ML20214Q162

Safety Evaluation Supporting Amends 98 & 88 to Licenses DPR-39 & DPR-48,respectively Re Tech Spec Change Allowing Installation of Steam Generator Repair Sleeving

ML20214Q162
Person / Time
Site:	Zion  File:ZionSolutions icon.png
Issue date:	11/18/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20214Q151	List: ML20214Q148 ML20214Q156 ML20214Q162
References
NUDOCS 8612050013
Download: ML20214Q162 (9)

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.,' /*'% UNITED STATES 3" t NUCLEAR REGULATORY COMMISSION j i #l .g WASHINGTON, D. C. 20555

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.....J SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 98 TO FACILITY OPERATING LICENSE NO. DPR-39 AND AMENDMENT NO. 88~ TO FACILITY OPERATING LICENSE NO. DPR-48 COMMONWEALTH EDISON COMPANY ZION NUCLEAR POWER STATION, UNITS 1 AND 2 DOCKET NOS. 50-295 AND 50-304 INTRODUCTION-Combustion Engineering Report CEN-331, Revision 1, was submitted by I Comonwealth Edison Company (Ceco) by letter dated June 16, 1986, (Ref. 1) to support a technical specification change allowing installation of steam generator repair sleeves in Zion Units 1 and 2.

Combustion Engineering (C-E) provides a leak tight sleeve which is welded to the steam generator tube near each end of the sleeve. The sleeve spans the degraded area of the parent steam generator tube in,the tube sheet region.

The operation of Pressurized Water Reactor (PWR) steam generators has in some instances, resulted in localized corrosive attack on the inside (primary side)

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or outside (secondary side) of the steam generator tubing. This corrosive attack results in a reduction in steam generator tube wall thickness. Steam generator tubing has been designed with considerable margin between the actual wall thickness and the wall thickness required to meet structural r'equirements.

Thus it has not been necessary to take corrective action unless structural' limits are being approached. '

Historically, the corrective action taken where steam generator tube wall degradatiorF has been severe has been to install plugs at the inlet and outlet of the steam generator tube when the reduction in wall thickness reached a calculated value referred to as a plugging criteria. -Eddy current testing (ECT) has been used to measure steam generator tubing degradation and the tube plugging criteria accounts for ECT measurement uncertainty.

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PDR ADOCK 05000295 P PDR g

2 Installation of steam generator tube plugs removes the heat' transfer surface of the plugged tube from service and leads to a reduction in the primary coolant flow rate available for core cooling. Installation of welded steam generator sleeves does not significantly affect the heat transfer capability of the tube being sleeved and a large number of sleeves can be installed without significantly affecting primary flow rate.

ACCEPTANCE CRITERIA The objectives of installing sleeves in steam generator tubes are twofold.

The sleeve must maintain structural integrity of the steam generator tube during normal operating and postulated accident conditions. Additionally, the "

I sleeve must prevent leakage in the event of a through hole in the wall of the steam generator tube. Tests and analyses were performed to demonstrate the capability of the sleeves to perform these functions under normal operating and postulated accident conditions.

Plugs are installed in the sleeved steam generator tubes when the tubes cannot be successfully repaired with sleeves.

SLEEVE DESIGN AND PROCESS DESCRIPTION The sleeve design qualified by CE is inserted in a degraded or defective tube to a point above the tubesheet but below the first support plate.

The sleeve material is thermally treated Inconel 690. The outside diameter of the sleeve was selected to provide a generous clearance between the sleeve and steam generator tube so that the sleeve slides freely through the tube during installation. There were two considerations in selecting the sleeve l thickness. First, the sleeve has sufficient thickness so that the steam  ;

generator tube with the sleeve bridging the degraded section of the tube meets the structural requirements of the undamaged portion of the steam generator tube. This consideration assumes no benefit from the tube behind the sleeve.

Second, there is a large margin in thickness over what is required structurally to allow for sleeve eddy current measurement uncertainty. the inside diameter of the sleeve is large enough so that the flow rate and heat transfer capability of the steam generator tube are not significantly affected by sleeve installation.

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The sle~ eve is chamfered at the upper end to prevent hang-up with equipment which is used to instaff the sleeve or inspect the sleeve and the tube. Means are

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provided for temporary mechanical support of tha sleeve prior to welding.

Inasmuch as each sleeve is welded at both ends to each tube being sleeved, this process is considered to provide a leak tig% repair. Essentially the process t consists of cleaning the tube to be sleeved at the areas of the welds, insertion of the sleeve, expansion in the upper weld area, welding and inspection.

t Plugs will be installed if sleeve installation is not successful or if there l

is unacceptable degradation of s'leeves or sleeved steam generator tubes.

Analyses and testing demonstrated that the welded plug design which is provided by C-E is leak tight and will meet structural requirements during nomal operating and postulated accident conditions.

MATEPIALS SELECTION AND CORROSION CONSIDERATIONS The tubing from which the sleeves are fabricated is Inconel 690. It was procured according to ASME Poiler and Pressure Vessel Code Case N-20. In addition, a themal treatment is also specified in order to impart greater corrosion resistance and to lower the residual stress level in the tube.

1 The primary selection criterion for the sleeve material was its corrosion resis-tant in primary and faulty secondary PWR environments. Specific resistant to pure water and caustic stress corrosion cracking were considered.

I Information published in the open literature indicates that the corrosion product release rates of alloy 690 is superior to Alloy 600 in both high temperature ammoniated and borated waters. The corrosion rate of Alloy 600 is significantly higher, especially in borated waters, with the concurrent formation of thicker oxides. The latter is a potential concern during thermal transients which could initiate crud bursts.

C-E has conducted a number of bench and autoclave tests to evaluate the corrosion resistance of the welded sleeve joint. Of particular' interest is the effect of the mechanical expansion / weld residual stresses and the condition of the weld and weld heat affected zone. Various tests have or are presently being conducted under accelerated conditions to assess the sleeve-tube joint performance under nominal and potential faulted I

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environmental conditions. Corrosion testing of typical sleeve-tube assemblies that have been completed reveal no evidence of sleeve or tube corrosion considered detrimental under anticipated service conditions. In addition, this repair method has been used, thus far successfully, for sleeving at the Swedish Ringhals 2 plant and at R. E. Ginna.

PROCESS AND INSERVICE INSPECTION Three types of nondestructive examination are used during the sleeving process. They are as follows: eddy current testing (ECT), ultrasonic testing (UT), and visual.

A dual cross wound probe and bobbin probe using the multifrequency eddy current method will be used to perform a baseline inspection of the installed sleeve for future reference. The ECT fixture is used on a manipulator arm to position the probe.

Ultrasonic testing using an immersion technique with demineralized water as a couplant is used to inspect the upper tube to sleeve weld. A transducer is positioned in the weld area and is rotated with an electric motor to scan the weld. The pulse echo tester has the ability to interface with an on-line data reduction computer to produce a display /hardcopy during radial and axial scanning.

5 An eddy current test has been qualified for the inspection of installed welded sleeves to detect flaws in the pressure boundary. Eddy currents circulating in the sleeve and steam generator tube are interrupted by the presence of flaws in the material with a resultant change in test coil impedance. This impedance change is processed and displayed on the test instrument to indicate the presence of a flaw.

The pressure boundary is considered to be the sleeve up to and including the upper weld joint and the steam generator tube above the weld. Consequently,

. there are the three distinct regions relative to the inspection methods: 1) the sleeve below the weld, 2) the steam generator tube behind the top section of the sleeve (above the weld), and 3) the steam generator tube above the

,' sleeve.

Using specialized probes and multifrequency eddy current techniques, it has been demonstrated that a 40 percent through wall flaw (ASME calibration standard) is detectable anywhere in the tube behind the sleeve and above the weld. These techniques are also capable of detecting and sizing a 33%.through wall flaw (ASME calibration standard) in the sleeve below the weld region and detecting a 33% through wall flaw in the weld region. These inspection a capabilities are documented in the Combustion Engineering Report CEN-331.

Ceco has agreed in a letter dated September 24,1986, (Ref. 4) to provide NRC with the results of further testing that they are undertaking to quantify minimum detection and sizing capabilities. The proposed technical specifications contained in the April 24, 1986, letter (Ref. 2) as revised by the September 2, 1986 letter, (Ref. 3) from CECO reflect these values in the plugging limits in TS Section 4.3.1.B.4.A.6. .The basis for the plugging limit of 33% is discussed in the Mechanical Evalaution section below.

Visual examinations can be performed on the upper welds to support UT results and are performed on the lower welds to determine their integrity and acceptance. The welds are examined using a boroscope examination system. The lighting is supplied as an integral part of the visual examination system.

Each examination is recorded on video tape for optional later viewing and to provide a permanent record of each weld's condition.

The inspections are performed to ascertain the mechanical and structural condition of each weld. Critical conditions which are checked include weld width and completenes's and the absence of' visibly noticeable indications such as cracks; pits, blow holes, and burn.thrcugh.

4 MECHANICAL EVALUATION

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Mechanical tests were performed on mockup stean generator tubes containing sleeves and plugs to provide qualified test data describing the basic properties-of the completed assemblies. These tests determined axial load,

. collapse, burst, and thermal cycling papability.

6 These tests demonstrate that the load capability of the upper and lower sleeve welds is sufficient to withstand thermally induced stresses in the weld resulting from temperature differential between the sleeve and the tube and the pressure induced stresses resulting from normal operating and postulated accident conditions. The burst and collapse pressures of the tested sleeve provided substantial margin over the limiting differential pressure. Mechanical testing indicated that the installed sleeve will withstand the cyclical loading resulting from power changes in the plant and other transients.

From testing it was determined that the welded plugs have sufficient load capacity to perform their function during normal operating and postulated accident conditions. The axial load required to loosen the plug from the sleeve-tube assembly is approximately four times greater than the design load.

The licensee has performed analyses to demonstrate the structural integrity of l the sleeve-tube assemblies and their compliance with design requirements. The methodology used is in accordance with the ASME Boiler and Pressure Vessel Code,Section III. .,

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. . 1 An evaluation was performed to NRC Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes." Based upon the allowable degradation limit calculated and allowing an appropriate margin for inservice inspection uncertainties and growth of degradation between inspections, the 33% plugging limit for steam generator tube sleeves contained in the proposed Technical Specifications for Zion 1 & 2 steam generators contained in the September 2, 1986 letter from Conrnonwealth Edison is adequate for the Combustion Engineeri.ng welded sleeve method.

Based upon test data to determine susceptibility to flow induced vibration, it was concluded that a sleeved tube is no more susceptible to vibration than a normal tube.

a Fatigue analysis of the upper and lower sleeve weld joint has been performed and shown to meet ASME Code allowables.

A comparison of the calculated failure modes and the test data indicate goo'd agreement. Safety factors were determined for hypothetical pipe break accidents, and the minimum safety factor demonstrated adequate conservatism.

The normal operations factor of safety, based on the full power restrained thermal expansion . loading, exceeded that required by Regulatory Guide 1.121.

CONCLUSION The staff concludes that the repair of the Zion steam generators utilizing the C-E welded sleeve design is acceptable. This conclusion is based upon 1) the analytical work performed by the sleeve vendor 2) the confirmatory testing performed by the sleeve vendor, and 3) the thus-far satisf(ctory performance of similar sleeves installed in Ringhals and Ginna.

The licensee has provided modifications to his Technical Specifications which contain appropriate surveillance requirements and plugging limits. The staff concurs with the licensee that sleeving is a satisfactory alternative to plugging and that the sleeves are satisfactory with regard to mechanical properties, corrosion resistance, and inspectability.

ENVIRONMENTAL CONSIDERATION These amendments involve a change in the installation or use of the fecilities. components located within the restricted areas as defined in 10 CFR 20. The staff has determined that these amendments involve no sigrificant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite and that there is no significant increase in individual or cumulative occupational radiation-exposure. The Commission has previously issued a proposed finding that these amendments involve no significant hazards consideration and there has been no public comment on such finding. Accordingly, these amendments meet the eligibility criteria for categorical exclusion set forth in 10 CFR Sec-51.22(c)(9). Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in connection with the issuance of these amendments.

t CONCLUSION We have concluded, based on.the considerations discussed above, that:

- (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (2) such. activities will be conducted in compliance with the Commission's regulations and the issuance of these amendments will not be inimical to the common defense and security or to the health and safety of the public.

Dated: November 18, 1986 PRINCIPAL CONTRIBUTOR:

David Sellers l

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REFERENCES

1. June 16, 1986, letter from P. C. LeBlond (CECO) to H. R. Denton (NRC) transmitting Combustion Enginering Report No. CEN-331, Revision 1.

2. April 24, 1986, letter from P. C. LeBlond to H. R. Denton transmitting proposed technical specification change.

3. September 2,1986,' letter..from P. C. LeBlond to H. R. Dent'on transmitting l ' ,- a revision to the techncial 'pecifications s change proposed in Ref. 2.

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, 4. September 24, 1986, letter from P. C. LeBlond to H. R. Denton transmitting supplementary information to Ref.1.

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