ML20085D522
| ML20085D522 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 10/09/1991 |
| From: | Devine J GENERAL PUBLIC UTILITIES CORP. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| 5000-91-2074, C321-91-2276, TAC-79166, NUDOCS 9110160252 | |
| Download: ML20085D522 (3) | |
Text
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l GPU Nuclear Corporation NMCIMr One Upter Ibnd Haad Pssippany, htw Jetsey 07054 201-31C 7000 TE L LX 13Cr482 Wnter's Direct Dial Nurnter i
l October 9, 1991
$000-91-2074 C321-91-2276 U. S. Nuclear Regulatory Commission Att: Document Control Desk Washington, DC 20555 Gentlemen:
Subject:
Oyster Creek Nuclear Generating Station (OCNGS)
Docket No. 50 219 facility Operating License No. DPR-16 Oyster Creek Drywell Containment
Reference:
(1) HRC Letter dated 9/3/91, "0CNGS - Staff Position on Evaluation of Structural Integrity of a Degraded Steel Containment (TAC No. 79166)."
Reference 1 provided the NRC staff position on the evaluation of structural integrity of a degraded steel containment for Oyster Creek Nuclear Generating Station (OCNGS).
This letter provides GPU Naclear's response and intended course of action on this matter._
We are in full agreement with the principle on which the NRC staff nosition is based, that is, that stress intensities for the containment structure as a whole should be governed by Code specified primary membrane stress, and that any local exceedences should be of minor magnitude and limited in extent. The Oyster Creek drywell complies with the NRC's intent in that regard.
However, we do not fully agree with all of the specifics of the NRC staff position, particularly with the staff's proposed application of the limits of NE-3213.10 of ASME Section III, Division 1, to areas in which calculated stress intensities are in the range of 1.0 to 1.1 Smc.
More importantly, it is our view that further precise mapping of corroded areas to evaluate compliance with those spatial limits is impractical and unnecessary for the Oyster Creek containment.
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C3212276,LLT GPU Nuclear Corporabon is a subsidiary of General Pubhc Utihbes Corporabon
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I C321 91-2276
'Page 2 Consistent with this conclusion, our point by-point response to the NRC staff position is as follows:
1.
We accept that a corroded or degraded area in the containment structure in which calculated stress exceeds 1.0 Smc but is less than 1.1 Smc can be considered to be a discontinuity.
However, limiting the permissible extent of such discontinuities to the spatial limits of Subsection NE-3213.10 would clearly be a misapplication of that Code Subsection.
In effect, the proposed NRC staff position would establish new limitations far more restrictive than those specified by the already conservative ASME Code.
2.
We agree that the extent of the corrosion in the containment shell should be detern.ined as accurately as practical. As you know, over the last several years we have been conducting an extensive program of drywell examination and analysis.
One which b s included many thousands of individual UT measurements of shell thickness, along with material samples, laboratory experiments, structural evaluations, statistical analyses of corrosion trends, etc. We havt s'.awi the methods and results of that work with the NRC in previous meetings and co.respondence.
Based j
on this assessment program, we now have a very thorough understanding of the condition of the structure and we conclude with high confidence that there are very few locations where the calculated stress intensities for design basis conditions, would exceed 1.0 Sme, and in these cases only slightly.
Considering first the upper regions of the drywell (that is, all areas above the sand bed region), we have identified seven 6" x 6" grid locations where the mean thickness is such that calct lated stress intensity for design basis conditions would exceed 1.0 Sme.
Of these seven locations, two in the cylinder (at elevation 87'5") are two feet apart and two others at mid-sphere (elevations 50'2" and 51'10") are about 20 inches apart.
The remaining three (3) locations are isolated.
As you know, our corrosion monitoring program has focused on repeated measurements at the same locations for ;7 rate analysis of carrosion rates.
For that reason, the corrosion in each of these locations has been very precisely ascertained within the 6" x 6" grid locations, but has not been mapped either in extent or proximity outside of those 6" x 6" l
squares. Nonetheless, the overall conclusion from this extensive data base, including tae augmented measurements taken during the 13R outage to assess the statistical adequacy of the entire set, is that these thinner locations represent a very small fraction of the overall shell.
The situation in the sand bed region is structurally more complex. To simplify the structural analysis, we conservatively assumed that the shell thickness in the entire sand bed region has been reduced uniformly to a thickness _ (0.736") well below the value observed at any single location.
i.
l Based on this conservative assumption, the analysis shows that for the most severe design basis load combinations, 1.0 Smc (and in fact 1.1 Smc) would be exceeded in the central band of the sand bed region.
- However, even in this very conservative case, the limits of Subsection NE 3213.10 are fully met.
Based on this existing thorough understanding of the condition of this structure, along with the very conservative nature of C3212276.uf I
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C321-91-22?o Page 3 our stress calculations, we conclude that further precise mapping of local discontinuities, a resource and dose intensive effort, would add essentially no value to the existing evaluation.
1.
We agree that primary membrane stress should be in accordance with the stress intensity limits as stipulated in Table NE-3221-1, and we assert that the Oyster Creek containment shell is in full compliance with those limits.
In conclusion, the Oyster Creek drywell meets the intent of the staff position presented in Reference 1, in that the areas where the calculated stress intensities for design basis combinations exceed 1.0 Smc are limited in magnitude and extent. We do not agree fully with the staff's proposed application of the limits of NE-3213.10 to locations of calculated stress intensity in the range of 1.0 Smc to 1.1 Sme, although we have high confidence that the vast majority of the Oyster Creek drywell surface area is in compliance with that criteria.
Based on the extensive UT measurements already taken, we do not intend to further map the existing areas of corrosion.
Finally, as before, it is our positien that the Oyster Creek drywell 4 fully in compliance with the ASME Section Ill Code.
We do plar, to continue our aggressive program of acting to arrest the corrosion, as previously presented to the staff. The next major step is removal of sand from the sand bed region to be accomplished later this year.
Also, we note that NRC action on our proposed change to the licensed design pressure for the drywell (TSCR 198) is important in establishing a more meaningful design basis for the structure.
If you have any questions or comments on this submittal or the overall drywell ccrrosion program, please contact Mr. Michael Laggart, Manager, Corporate Nuclear Licensing at (201) 316-7968.
~
Jirfcerely
/
J. C. DeVine, Jr.
Director, Technical Functions JCD/RTZ/ pip cc: Administrator, Region i Senior NRC Resident Inspector Oyster Creek NRC Project Manager I
O212276.LET
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