floor of the turbine
building on multiple occasions; however, it did not correct the condition because it had assessed
that the overflow of the water did not have a significant impact on plant operations.
The licensees corrective actions included (1) limiting the condensate storage tank level to
prevent another AFW standpipe siphon event, (2) implementing an operating configuration
change to valves controlling a condensate hotwell rejection, (3) developing a plan to identify
long-term water leaks, spills, other uncontrolled fluid conditions or any degraded or abnormal
conditions. In addition, the licensee planned to install a design feature on the AFW suction
standpipe to prevent recurrence of water spill events via that pathway.
More information is available in Braidwood Station, Units 1 and 2, NRC Special Inspection
Team (SIT) Report 05000456/2010010; 05000457/2010010, dated November 12, 2010
(ADAMS Accession No. ML103190505); Braidwood Station, Unit 2, LER 457-2010-003, Reactor Trip Caused by Phase to Ground Fault of a Failed Crossover Damper/Deionizer
Assembly due to an Inadequate Inspection Acceptance Criteria and Preventive Maintenance
Inspection Frequency, dated October 15, 2010 (ADAMS Accession No. ML102880320); and
Braidwood Station, Unit 1, LER 456-2010-001-01, Reactor Trip due to Water Intrusion in
Breakers Causing Circulating Water Pump Trips and Resulting in Loss of Condenser Vacuum, dated March 2, 2011 (ADAMS Accession No. ML110620091).
Grand Gulf Nuclear Station, Unit 1
On March 8, 2010, Grand Gulf Nuclear Station, Unit 1, experienced an automatic reactor scram
caused by low reactor water level. An erroneous signal caused the minimum flow valve on
reactor feedwater pump B (RFP B) to fail open. The erroneous signal was generated as a result
of an electrical short that was caused by condensation in a junction box that contained
feedwater suction flow transmitter cables. As RFPs A and B attempted to increase speed to
restore normal feedwater flow, the RFP A turbine control valve experienced mechanical binding, which caused RFP A to trip. The reduction in feed flow caused the reactor water level to
decrease to the reactor protection system actuation setpoint level.
The licensee found that the junction box where the feedwater suction flow transmitter fault
originated was full of condensed water from a steam leak from valve packing in the feedwater
heater room. An inadequate splice assembly allowed water to penetrate the cable jacket at the
spliced connection, thus causing a short that affected the signal from the flow transmitter and
driving the minimum flow valve open. The licensee discovered the steam leak approximately
1 month before the reactor scram event. However, the licensee had not thoroughly evaluated
the potential effects of the leak and, as a result, did not immediately correct the condition.
Before restarting the unit, the licensee inspected electrical boxes in the turbine building and
drained several boxes where water had accumulated. The licensee identified boxes that
contained water and inadequate cable splices and repaired the splices as necessary. More information is available in Grand GulfNRC Integrated Inspection Report 05000416/2010002, dated May 4, 2010 (ADAMS Accession No. ML101250383), and Grand
Gulf Nuclear Station, Unit 1, LER 416-2010-001, Automatic Reactor Scram on Decreasing
Reactor Water Level due to Inadvertent Reactor Feed Pump Trip, dated May 5, 2010 (ADAMS
Accession No. ML101250552).
DISCUSSION
In each event described in this IN, an electrical fault occurred as a result of water intrusion into
electrical equipment. In each case, the licensee had previously recognized the source of the
water but had not corrected it. For systems within the scope of Appendix B, Quality Assurance
Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, to 10 CFR Part 50,
Criterion XVI, Corrective Action, requires licensees to establish measures to assure that
conditions adverse to quality be promptly identified and corrected.
The timely corrective actions to assess and prevent water intrusion into electrical equipment can
help prevent significant events and ensure nuclear plant safety.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or to the appropriate NRC Office of Nuclear
Reactor Regulation project manager.
/RA/
Timothy J. McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: David Garmon, NRR
301-415- 3512
301-415- 3730
E-mail: David.Garmon@nrc.gov
E-mail: Vijay.Goel@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ML101250383), and Grand
Gulf Nuclear Station, Unit 1, LER 416-2010-001, Automatic Reactor Scram on Decreasing
Reactor Water Level due to Inadvertent Reactor Feed Pump Trip, dated May 5, 2010 (ADAMS
Accession No. ML101250552).
DISCUSSION
In each event described in this IN, an electrical fault occurred as a result of water intrusion into
electrical equipment. In each case, the licensee had previously recognized the source of the
water but had not corrected it. For systems within the scope of Appendix B, Quality Assurance
Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, to 10 CFR Part 50,
Criterion XVI, Corrective Action, requires licensees to establish measures to assure that
conditions adverse to quality be promptly identified and corrected.
The timely corrective actions to assess and prevent water intrusion into electrical equipment can
help prevent significant events and ensure nuclear plant safety.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contacts listed below or to the appropriate NRC Office of Nuclear
Reactor Regulation project manager.
/RA/
Timothy J. McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: David Garmon, NRR
301-415- 3512
301-415- 3730
E-mail: David.Garmon@nrc.gov
E-mail: Vijay.Goel@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
ADAMS Accession No.: ML110450487
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