ML19326E098

Forwards NRC Evaluation of Control Rod Guide Thimble Wear in Pwr.Issue Resolved for 15X15 Fuel Assemblies Designed by Westinghouse

ML19326E098
Person / Time
Site:	Haddam Neck File:Connecticut Yankee Atomic Power Co icon.png
Issue date:	07/10/1980
From:	Crutchfield D Office of Nuclear Reactor Regulation
To:	Counsil W CONNECTICUT YANKEE ATOMIC POWER CO.
References
NUDOCS 8007250672
Download: ML19326E098 (4)

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July 10, 1980 Docket No. 50-213 Mr. W. G. Counsil, Vice President Nuclear Engineering and Operations Connecticut Yankee Atocic Power Company Post Office Box 270 Hartford, Connecticut 06101

SUBJECT:

CONTROL R0D GUIDE THIMBLE WEAR ISSUE (B-14) -

HADDAM NECK PLANT 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 15x15 fuel assemblies designed by Westinghouse for the Haddam Neck Plant.

Sin rely, Dennis M. Crutchfield, Chief Operating Reactors Branch #

Division of Licensing

Enclosure:

Evaluation of Control Rod Guide Thimble Wear cc w/ enclosure:

See next page l

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Mr. W. G. Counsil July 10, 1980 cc w Day, / enclosure: Berry & Howard U. S. Environmental Protection Counselors at Law Agency One Constitution Plaza Region I Office' Hartford, Connecticut 06103 ATTN:

EIS COORDINATOR JFK Federal Building Superintendent Boston, Massachusetts 02203 Haddam Neck Plant RFD #1 Post Office Box 127E East Hampton, Connecticut 06424 Mr. James R. Himmelwright Northeast Utilities Service Company P. O. Box 270 Hartford, Connecticut 06101 Russell Library 119 Broad Street Middletown, Connecticut 06457 Board of Selectmen Town Hall Haddam, Connecticut 06103 Connecticut Energy Agency ATTN: Assistant Director Research and Policy Development Department of Planning and Energy Policy 20 Grand Street Hartford, Connecticut 06106 Director,' Technical Assessment Division Office of Radiation Programs

( AW-459)

U. S. Environmental Protection Agency Crystal Mall 12 Arlington, Virginia 20460

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Enclosure EVALUATION OF CONTROL ROD GUIDE THIMBLE WEAR IN FACILITIES DESIGNED BY 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 water reactors. Subsegently, it has been detemined that coolant flow up 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 fratting wear of the wall occurs. Significant wear has been found to be confined to the ralatively 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 femation of holes.

Guide thimble / tubes function principally as the main' structural members of the fuel i

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 scramability.

fn 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- ~

mation was submitted 'oa'arences 1 and 2) on their experience and understanding of the issue. This infor-

.sisted of guide thimble wear measurements taken on irradiated fuel assemblies fr --

stnt Beach, Units 1 and 2 (two-loop piants 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's wear predictions on the safety analyses of plant 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 flexi-i 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 the Westinghouse analysis probably accounts for all of the major variables that control this wear process. However, because of the complexities and uncertainties in (a) detemining contact forces, (b) surface-to-surface wear rates, (c) forcing functions, and (d) extrapolations of these variables to the riew 17 x 17 fuel assembly design, the staff required several near-term OL applicants to submit to a surva111ance program.

For acceptability, the minimum objective of such program was to demonstrate that there is no occurrence of hole formation in rodded guide thimbles.

• Plants using Westinghouse HIPAR. fuel assembly designs (stainless steel guide thimble 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 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 exsmine all guide thimbles in six rodded fuel assemblies having either one i

or two cycles of burnup.

It is our expectation that the program will confirm Westing-h:use predictions, and therefore this issue should be considered resolved for all Westing-house plants using the newer 17 x 17 fuel assembly design.

Th3 relevant primary system design differences in plants fueled w!th 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. Rubenstein, NRC, Dockets 50-327 and 50-328, dated November 27, 1979.

2.

Letter from T. M. Anderson, Westinghouse, to H. R. Denton, NRC NL 'MA-2238,"

dated April 29, 1980.

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