ML20101K548
| ML20101K548 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 06/29/1992 |
| From: | Shelton D CENTERIOR ENERGY |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 2055, TAC-M81397, NUDOCS 9207060011 | |
| Download: ML20101K548 (4) | |
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.1RIOR ENERGY Donced C. Shelton 300 Madtson Avenue Vice President + Nuclear Toledo, OH 436524001 Davis Besse (419)249 2300 Docket Number 50-346 License Number NPF-3 Serial Number 2055 June 29, 1992 United States Nuclear Regulatory Commission Document Control Desk Washington, D.
C.
20555 Subj ec t :
Follovup Corrosion Resistance Information for the INCO 82
_i Velded Steam Generator Tube Plug Installed at the Davis-Besse Nuclear Power Station Gentlemen:
By_ letter dated September 3, 1991 (Serial Number 1978),1 Toledo Edison requested relief from Section XI of the 1986 Edition of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
Specifically,_ relief was requested from the requirements of-Section XI, IVB-4400, Heat Exchanger Tube or Tubesheet Bore Hole Plugging.
Relief vas requested to permit machine velding of a single steam generator
-tube plug at the Davis-Besse Nuclear Power Station (DBNPS) during the l
seventh refueling outage (7RF0)._ The single velded tube plug was designed and fabricated from INCO 82 material by ABB/ Combustion Engineering (CE), Toledo Edison's contractor for steam generator-services during the 7RF0. The velded plug was installed as a back'up-for an explosive vcided Inconel 600 plug because-of concerns regarding thw susceptibility of Inconel 600 to primary water stress corrosion cracking (PUSCC).
During the review of the relief request, the NRC raised.several questions regarding the velded tube _ plug. The questions related to the material used in fabricating the plug (INCO 82), and the stress analysis of the plug design. Toledo Edison provided additional information to address the questions regarding the design analysis and INCO 82 mechanical properties used in the design analysis, by letter dated October 23, 1991 (Serial Number 1991).- In addition the NRC-requested information regarding.the corrosion _ behavior of INCO 82 material in high temperature, high pressure water environments. Toledo-Edison committed to monitor CE's-progress in generically addressing the:
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Docket Number 50-346 License Number NPP-3 Serial Number 2055 Page 2 corrosion resistance of INCO 82 material with the NRC staff.
By letter dated October 31, 1991 (Log Number 3614), the NRC approved Toledo Edison's request for automatic welding of the steam generator tube plug as an alternative to the ASME Code r_equirements. The NRC safety evaluation of the alternative to the ASME Code included in the NRC's October 31, 1991 letter, requested that the information regarding corrosion resistance be provided prior to the end of the DBNPS eighth refueling outage. The purpose of this letter is to summarize CE's assessment of corrosion resistance of INCO 82 steam generator tube plugs which was recently received by Toledo Edison.
Combustion Engineering identified that there are approximately 2,000 INCO 82 steam generator tube plugs currently in service at 20 pressurized water reactors.
In addition more than 2,000 INCO 82 tube plugs vere installed in the original Palisades steam generators.
Some-of the Palisades INCO 82 plugs had-been in service for approximately 15 years when the Palisades steam generators were replaced in 1990.
Additionally, another CE plant has plugs which have been in service since 1978. There have not been any reported tube plug material failures in this data base of over 4000 tube plugs. There have been some isolated tube plug leaks at some plants which vere all traced to veld deficiencies and not to tube plug base material problems.
Industry reported tube-plug failures due to PVSCC to date have been isolated to tube plugs fabricated from Inconel 600 material. Although INCO 82 shares many mechanical properties with Inconel 600, and is essentially Inconel 600 veld filler material, the elemental composition of INCO 82 differs significantly from that of Inconel 600 in areas important to resistance to PVSCC.
The ASME Code Section II chemistry compositions for INCO 82 and Inconel 600 are compared in Table 1.
INCO 82 has less nickel and carbon, and more chromium than Inconel 600, and includes carbon stabilizing elements, columbium and tantalum, which are not present in Inconel 600. INCO 82 has two to three percent stabilizing material in its makeup.
Consequently, for a common annealing temperature ou similar Inconel 600 and INCO 82 carbon heats, the Inconel 600 would be expected to have significantly more intragranular carbides than INC0 82.
In view of the large database of plugs which have operated since the mid-seventies without any reported material problems, it is reasonable to conclude that INCO 82 material is not susceptible to PVSCC in this
-application.
Since installation of the first plugs at the Palisades plant, improvements in plug design and the material heat treatment have been incorporated in the current INCO 82 plugs that further enhance their resistance to PVSCC. These improvements vere incorporated in the plug installed at the DBNPS.
Early plugs installed at Palisades vers essentially a solid cylindrical plug with a small veld lip.
Plugs of this design vere very stiff and tube sheet flexure would induce high stresses in and adjacent to the veld.
Notwithstanding this unfavorable stress condition, there were no reported PUSCC failures. These plugs were installed [only] in the original Palisades' steam generators and none is in service today.
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Docket Number 50-346 License Number NPP-3 Serial Number 2055 P, age 3 The current INCO 82 velded plug is a long thin valled cylinder. The cylinder is flexible and can deform elastically as the tube sheet flexes. This results in stresses not being concentrated in localized areas and reduced susceptibility to PVSCC.
The current annealing heat treatment of INC0 82 material used to fabricate tube plugs also improves resistance to PVSCC. _The anneal dissolves the carbon into solution and upon cooling the carbon precipitates at the grain boundaries in the form of chromium carbides.
These intergranular carbides (carbide decoration) are considered desirable in the microstructure from the standpoint of resistance to PVSCC. The anneal also provides a lover yield strength material which is desirable in both the vrought material form and veld joint.
Lover yield strength material results in lover residual stresses and reduced susceptibility to PVSCC.
Toledo Edison considers that the information provided by this letter addresses NRC questions regarding the long term corrosion resistance of the single INCO 82 steam generator tube plug installed at-the DBNPS.
As such, this letter satisfies Toledo Edison's commitment to provide information to the NRC staff addressing the corrosion behavior of INCO 82 material.
If you have any questions regarding the information provided by this letter, please contact Mr. Robert V. Schrauder, Manager - Nuclear Licensing, at (419) 249-2366.
Very tru ' yours, PVS/dle Attachment ec:
A. B. Davis, Regional Administrator, NRC Region III J. B. Hopkins, NRC/NRR DB-1 Senior Project Manager-V. Levis, NRC Region III, DB-1 Senior Resident Inspector J
- 11. Sodergren, ABB/ Combustion Engineering Utility Radiological Safety Board
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. Docket Number 50-346 License Number NPF-3 Serial Number 2055-Attachment Page 1 Table 1 Comparison of Inconel 600 and INCO 82 Elemental Composition Element inconel 600 INCO 82 Nickel 72.0 minimum 67.0 minimum Chromium 14.0 - 17.0 18.0 - 22.0 Iron 6.0 - 10.0 3.0 Manganese 1.0 maximum 2.5 - 3.5 Carbon 0.15 maximum 0.10 maximum Copper 0.50 maximam 0.50 maximum
)
Silicon 0.50 maximum 0.50 maximum Sulphur 0.015 maximum 0.015 maximum Cb - Ta 2.0 - 3.0
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