ML19098B475

Forwarding Tech Bulletins from Westinghouse, Furnishing Information Concerning Steam Generator Tube Phenomena Referred as Denting

ML19098B475
Person / Time
Site:	Surry
Issue date:	12/11/1975
From:	Stallings C Virginia Electric & Power Co (VEPCO)
To:	Moseley N NRC/RGN-II
References
Serial No. 794
Download: ML19098B475 (9)

Text

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e VIRGINIA ELECTRIC AND POWER COMPANY RIOHMOND,VIRGINIA 23261 December 11, 1975 Mr. Norman C. Moseley, Director Office of Inspection and Enforcement U.S. Nuclear Regulatory Com.mission Region II - Suite 818 230 Peachtree Street, Northwest Atlanta, Georgia 30303

Dear Mr. Moseley:

Serial No. 794 PO&M/JTB:clw Docket Nos. 50-280 50-281 License Nos. DPR-32 DPR-37 The purpose of this letter is to document discussions which have been held with members of your staff concerning the steam generator tube phenomena referred to as "denting" and to apprise you of the.current developments as we know them.

During the current refueling outage on Unit No. 1, inspections were con-ducted on the steam generator tubes to acquire additional information con-cerning "denting."* As you are probably aware, steam generator tube diameter reductions, i.e. "denting," have been experienced at a number of operating nuclear power stations.

Because of the generic nature of the phenomena, the nuclear steam supply system manufacturer, Westinghouse Electric Corporation, is doing extensive investigations and studies into the matter.

Enclosed here-with are two "Technical Bulletins" prepared by Westinghouse summarizing the "denting" phenomena.

A meeting between the Regulatory Staff and representatives from Westing-house was held in Bethesda, *Maryland on November 21, 1975 to discuss "denting" on a generic basis.

I understand that a representative from Region II was in attendance at that meeting..

As a result of the meeting, it was agreed that Westinghouse would prepare a topical report on "denting" and would apprise the Regulatory Staff of their proposed investigative program.

As related to our units, the inspections conducted on the Unit No. 1 steam generator tubes indicate that there is significant "denting." The most severe "denting" has occurred on steam generator "A".

The data obtained during the inspections are presently being evaluated to quantify the amount of denting which has occurred.

In an attempt to minimize further denting, we have changed our method for controlling feedwater pH by changing from ammonia to cyclo-hexylamine on both units.

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e VIRGINIA ELECTRIC AND POWER COMPANY TO Mr. Norman C. Moseley Page 2.

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• e Both the Station Nuclear Safety and Operating Committee and the System Nuclear Safety and Operating Committee have reviewed the matter of "denting."

We are continuing to follow the generic implications of this phenomena and will continue to apprise you and your staff of significant developments.

Attachments cc:

Mr. Robert W. Reid Very truly yours,

~. ll?. vdla/,t,,;F C. M. Sta_llings Vice President-Power Supply and Production Operations

advisory notice of a recent technical development pertaining to the installation or operation of Westinghouse-supplii,d Nucl~ar t equipment. Recipianti should c11aluate the information_ and rei:;ornmendation, and initiate action where appropriate.

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Occui"rence of Denting* in Steam Generator Tubes I System(s)

Steam Generator Affected Plants All ope.rating and construction References

.* None AC KG ROUND P.O. Box 2728, Pittsburgh, PA 15'230 Number N_SO-TB-75-12 Date 10/15/75 S.O.(s) 120 Sheet Of 1

During the scheduled refueling outage of the Turkey Point Unit 4 plant.in May 1975-;

3 ddy current inspections of the three steam generators were performed.

The purpos~ of he inspection was to provide a measure of tube integrity following the steam generator hemistry _change to AVT which occurred in September 1974.

This insepction revealed a.

onti.nuation of thinning due to the previous phosphate water chemistry treatment.

The CCJAOSion vias l*imitcd to the area immediately above the tube sheet in the ar,::!a of sludge

~~ulation. It has bP~~ attributed to residual phosphates retained in this sludge a.i:eri al which sur-*, ounds the tubes.

These obsei'Vati ons are i den ti ca 1 to tha fi°ndi ngs at several other plunts with a crnilpar*able operating history.

The continued corrosion i-n the sludge pile has provided the basis for increased emphasis on sludge removal by lancing techniques in these units.

Each of the steam generators \\*las sludge lanced during the shutdown.

Also during the M~y outage, steam generator mechanical

~odificati.ons were accomplished.

INFORnATIOi'-l Examination of the eddy current tapes reve?..led additional s.ignals at the intersection of the tube support plates. These signals were interpreted by Zetec. and -Westinghouse personnel to be due to physical deformation of the tube wall.

The unit was returned to full power in late June and*op2rated until Au~u~t-3, 1975 when shutdovm \\"as initiated to repair a primary to secondary leak which developed in the B steam generator. The leak was identified as a. peripheral tube and this tub2 was plugged.

The leak was between the second and sixth tube support plate on the hot leg side.. The axial locDtion could not be accurately determined since the standard eddy current probe could not pass.beyond these locations.

Adaiiicnal lnform;1ti:rn, if Req:.;ir;_*,~. r.rny be Ob:air.~d from the Ori9inator. Telephone 412 _ 256-5413.

or (WIN) 236-541-3 Originator Approval

-*I£. Bov;man, Lead En~ineer

. F. C. t*lellhofer, Manager Operating Pl ants se_rvices

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P.O. ~ox 2728, Pittsburgh. PA 15230

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iring the August shutdo0n, a tube sample was removed from the B steam generator for lboratory examination to mar~ fully explore the eddy current denting signals.

~mination of the tube is still in progress, and the findings thus far are

.,i:*erated below:

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~*: _ _:iThe tube has shown no significant loss in integrity.

• Wall thinning to the extent of 4 percent (maximum) was evident on the OD surface adjacent to the tube support plate.

Tube wall in1,1ard deformation up to the extent of 7 mils on the radius was measured by profilometry. The denting is rather uniform, slightly oval, and is the area adjacent to the 3/4 inch thick tube support plate.

No gross*

11working 11 of the tube surface is evident from metallography.

4lere is r,~ evidence of intergranular stress.corrosic11.

)th analytical and laboratory studies are in progress to pinpoint the possible cause

)r this finding.

Eddy current tape reviews are also in progress to define any patterns the denting which may exist.

• nee no gross surface working or metal grain deformation is evident in the.metallography,

'bration forces such as those which might be induced by flow and turbulence do not pear a likely cause for denting.

This is also consistent with analytical predictions dyna~ic forces in the tube bundle.

The definitive mechanism for denting, therefore, j not as yet readily explainable.

It is clear that the denting is not related to steam nerator mechanical modifications since the eddy current indications were evident prior implementation of these modifications.

ince restart of'the piant in August, another tube l~ak was detected in the B steam

?nerator. The plant entered into another shutdown on September 21.

• The l~akage was Jain identified as a peripheral tube and this tube was plugged in addition to several ther tubes in this wedge region.

It has not been ascertained as yet whether the tube

akage is related to the denting.

A section of tubing from the hot and cold legs v1as

_moved during the outage for further investigation into the denting phenomena.

revie~ of eddy current inspe:tion records is in progress for other plants.

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_nting signals are also found in this review to an extent yet to be finalized. This

_view will cover Beznau I and II, Prairie Island *, Ginna, Robinson, San Onofre, Ji-Be~ch 2, and others as i n~pe~ti ans are performe.d in the future.

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~ I 2 !,;ell 1..lk°~~3 advisory notice of a recent technical development pertaining to the installatiort or operation of Westinghouse-supplied Nuclear ant oquipment. Recipients should evaluate the* information and recommendation, and initiat::! action wh?.re appropriate..

P.O. Box 272S. Pittsburgh, PA 15230 It is important to note that no loss of tube integrity _is indicated by this inspection.

The eddy current signal from denting is relatively large compared to the usual amplitude of corrosion related phenomena, and calibration standards are being devised to better quantify the amount of physical deforma.ti on.

The ~pecific mechanism involved is being pursued with expeditious effort by responsfble division within Westinghouse.

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n advisory notice of a recent technical deveiopment pertaining to the installation*or operation of \\'/s?stinghouse-supplied Nucli!ar ant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate.

,ubject Localized Steam Generator Tub~ Diameter Reductions iystem~

Reactor Coolant System

\\ffocted Plants lefarnnces NSD Technical Bull~tin 75-12 nformation P.O. Box 2728, Pittsbursh, PA 1~230 Number NSO-TB-75-16 I

Date

• November 17, S.O.(s) 120 I

Shaet l

Of 1975 4

ocalized ste~m generator tube diameter reductions have been observ~d at some operating lants.

Evaluation of such anomalies with regard to the effects on the operation and 1

afety analyses of such plants.have been performed:

The evidence, available to date indicates that operation of affected units can continue without undue risk to public 1e~h and safety.

Pl ans have been made to obtain additi ona 1 data in order to better in~stand the causes and extent of such d~ameter reductions:

the intent being to 1in1mize their occurrence in the future.

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j-he follm*ling elaborates on what has been observed, the evaluations performed to date, Ind the plan~ made to better characterize such localized steam gen2rator tube diamet9r

!*ecluct i ans.

~xamination of eddy current inspection data obtained from the-Turkey Poi~t Unit 4

team generators during.the scheduled refueling in May, 1975 reveale~ the presence of inomalbus signals at the tube/tube support plate intersections.* These signals were interpreted as being a result of localized deformation of the tube wall in the area

>I'the tube support plates.

~llowin~ refueling, Turkey Point *4 was returned to power in early June and operated mtil J'\\ugust 3, "i975 v,hen the plant was *shut dm-m to repair* a primary-to-secondary leak 1hich* developed in the 118 11 steam generator. The leak was in a peripheral tube on the iot leg side at approximately the 2nd tube support. This tube was plugged and another

ube having a similar EC indication on the hot leg was removed for examination.

\\dditional lnhrm,Hion, if Required, r.iay be Oht~ined from the Originator. Tel~phone 412 - 256-7183 er (':JI~) 236 - 7183 161 r

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-Operations Support Mechanical Service

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• ory notice of a recent technical development pertaining to the installatioa or operation of Westinghouse-suppli~d Nucl,!ar iuipment. Recipients should evaluate ~he information and recornm~nclaticn, and initiate action wh,ue a~propriate.

P.O. Box 2728. Pittsburgh, PA 15230 lant was restarted in August and operated until September 21 when it 0as shut down se.of a primary-to-secondary 1 eak.

The 1 eakage was again i den ti fi ed as being in ipheral tube at the 2nd support plate, and this tube was plugged.

During this own, both a hot leg and cold leg tube exhibiting diameter reduction indications removed from the unit for examination.

Both tube sections extended from the primary\\

pf the tubesheet to beyond the 5th tube support.

r atory examination of these tubes is still in progress. The results available to indicate the following whic~ are essentially similar to the results of the examina-of the August tube:

11. Plastic defcirmation of the tube wall has occurred in the area

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reduction in diameter in the tubes examined in the labora.tory i

is approximately 20 mils~

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12..Me tall ographi c examination *.and mechanical property tests indicate no loss in tube integrity.

3. Wall thinning to the extent of 4 percent {maximum) was observed

. on the 0.D. of the *tube surface adjacent t6*the tube support pl ate...

4. There ~s no evidence of stress corrosion*cracking.

5.

There i~ no *evidence of extensive deformation of the surface

• layer within the dent region.

19 September, the Surry 1 unit was sh'..lt down just prior to a scheduled refueling. At time of shutdmoJn) primary-to-secondary, 1 eakage was e.xperi enced in steam generator 11,'\\.

11

~ leaking tubes were identified in this steam generator; all were in Row 2, which is tube removid from the divider lane. The leaks:were in the hot leg in the r~gion of

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current examination of this unit is still in progress. Initial results indicate

~l~t localized diameter reduction.

Plug gaging of the leaking tubes indicates r

\\'* not pass a standard EC probe beyond the 2bd tube support.

Tubes have been d from this unit for detailed laboratory examination. The leaking tubes were *

.ed and were not removed due to jnaccess i bi 1 ity and proximity to the d.ivi der p 1 ~te.

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,ment. Recipient5 should evaluate the mforrnat1on and recommendat1on, and 1:11t1ote action where apµrop, 1ate.

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P.O. Box 2728, Pittsburgh. PA 15230

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~isual examinations of the tube support plat~s were made through the tube lane handholes.

[his examination indicated inplane hour-glassing of the flow slots in the center of the tube support µlites.

No other physical changes in the tube support plates were observed.

The support plate, tubes, and anti-vibration bafs were examined in the U-bend region of the bundle. All components were intact a~d in the as-shipped condition.

ince the initial *evidence of the localized diameter reduction problem, analytical and experimental programs have been underway to establish the ~echanism responsible for the henomena.

Examination of affected tubes shows no evidence of surface deformation which

,;ould be expected if the problem was caused by flow induced tube vibrations. This is also consistent with analytical *preductjons of dynamic forces in the tube bundle.

~nalytica1 studies to explore the possibility of dia.. reduction resulting from thermal

. hettir.g induced by local thermal/hydraulic effects in the tube support region also i

ate th:1.t this is not a feasible rr:echa:1ism.

At the present tim2, it app!;ars that the most likely mechanism is the growth of deposits and/or corrosion product within the tube/i;ub2 support plate annulus, giving rise to forces sufficiently high to local1y-lastica11y deform the tubes.

~ethods and procedures are bein~ d~0eloped so ~hat a section of tube within the adjacent tube support plate area can be secured fer laboratory examination at some future date..

dditional EC examinations are planned to categorize the extent of the localized dia~eter eduction..

he eddy current inspections bf those ~nits which have converted to AVT after extended Jhosphate operation have indicated that the phenomenon is localized at the tube support late locations.sjnce the AVT conversion; It has been observed in the past essentially n a random basis and to a minimum e~tent in these units during phosphate operation.

dqy current inspections thus far on those pl ants 1r1hich operated for a short period with hosphates and switched to AVT indicated no* denting Has taking place.

I aboratory testing is under~*1ay to simulate conditions of the proposed mechanism and to id~ntify corrective r.12asures ~*1hich can be applied to pre*.rent f:Jrther*wal1 de*~or:7atic~.

his testing includes the exposure of tube/tube support simulations to aqueous envircn-ents containing contaminants corrosive to carbon steel. The intent is to determine if

~lsion products 1rlil1 accumulate and exp:rnd on the tube wall. Aho, 1,1eans fer neu-zing or removing deposits in the tube/support pl~te re_gion are b2ing investigat2d.

>ossible implications-of localized tube diarn2ter reduction have been considered with

.espcct to normal plant operation, expected transients, and postulated accident conditicns.

VVestingi1ouse

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1~fuclear *.

I ff S-,~n1ice Divjsion visory notice of a recent t*echnical devdlopment pertaining to the installation or operation of 'J\\'.!:stinghoma-supplied Nuclear equipm_ent. Recipients should evaluate the information and recommendatiori, *and initiate action wher~ apptopriate.

P.O. Box 2728,.Pittsburglr, PA 15230

~ding riormal operation with. expected transients and based upon experience at several*

• s which ha~e operated \\*1ith tubes in this condition, if primary-to-secondary leakage affected tubes occurs. it can be detected at low, controlled levels and progresses

y.

If necessary, shutdo~m and repairs can be acco:r.plished in an orderly manner 11th no undue risk to the public health and safety.

respect to postulated design basis accidents, the criterion of importance is that ficant tube damage or ruptures will not occur during the course of the accident result of tube diameter redwction.

Our evaluation, based on analyses previously 1cted, indicates that limiting stresses will not be exceeded,*and hence, tube ires are not expected to occur.

Also, collapse strength of tubes is considered

! adequate even with significant diameter reduction, based upon available test data:

ma,y, the assessment of postulated accidents as presented in the S:Jfety analysis t~ not expect2d to change as a result of localized tube diameter reduction..

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