Information Notice 2007-18, Operating Experience Regarding Entrainment of Gas or Debris Into Auxiliary Feedwater Systems
| ML071100075 | |
| Person / Time | |
|---|---|
| Issue date: | 05/13/2007 |
| From: | Michael Case NRC/NRR/ADRA/DPR |
| To: | |
| References | |
| IN-07-018 | |
| Download: ML071100075 (4) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 May 13, 2007 NRC INFORMATION NOTICE 2007-18: OPERATING EXPERIENCE REGARDING
ENTRAINMENT OF GAS OR DEBRIS INTO
AUXILIARY FEEDWATER SYSTEMS
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 operating experience regarding possible entrainment of air or debris into
auxiliary feedwater (AFW) systems, potentially affecting the operability of these systems. 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
Several events have occurred at nuclear power facilities involving possible entrainment of gas
or debris into AFW systems. Air or debris entrained in AFW systems can impact the capability
of the AFW pumps to perform their specified safety functions. Several recent events are
discussed below:
McGuire Nuclear Station
A design deficiency resulted in gas coming out of solution and collecting in the nuclear service
water system where the Train A AFW assured suction was located. When this was recognized, the licensee decided to use operator manual alignments to preclude gas entrainment into the
AFW pumps when alignment was to the standby nuclear service water pond. NRC inspectors
identified that the licensee was crediting non-seismic pipe for no less than 30 minutes after a
seismic event to allow time to perform the AFW valve lineup. Consequently, there was not
reasonable assurance that the non-seismic pipe would be available to support supplying water
to the AFW pumps. (NRC Integrated Inspection Report 05000369/2004005 and
05000370/2004005, October 8, 2004, Agencywide Documents Access and Management
System (ADAMS) Accession No. ML042860339)
The licensee subsequently initiated a piping modification and test program to address the issue.
(Duke Energy letters to NRC dated April 25, 2006, and September 7, 2006, ADAMS Accession
Nos. ML061230672 and ML062640070, respectively)
Davis-Besse Nuclear Power Station
NRC inspectors identified an inadequate procedure that could have resulted in sufficient air in
an AFW pump suction to make it inoperable. The procedure addressed the realignment of the
AFW pumps suction from the condensate storage tank to the service water system using a
pipe that was normally voided to preclude potential leakage through a closed valve that could
adversely affect steam generator chemistry. The voided 6-inch diameter pipe section was
11-feet long. The licensee responded with a procedure change to adequately fill the affected
piping prior to transferring the suction supply. (NRC Inspection Report 05000346/2005004, June 14, 2005, ADAMS Accession No. ML051660047)
Watts Bar Nuclear Plant
The Watts Bar backup safety-related water supply to the AFW system is provided by the
essential raw cooling water (ERCW) system. On January 13, 2006, the licensee discovered air
in a 12-inch ERCW pipe that supplies water to a motor-driven AFW pump and a turbine-driven
AFW pump and in an attached 30-inch pipe. The licensee determined that the air in the 12-inch
pipe was, in part, caused by inadequate filling and venting during work performed in the Cycle 6 refueling outage. The air in the 30-inch discharge pipe and a portion of the 12-inch pipe was
due to gas coming out of solution. Corrective actions included the establishment of continuous
vent paths for the 30-inch ERCW Train A and B discharge headers and the addition of
ultrasonic equipment for verification of the water level in the 30-inch headers and the 12-inch
header. (Licensee Event Report 2006-001-00, March 14, 2006, ADAMS Accession No.
Virgil C. Summer Nuclear Station
The backup source of water to the emergency feedwater pumps (the licensees designation for
the AFW pumps) is the service water system. In its inspection report, the NRC reported that
the safety-related water supply could become unavailable due to the accumulation of tubercles
or other debris and that the flow control valves were not designed to handle relatively unclean
service water. (NRC Inspection Report 05000395/2005007, March 10, 2005, ADAMS
Accession No. ML050700044)
San Onofre Nuclear Generating Station
The seismically qualified Enclosure Building surrounding the non-seismically qualified
condensate storage tank is designed to ensure sufficient condensate remains available if this
tank should fail during a design basis earthquake. An NRC inspection team found that the
licensee failed to follow procedural requirements and establish the necessary foreign material
exclusion areas. As a result, foreign materials could cause AFW operational problems
following a seismic event. Further, the licensee failed to properly address industry operating
experience related to foreign materials in AFW water sources. (NRC Inspection Report
05000361;362/2006009, December 8, 2006, ADAMS Accession No. ML063420342) Kewaunee Nuclear Power Plant
The AFW pump shaft seals require a small amount of leakage to maintain adequate cooling
and sealing. Leakage reduction or loss can cause loss of seal cooling and lubrication with
subsequent seal degradation that may allow air flow into the pump inlet if the inlet pressure is
sub-atmospheric, a condition that may occur following a steam line break. This may cause
pump damage due to air entrainment. (Licensee Event Report 2005-006-00, May 25, 2005, ADAMS Accession No. ML051530312 and NRC Inspection Report 05000305/2005010,
October 16, 2005, ADAMS Accession No. ML052290348)
BACKGROUND
Some of the issues of gas entrainment on AFW systems are similar to the concerns discussed
in NRC IN 2006-21, Operating Experience Regarding Entrainment of Air into Emergency Core
Cooling and Containment Spray Systems, dated September 21, 2006, ADAMS Accession No.
DISCUSSION
Licensees rely on backup water supplies to AFW systems in response to low probability events, such as seismic and station blackout events, that render normal and preferred water sources
unavailable. The above events indicate that use of backup water sources or other unique and
potentially overlooked conditions may introduce air or debris that could render AFW inoperable
and illustrate the importance of having developed a complete understanding of the implications
of gas or debris in AFW systems. This is particularly important for operational aspects that are
not tested, as illustrated by the above events where the likelihood of being in the condition was
small, but the condition impacted defense-in-depth and resulted in a failure to comply with the
plant design basis.
CONTACT
This information notice requires no specific action or written response. Please direct any
questions about this matter to the technical contact listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA/
Michael J. Case, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contact:
Warren C. Lyon, NRR
301-415-2897 e-mail: wcl@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 contact listed below or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.
/RA/
Michael J. Case, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contact:
Warren C. Lyon, NRR
301-415-2897 e-mail: wcl@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: ML071100075 OFFICE SPWB:DSS TECH EDITOR BC:SPWB:DSS DD:DSS D:DSS
NAME WLyon LCulp by e-mail GCranston JWermiel TMartin
DATE 04/23/07 04/27/07 04/23/07 05/10/07 05/10/07 OFFICE PGCB:DPR PGCB:DPR BC:PGCB:DPR D:DPR
NAME DBeaulieu CHawes CJackson MCase
DATE 05/08/07 05/10/07 05/11/07 05/13/07 OFFICIAL RECORD COPY