ML20062J552

Forwards Evaluation of Control Rod Guide Tube Thimble Wear in Facilities Designed by Westinghouse. Issue Resolved for 17x17 Fuel Assemblies Designed by Westinghouse for Facility

ML20062J552
Person / Time
Site:	Farley
Issue date:	10/03/1980
From:	Varga S Office of Nuclear Reactor Regulation
To:	Clayton F ALABAMA POWER CO.
References
TAC-8676, NUDOCS 8010300614
Download: ML20062J552 (3)

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y. o, i UNITED STATES i y f) c, (j j NUCLEAR REGULATORY COMMISSION ') ' l }~

WASHWGTON, D. C. 20555

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October 3,1980 Docket No. 50-348 Mr. F. L. Clayton Senior Vice President ]gis 00CuthgT COMTh@S pgt,3 Alabama Pcwer Company Post Office Box 2641 POOR QUEIII Birmingham, Alabama 35291

Dear Mr. Clayton:

The NRC staff has been reviewing the subject of control rod guide thimble wear in pressurized water reactors. The enclosure to this letter des-cribes our review and makes an assessment of this problem in facilities with fuel assemblies designed by Westinghouse. Based on our review, we have concluded that this issue is resolved for the 17x17 fuel assemblies designed by Westinghouse for the Joseph M. Farley Nuclear Plant, Unit No.1.

It is not acected that the above conclusion will be altered by the results of the sur<eillance program sponsored by a cooperative owners group to obtain post-irradiation examination data from Salem, Unit No.1. We will notify you of any additional concerns when the results of this examination are evaluated.

Sipcerely.

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h/[Ila nM.tVarga, 'hi f Operating Reacto,r Branch #1 Division of Lice', ng

Enclosure:

Evaluation of Control Rod Guide Thimble Wear cc: W/ enclosure See next page 8 010300f//[

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Enclosure EVALUATION OF CONTROL ROD GUIDE THIMBLE WEAR IN FACILITIES DESIGNED SY WESTINGHOUSE A degradation of control rod guide thimble / tube walls has been observed during post-irradiation examinations of irradiated fuel assemblies taken from several operating pressurized vater reactors. Subsequently, it has been determined that coolant flow up through the guide tubes and turbulent cross flow above the fuel assemblies have been responsible fer inducing vibratory motion in the normally fuiiy withdrawn (" parked")

controi rods position. When these vibratingereds are in contact wi:n the inner surface of the gu de tube wall, a fretting wear of the wall occurs. Significant wear has been i

found to be confined te the relatively sof: Zircaloy-4 guice tubes because :he control rod claddings--stainless steel for Westingnouse-NSSS designs--provide a relatively

. hard wear surface.' ine extent of the observed wear is both time and NSSS-design depen-den and has, in some non-Westinghouse cases, been observed to extend completely through the guide tube walls, thus resulting in the formation of holes.

Guide thimele/ tubes function princi; ally as the rain structural members of the fuel .-

asser61y and as channels to guide and decelerate con:rcl rod motion. Significant loss of mechanical in egrity cue to wear er hole formation could: (1) result in the inability of the gui:e thinbie to withstand their anticisated loadings for fuel hardling accicents anc conoition 1-4 events; and, (2) ninder scramacility. __ _

In response :: the staff's attempt to assess one susceptibility and impact of guide ~~

thimble wear in Westinghcuse plants, two meetings were held with Westinghouse and infor-_ '

mation was submitted (References 1 and 2) on their experience and understanding of tne i: sue. Thi: information consisted Of guide thimble wear measurements taken on irraciated fuel assemolies from Poin Beach, Uni s 1 and 2 (two-loop plants using la x la fuel a s s tm:l ie:) . Also descriced was a nechanistic wear model (developed from the Doint Beach data) and :ne imcact of the mocei's wear predictions en the safety analyses of plan: designs. '

repor-Vestinghouse believes that their fuel designs will experience less wear than thr:

ted in s:me other NSSS designs because the Westinghouse designs use thinner, more flexi-ble, control rods that have a relatively more lateral support in the guide thimole ,

assembly of the upper core structure. Such ccnstruction provides ne housing and guide '

cate #:r tne roc cluster contrcl assemblies (RCCAs) above the core and thus restricts cor:roi rod vipration cue to lateral exit flow. Also, Westingnouse believes :na: :neir wear model conservatively credicts guice nimble wear and :na: even with the wors anti-cicated wear conditiens (both in :ne degree of wear and the location of wear) tneir guide thim:les will be able to fulfili their design functions.

Tne staf# :cncluded that the Westingnouse analysis probably accounts for all of the major variables :na control this wear process. However, because of the ccmolexities anc uncertainties in (a) determining contac: fccces, (b) surface-to-surface wear ra es, (c) forcing fun:: ions, and (d) extrapolations of tnese variaties to the new 17 x 17 fuel cssemoly design, the staff recuired several near-term OL apclicants te submi: to a "

survaillance program. For acceptability, tne minimum objective of such program was to

r.;nstra e tna

there is no oc:urrence of hole formation in rodcec guide nimbles.

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=:icr s us ng nestingnouse HIPAR fuei assemoly designs (stainless steel guide thimele u es: are no: ::nsicered suscep-ible :c sigra-1 cant wear.

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r 1 Enclosure To satisfy this request for confirmation of the Westinghouse analytical predications, a cooperative owners group was established which is now sponsoring a program to obtain post-irradiation examination (PIE) data from the Salem, Unit No.1 facility. This PIE program will examine all guide thimbles in six rodded fuel assemblies having either one or two cycles of burnup. It is our expectation that the program will confirm Westing-house predictions, and therefore this issue should be considered resolved for all Westing-house plants using the newer 17 x 17 fuel assembly design.

The relevant primary system design differences in plants fueled with the 15 x 15 fuel assemblies as compared witn : nose of plants fueiec witn 14 x la fuel assemolies are miniaal. And certaini~v the extrapolation of wear prediction is less than :nat associated with the extrapolation to the newer plants using 17 x 17 fuel assenclies. Thus it is reasonable to conclude tha* tne wear in 15 x 15 fuel assectlies should be ecuivalent to that experienced and measured in 14 x 14 fuel assemolies, and therefore these designs are not likely to experience significant wear to the degree that the design capabilities will be impaired. Therefore, we conclude that the information that has been provided is sufficient to resolve the issue of guide thimbie/ tube wear in plants fueled with la x 14 and 15 x 15 fuel assemolies. . -

References

1. Letter from L. M. Mills, Tennessee ',' alley Authority, to L. 5. Rubens: tin, NR2, Dockets 50-327 and 50-328, dated Novetter 27, 1979. -

2. Letter from T. M. Anderson, Westingnouse, to H. R. Denton, NRC, NS-TMA-2238, catec April 29, 1980.

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