ML18139A647
| ML18139A647 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 07/07/1980 |
| From: | Varga S Office of Nuclear Reactor Regulation |
| To: | Ferguson J VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| References | |
| NUDOCS 8008130614 | |
| Download: ML18139A647 (5) | |
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JULY 7 1980 and 50-1 Docket Nos. 50-280 and 50-281 Mr. J. H. Ferguson Executive Vice President - Power Virginia Electric and Power Company Post Office Box 26666 Richmond, Virginia 23261
Dear Mr. Ferguson:
NRC PDRs (2)
Local PDR TERA NSIC NRR Reading ORB1 Reading T. Novak S. Varga D. Neighbors E. Conner C. Parrish I &E ( 3)
Attorney, OELD J.Heltemes The NRC staff has been reviewing the subject of control rod guide thimble wear in pressurized water reactors. The enclosure to this letter describes 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'15 x 15 fuel assemblies designed by Westinghouse for the Surry Power Station, Units 1 and 2.
Enclosure:
Evaluation of Control Rod Guide Thirnbl e ~!ear cc: w/enclosure See next page Sincerely, Original *signed by:*
S. A. Var_ga Steven A. Varga, Chief Operating Reactors Branch #1 Division of Licensing p
N~C FORM 318 (9-76) NRCM 0240
-1:ru.s. GOVERNMENT PRINTING OFFICE: 1979-289-369
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UNITED STATES e
NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 Docket Nos. 50-280 and 50-281 Mr. J. H. Ferguson Executive Vice President - Power Vfrginia Electric and Power Company Post Office Box 26666 Richmond, Virginia 23261
Dear Mr. Ferguson:
J_~lY, 7, 1980 The NRC staff has been reviewing the subject of control rod guide thimble wear in pressurized water reactors.
The enclosure to this letter describes 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 15 x 15 fuel assemblies designed by Westinghouse for the Surry Power Station, Units 1 and 2.
Enclosure:
Evaluation of Control Rod Guide Thimble Wear cc:
w/enclosure See next page t:y~~~
.i Steven A\\ Varga, ~
Operating Reactort~nch #1 Division of Licensing
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- -'.r. J. H. Ferguson
\\/frgi nia Electric and Power Cor.ipany cc:
Mr. Mi chae 1 W. t*'.aupi n Hunton and Hi 11 i a;.:s Post Office Box 1535 Richmond, Virginia 23213 Mr. W. L. Stewart, Manager P. 0. Box 31 5 Surry, Virginia 23883 S_wem Library
... _College of William and Mary
'°1-.'illiamsburg, Virginia 23185 Donald J. Burke, Resident Inspector Surry Power Station U. S. Nuclear Reg'Jlatory CofITilission Post Office Box 959
'..Ji 11 i ams burg, Vi rgi ni a 23185 July7,1980
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e EVALUATION OF CONTROL ROD GUIDE THIMBLE WEAR IN FACILITIE~ DESIGNED BY WESTINGHOUSE Enclosure A degradation of control rod guide thimble/tube wa)ls has been observed during post-irradiation examinations of irradiated fuel assernbJies taken from several operating pressurized water reactors.
Subsequently, it has [.been detennined that coolant flow up through the guide tubes and turbulent cross flow ~bove the fuel assemblies have been res pons i bl e for inducing vibratory rnoti on in the nci"rma lly fully withdrawn ( "parked 11
)
control rods position.
When these vibrating rods ~re in contact with the inner surface of the guide tube wall, a fretting wear of the wa11 -occurs. Significant wear has been found to be confined to the relatively soft Zircalny-4 guide tubes because the control rod claddings--stainless s*teel for Westinghouse-NS!SS designs--provide a relatively hard wear surface.* The extent of the observed wear is both time and NSSS-design depen-dent and has, in some non-~lestinghouse cases, been observed to extend completely through the guide tube walls, thus resulting in the for.nation of holes.
Guide thimble/tubes function principally as the rrain*structural members of the fuel assembly and as channels to guide and decelerate control rod motion.
Significant loss of mechanical integrity due to wear or hole formation could:
(1) result in the inability of the guide thimble to withstand their anticipated loadings for.fuel handling accidents and condition 1-4 events; and, (2) hinder scramabil i ty.
In response to the staff's attempt to assess the susceptibility a.rid impact of guide thimble wear in Westinghouse plants, two meetings were held with \\~estinghouse an9 infor-ii~tion was submitted (References 1 and 2) on their experience and understanding of the issue.
This information consisted of guide thimble wear measurements taken on irradiated fuel asse~blies from Point Beacn, Units 1 and.2 (two-loop plants using 14 x 14 fuel assemblies). Also described was* a mechanistic wear model (developed from the Point Beach data) and the impact of the'model 1s wear predictions on the safety analyses of p1ant designs.
Westinghouse believes that their fuel designs will experience less wear than that repor-ted in some other NSSS designs because the Westinghouse designs use thinner, more fiexi-ble, control rods that have a relatively more lateral support in the guide thimble
.assembly of the upper core structure,* Such construction provides the housing and guide path for the rod cluster control assemblies (RCCAs) above the core and thus restricts control rod vibration due to lateral exit flow.
Also, Westinghouse believes that their wear model conservatively predicts guide thimble wear and that even with the worst anti-cipated wear conditions (both in the degree of wear and the location of wear) their guide thimbles will be able to fulfill their design functions.
The staff concluded that -:.1,e Westinghouse analysis probably accounts ~for al 1 of the major variables that con:~ol this wear process. However, because of..the complexities and uncertainties in (a) Jeterrnining contact forces, (b) surface-to-surface wear rates, (c) forcing functions, and (d) extrapolations of these variables to tbe,new 17 x 17 fuel assembly design, the staff required several near-tenn OL appl icarits to submit to a survaillance program.
For acceptability, the minimum objective of such pr,ogram was to demonstrate that there is no occurrence of hole formation in rodded gui"de thimbles.
- Plants using Westinghouse HIPAR fuel assembly designs (stainless steel guide thimb1e tubes) are not considered susceptible to significant wear.
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,
- Enclosure To satisfy this request for confirmation of the Westinghouse analytical predications, a cooperative owners group was estabiished 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 with those of plants fueled with 14 x 14 fuel assemblies are minimal.
And certainly the extrapolation of wear prediction is less than that associated with the extrapolation to the newer plants using 17 x 17 fuel assemblies.
Thus it is reasonable to conclude that the wear in 15 x 15 fuel assemblies should be equivalent to that experienced and measured in 14 x 14 fuel assemblies, 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 inforrr~tion that has been provided is sufficient to resolve the issue of guide thimble/tube wear in plants fueled with 14 x 14 and 15 x 15 fuel assemblies.
References
- 1.
Letter from L. M. Mills, Tennessee Valley Authority, to L. S. Ruben~in, NRC,
.Dockets 50-327 and 50-328, dated November 27, 1979.
- 2.
Letter from T. M. Anderson, Westinghouse, to H. R. Denton, NRC, NS-Tf-1A-2238, dated April 29, 1980.
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