ML13322A848
| ML13322A848 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 07/10/1980 |
| From: | Crutchfield D Office of Nuclear Reactor Regulation |
| To: | Dietch R Southern California Edison Co |
| References | |
| NUDOCS 8007280098 | |
| Download: ML13322A848 (5) | |
Text
OfN:
GLai nas FocketJHeltemes NRC PDR JOlshinski LPDR JWetmore TERA RDiggs Docket No. 50-206 NSIC MConner NRR Reading OELD JULY 1 o 1980 ORB #5 Reading OI&E (3)
DEisenhut SNowicki Mr. R. Dietch RPurple Gray File Vice President TNovak RTedesco Nuclear Engineering and Operations DCrutchfield HSmith Southern California Edison Company 2244 Walnut Grove Avenue P. 0. Box 800 Rosemead, California 91770
Dear Mr. Dietch:
SUBJECT:
CONTROL ROD GUIDE THIMBLE WEAR ISSUE (8-14)
SAN ONOFRE NUCLEAR GENERATING STATION, UNIT NO. 1 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 14x14 fuel assemblies designed by Westinghouse for the San Onofre Nuclear Generating-Station, Unit No. 1.
Sincerely, Original signed by Dennis M. Crutchfield Dennis M. Crutchfield, Chief Operating Reactors Branch #5 Division of Licensing
Enclosure:
Evaluation of Control Rod Guide Thimble Wear cc w/enclosure:
See next page S DL:ORB
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SNowicki DC 1 Id
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//(,/80 N=r PORN 318 (9-76) NRn 0240
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GOVERNMENT PRINTING oFFIC: 1978 - 265 -769
0 UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D. C. 20555 July 10, 1980 Docket No. 50-206 Mr. R. Dietch Vice President Nuclear Engineering and Operations Southern California Edison Company 2244 Walnut Grove Avenue P. 0. Box 800 Rosemead, California 91770
Dear Mr. Dietch:
SUBJECT:
CONTROL ROD GUIDE THIMBLE WEAR ISSUE (B-14)
SAN ONOFRE NUCLEAR GENERATING STATION, UNIT NO. 1 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 14x14 fuel assemblies designed by Westinghouse for the San Onofre Nuclear Generating Station, Unit No. 1.
Sinc rely, D nnis M. Crutchfield, Chi Operating Reactors Branch #5 Division of Licensing
Enclosure:
Evaluation of Control Rod Guide Thimble Wear cc w/enclosure:
See next page
Mr. ^. Dietch 2 -
July 10, 1980 cc w/enclosure:
Charles R. Kocher, Assistant Director, Technical Assessment General Counsel Division Southern California Edison Company Office of Radiation Prograrm Post Office Box 800 (AW-459)
Rose-,ead, California 91770 U. S. Environmental Protection Agency David R. Pigott Crystal Mall 12 SSamuel B. Casey Arlington, Virginia 20460 Chickering & Gregory Three Embarcadero Center U. S. Environmental Protection Twenty-Third Floor Agency San Francisco, California 94111 Region IX Office ATTN:
EIS COORDINATOR Jack E. Thomas 215 Freemont Street Harry B. Stoehr San Francisco, California 94111 San Diego Gas & Electric Company P. 0. Box 1831 San Diego, California 92112 Resident Inspector c/o U. S. NRC P. 0. Box AA Oceanside, California 92054 Mission Viejo Branch Library 24851 Chrisanta Drive Mission Viejo, California 92676 Mayor City of San Clemente San Clemente, California 92672 Chai rman Board of Supervisors County of San Diego San Diego, California 92101 California Department of Health ATTN: Chief, Environmental Radiation Control Unit Radiological Health Section 714 P Street, Room 498 Sacramento, California 95814
Enlosure 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.
Subsequently, it has been determined 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 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.
Guide thimble/tubes function principally as the main 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 scramability.
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 mation 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 assemblies from Point Beach, 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'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 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) determining 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 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.
- Plants using Westinghouse HIPAR fuel assembly designs (stainless steel guide thimble tubes) are not considered susceptible to significant wear.
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 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 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, NS-TMA-2238, dated April 29, 1980.