ML20003F971
| ML20003F971 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/20/1981 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20003F970 | List: |
| References | |
| NUDOCS 8104270295 | |
| Download: ML20003F971 (2) | |
Text
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING TH'E ACTION TAKEN ON FUEL ASSEMBLY HOLDDOWN SPRING FAILURES METROPOLITAN EDISON COMPANY JERSEY CENTRAL POWER AND LIGHT COMPANY PENNSYLVANIA ELECTRIC CDMPANY THREE MILE ISLAND NUCLEAR STATION, UNIT NO.1 2.
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DOCKET NO. 50-289 Introduction z..
By our letter dated July 1,1980, we requested Metropolitan Edison Company (the licensee) to furnish 1nfomation on the holddown springs used in the fuel assemblies at the Three Mile Island Nuclear Station, Unit No.1.
Our concerns l
stemed from the discovery of the fuel assenbly holddown spring failures that occurmd at. Davis-Besse4 nit:No.1, Crystal River Unit No. 3 and Oconee Unit No. l.. The holddown-springs which are located in the upper fitting of,
the Babcock and Wilcox (B8H) Mark B-4 fuel assenbly accomodate length changes due to themal expansion and. irradiation growth while still providing a positive ~
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i holddown force for the assembly.
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- - 1 Disecssion and Evaluation
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In a letter dated September i0,1980 (TLL 447), the licensee responded to our concem which indicated that no failed holddown springs have been identified at THI-l resulting from reviewing Cycle 5 verification tapes of the fuel assemblies now in the core and the special video examinations of fuel assemblies in the spent fuel pool.
In addition, an evaluationjperformed by 9&W indicated that no significant safety concems exist for TMI-l operation even if.an unlikely event of broken holddown springs were to occur.
1 An archival material.-examination of the spring material perfomed by B&W 'revebled a coarse grain structure on.the outer surface and a " duplex" material grain size (i.e., large variation between interior grain sizes). A coarse outer grato structure is indicative of reduced resistance to fatigue crack initiation from which we conclude that the failures were initiated from fatigue cracks, except
'for one of the failures (at Crystal River), however, these fatigue cracks appeared to have propagated by stress corrosion cracking (SCC). The conclusion that these cracks propagated by stress corrosion was supported by the inter-granular nature of the cracks, coupled with etch tests indicating that the l
suspect material had lower than desired resistance to SCC.
Examination of the heat treatment records revealed that there has been some variations in solution anneal and drawing schedules in two batches of springs, which have resulted in the " duplex" grain structure with lowered SCC resistance. Thus, the cause of I
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. the failures was attributed to an improper material conditioning characterized by a coarse outer grain structure (which is indicative of less fatigue resistance) and a duplex inner structure (which is indicative of less resistance to stress corrosion cracking).
In'tne case of the single Crystal River failure, we have judged this failure to have occurred solely by fatigue. Improper component design did not contribute to these failures for two basic reasons: 1) stress analysis indicated that the spring design was adequate for the loads involved and 2) except for the one failed spring at Crystal River 3, all the failed springs (at Davis-Besse 1 and Oconee 1) come from the batches with suspect irsproper material conditioning.
The material that was improperly conditioned was not used in fabricating the holddown springs for the fue? assemblies at TMI-l and therefore such failures are not expected to occur. However, to further insure that such failure will not occur in the future, the licensee has committed that future reload fuel assemblies will contain holddown springs fabricated from materials which speci-fications call for improved material and process controls. The improved material will be more resistant to fatigue and stress corrosion cracking.
In aB3TtTon, the licensee has committed to establishing a fuel surveillance program to inspect fuel assemblies during each outage for indications of holddown spring failures or damage to other assembly components.
Based on our evaluation, we conclude that the licensee has adequately responded to our concerns in regard to the potential failures of the holddown springs of the fuel assemblies at TMI-1.
In addition, we find the proposed commitments to a surveillance program described -above insures against future failures and is acceptable. This action is not hazardous to public heelth and safety and therefore is acceptable.
Dated:
April 20,1981
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