ML20046B449
| ML20046B449 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 07/14/1993 |
| From: | Liparulo N WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | Murley T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM), Office of Nuclear Reactor Regulation |
| Shared Package | |
| ML19310D556 | List: |
| References | |
| CAW-93-490, NUDOCS 9308040217 | |
| Download: ML20046B449 (25) | |
Text
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Westinghouse Electric Corporation Energy Systems gyg,,,3g g3g j
l l l July 14,1993 l CAW-93-490 l Document Control Desk l US Nuclear Regulatory Commission Washington, DC 20555 l Attention: Dr. Thomas Murley, Director 1
APPLICATION FOR WITHHOLDING PROPRIETARY l INFORMATION FROM PUBLIC DISCLOSURE
Subject:
WCAP-13619, Revision 1, " Specific Application of Laser Welded Sleeves for North Anna Unit 2 Steam Generators" (Proprietary)
Dear Dr. Murley:
l i The proprietary information for which withholding is being requested in the above-referenced letter is l
! further identified in Affidavit CAW-93-490 signed by the owner of the proprietary information, l Westinghouse Electric Corporation. The affidavit, which accompanies this letter, sets forth the basis on which the information may be withheld from public disclosure by the Commission and addresses with specificity the considerations listed in paragraph (b)(4) of 10 CFR Section 2.790 of the Commission's regulations.
Accordingly, this letter authorizes the utilization of the accompanying Affidavit by Virginia Power.
Correspondence with respect to the proprietary aspects of the application for withholding or the Westinghouse affidavit should reference this letter, CAW-93-490, and should be addressed to the undersigned. l Very truly yours, 4W F h4/p ,7 Mr. N. J. Liparulo, Manager Nuclear Safety and Regulatory Activities
/cid Enclosures cc:K. Bohrer/NRC (12H5) i
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CAW-93-490 AFFIDAVIT j i
COMMONWEALTH OF PENNSYLVANIA:
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Before me, the undersi;ned authority, personally appeared ' John J. McInerney, being by me !
t duly sworn according to law, deposes and says that he is authorized to execute this Affidavit on-l behalf of Westinghouse Electric Corporation (" Westinghouse") and that the ave'r ments of fact set forth
!. in this Affidavit are true and correct to the best' of his knowledge, information, and belief:
h VLt' John J McInerney," Acting Manager l Nuclear Safety Licensing -
Sworn to and subscribed before me this l'/ e day i of M ,1993 i
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Notary Public i
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CAW-93-490 1
l (1) I am Acting Manager, Nuclear Safety Licensing, in the Nuclear and Advanced Technology Divisions, of the Westinghouse Electric Corporation and as such, I have been specifically delegated the function of reviewing the proprietary information sought to be withheld from public disclosure in connection with nuclear power plant licensing and rulemaking proceedings, and am authorized to apply for its withholding on behalf of the Westinghouse Energy Systems Business Unit.
(2) I am making this Affidavit in conformance with the provisions of 10CFR Section 2.790 of the Commission's regulations and in conjunction with the Westinghouse application for withholding accompanying this Affidavit.
(3) I have personal knowledge of the criteria and procedures utilized by the Westinghouse Energy Systems 13usiness Unit in designating information as a trade secret, privileged or as confidential commercial or financial information.
(4) Pursuant to the provisions of paragraph (b)(4) of Section 2.790 of the Commission's t
regulations, the following is furnished for consideration by the Commission in determining whether the information sought to be withheld from public disclosure should be withheld.
(i) The information sought to be withheld from public disclosure is owned and has been held in confidence by Westinghouse.
(ii) The information is of a type customarily held in confidence by Westinghouse and not customarily disclosed to the public. Westinghouse has a rational basis for determining the types of information customarily held in confidence by it and, in that connection, utilizes a system to determine when and whether to hold certain types of information in confidence. The application of that system and the substance of that system constitutes Westinghouse policy and provides the rational basis required.
l Under that system, information is held in confidence if it falls in one or more of i l' several types, the release of which might result in the loss of an existing or potential l
competitive advantage, as follows: l l
(259C-DLC-2:071373
i CAW-93-490 4
(a) The information reveals the distinguishing aspects of a process (or component, structure, tool, method, etc.) where prevention of its use by any of Westinghouse's competitors without license from Westinghouse constitutes a competitive economic advantage over other companies.
(b) It consists of supporting data, including test data, relative to a process (or component, structure, tool, method, etc.), the application of which data secures a competitive economic advantage, e.g., by optimization or improved marketability.
l (c) Its use by a competitor would reduce his expenditure of resources or improve 4
his competitive position in the design, manufacture, shipment, installation, 4 assurance of quality, or licensing a similar product.
(d) It reveals cost or price information, production capacities, budget levels, or commercial strategies of Westinghouse, its customers or suppliers.
(e) It reveals aspects of past, present, or future Westinghouse or customer funded -
development plans and programs of potential commercial value to Westinghouse.
t (f) It contains patentable ideas, for which patent protection may be desirable.
There are sound policy reasons behind the Westinghouse system which include the following:
(a) The use of such information by Westinghouse gives Westinghouse a competitive advantage over its competitors. It is, therefore, withheld from disclosure to protect the Westinghouse competitive position. i (b) It is information which is marketable in many ways. The extent to which such information is available to competitors diminishes the Westinghouse ,
ability to sell products and services involving the use of the information. l Op9C-Dir-3971373
. 1 CAW-93-490 i (c) Use by our competitor would put Westinghouse at a competitive disadvantage by reducing his expenditure of resources at our expense. !
(d) Each component of proprietary information pertinent to a particular competitive advantage is potentially as valuable as the total competitive advantage. If competitors acquire components of proprietary information, any one component may be the key to the entire puzzle, thereby depriving Westinghouse of a competitive advantage.
(e) Unrestricted disclosure would jeopardize the position of prominence of Westinghouse in the world market, and thereby give a market advantage to the co*npetition of those countries.
(f) The Westinghouse capacity to invest corporate assets in research and development depends upon the success in obtaining and maintaining a competitive advantage.
(iii) The information is being transmitted to the Commission in confidence and, under the provisions of 10CFR Section 2.790, it is to be received in ecmtidence by the Commission.
(iv) The information sought to be protected is not available in public sources or available information has not been previously employed in the same original manner or method to the best of our knowledge and belief.
(v) The proprietary information sought to be withheld in this submittal is that which is appropriately marked in " Specific Application of Laser Welded Sleeves for North t Anna Unit 2 Steam Generators", WCAP-13619 Rev.1 (Proprietary), July,1993 being transmitted by the Virginia Electric and Power Company (VEPCO) letter and Application for Withholding Proprietary Information from Public Disclosure, Mr. W. L. Stewart, Senior Vice President, Nuclear, VEPCO, to Attention Dr. Thomas Murley. The proprietary information as submitted for use by Virginia Electric and Power Company for North Anna Unit 2 is expected to be applicable in l
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CAW-93-490 i
other licensee submittals in response to certain NRC requirements for justification of use of laser welded sleeving in steam generator tubes.
l This information is part of that which will enable Westinghouse to: )
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(a) Provide documentation of the methods for laser welded sleeving of steam I generator tubes.
(b) Establish applicable testing methods. <
l (c) Establish the use of fiber optics in laser welded sleeving applications.
(d) Establish applicable codes and standards which are to be applied to the l process.
(e) Assist the customer to obtain NRC approval.
l Further this information has substantial commercial value as follows:
(a) Westinghouse plans to sell the use of similar information to its customers for !
purposes of meeting NRC requirements for licensing documentation.
(b) Westinghouse can sell support and defense of the technology to its customers in the licensing process.
Public disclosure of this proprietary information is likely to cause substantial harm to the competitive position of Westinghouse because it would enhance the ability of competitors to provide similar sleeving services and licensing defense services for commercial power reactors without commensurate expenses. Also, public disclosure of the information would enable others to use the information to meet NRC i i
requirements for licensing documentaticm without purchasing the right to use the i information. l l
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l OR59C-DIE 5271393
,. l CAW-93-490 ,
The development of the technology described in part by the information is the result i of applying the results of many years of experience in an intensive Westinghouse effort and the expenditure of a considerable sum of money. t In order for competitors of Westinghouse to duplicate this information, similar technical programs would have to be performed and a significant manpower effort, having the requisite talent and experience, would have to be expended for developing l
testing and analytical methods and performing tests.
Further the deponent sayeth not.
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DR$9C-DLC4:071393 l
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Proprietary Information Notice l
l Transmitted herewith are proprietary and/or non-proprietary versions of documents furnished to the NRC l
in connection with requests for generic and/or plant-specific review and approval. i In order to conform to the requirements of 10 CFR 2.790 of the Commission's regulations concerning the 1 protection of proprietary information so submitted to the NRC, the information which is proprietary in the proprietary versions is contained within brackets, and where the proprietary information has been deleted in the non-proprietary versions, only the brackets remain (the information that was contained within the brackets in the proprietary versions having been deleted). The justification for claiming the information so designated as proprietary is indicated in both versions by means of lower case letters (a) through (f) contained within parentheses located as a superscript immediately following the brackets enclosing each I item of information being identified as proprietary or in the margin opposite such information. These lower case letters refer to the types of information Westinghouse customarily holds in confidence identified in Sections (4)(ii)(a) through (4)(ii)(f) of the affidavit accompanying this transmittal pursuant to 10 CFR 2.790(b)(1).
l cimso7mtrAm393 l
Copyright Notice The reports transmitted herewith each bear a Westinghouse copyright notice. The NRC is permitted to make the number of copies of the information contained in these reports which are necessary for its internal use in connection with generic and plant-specific reviews and approvals as well as the issuance, denial, amendment, transfer, renewal, modification, suspension, revocation, or violation of a license, permit, order, or regulation subject to the requirements of 10 CFR 2.790 regarding restrictions on public disclosure to the extent such information has been identified as proprietary by Westinghouse, copyright protection notwithstanding. With respect to the non-proprietary versions of these reports, the NRC is permitted to make the number of copies beyond those necessary for its internal use which are necessary in order to have one copy available for public viewing in the appropriate docket files in the public document room in Washington, DC and in local public document rooms as may be required by NRC regulations if ,
the number of copies submitted is insufficient for this purpose. The NRC is not authorized to make copies for the personal use of members of the public who make use of the NRC public document rooms.
Copies made by the NRC must include the copyright notice in all instances and the proprietary notice if the original was identified as proprietary.
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Significant Hazards Consideration :
Steam Generator Tube Repair Using Laser Welded Sleeves North Anna Power Station Unit 2 INTRODUCTION A license amendment is proposed to permit the installation of Alloy 690 laser welded tube sleeves at degraded tube support plate intersections and within the tubesheet area of the North Anna Power Station Unit 2 steam generators. Per the current Technical Specifications, steam generator tubes with eddy current indications of 40%
through wall or greater must be removed from service. Laser welded tube sleeves can be installed to repair degraded steam generator tubes either at the tube support plate inte sections, within the tubesheet area, or a combination of both within the same tube.
DESCRIPTION OF THE AMENDMENT REQUEST As required by 10 CFR 50.91(a)(1), this analysis is provided to demonstrate that a proposed license amendment to implement repair of tubes using laser welded tube sleeves for the North Anna Unit 2 steam generators represents no significant hazards consideration. In accordance with 10 CFR 50.92(c), implementation of the proposed license amendment was evaluated and found not to: 1) involve a significant increase in the probability or consequences for an accident previously evaluated,2) create the possibility of a new or different kind of accident from any accident previously evaluated, or 3) involve a significant reduction in a margin of safety.
Maintenance of steam generator tube integrity is the prime objective of regular steam generator tube inspection programs. Due to the high impact that removal of tubes from service can have on overall plant availability and operability, a repair method has been developed which secures to the original tube a short length of tubing with an outer diameter slightly smaller than the inside diameter of the tube, spanning the degraded area of the parent tube. The tube support plate sleeve is attached to the degraded tube by producing an autogenous weld between the original tube and !
sleeve. Tube support plate sleeve welds are produced in the free span sections of the tube. The free span welds provide the structural joint between the tube and sleeve and also provide positive (leaktight) leakage integrity. The tubesheet sleeve is i secured and supported structurally at the upper section by a free span autogenous I weld performed identically to the tube support plate sleeve welds while the lower joint l is secured by a mechanical hardroll. A seal weld can also be included within the l tubesheet sleeve lower joint at an elevation coincident with the approximate midpoint j of the tubesheet cladding. However, the hardroll area supplies the necessary structural requirements for the lower joint. Both the lower hardroll and free span laser ,
weld joints (LWJ) provide structural integrity characteristics which exceed the structural l requirements for the sleeve. Therefore, it can be postulated that a loss of structural I
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Pago 2 of 9 integrity in one of the sleeve joints will not result in a loss of structural integrity for the sleeve. The sleeve structural integrity requirements include safety factors inherent to the requirements of the ASME Code. Installation of tube support plate sleeves and/or tubesheet sleeves restores the integrity of the primary pressure boundary to a condition consistent with that of the originally supplied tubing. All welds must be produced a minimum distance of 1 inch from any detected tube degradation.
Tubes with indications of degradation in excess of the plugging criteria would have to be removed from service, according to Technical Specification tube plugging criteria without provision for tube repair by sleeving. Removal of a tube from service results in a reduction of reactor coolant flow through the steam generator. This small reduction in flow has an impact on the margin in the reactor coolant flow through the steam generator in loss-of-coolant accident (LOCA) analyses and on the heat transfer efficiency of the steam generator. Repair of a tube with sleeving maintains the tube in service and results in a much smaller flow reduction. Therefore, the use of sleeving in lieu of plugging would minimize loss of margin in reactor coolant system flow and assist in assuring that minimum flow rates are maintained in excess of that required for operation at full power. Any combination of sleeving and plugging utilized at North Anna Unit 2 up to a level such that the effect of sleeving will not reduce the minimum reactor coolant flow rate to below the current Technical Specification limit is acceptable. Also, minimizing the reduction in flow has operational benefits by limiting the increase in heat flux across the tubes remaining in service. Increased heat fluxes have been associated with an increased potential for tube corrosion.
The proposed amendment would modify Technical Specifications 3/4.4.5 " Steam Generators," and Bases B 3/4.4.5, " Steam Generators," to provide the sleeve / tube inspection requirements and acceptance criteria to determine the level of degradation that would require the sleeve to be removed from service.
EVALUATION Sleeve / Tube Intearity During the development of laser welded sleeving, Section _Ill of the ASME Code was used for the minimum wall thickness determination and bounding stress and fatigue levels for the sleeve. By showing that the sleeve design meets all facets of the applicable subsections of Section 111 of the Code, the sleeve design meets the design requirements of the original tubing. Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," and ASME Code, Section lil, material strength limits are used to develop the plugging limit of the sleeve determined by NDE, should sleeve wall degradation occur. Potentially degraded sleeves at the plugging limit were shown (by analysis) to retain burst strength in excess of three times the normal operating pressure differential at end of cycle conditions, per Regulatory Guide 1.121 guidelines. The structural analysis utilized a generic set of loading inputs which conservatively bound the operating regimes of all plants with Westinghouse Series 44 and 51 steam generators. In actuality, the operating pressure differential for North
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Anna Unit 2 results in a slightly larger sleeve degradation plugging limit than i determined by WCAP-13088, Rev. 2. The requirements of Regulatory Guide 1.83, l
" Inservice Inspection of PWR Steam Generator Tubes," are implemented, and a i baseline eddy current inspection of the installed sleeves is performed prior to !
operation. An ultrasonic inspection of the free span weld joints is also performed prior to operation. The ultrasonic inspection is used to verify that the minimum acceptable fusion zone thickness of the weld is achieved. This minimum weld fusion zone thickness has been shown by analysis to satisfy the requirements of the ASME Code with regard to acceptable stress levels during operating and accident conditions. As stated previously, a generic set of loading conditions was used for structural analysis of the sleeved tube assembly. The values for primary-to-secondary pressure differential and Thot and Tcold represented bounding or design values and are considered conservative for all plants with Series 44 or 51 steam generators. In addition, a fatigue analysis was performed for the assembly, the critical location being the free span laser weld. The loading cycles that were applied to the sleeve assembly analysis were those for a 40 year plant life cycle. Therefore, the fatigue analysis is conservative for an operating plant. The results of the fatigue analysis indicate acceptable usage factors for the entire range of permitted weld thicknesses. A comparison of the number of loading cycles used in the generic analysis indicates that the generic conditions are conservative compared to North Anna Unit 2. For normal plant conditions, which include plant heatup/cooldown cycles, small step load changes, large step load changes, and hot standby operations, and all Upset and Test conditions, the generic conditions bound North Anna values. Only the number of turbine roll tests and steady state fluctuations were consistent between the generic and North Anna values. Using the generic inputs, the cumulative fatigue usage factor was found to be much less than 1.0. The pressure differentials used for the generic analysis were found to be conservative compared to North Anna values in all categories.
Leakage testing under conditions considered to be more severe than expected during all operating plant conditions has shown that the laser welded sleeve does not introduce additional primary-to-secondary leakage during a postulated steam line break event. Laser welded sleeved tube assemblies were subjected to thermal and fatigue cycling and then leak tested at pressure differences of up to 3110 psi, which far exceeds the expected North Anna Unit 2 steam line break pressure differential of 2335 psi to 2500 psi. No leakage was detected in any welded joint (both free span and tubesheet joints). Leakage testing has also shown that the seal weld of the tubesheet sleeve lower joint is not required in order to preclude leakage during normal operation or accident conditions at 600 F. Non-welded lower joint tubesheet sleeve / tube leakage test specimens were subjected to both fatigue and thermal cycling tests prior to final leak rate evaluation testing. The load level applied during the fatigue testing exceeded the maximum axial load applied to the sleeve during the most severe pressure loading condition. Thermal cycling tests simulated a standard plant heatup/cooldown cycle. No leakage was detected in any non-welded tubesheet sleeve lower joint at 600 F after both thermal and fatigue loading. Primary-to-
Page 4 of 9 secondary leakage through non-welded tubesheet sleeve lower joints would not be expected at 0% power (Thot = 547 F).
Sleeving of Previousiv Plugged indications The sleeve installation requirements applicable to active tubes which have been identified as containing degradation which exceeds the repair limit are no different for the sleeving of previously plugged tubes. A new " baseline" inspection of the entire tube length must be performed prior to sleeve installation in a previously plugged tube.
The location of the identified tube degradation must be verified to be a minimum distance of 1 inch from the weld joints (same for active tubes). Historically, the areas of the tube which have suffered corrosion degradation are the tube support plate intersections, the expansion transition and the sections of tube within the thickness of the tubesheet where secondary side contaminants have collected due to the operating crevices. The sleeve free span (structural) weld joints are not located in these areas, and should not be affected by any previously identified degradation mechanism which caused the tube to be removed from service. The analysis has also supported sleeve installation in a separated tube, therefore, the extent of the originally identified degradation should not affect sleeve installation. Additionally, the area of the tube where the tube plug was installed must be visually inspected prior to sleeve installation. Surface finish requirements for this area have been developed which help to maintain the ability of the joint to form a leaktight seal. Conformance to the surface finish requirements for the lower joint will help to ensure a leaktight sleeve joint, regardless of whether or not the seal weld has been produced. The ability of the weld to sufficiently penetrate the tube wall has been shown by test in cases where a localized gap of several (up to 2 mils) mils existed between the tube and sleeve. The penetrating capabilities of the weld will also help to ensure a leaktight joint in cases where slight surface imperfections due to tube plug removal may be present.
LOCA Flow Margin By reducing the numbers of tube plugs installed in the steam generator, the proposed amendment would minimize the loss of margin in reactor coolant flow through the steam generator during a postulated LOCA. Also, sleeving will provide margin above the required minimum flow for full power operation, than if equal numbers of tubes were plugged as opposed to sleeved.
ANALYSIS Conformance of the proposed amendments to the standards for a determination of no significant hazard as defined in 10 CFR 50.92 is shown in the following:
- 1) Operation of the North Anna Power Station Unit 2 in accordance with the proposed license amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated.
Paga 5 of 9 The tubesheet and/or tube support plate intersection laser welded sleeve configuration has been designed and analyzed in accordance with the requirements of the ASME Code and Regulatory Guide 1.121. Fatigue and stress analyses of the sleeved tube assemblies produced acceptable results.
Mechanical testing has shown that the structural strength of the Alloy 690 sleeves under normal, faulted and upset conditions is within acceptable limits. Leak testing has demonstrated that primary-to-secondary leakage is not expected during all plant conditions, including the case where the seal weld is not produced in the lower joint of the tubesheet sleeve.
A conservative leak-before-break evaluation has been performed for the sleeved tube assembly, using bounding values for the North Anna Unit 2 steam generators.
The evaluation is considered conservative in that no credit for the parent tube is assumed in determining the burst pressure of the sleeved tube assembly. The leak-before-break criteria compares the postulated through-wall crack length which will leak at a specified value at normal operating conditions, thereby permitting adequate leakage detection and safe shutdown of the plant prior to the crack achieving a length equal to the critical crack length which could be postulated to burst at steam _ line break conditions. The North Anna Unit 2 Technical Specifications limit primary-to-secondary leakage. Additionally, North Anna Power Station maintains an administrative maximum allowable leak rate limit which bounds leak-before-break concerns during all plant operating conditions (References 4 and 8). This administrative limit was previously accepted by the NRC in letters dated March 7,1991 and August 3,1992 (References 5 and 6). These primary-to-secondary leak rate limits provide acceptable leak-before-break margin. Thus, using tha bounding conditions for the North Anna Unit 2 steam generators, the limitint; leak rate satisfies the leak-before-break criteria for the Alloy 690 sleeved tubes.
Despite the fact that leak-before-break is considered to be applicable (historically no primary-to-secondary leakage or degradation has been evidenced in Westinghouse sleeves) to the sleeved tube assembly, the hypothetical consequences of failure of the sleeve would be bounded by the current steam generator tube rupture analysis included in the North Anna Power Station UFSAR.
Due to the slight reduction in diameter caused by the sleeve wall thickness, it is expected that primary coolant release rates would be slightly less than assumed for the steam generator tube rupture analysis, and therefore, would result in lower total primary fluid mass release to the secondary system. Additionally, further conservatisms would be included if the break were postulated to occur at a .
location higher than a location where sleeves are installed. Combinations of tubesheet sleeves and tube support plate sleeves would reduce the primary fluid flow through the sleeved tube assembly due to the series of diameter reductions the fluid would have to pass on its way to the break area. The overall effect would be reduced steam generator tube rupture release rates.
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Pago 6 of 9 The proposed Technical Specifications change to support the installation of Alloy 690 laser welded sleeves does not adversely impact any other previously evaluated design basis accident or the results of LOCA and non-LOCA accident analyses for the current Technical Specification minimum reactor coolant system flow rate. The results of the qualification testing, analyses, and plant operating experience demonstrate that the sleeve assembly is an acceptable means of maintaining tube integrity. Plugging limit criteria are established using the guidance of Regulatory Guide 1.121. Furthermore, per Regulatory Guide 1.83 recommendations, the sleeved tube can be monitored through periodic inspections with present eddy current techniques. These measures demonstrate that installation of sleeves spanning degraded areas of the tube will restore the tube to a condition consistent with its original design basis.
Conformance of the sleeve design with the applicable sections of the ASME Code and results of the leakage and mechanical tests, support the conclusion that installation of laser welded tube sleeves will not increase the probability or consequences of an accident previously evaluated. Depending upon the break location for a postulated steam generator tube rupture event, implementation of tube sleeving could act to reduce the radiological consequences to the public due to reduced flow rate through a sleeved tube compared to a non-sleeved tube based on the restriction afforded by the sleeve wall thickness.
- 2) The proposed license amendment does not create the possibility of a new or different kind of accident from any accident previously evaluated.
Implementation of laser welded sleeving will not introduce significant or adverse changes to the plant design basis. Stress and fatigue analysis of the repair has shown the ASME Code and Regulatory Guide 1.121 allowable values are met.
Implementation of laser welded sleeving maintains overall tube bundle structural and leakage integrity at a level consistent to that of the originally supplied tubing during all plant conditions. Leak and mechanical testing of sleeves support the conclusions of the calculations that the sleeve retains both structural and leakage integrity during all conditions. Sleeving of tubes does not provide a mechanism resulting in an accident outside of the area affected by the sleeves. Any hypothetical accident as a result of potential tube or sleeve degradation in the repaired portion of the tube is bounded by the existing tube rupture accident analysis. Since the sleeve design does not affect any other component or location of the tube outside of the immediate area repaired, in addition to the fact that the installation of sleeves and the impact on current plugging level analyses is accounted for, the possibility that laser welded sleeving creates a new or different type of accident is not supported.
The design of thermally treated Alloy 600 and Alloy 690 sleeved tube assemblies have performed well historically with regard to corrosion. There are no reported instances of sleeve degradation for the greater than 25,000 sleeves that Westinghouse has installed in the U.S. Accelerated corrosion test results show
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Pag @ 7 of 9 l the free span laser welded joint (with post weld heat treatment) is capable of exhibiting a resistance to corrosion of greater than 10 times that of rolled tube transitions. Accelerated corrosion tests also show that non-heat treated laser l
welded free span joints exhibit resistance to stress corrosion cracking equal to or greater than rolled tube transitions. Corrosion testing of the tubesheet sleeve lower laser welded joints exhibits a resistance to corrosion cracking of three to four times that of rolled tube transitions. These factors suggest postulated sleeve degradation would occur at a rate less than rolled transitions, and the potential for a sleeve with accelerated degradation rate characteristics more severe than roll transitions is negligible. The North Anna Unit 2 steam generator tubes employ a partial depth hardroll expansion with full depth WEXTEX explosive expansion.
Corrosion rates for WEXTEX expansion transitions are considered to be bounded by that of rolled transitions.
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- 3) The proposed license amendment does not involve a significant reduction in a margin of safety.
The laser welded sleeving repair of degraded steam generator tubes as identified in WCAP-13088, Rev. 2 has been demonstrated to restore the integrity of the tube l
bundle under normal and postulated accident conditions. Tne safety factors used l in the design of sleeves for the repair of degraded tubes are consistent with the I safety factors in the ASME Boiler and Pressure Vessel Code used in steam
! generator design. The plugging limit criteria for the ' sleeve have been established l' using the methodology of Regulatory Guide 1.121. The design of the sleeve joints has been verified by testing to preclude leakage during normal and postulated accident conditions. Implementation of laser welded sleeving will reduce the potential for primary-to-secondary leakage during a postulated steam line break while maintaining available primary coolant flow area in the event of a LOCA. By removing from service degraded intersections through repair, the potential for steam line break leakage is reduced. These degraded intersections novv are returned to a condition consistent with the Design Basis. While the installation of a sleeve causes a reduction in flow, the reduction is far below the reduction incurred by plugging. Therefore, far greater primary coolant flow area is maintained through sleeving. Use of Regulatory Guide 1.121 criteria assures that the margin of safety with respect to structural integrity is the same for the sleeves as for the original steam generator tubes.
The portions of the installed sleeve assembly which represent the reactor coolant pressure boundary can be monitored for the initiation and progression of !
sleeve / tube wall degradation, thus satisfying the requirements of Regulatory Guide 1.83. Portions of the tube bridged by the sleeve joints are effectively removed from the pressure boundary, and the sleeve then forms the pressure boundary in these areas. The areas of the sleeved tube assembly which require inspection are ;
defined in the Bases to the North Anna Unit 2 Technical Specifications.
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- n addition, since the installed sleeves represent a portion of the pressur?
boundary, a baseline inspection of these areas is required prior to operation with sleeves installed.
The effect of sleeving on the design transients and accident analyses have been reviewed based on the installation of sleeves up to the level of steam generator tube plugging coincident with the minimum reactor flow rate. Currently the North Anna Technical Specifications limit minimum reactor coolant flow rate at 284,000 gpm total. Virginia Electric and Power Company has submitted a proposed I license amendment to lower the minimum measured flow rate to 275,300 gpm (Reference 3). The installation of sleeves is to be evaluated as the equivalent of some level of steam generator tube plugging. The North Anna Unit 2 steam generator plugging level is approximately 7% (average), and loop flow was last measured to be 293,321 gpm in April 1992. Evaluation of the installation of sleeves is based on the determination that LOCA evaluations for the licensed minimum reactor coolant flow bound the effect of a combination of tube plugging and sleeving up to an equivalent of the actual steam generator tube plugging limit.
Information provided in WCAP-13088, Rev. 2, describes the method to determine the flow equivalency for all ccmbinations of tubesheet and tube support plate sleeves in order that the minimum flow requirements are met.
For North Anna Unit 2, a AP of 1490 psi (Psteam = 760 psia) at a bounding hT otf or normal operating condit! ns was used in the sleeve minimum wall thickness calculations. The geneiic evaluation determined the level of sleeve wall degradation (depth by eddy current determination) that would require the sleeve to be removed from service is 25%, which includes an allowance of 10% for eddy current uncertainty and 10% for growth, although sleeve wall degradation has not been observed to date in Westinghouse sleeves. The sleeve plugging limit based on through-wall penetration of degradation for North Anna Unit 2 is determined to be 27%. As stated previously a primary-to-secondary pressure differential of 1490 psi across the steam generator tubes with a steam pressure of 760 psia was used.
This value is taken from an extended tube plugging analysis performed for Unit 1, and is considered to be bounding for North Aana Unit 2 at a minimum reactor coolant system flow rate of 275,300 gpm. Tha pressure stress equation of Section NB-3224.1 of the ASME Code was used to establish the minimum wall thickness which helps to define the sleeve plugging limit as determined by eddy current.
The minimum wall thickness established for the normal operation case bounds the i calculated rninimum wall thickness _ values for Upset and Faulted conditions at a minimum measured reactor coolant system flow rate of 275,300 gpm. Therefore, .
the 27% plugging limit is still bound;ng for all anticipated plant conditions upon issuance of the Technical Specifications change supporting a minimum measured reactor coolant flow of 275,300 gpm. '
Page 9 of 9 CONCLUSION Based on the preceding analysis, it is concluded that operation of North Anna Power Station Unit 2 following the installation of Alloy 690 laser welded sleeves at the tube support elevations and within the tubesheet region of the steam generators, in accordance with the proposed amendment does not result in the creation of an unreviewed safety question, an increase in the probability of an accident previously evaluated, create the possibility of a new or different kind of accident from any accident previously evaluated, nor reduce any margins to plant safety. Therefore, the license amendment does not involve a significant hazards consideration as defined in 10 CFR 50.92.
REFERENCE DOCUMENTS
- 1) WCAP-13088, Rev. 2, " Westinghouse Series 44 and 51 Steam Generator Generic Sleeving Report - Laser Welded Sleeves," July 1993.
- 2) WCAP-13619, Rev.1, " Specific Application of Laser Welded Sleeves for North Anna Unit 2 Steam Generators," July 1993.
- 3) Virginia Electric and Power Company letter, Serial No.92-721, " North Anna Power Station Unit 2 Proposed Technical Specifications Changes - Reduction in Minimum Measured RCS Flow Rate" (to 275,300 gpm), dated December 4,1992.
i 4) Virginia Electric and Power Company letter, Serial No. N-92-11, Licensee Event Report No. 50-339/92-005-00, dated April 9,1992.
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- 5) NRC letter to Virginia Electric and Power Company, " Approval for Restart - l Category C-3 Status of Steam Generators, North Anna Unit No.1 (TAC No. !
79783)," dated March 7,1991.
s 6) NRC letter to Virginia Electric and Power Company, " North Anna Power Station, Unit No.1 - Steam Generator Operating Cycle Evaluation (TAC No. M83715)," ;
dated August 3,1992. I l
- 7) Virginia Electric and Power Company letter, Serial No.93-112, " North Anna Power Station Unit 2 Proposed Technical Specifications Changes - Steam Generator Tube Repair Using Laser Welded Sleeves," dated April 8,1993. ;
- 8) Letter from Westinghouse Steam Generator Design and Analysis to R. S. Lapides, Westinghouse Nuclear Safety and Licensing, " Summary of Calculations to Determine Minimum Wall Thickness Requirement and Allowable Leakage Iin.; lier j Laser Welded Sleeves for North Anna Unit 2 Steam Generators," NSD-JLH-3257, dated July 12,1993. j l
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ATTACHMENT 6 i
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i PREVIOUS SUBMITTAL COMPARISON ;
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VIRGINIA ELECTRIC AND POWER COMPANY ,
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Previous Submittal Comparison (Differences From April 8,1993 Submittal)
Steam Generator Tube Repair Using Laser Welded Sleeves !
North Anna Power Station Unit 2 INTRODUCTION By letter dated April 8,1993, Virginia Electric and Power Company requested an amendment, to the Technical Specifications, to Facility Operating Ucense NPF-7 for North Anna Power Station Unit 2 to allow steam generator tube sleeving in accordance with the Westinghouse laser welding process. This proposed change i request supplements the April 8,1993 Technical Specification change request. The intent of this change package remains the same, however, the information being provided has been revised per discussions with the NRC staff. The substantive change are within the Westinghouse generic report on laser welded sleeving of steam generator tubes. The differences between our April 8,1993 submittal and this submittal are documented in this attachment to facilitate the NRC staff's review of our proposed Technical Specifications change request.
GENERAL DIFFERENCES t North Anna Unit 2 Laser Welded Steeve Plugaing Limit Westinghouse Electric Corporation's generic report on steam generator tube sleeving using laser welded sleeves has been revised to use the ASME Code allowable strength values for the sleeve material properties instead of the lower bound values determined by Westinghouse from their testing and research. These changes are reflected in the revised report, WCAP-13088, Rev. 2, " Westinghouse Series 44 and 51 Steam Generator Generic Sleeving Report - Laser Welded Sleeves," dated June 1993.
Use of the more conservative ASME Code maximum strength values changes the repair limit determined for the sleeves. The percentage of degradation affecting the sleeve wall which requires removal of the sleeve from service is changed from 33% to 27%.
North Anna Unit 2 is currently licensed for a minimum measured flow of 284,000 gpm, but a license submittal has been made to lower this value to 275,300 gpm. Therefore, ;
the 27% sleeve through-wall degradation limit has been evaluated to be conservative for the estimated plant conditions at a minimum reactor coolant flow rate of 275,300 gpm. j Leak Before Break Discussion A leak-before-break evaluation for the sleeved tubes was provided in the previously submitted Westinghouse reports (WCAP-13088, Rev.1, and WCAP-13619). The leak-
Page 2 of 4 before break discussion has been removed from the current version of these reports ]
(WCAP-13088, Rev. 2, and WCAP-13619, Rev.1). However, for continuity, a summary !
of the leak-before-break evaluation has been provided in the enclosed Discussion of Changes. The North Anna Unit 2 Technical Specifications and the station administrative controls for primary-to-secondary leakage, combined with the conservatisms of the Westinghouse leak-before-break evaluation model, continue to support leak-before-break for the implementation of laser welded tube sleeves at North Anna Unit 2.
SPECIFIC DIFFERENCES Discussion of Chanoes
- Added introduction describing this supplemental change.
- Changed reference to Westinghouse generic sleeving evaluation from WCAP-13088, Rev.1 to WCAP-13088, Rev. 2 throughout the document.
- Changed reference to Westinghouse site-specific report for Nor1h Anna Unit 2 sleeving evaluation from WCAP-13619 to WCAP-13619, Rev.1, throughout the document.
- Changed the sleeve plugging limit from 33% to 27% through-wall throughout the document.
- Changed the Licensing Basis and the Sleeve Minimum Wall Thickness Determination discussions to reference that ASME Code allowable strength values were used for determining sleeve rainimur,i wall thickness and plugging limit.
Reformatted and revised the Leak-Before-Break discussion to summarize the site-specific evaluation performed by Westinghouse (Reference 8).
- The comparison between laser welded sleeve hybrid expansion joint testing and the F* and L* tube plugging limits mechanical loading criteria was removed from the discussion on Laboratory Leak Testing to eliminate confusion. A statement was added discussing industry use and acceptance of the hybrid expansion joint mechanical loading criterion.
- Additional minor wording changes were made to improve readability and provide clarification.
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Pago 3 of 4 Pronosed Technical Soecifications Changed plugging limit for Westinghouse laser welded sleeve wall from 33% to 27%. (Technical Specification 4.4.5.4.a.6 and Bases Section 3/4.4.5) l
- Changed reference to Westinghouse generic sleeving evaluation from.WCAP-13088, Rev.1, to WCAP-13088, Rev. 2 and changed reference to Westinghouse - :
site-specific report for North Anna Unit 2 sleeving evaluation from WCAP-13619 to-WCAP-13619, Rev.1. (Technical Specification 4.4.5.4.a.9)
Sionificant Harnrds Consideration
- Changed reference to Westinghouse generic sleeving evaluation from WCAP- j 13088, Rev.1, to WCAP-13088, Rev. 2, throughout the document. 1 1
- Changed reference to Westinghouse site-specific report for North Anna Unit 2 I sleeving evaluation from WCAP-13619 to WCAP-13619, Rev.1, throughout the document. j i
- Changed the sleeve plugging limit from 33% to 27% through-wall throughout the l document. {
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- Revised ~ the Leak Before Break discussion to summarize the site-specific ;
evaluation performed by Westinghouse (Reference 8). -1 l
- Changed the Licensing Basis and the . Sleeve Minimum Wall Thickness i Determination discussions to reference that ASME Code allowable strength values were used for determining sleeve minimum wall thickness and plugging limit.-
Additional minor wording changes were made to improve readability and provide clarification.
WCAP-13088 -- Generic Sleevino Reoort (from Rev.1 to Rev. 2) f Abstract - Removed leak-before-break' discussion from the third paragraph.
3.1 -
Deleted last sentence.
3.1.10 - Deleted first five paragraphs.
Deleted seventh paragraph.
Deleted first sentence of paragraph 8. -l Revised minimum wall numbers as appropriate.
Revised stress limits as appropriate.
3.1.10.3 -
Deleted entire section on leak-before-break evaluation.
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. . _ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ . _ _ _ . _ _ _ - _. . . . . ., . . . . . . L .,_. ,. . , ~ , . , . ..,m , .,_.,,,,_,_.m. , m. .,c._,..,..,
Paga 4 of 4 3.1.11 -
Revised sleeve plugging limits. l 3.1.13 -
Revised sleeve plugging limit.
Deleted last two sentences.
3.1.14 -
Moved / Combined at end of Section.
Table 3 Removed.
Table 3-16 -
Removed.
Table 3 Removed.
Figure 3-18 -
Removed.
Figure 3 Removed.
Figure 3-20 -
Removed.
Figure 3-21 -
Removed.
3.2.1 -
Reference 1 is now Reference 6 in third line.
Page 3-52 (old doc.) - Revised Table #'s in last paragraph.
Page 3-53 (old doc.) - Revised Figure #'s.
3.2 - Revised remaining Table #'s and Figure #'s.
New 3.3 -
References.
WCAP-13619 -- Site-Soecific Sleevina Reoort (from Rev. O to Rev.1)
Abstract - Changed reference from WCAP-13088, Rev.1, to WCAP-13088, Rev. 2, throughout the document.
- Modified first sentence of third paragraph.
- Removed leak-before-break evaluation reference.
TOC - Deleted Burst Strength Section. (Leak-before-break evaluation.)
List of Tables - Deleted Table 7.
I 1.0 - Removed leak-before-break discussion from first paragraph.
3.2 - Revised first sentence in second paragraph.
3.4 - Removed first four paragraphs.
3.5 - Deleted entire section on Burst Strength Requirements.
3.6 (old doc.) - Revised sleeve plugging limit.
- Deleted item no. 3.
8.0 - Revised references as appropriate.
Table 6 - Revised minimum wall thickness.
Table 7 - Removed.
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