ML20197C776
| ML20197C776 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 12/19/1997 |
| From: | Nunn D SOUTHERN CALIFORNIA EDISON CO. |
| To: | Howell A NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| References | |
| 50-261-97-15, 50-262-97-15, NUDOCS 9712240269 | |
| Download: ML20197C776 (6) | |
Text
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SoUf HHrN C.Auf ORMA EDISON
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Aa (Dilov INTLAN4Tf0N4L Coenpnp.
s-o' December 19,1997 i
Arthur T. Ilowell 111 Director Division of Reactor Safety U.S. Nuclear Regulatory Comtnission Region IV 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 7601180$4
Dear Sir:
Subject:
Docket Nos. 50 361 and 50 362 Pre decisional Enforcement Conference - Reactor Cool nt System Alloy 600 Nozzle Penetrations San Onofre Nuclear Generating Station, Wts 2 and 3
References:
1)
Letter, Mr. Arthur T.119well 111, (USNRC) to Mr. Harold B. Ray (SCE),
dated December 9,1997 2)
Letter, Mr. Dwight E. Nunn (SCE) to Mr. E. W. Merschoff(USNRC),
dated October 3, '.997 On December 17,1997, SCE received Reference 1, which provided your response to SCE's comments (Reference 2) concerning certain characterizations and conclusions provided in NRC Insp.: tion Report No. 50-361/9715 and 50 362/97-15. Out of the 16 SCE comments and
)
clarifications submitted to the NRr' in Reference 2, you accepted all but three. Accordingly, we understand you plan to submit con aed inspection report pages.
Of the three SCE corunents which you disagreed with, we have carefully reviewed these issues.
Two issues do not appear significant and do not require any additional discussion. However, one issue is significant in regards to whether licat NX7630 is more susceptible to primary water stress corrosion cracking (PWSCC) than other heats. Although this aspect was discussed with you today by Mr. Greg Gibson, Manager, Compliance, we wish to ensure the docket clearly reflects
. SCE's perspective, in order to support appropriate disposition of the proposed escalated enforcement item.
K P. O.l u l2p Ckmeme. CA 926744128 r.a u.
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i Aphur T,..llowell til December 19,1997 Specifically, Reference I stated:
"With respect to your comment on the first paragraph of page 14 of the inspection report, we do not consider the text to be in error. This paragraph contained an inspector view that the detection in a nozzle (manufactured from Heat NX7630) of the presence of shallow intergranular cracking, at a location remote from the more highly stressed material adjacent to the hweld, was indicative that the Heat NX7630 material was highly susceptible to primary water stress corrosion cracking. Your response to this view indicates that you believe that while Heat NX7630 microstructure may increase susceptibility to primary water stress corrosion cracking, other factors (e g., temperature, yield strength, environment, residual stresses, etc.) result in Heat NX7630 being no more susceptible to primary water stress corrosion cracking than other heats. We do not consider that this response provides a suflicient technical basis for recharacterizing our originalinspection observation."
It is not clear whethei the last sentence above refers only to the inspector's siew, or if the NRC has finalized their position on this item. It would be beneficial to tne Industry if the NRC would provide a suflicient technical basis for concluding that shallow intergranular micros:meture indications (observed in a micrograph from a single nozzle) dominates in PWSCC susceptibility over other factors such as: time, temperature, residual stress in the nozzle, weld, and water chemistry.
In the absence of such a technical basis, as noted in Reference 2, SCE strongly believes that
" Industry and SCE data demonstrate that NX7630 is no more susceptible to PWSCC than many other heats. There is no reliable way to predict if or when a particular nozzle will exp:rience PWSCC. Accei Jingly, there was no reason for special replacer,ient activities on Heat NX7630; in fact, such a program would have had significant ALARA impact. SCE can not identify corrective action in response to the apparent violation. other tSan replacement of all Alloy 600 RCS penetrations, which would preclude recurrence "
The facts and circumstances of Alloy 600 performance demonstrate that:
1.
Both at San Onofre and in the Industry, of the total population of Alloy 600 RCS hot leg and pressurizer penetration heats, there are: 32 total heats of Alloy 600 used in the industry (11 of which had detected PWSCC by 1995); and 11 total heats of Alloy 600 used at San Onofre (6 of which had detected PWSCC by 1995). See attached graphs.
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- 2.
SCE has responded to Alloy 600 PWSCC consistent with the NRC's historical positions that: Alloy 600 RCS penetration PWSCC has a low safety significance; PWSCC leaks will not propagate rapidly; and any PWSCC leakage will be detected by inspections and repairs effected by licensees.
l
Arthur T. Ilowell 111 3.
December 19,1997 3.
Some NX7630 nozzles in the most aggressive environment (Pressurizer), are still performing without visual indications of PWSCC.
4.
Nozzle PWSCC is a function of time, temperature, residual stress in the nozzle, weld, micro structure, and water chemistry.
5.
San Onofre Root Cause Evaluation 92 019 provides no evidence that NX7630 was more susceptible than other heats of Alloy 600, stating "The presence of high density carbide bands may explain the -tr king observed on this heat despite its consideinble low yield strength."
If any additional data or information would be helpful, we will be pleased to discuss them with you.
Sincerely, i
Attachment:
as stated cc:
E. W. Merschoff, Regional Administrator, NRC Region IV K. E. Perkins, Director, Walnut Creek Field Omce, NRC Region IV J. A. Sloan, NRC Senior Resident inspector, San Onofre Units 2 and 3 l
M. II. Fields, NRC Project Manager, San Onofre Units 2 and 3 I
NRC Document Control Desk l
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Alloy 600 Nozzle PWSCC History
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H When compared to other heats of Alloy 600, beat NX 7630 does not exhibit an unusual rate of PWSCC i
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incomet 600 Nozzle PWSCC Indications CE Flames (T > 600'F) n. Heat NX7630 (Dasa smilable thromek 1995).
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The Weitmill plot for NX7630 inchcates performance omsassent with the industry data. This can be seen ty the close weg of both the Characteristic Life (Eta), and the Slope Factor (Beta) for both pop w-=
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Attachment Figure 3
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