Information Notice 2005-29, Steam Generator Tube and Support Configuration

Steam Generator Tube and Support Configuration

ML052280011
Person / Time
Issue date:	10/27/2005
From:	Hiland P NRC/NRR/DIPM/IROB
To:
Yoder M, NRR/DE/EMCB, 415-4017
References
IN-05-029
Download: ML052280011 (6)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 October 27, 2005 NRC INFORMATION NOTICE 2005-29: STEAM GENERATOR TUBE AND SUPPORT

CONFIGURATION

ADDRESSEES

All holders of operating licenses or construction permits for pressurized water reactors (PWRs)

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 to describe

recent experience in which the configuration of steam generator tube supports or expansions

was different than expected. It is expected that recipients will review the information for

applicability to their facilities and consider actions, as appropriate, to avoid similar problems.

However, suggestions contained in this information notice are not NRC requirements; therefore, no specific action or written response is required.

DESCRIPTION OF CIRCUMSTANCES

The following describes the as-found condition of the steam generators at Byron Unit 1, Davis- Besse, and Waterford Unit 3 during inspections performed during 2005.

Byron Unit 1

Byron Unit 1 replaced the four recirculating steam generators in 1997 with steam generators

designed and fabricated by Babcock and Wilcox International. Each steam generator has

6,633 thermally treated Alloy 690 U-tubes. The U-tubes are supported by stainless steel lattice

grid structures along the straight portion of the tube and by a collector bar and fan bars in the

U-bend region. All fan bars are connected to the collector bar.

During the 2005 steam generator tube inspections, an evaluation of the bobbin coil eddy current

data revealed that the collector bar did not completely engage all of the row 1 tubes in steam

generator B as expected. Investigations discovered that the collector bar engaged, or partially

engaged, only 10 of the 67 row 1 tubes on the hot-leg side of the steam generator, rather than

engaging all 67 of the row 1 tubes. The collector bar was verified to engage all row 1 tubes on

the cold-leg side of the steam generator. Additionally, the collector bar was verified to be

engaged in all row 1 hot-leg and cold-leg tubes in the other three steam generators.

The nonengaged collector bar of the row 1 hot-leg tubes was identified by eddy current data

analysis software that was programmed to compare the existing support structure location with

the design locations of the support structures (landmarks). Following identification of this

condition, a review of past inspection data revealed that this condition existed before the

replacement steam generators were placed into service. The licensee concluded that the

condition was not the result of inservice degradation. Since the as-found condition of the steam

generator was different than the condition analyzed during the design of the steam generator, an analysis was performed to show that the tubes remain adequately supported despite the

mispositioning of the collector bar. Concerned that the "disengaged" tubes would vibrate and

become worn at the lattice support or at the other collector bar location on the cold leg side, the

licensee reviewed eddy current inspections at those locations and found no wear to date. The

licensee concluded that flow-induced vibration is not increased due to the lack of contact with

the collector bar.

Davis-Besse

Davis-Besse has two once-through steam generators designed and fabricated by Babcock and

Wilcox International. Each steam generator contains approximately 15,500 Alloy 600 mill- annealed tubes. The tubes are partial-depth-expanded into both the upper and the lower

tubesheet.

Based on recent operating experience at other similarly designed plants, a significant number of

rotating probe inspections were performed at the tube end in the lower tubesheet (cold-leg) for

the first time during the 2005 outage at Davis-Besse. While performing these inspections, most

tubes (all but approximately 100 tubes) were identified as having two roll-expanded regions in

the lower tubesheet. The presence of two rolls (rather than one) in the lower tubesheet region

was not known by personnel responsible for determining potential forms of degradation that

could affect the tubes. A review of fabrication records indicated the second roll was installed

during the original manufacture of the steam generators since there was inadequate control of

the original roll in the lower tubesheet and many of the original rolls were too short. The tubes

were rerolled prior to annealing the steam generator. The second rolls are referred to as shop

rerolls to distinguish them from reroll repairs performed subsequent to commercial operation.

Approximately 30 tubes were plugged during the 2005 outage as a result of crack like

indications detected at the shop rerolls in the lower tubesheet. None of the crack like

indications were safety significant.

Waterford Unit 3

Waterford Unit 3 has two recirculating steam generators designed and fabricated by

Combustion Engineering. The mill-annealed Alloy 600 steam generator tubes are supported in

the straight portion of the tube by a number of carbon steel lattice grid (i.e., eggcrate) tube

supports and in the U-bend region by diagonal bars (also called batwings) and vertical straps.

Several of the lattice grid tube supports are referred to as partial eggcrate supports since they

only support some of the tubes. The tubes in rows 1 through 18 are U-bends and the tubes in

rows 19 through 147 are square bends (i.e., there are two 90-degree bends for tubes in rows

19 through 147). Routine eddy current testing of the steam generator tubes in 2005 identified that two diagonal

batwing supports in steam generator 2 had moved. The two batwings were displaced from their

nominal locations on the cold-leg side of tubes in columns 82, 83, and 84. These batwings

were at their nominal locations during the previous inspection. Wear scars were observed for

tubes in these columns during the 2005 inspections. These wear scars occurred in the free

span of the tube at the nominal axial location of the batwing and thus were apparently formed

before the displacement of the batwings. These wear indications were not observed during the

previous inspection. The depth of these wear indications ranged from 7- to 30-percent of the

tube wall thickness.

The batwing assembly is formed by two opposing diagonal bars connected by a short horizontal

bar. A visual inspection of the lower portion of the batwings confirmed that two batwings had

failed at the intersection of the horizontal bar and a slotted bar, which runs perpendicularly to

the horizontal bar and is keyed to the horizontal bar. The slotted bar holds the lower portion of

the batwing in place. The licensee concluded that the failure mechanism was fatigue, based on

the location of the failure, the length of the batwing (one of the longest in the steam generator),

and the flow in this region of the tube bundle. The loads on the batwing in this region are not

high enough to cause an overload-type failure.

As a result of these findings, several corrective actions were taken, including plugging and

stabilizing many tubes, performing analyses, and evaluating the integrity of the batwing-to- wrapper bar welds. The analyses were done to confirm that tube integrity will not be

compromised for the period of time between tube inspections if additional batwings fail. The

evaluations were done to ensure that the failed batwings would not become free to move

throughout the steam generator, (i.e., become loose parts). The batwing-to-wrapper bar welds

connect all of the batwings and are located on the outside of the tube bundle to permit access

for visual inspection.

BACKGROUND

High-cycle fatigue due to inadequate tube support (antivibration bar location anomalies) was

the cause of a tube rupture in 1987 (Bulletin 88-02, Rapidly Propagating Fatigue Cracks on

Steam Generator Tubes).

Steam generator internals can be important in ensuring that the tubes are capable of

performing their intended safety function (Generic Letter 97-06, Degradation of Steam

Generator Internals).

DISCUSSION

The positioning of the tube supports (e.g., antivibration bars, batwings, vertical straps, collector

bars, fan bars, lattice grids, support plates) is important in ensuring the tubes are adequately

supported. Inadequate tube support can result in increased tube vibration and increased tube

wear and fatigue. At Byron Unit 1, the nonengaged collector bar in the U-bend region of many row 1 tubes was

discovered through a review of inspection data. The data indicated that the support was not in

the correct position. This condition was identified by applying a computerized data-screening

algorithm to the inspection data. In the case of Waterford 3, the failed batwing support was

discovered by investigating several indications of wear that were apparently occurring at a

location not associated with a support structure. In both cases, the actual tube support

conditions were evaluated to identify any tube integrity concerns (e.g., flow-induced vibration).

Prompt identification of supports that are not in their proper position (as a result of fabrication or

from service-induced conditions) is important to ensure that appropriate actions are taken in a

timely manner before tube integrity is compromised.

At Davis-Besse, the presence of a second roll expansion in the lower tubesheet was not known

by responsible plant personnel. Roll expansions are locations of high stress and are more

susceptible to degradation. As a result, it is important to know all locations of high stress to

ensure that appropriate inspection techniques are used at those locations to detect possible

degradation.

At Waterford Unit 3, additional evaluations and inspections were performed to ensure that the

actual configuration of the failed batwings was stable. Since one end of each failed batwing

was free to move, the welds securing the other ends were evaluated to ensure the failed

batwings would not become free during operation and potentially impact other tubes.

Evaluating the as-found condition to ensure that continued degradation will not result in the

generation of loose parts is also important for ensuring tube integrity.

In summary, it is important to compare the as-found condition of the steam generator to the

steam generator design. This includes confirming that the as-found location of the steam

generator tube supports is consistent with the original design. Significant differences between

the as-found and as-designed location of the tube supports can result in increased tube

vibration and increased tube wear and fatigue. Promptly identifying significant differences

between the as-found and as-designed configuration may prevent a loss of tube integrity.

Similarly, monitoring the current condition and location of the tube supports is important to

ensure that any service-induced degradation or movement of the supports is promptly detected

and evaluated. In addition, it is important to know the actual configuration of the steam

generator tubes to ensure that all high-stress locations are identified in the degradation

assessment and inspected with techniques capable of detecting the forms of degradation that

may occur at those locations.

CONTACT

Please direct any questions about this matter to the technical contact(s) or the Lead Project

Manager listed below, or to the appropriate Office of Nuclear Reactor Regulation (NRR) project

manager.

/RA/

Patrick L. Hiland, Chief

Reactor Operations Branch

Division of Inspection Program Management

Office of Nuclear Reactor Regulation

Matthew Yoder, NRR

301-415-4017 E-mail: mgy@nrc.gov

Note: NRC generic communications may be found on the NRC public Website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ML052280011 OFFICE EMEB:DE Tech Editor EMCB:DE BC:EMCB:DE

NAME VHodge PKleene MGYoder WHBateman

DATE 10/13/2005 8/18/2005 10/13/2005 10/18/2005 OFFICE ADPT:DLPM ADPT:DLPM ADPT:DLPM TL:OES:IROB:DIPM

NAME GDick WAMacon KKalyanam IJung

DATE 10/21/2005 10/21/2005 10/20/2005 10/25/2005 OFFICE SC:OES:IROB:DIPM C:IROB:DIPM

NAME MJRoss-Lee PLHiland

DATE 10/27/2005 10/27/2005