ML18081A831
| ML18081A831 | |
| Person / Time | |
|---|---|
| Site: | Salem  |
| Issue date: | 12/18/1979 |
| From: | Parr O Office of Nuclear Reactor Regulation |
| To: | Mittl R Public Service Enterprise Group |
| References | |
| NUDOCS 8001030853 | |
| Download: ML18081A831 (6) | |
Text
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DISTRIBUTION:
NRC PDR bee:
DEC 18 1979 Local PD~
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Docket. F11 e~-1-c~&
LWR #3 File TERA Docket No.:
50-311 Mr. R. L. Mittl, General Manager licensing and Environment Engineering and Construction Department Public Service Electric and Gas Company 80 Park Place Newark~ New Jersey 07101
Dear Mr. Mittl:
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- BMoore OParr ADromerick MRushbrook Attorney, ELD I&E (3)
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SUBJECT:
DEGRADATION OF GUIDE THIMBLE TUBE WALLS Enclosed are the results of our review of the subject matter for Salem, Unit 2.
L~e believe that the analysis accounts for the major variables that control the wear process on guide thimble tube walls. Nevertheless, due to the complexities and uncertainties in the wear process, it is necessary to establish a surveillance program for guide tube wear.
This issue is resolved for Salem, Unit 2 for the first cycle of operation, if TVA agrees to perform a surveillance program that meets our minimum objective as described in the enclosure. A cooperative owner's group commit-ment would also be acceptable, provided a commitment is made prior to January 1, 1980. If neither of these parties commit to this program, it may be required that each near term ap'plicant commit to this program.
Enclosure:
As Stated cc: See Next Page Sincerely, Olan D. Parr, Chief Light Water Reactors, Branch No. 3 Division of Project Management OFFICE~,---~,~-~~-~~-... ~-~~~~-~~-........................ * * * * * * * * * * * * * * * ** * * * * * * ** * * * * * * * * * * * * * * * * * * * * * *
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NRC FORM 318 (9*76) NRCM 0240
{:ru.s. GOVERNMENT PRINTING OFFICE: 1979-289-369
Mr. R. L. Mittl, General* Manager cc:
Richard Fryling, Jr., Esq.
Assistant General Counsel Public Service Electric~ Gas Company 80 Park Place Newark, New Jersey 07100 Mark Wetterhahn, Esq.
Conner, Moore & Caber 1747 Pennsylvania Avenue, N.W.
Suite* 1050 Washington, D.C.
20006 Mr. Leif J. Northolm U. S. Nuclear Regulatory Commission Region I Drawer I Hancocks Bridge, New.Jersey 08038
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.. __.. "J ENCLW~URE GUIDE TUBE THH\\l3LE \\~ALL WEAR An unexpected degradation of guide thimble tube walls has been observed during post-irradiation examinations of irradiated fue.l assemblies taken from several operating pressurized water r~actors: Subsequently it has been determined that coolant flow up through the guide thimble tubes and turbulent cross flow above the fuel assemblies have been responsible for.
inducing vibratory motion in the normally fully 1vilhdra1-m
("pi1r~ed")
When these vibrating rods are in contact 1vith the inner surface of the thimble 1..,rall, a fretting 1oJear of the thimble 1*:all occurs.
Significant wear has been found to be confined to the relatively soft Zircaloy-4 thimble tubes because the control rod claddings -- stainless steel for Westinghouse-NSSS designs -- provide a relatively hard wear surface.
The extent of the observed wear is both time and NSSS-design dependent and has, in some cases, been observed to extend.completely through the guide thimble tube \\'/alls, thus resulting in the formation of ho 1 es.
Guide thimble tubes functfon principally a,s the main structural members of the fuel assembly and as channels to guide and decelerate control rod motionA Significant loss of mechanical integrity due to wear or hole formation could (1) result in the in~bility of the guide thimble tubes to withstand their inticipated loadings for fuel handling accidents and condition 1-4 events and (2) hinder scramability.
rn response to the staff's attempt to assess the susceptibility and impact of.guide 'thimble tube 1vear in \\.Jestinghouse plants, \\*:estinghouse and the applicant have submitted information (References l-4) on their experience and understanding of the issue. This information consisted of guide thimble tube wear measurements taken on irradiated fuel as~e~blies from Point Beach Units 1 and 2 (two-loop plants using 14xl4 fuel,1ssemblies).
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Also described was a mechanistic wear model (developed fro~ the Point Beach data) and the impact of the modrl 1 s wear predi~tions on the safety analyses of,plant*designs such as those ut1lizing 17xl7 fuel assemblies.
Westinghouse believes that their fuel designs will experience less wear than that reported in other NSSS designs because the Westinghouse designs use thinner, more flexible, control rods that have relatively more lateral support in the guide tube assembly of the upper core strutture.
Sych construction provides the housing and guide path for the RCCA's above the core and thus restricts control rod vibration due to lateral exit flow.
Also, Westinghouse believes that their wear model conservatively predicts guide thi.mble tube 1<1ear and that even with the 1*1orst u.nticipated 1*:ear conditions (both in the degree of wear and the location of wear) their guide thimble tubes will be able to fulfill* their design functions.
- The staff concludes that the Westinghouse analysis probably accou11ts for all of the major variables that control this wear process.
- However, because of the complexities and uncertainties in (a) determining contact forces, (b) surface-to-surface wear rates~ (c) forcing functions, and (d) extrapolations of these variables to other fuel designs (such as the 17x17 design used in Sequoyah), we believe that it is prudent for the applicant to make a commitment, before issuance of the OL, to submit for review a surveillance plan and schedule for the examination of guide thimble tube wear.
The specifics of such a surveillance program have not yet been det6rmined, but sin~e the wear phenomenon is a ti1ne-dependent process the details of such an inspection program do not need to be specified prior to the first Sequoyah refueling outage.
Furthermore, such inspection may not have to be conducted at.Sequoyah.
For example, the applicant could join in a cooperative 01mer 1s group and thereby submit applicable information derived fro_m a similar typQ of plant usi.ng 17xl7 fuel assemblies.
For acceptability, the minimum objectiv~ of such progfam should be to demon-strate that there is no occurrence of hole formation in rodded guicte thimble tubes.
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- Provided that the applicant agrees to commit to the performance of the surveillance described above, this issue is adequately resolved for the first cycle of operation. This issue will be resolved for later cycles of operation provided that surveillance results confirm the predictions of ~he analysis described above.
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REFEREricES
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Letter from T. ~-Anderson, Westinghouse, to D. G. Eisenhut, *NRC, NS-TMA-1936, dated September 12, 1978.
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Letter from T. M. Anderson, Westinghouse, to D. G. Eisenhut, NRC, NS-TMA-1992, dated December 15, 1978.
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Letter from T. M. Anderson, Westinghouse, to D. G. Eisenhut, NRC, NS-TMA-2102, dated June 27, 1979.
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Letter from L. M. Mills, Tennessee Valley Authority, to L. S. Rubenstein, NRG, Dockets 50-327*and 50-328, dated November 27, 1979.
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