Information Notice 2010-20, Repetitive Failures of Turbine Driven Auxiliary Feedwater Pumps Due to Ineffective Corrective Actions
| ML101670005 | |
| Person / Time | |
|---|---|
| Issue date: | 09/24/2010 |
| From: | Mcginty T Division of Policy and Rulemaking |
| To: | |
| Beaulieu, D P, NRR/DPR, 415-3243 | |
| References | |
| IN-10-020 | |
| Download: ML101670005 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 September 24, 2010
NRC INFORMATION NOTICE 2010-20: TURBINE-DRIVEN AUXILIARY FEEDWATER
PUMP REPETITIVE FAILURES
ADDRESSEES
All holders of an operating license or construction permit 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 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 recent operating experience involving repetitive failures of turbine-driven auxiliary
feedwater (TDAFW) pumps. The lessons learned from these events may apply to turbine- driven pumps in other systems such as reactor core isolation cooling and high-pressure coolant
injection systems. The NRC expects recipients to review the information for applicability to their
facilities and consider actions, as appropriate, to avoid similar problems. The suggestions in
this IN are not NRC requirements; therefore, no specific action or written response is required.
DESCRIPTION OF CIRCUMSTANCES
Fort Calhoun Station
On February 17, 2010, the TDAFW pump at the Fort Calhoun Station (Fort Calhoun) failed a
surveillance test when it tripped on high turbine exhaust backpressure approximately
20 seconds after starting. In the unique design of the Coffin turbine pump at Fort Calhoun, the
turbine has an exhaust backpressure trip mechanism consisting of a trip piston that is actuated
from the turbine exhaust line that actuates a trip latch and reset lever. A high exhaust
backpressure causes the trip piston to extend and push up on the trip latch, unlatching it from
the reset lever. The reset lever, through linkages, depressurizes the TDAFW pump control oil
pressure and closes the turbine steam inlet valve.
In five instances between 2001 and 2010, the licensee found that the trip latch had unlatched
from the reset lever, most likely due to personnel working near the backpressure trip
mechanism bumping into the latch. However, the licensees actions to understand the cause
and prevent additional instances of the mechanism unlatching were ineffective. When the
mechanism becomes fully unlatched, a control room alarm alerts operators to the condition.
The licensee had not recognized that the mechanism was susceptible to a partial unlatched
condition in which sufficient engagement of the trip latch and reset lever existed to prevent the
actuation of the alarm but not enough to prevent the TDAFW pump from tripping when started.
In light of the February 17, 2010, failure, the licensee reexamined two other recent failures of
the TDAFW pump that occurred on February 26, 2009, and April 6, 2009. The licensee was
unable to duplicate the exact conditions that caused any of these three failures. Instead, the
most likely root cause of the events was determined from data collected during the failure
analysis that occurred following each of the pump trips.
The cause of the first failure, on February 26, 2009, was air entrainment in the pump oil system, following a maintenance activity from earlier that day. A high point in the oil system was created
during the oil tubing replacement modification in 2001. The licensee was not aware that air
could collect in the high point of the oil system and affect the starting of the pump, and therefore
provided no procedural guidance for ensuring that air was vented following maintenance that
affected the tubing. The April 6, 2009, TDAFW pump trip was due to an actuation of the pump
high discharge pressure switch. Following a pump configuration change on February 26, 2009, the operating discharge pressure of the pump was much closer to the high discharge pressure
setpoint. The licensee did not sufficiently account for the even higher discharge pressure that
occurs during pump startup.
The three TDAFW pump failures resulted from inadequate design changes or involved an
insufficient understanding of the control systems and the baseline transient performance
characteristics of the turbine. Both internal and industry operating experience were available, and if properly analyzed and applied could have prevented the pump trips. Following a
comprehensive review of the three trips, the licensee took corrective actions, which included
providing additional training for engineering personnel on the control systems for the TDAFW
pump. They also took actions to prevent the inadvertent bumping of the TDAFW pump
backpressure trip mechanism.
Additional information is available in Fort Calhoun/NRC Special Inspection Report 05000285/2010-006, dated August 12, 2010, and can be found on the NRCs public Web site
under Agencywide Documents Access and Management System (ADAMS) Accession
No. ML102250215.
Robert E. Ginna Nuclear Power Plant
On December 2, 2008, the TDAFW pump at the Robert E. Ginna Nuclear Power Plant (Ginna)
failed a surveillance test when it was unable to develop the acceptable minimum discharge flow
and pressure. Following review of the incident, the licensee determined that binding of the
governor control linkage caused the failure. The binding occurred because the licensee had
incorrectly removed the task of cleaning and lubricating the linkage from the work scope of the
previous maintenance window.
On May 26, 2009, the TDAFW pump tripped on overspeed during a surveillance test. The
licensee replaced several components in the pumps lube oil system and adjusted the governor
valve linkage. However the licensee had not yet completed their root cause evaluation to
identify the definitive cause for the pump trip when the TDAFW pump again tripped on
overspeed during a surveillance test on July 2, 2009. The licensee evaluation determined that the stem of the governor control valve had become bound to its bushing because of corrosion
buildup.
In April 2005, the licensee found corrosion on the stem of the governor control valve during a
routine scheduled maintenance inspection and replaced the valve stem as part of the
maintenance activity, but took no further action to determine the cause of the corrosion. In
July 2005, the licensee identified that the TDAFW pump steam admission valves were leaking.
Attempts to stop the leakage by cycling the valves in October 2006 and attempts to rebuild the
valves during the subsequent refueling outage reduced the leakage but were unsuccessful at
completely stopping it. At this time, the licensee did not make the connection between the
leaking steam admission valves and the earlier corrosion on the stem of the governor control
valve; therefore, it took no measures to increase the frequency of inspections and maintenance
on the governor control valve stem. The NRC conducted a special inspection of this event and
determined that the corrosion on the governor valve stem was the likely cause of the
May 26, 2009, surveillance test failure and may have contributed to the December 2, 2008, failure.
The NRC issued IN 94-66, Overspeed of Turbine-Driven Pumps Caused by Governor Valve
Stem Binding, on September 19, 1994, and IN 94-66, Supplement 1, Overspeed of Turbine- Driven Pumps Caused by Binding in Stems of Governor Valves, on June 16, 1995 (ADAMS
Accession Nos. ML031210648 and ML031060370, respectively) to describe binding in the
stems of governor control valves caused by corrosion that has been accelerated by leaking
steam admission valves. When the licensee first encountered corrosion on the stem of the
governor control valve in 2005, they missed the opportunity to use previous operating
experience to help fully resolve the problem. The licensee inspected and reworked the steam
admission valves and the governor control valve and enhanced its TDAFW surveillance
program. To further address the issue, the licensee worked with the original equipment
manufacturer to redesign the governor control valve to be less susceptible to corrosion.
Additional information is available in Ginna/NRC Special Inspection Team
Report 05000244/2009008, dated November 12, 2009, and on the NRCs public Web site under
ADAMS Accession No. ML093160122.
Tihange Nuclear Power Station, Unit 2 (Belgium)
During a 5-month period beginning in October 2005, Tihange Nuclear Power Station, Unit 2, experienced three overspeed trips of the TDAFW pump; the second and third trips occurred
during demand starts following a reactor trip. After the first two pump trips, plant personnel
attributed the excessive moisture accumulation in the turbine during the turbine start as the
cause of the overspeed trip. However, the plant initiated a more comprehensive analysis only
after the third trip. Further tests indicated a lack of synchronization in the operation of the steam
inlet valve and the speed regulation valve. Plant personnel also noted that the accumulation of
moisture was strongly dependent on the external temperature. The plant addressed this
moisture accumulation issue by adding insulation, adjusting the opening times of the steam inlet
valve and speed regulating valve, and increasing the surveillance frequency from quarterly to
daily. As surveillances were successfully completed, the plant gradually relaxed the frequency
to the previous quarterly schedule. The pump functioned properly until it tripped on overspeed during a surveillance test on
October 16, 2006, and on October 30-31, 2006. Plant personnel noted that the external
temperature on these days was much colder than usual, but took no other actions except to
establish an auxiliary feedwater working group in early 2007.
In September 2008, the pump failed to start during its quarterly surveillance when the steam
inlet valve failed to open. Plant personnel performed testing and replaced the packing for the
steam inlet valve, but the surveillance failed two more times in October 2008. They then
identified a failed spring in the pneumatic servomotor of the steam inlet valve. Replacing the
spring was effective in correcting the overspeed trip problem.
Plant personnel addressed the direct causes of the pump failure several times but failed to
identify the possible underlying causes of the failures. If the plant had followed a
recommendation, made after the first series of failures, to install instrumentation to monitor the
transient behavior of the steam inlet valve, it might have been possible to identify the problem
earlier. However, this recommendation was not implemented before plant personnel resolved
the issue. The plant has since determined that establishing a multidisciplinary working group to
address failures of complex safety systems is a successful approach for determining the root
cause of the failure and for making appropriate changes to maintenance, surveillance, and
training programs to ensure the continued availability and reliability of the system.
DISCUSSION
This IN discusses operating experience involving repetitive failures of TDAFW pumps
risk-significant components that must be operable as specified in technical specifications. The
licensees in the above examples did not meet Criterion XVI, Corrective Action, of Appendix B,
Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing Plants, to
10 CFR Part 50, which requires licensees to establish measures to assure that conditions
adverse to quality be promptly identified and corrected. This failure in the area of problem
identification and resolution by both licensees resulted from a failure to properly implement their
corrective action program.
The above examples illustrate the importance of ensuring that any condition adverse to quality
affecting the TDAFW system is fully understood so that appropriate corrective actions can be
taken. Repetitive failures, even if the physical failure mechanisms are different, may indicate
that although the direct cause of the original condition was addressed, the root cause remains
uncorrected. A thorough review of previous industry operating experience, not only for TDAFW
systems but also for turbine-driven pumps in the reactor core isolation cooling and
high-pressure coolant injection systems, can assist in determining a course of action. Likewise, a thorough knowledge of plant operating experience, including the expected baseline
characteristics of the systems, may allow the licensee to diagnosis impending problems before
a failure actually occurs. A multidisciplinary root cause analysis that explores all possible
causes of the failure, not only to determine what actually failed but also to determine how that
failure may have occurred, could be especially useful for cases of intermittent or seemingly
unconnected failures that have different direct causes but may have a deeper, unresolved
problem. Corrective actions that address not only the failed component but also the inadequate
processes that allowed the component to fail are more likely to be effective in preventing
recurrence.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or to the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
/RA/
Timothy J. McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contact:
Rebecca Sigmon, NRR Michael Chambers, RIV
301-415-4018 402-825-5657 E-mail: Rebecca.Sigmon@nrc.gov E-mail: Michael.Chambers@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.
CONTACT
This information notice 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/
Timothy J. McGinty, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: Rebecca Sigmon, NRR Michael Chambers, RIV
301-415-4018 402-825-5657 Rebecca.Sigmon@nrc.gov Michael.Chambers@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.
ADAMS Accession Number: ML101670005 ME3966 OFFICE IOEB:DIRS RIV:DRP TECH EDITOR BC:IOEB:DIRS
NAME RSigmon MChambers (email) KAzariah-Kribbs(e-mail) JThorp
DATE 7/29/10 9/20/10 7/7/2010 8/12/10
NAME SMoore GCasto WRuland
DATE 9/22/10 8/12/10 9/14/10
OFFICE LA:PGCB:NRR PM:PGCB:NRR BC:PGCB:NRR D:DPR:NRR
NAME CHawes DBeaulieu SRosenberg TAlexion for TMcGinty
OFFICE 09/22/10 9/20/10 9/23/10 9/24/10
OFFICIAL RECORD COPY