ML19330C296
| ML19330C296 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/25/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19330C294 | List: |
| References | |
| NUDOCS 8008080195 | |
| Download: ML19330C296 (2) | |
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3 Enclosura EVALUATION OF CONTROL R0D GUIDE THIMBLE WEAR IN FACILITIES DESIGNED BY WESTINGHOUSE 1
A degradation of control rod guide thimble / tube walls has been observed during post-irradiation examinations'of irradiated fuel assemblies taken from several operating Subsequently, it has been detemined that coolant flow up pressurized water reactors.
through the guide tubes and turbulent cross flow above the fuel assemblies have been responsible for inducing vibratory motion in the normally fully withdrawn (" parked")
control rods position. When these vibrating rods are in contact with the inner surface of the guide tube wall, a fretting wear of the wall occurs. Significant wear has been found to be confined to the relatively soft Zircaloy-4 guide tubes because the control rod claddings--stainless stell.for Westinghouse-NSSS 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-Westinghouse cases, been observed to extend completely through the guide tube walls, thus resulting in the formation of holes.
4 Guide thimble / tubes function principally as the main' structural members of the fuel assenbly 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'f~uel handling accidents andcondition1-4 events;and,(2)hinderscramability.
In response to the staff's attempt to assess the susceptibility and impact of guide thimble wear in Westinghouse plants, two meetings were held with Westinghouse and infor. '-
i mation was submitted (P.eferences 1 and 2) on their experience and understanding of the issue. This information consisted of guide thimble wear measurements taken on irradiated fuel assemblies from Point Beach, Units 1 and 2 (two-loop plants using 14 x 14 fuel 1
assemblies. Also described was a mechanistic wear model (developed from the Point s
Beach data and the impact of the model's wear predictions on the safety analyses of plant designs.
l!estinghouse believes that their fuel designs will-experience less wear than that repor-l ted in some other NSSS designs because the Westinghouse designs use thinner, more flext-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 cot.t.'ci assemblies (RCCAs above the core and thus restricts control rod vibration due to lateral exit flow. A so, 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 weer) their guide thimbles ~ will be able to fulfill their design functions.
i The staff concluded that the Westinghouse analysis probably accounts for all of the l
major variables that control this wear process. - However, because of the complexities and nr. certainties in (a) detennining contact forces, (b) surface-to-surface wear rates, (c) forcing functions, and (d) extrapolations of these variables to the new 17 x 17 fuel assembly design, the. staff required several near-term OL applicants to submit to a
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survaillance program. For acceptability, the minimum objective of such program was to demonstrate that-there is no occurrence of hole formation in rodded guide thimbles.
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- Plants using Westingnouse HIpAR fuel assembly designs.(stainless steel guide thimble tubes) a.' e not considered susceptible to significant wear.
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'l f. Enclosure s
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 with those of plants fueled with 14 x 14 fuel assemblies are mininal.
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 information 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. Rubensitin, NRC, Dockets 50-327 and 50-328, dated November 27, 1979.
2.
Letter from T. M. Anderson, Westinghouse, to H. R. Denton, NRC, NS-TMA-2238,'
dated April 29, 1980.
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