ML19095A703

Documenting Discussions Held with NRC Staff Concerning Steam Generator Tube Phenomena Referred to as Denting & Apprising Current Developments

ML19095A703
Person / Time
Site:	Surry
Issue date:	12/11/1975
From:	Stallings C Virginia Electric & Power Co (VEPCO)
To:	Moseley N NRC/IE, NRC/RGN-II
References
Serial No. 794
Download: ML19095A703 (9)

Text

,-- -----..::

't e

.'fflegulatory. Docket* Bil.

... _--.-:.....-.-*-.-~

ELECTRIC.A.ND POWER COMP.A.NY December. 11, 197 5 Mr. Norman C. Moseley, Director Office of* Inspection and *Enforcement U.S. Nuclear Regulatory Commission...

Region II - Suite 818.

230 Peachtree Street, Northwest.

• Atlanta, Georgia 30303 *

Dear Mr. Moseley:

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" *anc.i to apprise you of'*tli.e current developments. ~8.

we know them.

During the current refuel~ns. outage on Unit 0 No. _l, inspections were c~m-ducted on the steam generator tubes.:to acquire additional information con-cerning 11d_enting. II. As you. are: prC>bably; awar'e, / &tf¥.llli generator tube diameter reductiOllEl, i o 0 o "denting~ II-have. t>eell. experienced', at a number, Of Operating nuclear**power stations.

Because of the generic nature of the phenomena~ the nuclear:* steam supply *system, manufactu:r;~r-, Westinghe>use Electric Corporation,.

is doing. extensive investigations and studies. into the matter~.. Enclosed here-with are.two "Technical Bulletin~" prepared.by Westinghouse s~r:l.zing the 11denting 11 : phenomena~--

A meeting between the*: Regulatory. St:s.££° and rep*resentatives from Westing-house was held in Bethesda, Maryland.on November 21, 1975 to discuss 11denting 11 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 topic~.! report on.. "denting" and. would apprise the*

Regulatory Staff of their proposed investigativ.e program.

• As related to our unite, the inspections*conducted ori the Unit No. 1

• steam generator.tubes indicate *that there is significant "denting." The most severe l!denting'i has occurred on steani generator "A".

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

In*ari attempt to minimize further denting, we have changed our method for controlling feedwater*pH by.changirig*from ammonia. to cyclo-he~-ylamine on both units.

I 3-CJ 3/;

~

W.

~

',.(/

)

... Mr~ Norman *C. Moseley-*.

Page 2

• • .Both the ;StationNuclear Safety and Opera.ting Committee.and the System

. : Nuclear Safety and Operating. Committee ha,;.e. re~iewe~ the. mat.tar of. "denting. II

. We *are colitinuing

• to foliow. :the _generic\\:tmplications of thi~

• ph~nomena and. *.

. 'will,contiilue to apprise you and.' your staff of significanf developments.

Attachments

• cc:*. Mr~ Robert JJ-~ _: Re~d ;\\//

• __ b,.)l}?_0Mr~;;r1/'

. c*. M. Stallings *.

• Vice 'President~Power ';supply

• *-and. Production Operations

~. '

• .An.advisory notice of a recent technical development pertaining to the installation or operation of _Westinghouse-supplied N*.iclear

• Plant equipment. Recipients should evaluate_ the information and recommendation, and initiate.action,*,her:? app~?priate.

C11hiPCt v....,J,J

- Occufrence of Oe~ting i~ Ste~m Generator Tubes P.O. Box 2728, Pittsburgh, PA 15233 Number N_SD-TB-75-12 Date Systam(s)

Steam Generator 10/15/75

• Affected Plants I

S.0.(s) 120 All ope.rating and construction Of

_R_e*-:e-re_~_ce_t~_*o_n_e _______________, _____________.... ls_t:~--1*

3 BACKGROUND During the scheduled refueling outage of the Turkey Point Unit 4 plant in May 1975; eddy current inspections of the three steam generators were performed.

The purpos~ of

-the inspection was to provide a measure of tube integrity following the steam generator chemistry change to AVT vJhich occurred in September 1974.

This insepction revealed a conti.nuati on of thinning due to the previous phosphate \\'later chemistry treatment.

The corrosion was limited to the area immediately above the tube sheet in the area of sludge

~ccumulation.

It has been attributed to residual phosphates retained in this sludge material which surrounds the tubes. These observations are identical to the ftndings _*

• at several other plants with a comparable operating history. 'The continued corrosion in the sludge pile has provided the basis for increased emphasis on sludge removal by iaricing techniques in these units.

Each. of the steam ge_nerators was sludge lan~ed

• during the shutdown.

Also during the May outage, steam generator mechanical

~odificati.ons were accomplished.

Examination of the eddy current tapes revealed additional signals at the intersection of the* tube support plates.

These signals were interpreted by Zetec and -~1estinghouse

.personnel to be due ~o physical deformation of the tube wall.

\\ ~he unit was-retufned to full power in late June and operated until August 3, 1975

-*

• when shutdown \\'las initiated to repair a primary to secondary 1 eak \\*1hi ch developed in

• ~:the B steam generator. The leak was identified as a peripheral tube and this tub2 was plugged. Jhe leak was betwe~n the second and sixth tube support plate on the hot leg

• side.. The axial location could not be accurately determined since the. standard eddy

\\current ~robe could not pass beyond these locations.

2s~ s~13

. 2"r ~,-,

Add:tionaJ Information, if Required, may be Obtained from the Originator.. Telephone 412 -

-o-' -r or {WIN)

.:.iO-:J.lt 1*..)

Originator Approval

-T. E. 80\\*Jman, Lead Engineer F. C. Wellhofer, Manager G.

• Operating Pl ants Services 02-

• lechan~~Techno'. ogyr---

~

... ;;9

[5_* Id

~

1

~

/~

.,.A'\\

~II

".1/

,? -,].../

• .-:-if~-~~"'\\."""'

'-*' :.,,.c::#i~-

• • .. * -~,£either ~Veslinghouse Electric Corporation nor i:s ernployes rnakc any warranty or reprcsentat;on,~ith respe~t.-to ;h.e accur;;-::.. compl~ten,.-s; er u~eful-

• .- =:;.he~~ ol_ rhe information contilined in this report or assume ar,y responsibility IN liability or dama9e which may _rese1lt from th!? u~e of rnch inforrn~:,on.

~ESl"INGHOUSE FOnM 55378

. -An advisor/ notice of a recent nichnical development pertaining to the installation o*r operation of w~stinghoma-supplied Nuclear

• Plant equipm,mt. Recipients should evaluate the information and recommendation, and initiate action where appropriate.

P.O. Box 2723, Piruoursh. ?A 15230

• Durin~ the August shutdo~n, a tube sample was removed frorn the B steam generator for laboratory examination to more fully explore the eddy current denting signa*1s.

• -Examination of the tube is still in progress, and th2 findings thus far are enumerated below:

1.

• The tube has shown no significant loss in integrity.

2.

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

3.

Tube wa 11 i ni,.1ard deformation up to the extent of 7 mi 1 s on the radfos was measured by profil ometry.

The denting is rather un if arm. s 1 i ghtly ova 1, ar1d is the area adjacent to the 3/4 inch thick tube support piate.

4.

No gross 11working 11 of.the tube surface is evident from metallography.

5.

There is n6 evidence of intergranular stress ~orrosion.

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

~for this finding.

Eddy current tape reviews are also in progress to define any patterns

• to the denting which may exist.

• . Since no gross surface working or metal grain deformation is evident in the ~etallography, vibration forces such as those which might be induced by flow and turbulence do not

.appeaf a likely cause for denting.

This is also consistent with analytical predictions

.of dynamic forces in the tube bundle.

The definitive mechanism for denting, therefore,

• • is not as yet readily explainable. It is clear that the denting is not related to steam

. tenerator mechanical modifications since the eddy current indications were evident prior to implementation of these modifications.

Since restart of the plant in August, another tube leak was detected in the B steam

• generator.

The plant entered into another shutdown on September 21. - The leakage was

• again identified as a periph~ral tube and this tube was plugged in addition to several other tubes in this wedge region.

It has not been ascertained as yet whether the tube*

leakage is related to.the denting.

A section of tubing from the hot and cold legs was removed during the outage for further investigation into the denting phenomena.

. *A review of eddy current inspection records is in pfogress for o~h~~ plants.

So~e

• denting signals are also found in this reviev, to an extent yet to be finalized. This

• review will cover Beznau I and II, Prairie Island

, Ginna, Robinson, San Onofre~

.. Point Beach 2, and others as inspections are performed in the future.

. 2 -.

l

• IJ,.

Division

. ~.

~"i:i\\.

V:.)

. An advi;ory notice of a recent technical development pertaining to the installatio11 or operation of Westinghouse-supplied Nuclear

• .,;Plant equipment. Re_cipients should evaluate the information and recommendation, and initiate action where appropriate..

P.O. Box 2728, Pittsburght 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 deformation.

The specific mechanism involved is befog pursued with exped'itious effort by responsible division within Westinghouse.

.,- 3-,-:

-~- '3

'VJest~nghouse Nuclear Division

. An m;lvisory noti.ce of a recent technical development pertaining to the installation or operation of Westinghouse-supp!ied Nuclear

• *: : ' Plant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate.

Subject Localized Steam Generator Tub~ Diameter Reductions System(s)

P.O. Box 2728, Pittsburgh, PA 15230 Number*

NSD-TB-75-16.

Date Reactor Coolant System

• -A~fi"-~ c~te_d_P-lan ts November 17, 1975 S.O.{s) 120 Refernnces NSO Technical Bull~tin 75-12 Sheet Of 4

Information Localized steam generator tube diameter reductions have been observed at some operating plants.

Evaluation of such anomalies with regard to the effects on the operation and safety analyses of such plants have been performed:

The evidence, avaflable to date.

. indicates that operation of affected units can cbntinue without undue risk to public

• . health and safety.

Plans have been made to obtain additional data in order to better understand the causes and extent of such diameter reductions:

the intrnt being to minimize their occurrence in the future.

The following elaboiates on what has been observed,.the ev~luations performed to date,

• and the plan~ made to better characterize such localized steam generator tube diameter

• reductions.

• Examination of eddy current inspection data obtained from the.Turkey Point Unit 4 steam g~nerators during the scheduled refueling in May, 1975 revealed the presence of anomalous signals at the tube/tube support plate intersections.* These signals were interpreted as being a result of localized deformation of the tube wall in th~ area of the tube support plates.

FollovJing refueling, Turkey Point *4 was returned to. pmver in e.arly June and operated

. until August 3, 1975 when the plant was shut down to repair *a pr'imary-to-se.condary leak whi,ch* developed in the 118 11 steam generator. The leak*,was in a peripheral tube on the hot leg side at approximately t'he 2nd tube support. This tube was plugged and another tube having a similar EC indication on the hot leg was remcived for*examination.

• Additional Information, if Required, m;y be Obtained from the Originator. Telephone 412-256-,7183 or (WIN) 236 - 7183

/ti,., r-\\_ 9rig:nator

.~/,,

• ';,(*pr al

, w: ~

,_,,vu{A,.11,

..e.-.../

_..;:c.-...-~--

. V. W, Doutt, Manager f /J.C. We 11 ho fer, Mar.ager __

_*operations Support

• l Mechanical Service Ne11h~rWestingnouse Electric Corporation nor its ernployes make ;my warranty or representation with respect to the accuracy, com;:,iecenils; or useful*

.ness of che*information con Wined in this report or assume any responsibility for liability or damage which inay result frorn the use ol such informat,on.

. ~'VE.!lTINGHOUS~ FORM 55378

VVestinghoua.

s\\luch9ar.

Technica] BuBetki

..- An advisory notice of a recent technical development pertaining to the installation or operation of-Westinghousa-SUfJpli~ Nucl-:ar

• 'Plant equipment. Recipiants should evaluate ~he information and recommendation, and initiate action 1,vhere appropriate.

P.O. Box 2728, Pittsburgh, PA 152:!D The plant was restarted in August ~nd operated until September 21 when it was shut down because of a piimary-to-secondary leak.

The leakage \\'las again identified as being in a peripheral tube at the 2nd support plate, and this tube was plugged.

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

Both tube sections extended from the primary face of the tubesheet to beyond the 5th tube support.

Laboratory examination of these tubes is still in progress. The results available to date indicate the following which are essentially similar to the results of the examina-tion of the August tube:

1.

Plastic defrirmation of the tube wall has occurred in the area adjacent to the 3/4 11 thick tube support plates, resulting in

• a constriction of the tube diameter in that area.

Maximum reduction in diameter in the tubes examined in the laboratory is approximately 20 mils.

2.* Metallographic examination and mechanical property tests indicate no 1 ass in tube i ntegr"ity.

3.

Wall thinning to the extent of 4 pe~cent (m~ximum) was observed on the 0.D. ~f the tube surface adjacent to*the tube support p1 ate..

4. There is no evidence of stress corrosion*cracking..

5.

There is no evidence of extensive deformation of the surface

• 1 ayer within the den'!: region.

. _ *ouring September, the Surry 1 unit \\<Jas shut down just prior to a scheduled refueling.

At

. the time of shutdown, primary-to-secondary leakage 't!~s experienced in steam generator 11 A. n

'Three leaking tubes were identified in this steam generator; all were in Row 2, which is

-one tube removed from *the div*ider lane. The leaks*were in the hot leg in the region of

• *_./t.he 6th and 7th support p 1 ate.

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

• prevalent localized diameter reduction.

Plug gagi'ng of the leaking tubes indicates

• *they vii 11 not pass a standa,:.d EC probe beyond the 2nd tube support. Tubes have bei2n

._pulled from this unit for detailed 1 abor-atory examination.

The leaking tubes w2r2

• *plugged and were not removed due to 1naccessibility apd proximity to the divider plate.

-2.;.

• 1 V\\Jestinghou4t

~1uc!ear

-Q>-:::irvicc.::.

.._.,,t;

• 'if*

v Division Technical BuHatin An advisory notice of a recent technical development pertaining to the installation or operation of \\11/e'stinghouse*suppli.?d Nuclear

• Plant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate.

P.O. Box 2728, Pittsburgh, PA 15230

. Visual examinations of the tube support plates were made through the tube lane handholes.

This examination indicated inplane hour-glassing of the flow slots in the center of the

• tube support pl cites.

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

The support plate, tubes, and anti-vibration bars wefe examined in the U-bend regiorr of the bundle.

All components 1tJere intact an*d in the as-shipped condition.

• .. Since the initial evidence of the localiz~d diameter reduction proble~, analytical and experimental programs have been underway to establish the mechanism responsible for the phenomena.

Examination of affected tubes shows no evidence of surface deformation which would be expected if the problem was caused by flow induced tube vibrations. This is also consistent with analytical preductions of dynamic forces in the tube bundle.

• Analytical studies to explore the possibility of dia.. reduction resultihg from thermal ratchetting induced by local thermal/hydraulic effects in the tube support region also indicate that this is not a feasible mechanism.

At the present time, it 0ppears that the most l*ikely mechanism is the growth of deposits and/or corrosion product within

• the tube/tube support plate annulus, giving rise to forces sufficiently high to locally-

. plastically deform the tubes.

Methods and procedures are being devel.oped so.that a section of tube within the adjacent

• tube support pl ate area can be secured for 1 aboratory examination at some future date..

. * *Additional EC examinatiohs are planned to categorize the extent of the localized diameter reduction...

The eddy current inspections bf those units which have converted to AVT after extended phosphate operation have indicated that the phenomenon is localized at the tube support

plate locations since the AVT conversion.- It has been observed* iri the past ess,entially

• on a ran~om basis and to a minimum extent in these units during phosphate operation.

• Edcly current inspections thus far on those plants which operated for a short period \\,1ith

.phosphates and switched to p,..v} indicated no* denting was taking place.

. Laboratory testing is unden,1ay to simulate conditions of the proposed mechanism and to "d '". -.

.i.*

l l

1

..J

.j;'

11 d -

,1 :2nt.1*TY correc1.,1ve measures \\*:n,cn canoe app11eu ;,O prevent.. ilJr"t.neY' via, *<.:..-Or~a1:1on.

• This testing includes the exposure of tube/tube support simulations to aqueous environ-ments containing contaminants corrosive to carbon steel. The intent is to determine if

• corrosion products will accumulate and expand on the tube wall.

Al5o, means for neu-

. tral iz"ing or removing deposits "in the tube/support plate region are being investigated.

• . Possible implications of localized tube diameter reduction have been considered \\'lith r_espect to normal plant operation, expected transients, and postulated accident conditions.* -

_.'An advisory notice of a recent t*echniC3l development pertaining to the installation or operation of WBstinghousa-supplied Nuclear Plant equipment. Recipients should evaluate the information and recommendation, and initiate action where appropriate.

P.O. Box 2728. Pittsburgh. PA 15230

• =Regarding riormal operation wit~ expected transients and based upon experience at several p 1 ants which ha \\'.e operated 1*1i th tubes in this condition, if primary-to-secondary leakage from affected t8bes occurs, it can be detected at low, controlled levels and progresses slowiy.

If necessary, shutdovm and repairs can be accomplished in an orderly manner and ~-,ith no undue risk to the pub 1 i c hea 1th and safety.

With respect to postulated design basis accidents, the criterion of importance is that significant tube d~mage or ruptures will not occur during the course of the accident as a result of tube diameter reduction.

Our evaluation, based on analyses previously conducted~ indicates that limiting stresses will not be exceeded, and hence, tube failures are not expected to occur.

Also, collapse strength of tubes is* considered to be adequate even with significant diameter reduction, based upon available test data.

In summary, tho. assessment of postulated accidents as prPsented in the* safety analysis repo~t i~ not expected to change as a result of localized tube diameter reduction.