ML20204G341
| ML20204G341 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 03/10/1987 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20204G328 | List: |
| References | |
| NUDOCS 8703260381 | |
| Download: ML20204G341 (5) | |
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UNITED STATES 8
NUCLEAR REGULATORY COMMISSION o
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SAFETY EVALUATION RY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 59 TO FACILITY OPERATING LICENSE NO. NPF-17 SOUTH CAROLINA ELECTRIC & GAS COMPANY SOUTH CAROLINA PUBLIC SERVICE AUTHORITY VIRGIL C. SilMMER NUCLEAR STATION,flNIT NO. 1 DOCKET NO. 50-395 INTRODUCTION By letter dated January 16, 1986, as supplemented March 18, May 18 and July 22, 1986, and as clarified December 4,1986, South Carolina Electric and Gas Company (SCE&G or the licenseel requested a change to Technical Specifications which would allow the repair of steam generator tubes. Combustion Engineerina Report CEN-337 was submitted by SCE8G to support a Technical Specification (TSI change allowing installetion of steam generator repair sleeves in the Summer Muclear Station. Combustion Engineering (CE) 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) 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 requirements.
Thus, it has not been necessary to take corrective action unless structural i
limits are being approached.
Historically, the corrective action taken where steam generator tube wall l
degradstion 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 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.
Installation of steam generator tube pluas removes the heat transfer surface of the plugged tube from service and leads to a reduction in tre 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.
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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 nomal operating and postulated accident conditions. Additionally, the 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 perfom these functions under nomal 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 themally 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 F
installation. There were two considerations in selecting the sleeve thickness.
First, the sleeve has sufficient thickness so that the steam generator tube l
with the sleeve bridging the degraded section of the tube meets the structural requiremcnts 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, l
l The sleeve is chamfered at the upper end to prevent hang-up with equipment l
which is used to install the sleeve or inspect the sleeve and the tube. Means are provided for temporary mechanical support of the sleeve prior to welding.
Inasmuch as each ileeve is welded at both ends to each tube being sleeved, this process is considered to provide a leak tight repair. Essentially, the process 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.
Plugs will be installed if sleeve installation is not successful or if there is unacceptable degradation of sleeves or sleeved steam generator tubes.
Analyses and testing demonstrate that the welded plug design which is provided by CE is leak tight and will meet structural requirements during nomal I
operating and postulated accident conditions.
MATERIALS SELECTION AND CORROSION CONSIDERATIONS The tubing from which the sleeves are fabricated is Inconel 690 procured to ASME Boiler and Pressure Vessel Code Case N-20.
In addition, a thermal treatment is also specified in order to impart greater corrosion resistance and lower the residual stress level in the tube.
, i The primary selection criterion for the sleeve material was its corrosion resistance in primary and faulty secondary PWR environments. Specific resistance to pure water and caustic stress corrosion cracking were considered.
Information published in the open literature indicates that the corrosion product release rates of Alloy 690 are superict to Alloy 600 in both hioh 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.
CE 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 been or are presently being conducted under accelerated conditions to assess the sleeve-tube joint i
performance under nominal and potential faulted environmental conditions.
Corrosion testing of typical sleeve-tube assemblies that have been completed reveals 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 i
Three types of nondestructive examinations are used during the sleeving process.
They are as follows: eddy current testing (ECT), ultrasonic testing (UT1, 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 tranducer 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.
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 interruoted 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. Consecuentiv, there are the three distinct regions relative to the inspection methods: li 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.
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Using specialized probes and multifrequency eddy current techniques, it has been demonstrated that a 40% throuah 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 40% through wall flaw (ASME calibration standard) in the sleeve below the weld region and detecting a 40% through wall flaw in the weld region. These inspection capabilities are documented in the Combustion Enaineering Report CEN-337. The basis for the plugging limit of 40% is discussed in the Mechanical Evaluation 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 examinetton 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 completeness and the absence of visibly noticeable indications such as cracks, pits, blow holes, and burn through.
MECHANICAL EVALUATION Mechanical tests were performed on mockup steam 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, and burst capability.
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.
Analyses have been performed to demonstrate the structural integrity of the sleeve-tube assemblies and their compliance with design requirements. The methodology used is in accordance with the ASME Roiler and Pressure Vessel Code,Section III.
An evaluation was performed to NRC Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes." Based upon the allowable degradation l
limit calculated and allowing an appropriate margin for inservice inspection uncertainties and growth of degradation between inspections, the 40% pluggina limit for steam generator tube sleeves in the Technical Specifications is adequate for the Combustion Engineering welded sleeve design to-be employed in the V.C. Summer steam generators.
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Based upon test data to determine susceptibility to flow induced vibration, it was concluded that a sleeded tube is no more susceptible to vib' ration than a nomal tube.
Fatigue analysis of the upper and lower sleeve weld joint has been perfomed and shown to meet ASME Code allowables.
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A comparison of the calculated failure ino'de's and the test data indicate good agreement. Safety factors were detemined for hypothetical pipe break accidents, and the minimum safety factor demonstrated adequate conservatism.
The nomal operations factor of safety, based on the full power restrained thermal expansion loading, was also demonstrated to be adequate.
FINDINGS l
l The staff concludes that the repair of the V.C. Summer steam generators utilizing the CE welded sleeve design is acceptable. This conclusion is based upon 1) the analytical work perfomed by the sleeve vendor, 2) the confimatory testing perfomed by the sleeve vendor, and 3) the thus-far satisfactory performance of similar sleeves installed in Ringhals and Ginna.
The licensee has provided modificatic'.s 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 ths sicavas are satisfactory with regard to mechar.ical properties, corrosion, resistance, and inspectability.
ENVIRONMENTAL CONSIDERATION Pursuant to 10 CFR 51.32, the Comission has detemined that issuance of the amendment will have no significant impact on the environment (51 FR 29727, dated August 20, 1986).
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 Comission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
Dated: March 10, 1987 Principal Contributors:
J. B. Hopkins, Project Directorate #2, DPLA E. J. Sullivan, Jr., Engineering Branch, DPLA C. D. Sellers, Engineering Branch, DPLA