Information Notice 2004-10, Loose Parts in Steam Generators
| ML041170480 | |
| Person / Time | |
|---|---|
| Issue date: | 05/04/2004 |
| From: | Beckner W NRC/NRR/DIPM/IROB |
| To: | |
| Hodge, CV, NRR/DIPM/IROB, 415-1861 | |
| References | |
| IN-04-010 | |
| Download: ML041170480 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555 May 4, 2004 NRC INFORMATION NOTICE 2004-10: LOOSE PARTS IN STEAM GENERATORS
Addressees
All holders of operating licenses for pressurized-water reactors (PWRs), except those who have
permanently ceased operations and have certified that fuel has been permanently removed
from the reactor.
Purpose
The U.S. Nuclear Regulatory Commission is issuing this information notice to inform
addressees about loose parts found in steam generators. This notice discusses only a few
illustrative examples of loose parts found in steam generators. The NRC anticipates that
recipients will review the information for applicability to their facilities and consider taking
actions, as appropriate, to avoid similar issues. However, no specific action or written response
is required.
Description of Circumstances
In its fall 2003 inspection, Exelon Generation Company found several loose parts in its
Braidwood Unit 2 steam generators. For example, an object 3.18 cm (1.25 inch) long and
1.91 cm (0.75 inch) in diameter was discovered during a secondary-side visual inspection of
one steam generator. Exelon then reviewed the eddy current data for the tubes surrounding
the part and in hindsight identified tube wear in one tube and a possible loose part signal in
three tubes. The tube wear caused by the loose part measured 38 percent through-wall as
determined by a +PointTM coil. The indication of tube wear was present in prior cycles, but was
not reported because the indication was distorted due to its location (i.e., near an expanded
preheater baffle plate support).
In another steam generator, Exelon identified a piece of weld slag measuring 2.86 cm (1.125 inches) by 2.54 cm (1 inch) by 0.89 cm (0.35 inch) on the top of the cold-leg tubesheet. The
weld slag was originally identified during a foreign object search-and-retrieval inspection in
2002, but could not be retrieved. No tube wear was associated with this object. Four tubes
surrounding this object were plugged and stabilized in 2003. In addition, Exelon found two
manufacturing fitup bars (also referred to as backing bars) measuring 2.54 cm (1 inch) by
2.54 cm (1 inch) by 7.62 cm (3 inches) on top of a preheater baffle plate. These bars were
used to assist in the assembly of the steam generator and were welded on the bottom of
another preheater baffle plate. These fitup bars serve no structural or operational function.
After visually identifying the presence of these fitup bars, the licensee could ascertain from
eddy current data that one of these bars had been present on the top of the preheater baffle
plate since the spring of 1990 and the other since the fall of 1994. These bars resulted in tube
wear.
One bar caused two wear scars (with maximum depths of 28 percent and 21 percent
through-wall) in one tube and the other bar caused one wear scar (maximum depth of 5 percent
through-wall). After visually identifying these fitup bars in 2003, Exelon attributed a 39-percent
through-wall volumetric indication detected in a neighboring tube in 1994 and plugged in 1997 to wear associated with the fitup bar. These two backing bars could not be removed from the
steam generator, so Exelon plugged and stabilized all of the tubes surrounding them. As part
of its evaluation, Exelon also assessed the potential for other fitup bars to cause tube damage
(refer to NRC Letter to Exelon Nuclear, Subject: Summary of Conference Call With Exelon
Nuclear Regarding the 2003 Steam Generator Inspections at Braidwood Unit 2, January 15,
2004, ADAMS Accession No. ML033580377).
In its fall 2003 inspection, Nuclear Management Company (NMC) reported several potential
loose part indications on the tubes in the hot leg top of the tubesheet region in the Prairie Island
Unit 2 steam generators. These potential loose part indications were present in previous
inspections and have not resulted in any tube wear. NMC typically performs visual inspection of
the top of the tubesheet and a search for loose parts in the annulus region of one steam
generator during each outage regardless of indications of possible loose parts. Before
resuming plant operation, NMC evaluates and resolves possible loose part indications which
include an inspection of these indications with a rotating probe. (NRC Letter to NMC, Subject:
Prairie Island Nuclear Generating Plant, Unit 2 - Summary of Conference Call With Nuclear
Management Company, LLC Regarding the 2003 Steam Generator Inspections, November 26,
2003, ADAMS Accession No. ML033210569).
On June 22, 2002, Exelon shut down its Byron Unit 2 plant when the steam generator primary- to-secondary leak rate reached a level of 284 liters (75 gallons) to 303 liters (80 gallons) per
day. A subsequent investigation into the source of the leak identified one leaking tube. In this
leaking tube, a volumetric flaw was identified above a cold-leg tube support near the feedwater
inlet. Inspections performed around the leaking tube resulted in the identification of several
other flaws. Given the nature (volumetric) and location of the signals (periphery near feedwater
inlet), the flaws were attributed to wear from a loose part; however no loose parts were
observed during the eddy current inspection. The leaking tube and other degraded tubes were
plugged and stabilized. In its September 2002 refueling outage inspection, Exelon performed
sludge lancing and foreign object search-and-retrieval inspections and confirmed the presence
of loose parts in the Byron Unit 2 steam generators, including the parts believed to have
resulted in the leaking tube (NRC Letter to Exelon, Subject: Byron Station, Unit 2, Summary of
Conference Calls with Exelon Generation Company Regarding Its 2002 Steam Generator Tube
Inspection Results, November 8, 2002, ADAMS Accession No. ML022950043).
On May 10, 2002, Wolf Creek Nuclear Operating Corporation detected indications of an
unusual noise coming from steam generator D followed by an alarm in the plants loose parts
monitoring system (NRC Memorandum, Subject: Response to Request for Information on
Loose Parts Found In the Steam Generator at Wolf Creek Generating Station, August 27, 2002, ADAMS Accession No. ML022340708). After a detailed evaluation of available information, plant management directed the shutdown of Wolf Creek on May 13, 2002, in order to identify
the source of the noise. During its inspections, the licensee retrieved two loose parts, a guide
tube support pin nut, and a locking device from the primary side of steam generator D. There
were no indications within the steam generator of serious damage to the tubes, tubesheet, welds, or divider plate. The loose parts did not result in any primary-to-secondary tube leakage.
In the subsequent refueling outage (fall 2003), detailed inspections of steam generator D
resulted in repairs to the tube sheet and a mechanical plug. An additional loose part from the
split pin was identified to be lodged in one of the steam generator tubes which resulted in the
plugging of that tube. Additionally, all guide tube split pins were replaced with stainless steel
split pins.
Discussion:
Loose parts have been found in steam generators for a number of years (refer to NRC
Information Notice 83-24, Loose Parts In the Secondary Side of Steam Generators at
Pressurized Water Reactors, dated April 28, 1983). These parts may be introduced into the
steam generators from maintenance activities or degradation in primary- or secondary-system
components. Some of these parts have resulted in degradation of the steam generator tubes, and in some cases have led to tube leaks. Tube degradation may be the result of the
mechanical interaction between the loose part and the tube or may be the result of chemical
impurities (e.g., lead, chlorides) introduced into the steam generator by the part. Licensees
have found that to ensure that loose parts do not impair tube integrity, it is important to take
measures to limit their introduction into the steam generator. In addition, it is important to have
programs for detecting and monitoring loose parts in case loose parts are introduced into steam
generators. Recent operating experience of most plants indicates that loose parts have not
significantly affected tube integrity; however, they have resulted in tube degradation. Since
many licensees are beginning to extend the operating interval between tube inspections
(especially at plants with advanced tube materials), it is important to ensure that programs
continue to effectively limit the introduction of loose parts, promptly detect loose parts that do
enter, and implement appropriate corrective action upon identification of loose parts in steam
generators.
Licensees can take actions to prevent the introduction of loose parts into the steam generators.
By preventing the introduction of loose parts into the primary and secondary system, damage
from loose parts will be avoided. Procedures for precluding the introduction of loose parts into
the primary and secondary system typically include (a) detailed accountability for all tools and
equipment used during a maintenance operation, (b) appropriate controls on foreign objects
such as eyeglasses and film badges including barriers to keep material from dropping into a
system (e.g., feedwater), (c) cleanliness requirements, (d) accountability procedures for
components and parts removed from major components (e.g., reassembly of cut and removed
components), and (e) post-maintenance inspections.
Actions taken in response to the detection of a loose part in a steam generator vary depending
on the nature and location of the part and its effect on tube integrity. In many cases, parts are
removed from a steam generator upon detection. However, retrieval of a loose part is not
always practical or possible. In these cases, an engineering evaluation will determine whether
the part will impair tube integrity if it is left in service. This evaluation may result in preventive tube plugging and stabilization of the affected and surrounding tubes or may result in other
corrective actions such as insertion of additional access points in the steam generator shell to
provide access for removing the part.
The above examples and other operating experience illustrate several important points with
respect to managing steam generator loose parts. For example, verifying the integrity of
primary- and secondary-system components (such as support pins and fitup bars) may prevent
the introduction of loose parts into the steam generator from these sources. Operating
experience also indicates that loose part monitoring systems may provide an early indication
that a part is in a steam generator, permitting prompt corrective action to be taken before any
significant tube damage occurs. The importance of supplementing the steam generator tube
eddy current examinations with secondary side visual inspections is also supported by recent
operating experience indicating that some loose parts may not be detected during the eddy
current examination. In addition, operating experience indicates that tube damage from minor
loose parts may not always be detectable due to the presence of interfering signals.
This information notice does not require any specific action or written response. If you have
any questions about the information in this notice, please contact one of the technical contacts
listed below or the appropriate project manager in the NRCs Office of Nuclear Reactor
Regulation (NRR).
/RA/
William D. Beckner, Chief
Reactor Operations Branch
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical Contact:
John Tsao, NRR Kenneth Karwoski, NRR
301-415-2702 301-415-2752 E-mail: jct@nrc.gov E-mail: kjk1@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
OFFICE OES:IROB:DIPM TECH EDITOR EMCB:DE EMCB:DE
NAME CVHODGE JCTSAO ALLUND
DATE 04/06/2004 02/12/2004 04/06/2004 04/19/2004 OFFICE SC:OES:IROB:DIPM C:IROB:DIPM C:EMCB:DE
NAME CJACKSON WDBECKNER WHBATEMAN
DATE 04/27/2004 05/04/2004 04/20/2004
Attachment LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2004-09 Corrosion of Steel 04/27/2004 All holders of operating licenses
Containment and Containment for nuclear power reactors except
Liner those who have permanently
ceased operation and have
certified that fuel has been
permanently removed from the
reactor vessel.
2004-08 Reactor Coolant Pressure 04/22/2004 All holders of operating licensees
Boundary Leakage Attributable for nuclear power boiling-water
to Propagation of Cracking in reactors (BWRs), except those
Reactor Vessel Nozzle Welds who have permanently ceased
operations and have certified that
fuel has been permanently
removed from the reactor vessel.
2004-07 Plugging of Safety Injection 04/07/2004 All holders of operating licenses
Pump Lubrication Oil Coolers or construction permits for
with Lakeweed nuclear power reactors, except
those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel.
2004-06 Loss of Feedwater Isokinetic 03/26/2004 All holders of operating licensees
Sampling Probes at Dresden for nuclear power reactors except
Units 2 and 3 those who have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor vessel.
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To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following
command in the message portion:
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