ML20216D096

Submits Response to 970822 RAI on License Amend Request Requesting to Allow Use of New SG Tube Sleeve Designs & Installation & Exam Techniques

ML20216D096
Person / Time
Site:	Prairie Island
Issue date:	09/02/1997
From:	Sorensen J NORTHERN STATES POWER CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
Shared Package
ML20216D101	List: ML20216D096 ML20216D114
References
TAC-M97367, TAC-M97368, NUDOCS 9709090214
Download: ML20216D096 (8)

Text

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Northern States Power Company

, Prairie Island Nuclear Generating Plant 1717 Wakonado Dr. East Welch, Minnesota 55089 I

September 2,1997 U S Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 PRAIRIE ISLAND NUCLEAR GENERATING PLANT Docket Nos. 50-282 License Nos. DPR-42 50-306 DPR-60 Response to Request for AdditionalInformation Related to License Amendment Request Dated November 27,1996 Incorporation of Combustion Engineering Steam Generator Welded Tube Sleeve Topical Report (TAC Nos. M97367 and M97368)

By letter dated August 22,1997, the NRC Staff requested additional information regarding our License Amendment Request Dated November 27,1996 which proposed changes to Section 4.12 of the Prairie Island Technical Specifications that would allow the use of new steam generator tube sleeve designs and installation and examination techniques. Our response to the August 22,1997 request for additional information is provided as Attachment 1.

Additional changes to the Prairie Island Technical Specifications are being proposed in response to the August 22,1997 request for additional information. Attachments 2 and 3 provide revised Prairie Island Technical Specification and Bases pages incorporating the changes proposed by this response, A revised Safety Evaluation, Significant Hazards Determination and Environmental Assessment have not been submitted for the additional Technical Specification changes proposed in this response since those evaluations, as originally presented in the November 27,1996 License Amendment Request, continue to bound the proposed license amendment as revised by this l'

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response.

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0 USNRC NORTHERN STATES POWER COMPANY September 2,1997

- Page 2 In this letter we have made new Nuclear Regulatory Commission commitments which are identified as such in Attachment i as the statements which are in italles. Please contact Gene Eckholt (612 388-1121)if you have any questions related to the attached response.

oel P Sorensen Plant Manager Prairie Island Nuclear Generating Plant c: Regional Administrator - Region Ill, NRC Senior Resident inspector, NRC NRR Project Manager, NRC J E Silberg Attachments:

Affidavit Attachment 1 - Response to August 22,1997 NRC Request For Additional Information Attachment 2 - Proposed Changes Marked Up on Existing Technical Specification Pages Attachment 3 - Revised Technical Specification Pages i

UNITED STATES NUCLEAR REGULATORY COMMISSION NORTHERN STATES POWER COMt)ANY PRAIRIE ISLAND NUCLEAR GENERATING PLANT DOCKET NO. 50 282 50-306 RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION AND REVISION TO REQUEST FOR AMENDMENT TO OPERATING LICENSES DPR-42 & DPR40 LICENSE AMENDMENT REQUEST DATED NOVEMBER 27,1996 Northern States Power Company, a Minnesota corporation, by this letter dated September 2,1997, with Attachments 1 through 3, provides a response to an NRC Staff request for additional information in support of the subject License Amendment Request dated November 27,1996. Attachment 1 provides the response to the August 22,1997 NRC Request for Additional information. Attachments 2 and 2 provide revised Prairie Island Technical Specification and Bases pages incorporating changes l proposed as part of the response to the request for additional information.

This letter contains no restricted or other defense information.

NORTHERN STATES POWER COMPANY By N 46el P' Sore'nsen Plant Manager Prairie Island Nuclear Generating Plant On this .2 day of Sw66 M7 before me a notary public in and for said County, personally appe'ared Joel P Sorensen, Plant Manager, Prairie Island Nuclear Generating Plant, and being first duly sworn acknowledged that he is authorized to executo this document on behalf of Northern States Power Company, that he knows the contents thereof, and that to the best of his knowledge, information, and belief the statements made in it are true and that it is not interposed for delay.

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ATTACHMENT 1 l 4

Response to Auaust 22.1997 NRC Reauest For Additionalinformation The following information is provided in response to the Request for Additional Information regarding our License Amendment Request Dated November 27,1996 which proposed changes to Section 4.12 of the Prairie Island Technical Specifications that would allow the use of new steam generator tube sleeve designs and installation and examination techniques:

RESPONSE TO NRC STAFF QUESTJ'2N_1 Question 1:

A recent welded sleeve Irvservice Inspection at Kewaunee identified weld zone Indications, using a visual technique, that were not detected by either ultrasonic or eddy current technique. The proposed amendments do not specifically require a visual inspection of the upper weld for tubesheet sleeves. Discuss the basis for not requiring an upper weld visual Inspection during the Installation process in light of the findings at Kewaunee.

Response

The missed defect in a sleeve weld at Kewaunee involved a missed eddy current indication which would have required re-evaluation by visual examination of the sleeve weld. This supports the ability of eddy current examinations to screen for sleeve weld defects such as seen at Kewaunee. In light of the Kewaunee experience and previous experience at Pralrie Island, ABB CENO and NSP commit to a 100% post weld visual examination of the sleeve welds for the next sleeving installation at Prairie Island. This commitment is made even though visualinspection of sleeve welds is specified as an option for other than the lower edge welds in the sleeve licensing report (CEN-629-P).

The sleeve procedures for the Fall 1997 Prairie Island Unit 1 refueling outage include 100% visual weld examination. NSP reserves the right to relax this requirement in future outages after a high degree of acceptable welds has been demonstrated.

This is consistent with the following statement from near the top of Page 6 of Exhibit A to the Prairie Island License Amendment Request dated November 27,1996:

" Currently, the extent of the VT examination of the free span sleeve welds is 100%

of the new sleeves installed at Prairie Island. Relaxation of this requirement depends on demonstrating a high degree of acceptable welds in the future."

Att: chm:nt i S:pt:mb:r 2,1997 e Pags 2 Question 2:

Page 6, Exhibit A, inservice Examination, it is stated that a 20 percent sample of all installed sleeves in each steam generator will be inspected each refueling outage, and an expansion of 100 percent of sleeves,in affected steam generator, l will be inspected if one pluggable indication has been found. Please clarify whether these requirements have been incorporated into technical specifications or as a license condition to perform the sleeve examinations in accordance with these specifications.

Response

i

! A requirement is being adding to Technical Specification 4.12.B. ' Steam Generator l Tube Sample Selection and Inspection" which requires at least 20% of the total number '

l of repaired tubes in all steam generators he inspected. A new table (Table TS.4.12-2) l ls also being incorporated in to the Technical Specifications which specifies the requirements for minimum sample size, inspection result classification and the corresponding required actions for inspection of steam generator tubes repaired by sleeving. The revised Technical Specification pages are provided in Attachments 2 and 3 of this response. A discussion cf the Technical Specification changes proposed by this response is being edded to the bases for Specification 4.12 as shown in Attachments 2 and 3.

Question 3:

Table 81 of the CEN-629 P, Revision 1," Repair of Wettinghouse Series 44 and 51 Steam Generator Tubes Using Leak Tight Sleeves," report specifies that a tensile strength of 90 ksi for the sleeve material was assumed rather than the American Society of Mechanical Engineers (ASME) Code minimum value of 80 ksi for material procured to requirements of Section ll SB 163. The staff does not, generally, approve of the use of material property values differing from the ASME Code Section ll properties. Provide the basis for assuming that the sleeves to be installed in the Prairie Island steam generators would have an ultimate strength greater than or equal to the 90 ksl value assumed in the structural assessment.

Response

Initial structural analysis on the Combustion Engineering welded sleeve was performed in 1984, which preceded any ASME Code sleeve qualification requirements. Lacking any formal code requirements, actual sleeve material properties (90 ksi vs. ASME 80 ksi ultimate) were used in the early calculations. These are the traditional values used by Combustion Engineering, proposed in the original Combustion Engineering Sleeve Topical Report for Prairie Island, CEN 294-P, and accepted by the NRC. The logic for

i Attachm:nt 1 i S:pt:mb:t 2,1997

, Page 3 I

this evaluation is provided in the following footnote to Table 81 of CEN-629 P, l ' Revision 1:

! *While the minimum tensile strength and yield strength are listed in Reference 8.15 1 l (ASME Code Case N 20-3) as 80.0 ksi and 40 ksi, respectively, the actual material i properties were found to be higher based on Reference 8.18 (Mill Test Results for l-

) 690 Sleeve Material) Typically, So > 100 ksi and Sy > 50 ksi at room temperatures.

i Based on the trending data curves in Reference 8,19 (Nuclear Systems Materials Handbook) for the above room temperature allowables, it can be expected that So is 1

greater than or equal to 90 ksi at 650 'F. This value will be used to evaluate

accident conditions and the allowable sleeve wall degradation in Section 8.3."

e Per Appendix F of ASME Code Section Ill, Section F 1322.3 (a),"If So values are not tabulated in Appendix 1, the value used shall be included and justified in the Design Report." Since So values are not supplied at different temperatures for alloy 690, a ,

comparison is done to alloy 600. The 1989 Edition of ASME Code Table 1-3.2 does provide So values for SBA 163 Grade 600 pipe and tubing. This value is constant at 80 i ksi over the temperature range of 100 to 1000 degrees Fahrenheit. Since Code Case

N 20-3 treats the mechanical properties of Alloy 600 and 690 the same, it is concluded .

that the effects of temperature on Alloy 690 is similar to alloy 600, i.e., for design j calculations, the tensile strength at 650

• F is the same as at room temperature.

i Under the authority of the NRC to approve applications which were not covered by the ASME Code, precedent was set by the early sleeve licensing submittals. Therefore, the use of 90 ksi as ultimate strength is acceptable.

- The design of the sleeve for primary stress under design operating conditions does in

, fact use the Code allowable values for acceptance criteria. The design for the accident

conditions uses 90 ksi instead of 80 ksi for So. Acceptable margins still exist for faulted conditions when Su equals 80 ksi. For example, on Page 8-5 of CEN-629-P, the stress intensity under Main Steam Line Break conditions is 28.2 ksi which is less than the allowable value of 0.7So(0.7 X 80 = 56 ksi).

, The basis for assuming that the sleeves to be installed in the Prairie Island steam

generators would have an ultimate strength greater than or equal to the 90 ksi value

assumed in the structural assessment is review of the material certifications for the sleeves previously installed or to be installed at Prairie Island.

i Heat Treatment No Supplier installed / Future Ultimate Tensile Strength, ksi

! 750722 Sandvik Installed 99.3 752245 Sandvik Installed 111 752455 Sandvik Installed / Future 111 929 Valinox Future 96.1 4

l Attachment i S:pt:mb:r 2,1997

, Page 4 All steam generator sleeves installed to date by Combustion Engineering at Prairie Island have had an ultimate tensile strength greater than 90 ksi. Allsteam generator sleeves installed by Combustion Engineering at Prairie Island in the future will have ultimate tensile strength greater than 90 ksi unless the Combustion Engineering sleeving topical report is revised to reflect the ASME Code values.

Since draft RG 1.121 constitutes an operating basis rather than a design basis, the allowable stress limits for the RG 1.121 evaluation are based on expected material properties rather than the ASME Code minimum values.

Question 4:

Although NSP Indicates in the proposed amendment request that post weld heat treatment (PWHT) of sleeve welds would be performed in the upcoming refueling outage, NSP, does not specifically require PWHT in future outages. The i

elimination of the PWHT step from the Installation would theoretically increase the susceptibility of sleeve welds to stress corrosion cracking. Provide the i

technical basis for omitting PWHT in the sleeve Installation process Specifically address the potential for the initiation and growth of sleeve weld's degradation over a cycle of operation that could challenge the structural and leakage integrity margins for a sleeved tube.

Response

This is also true for tube support plate sleeves. The justification for not using post weld heat treatment is based on 1)oxperience at Prairie Island and Zion with ABBCE sleeves which have been in service since 1987 without post weld heat treatment and 2) the low hot leg operating temperature of 590 *F at Prairie Island. At Prairie Island 27 sleeves were installed in 1987,71 in 1988, and 47 in 1990 without post weld heat treatment.

Examination with + Point eddy current probes has found no degradation of the sleeve or parent tube due to corrosion. The oldest sleeves without heat treatment have approximately 9 Effective Full Power Years operational experience without degradation.

Both Prairie Island units have operated at a hot leg temperature of 590 'F since startup in 1973. The first indication of primary water stress corrosion cracking was identified in 1989 after 11.9 effective full power years of operation. The Prairie Island history combined with industry experience shows slower initiation and growth rates for plants operating with a lower hot leg temperature.

Based on data presented in Section 6.0 of the referenced report, the magnitude of stresses (applied plus residual) in the parent tube at the sleeve / tube joint are no greater than those found at the tube roll transition. Consequently the potential for the initiation and growth of sleeve weld degradation is no greater than that of the parent tube. Further, in the locked tube scenario, the sleeve imparts a compressive stress on

Attachment 1 september 2,1997

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the tube as a result of the differential thermal growth between the two members.

Although the potential for initiation of stress corrosion cracking may be comparable to that at the roll transition, this compressive load will most likely result in a lower oropagation rate, b

Tharefore, there is reasonable assurance that an ABBCE welded sleeve can operate at Prairie bland without corrosion degradation for several operating cycles.

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