ML20214F625
| ML20214F625 | |
| Person / Time | |
|---|---|
| Site: | Summer  |
| Issue date: | 11/05/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20214F612 | List: |
| References | |
| NUDOCS 8611250372 | |
| Download: ML20214F625 (8) | |
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.N UNITED STATES h~-
k NUCLEAR REGULATORY COMMISSION wasmNGTON, D. C. 20555 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 54 TO FACILITY OPERATING LICENSE NO. NPF-12 SOUTH CAROLINA ELECTRIC & GAS COMPANY SOUTH CAROLINA PUBLIC SERVICE AUTHORITY VIRGIL C. SUMMER NUCLEAR STATION, UNIT NO. 1 DOCKET NO. 50-395 INTRODUCTION By letter dated January 16, 1986, as revised August 15, and September 15, 1986, and supplemented May 8, and October 20, 1986, South Carolina Electric and Gas Company (the licensee for V. C. Summer Nuclear Station) requested a revision to the Technical Specifications (TS), Section 3/4.4.5., " Steam Generators."
This revision seeks to change the plugging limit definition in Item 4.4.5.4.a.
and would exclude from plugging those tubes with indications approximately 1.6 inches or greater below the top of the tubesheet provided that the top 1.6 inches of the tube within the tubesheet is not degraded. Westinghouse Reports WCAP 11228 and WCAP 11229 "Tubesheet Regica Plugging Criterion," which were part of the TS amendment request, address the issue of repairing ce plugging full depth hardroll expanded steam generator tubes which may have experienced degradation within the tubesheet area and provide the technical justification for the licensee's TS change request. WCAP 11229 is a nonproprietary version of WCAP 11228.
Existing plant TS tube plugging criteria apply throughout the tube length and do not take into account the reinforcing effect of the tubesheet on the external surface of the tube. The presence of the tubesheet will constrain the tube and will complement its integrity in that region by precluding tube deformation beyond its expanded outside diameter. The resistance to both tube rupture and tube collapse is significantly strengthened by the tubesh.eet.
In addition, the proximity of the tubesheet significantly affects the leak behavior of throughwall tube cracks in this region, i.e., no significant leakage relative to plant TS allowables is to be expected.
Based on these these considerations, the use of an alternate criterion for plugging is iustified.
The purpose for the development of the proposed criterion is to obviate the need to remove a tube from service (by plugging) due to detection of indications, generally by eddy current testing (ECT), in a region extending over most of the length of tubing within the tubesheet. This safety evaluation assesses the integrity of the tube bundle with ECT indications on tubes within the tubesheet under normal operating and postulated accident conditions.
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. The proposed criterion identifies a distance, designated P* in the original submittal, now designated as F* and referred to as the F* criterion, below the i
top of the tubesheet below which tube degradation of any extent does not necessitate plugging. The criterion, according to the licensee's evaluation, provides the same level of protection for tube degradation in the tubesheet region as that afforded by Regulatory Guide (RG) 1.121 for degradation located outside the tubesheet region. Limitations on the use of the criterion have, also been discussed by the licensee.
The Federal Register Notice (51 FR 9907, March 21,1986) stated that under the amendment steam generator tube imperfections would be addressed by the P-STAR evaluation method. This evaluation method relied on the combination of an adjacent tube and the tube-to-tubesheet interface to detemine the distance '
below which tube imperfections need not be plugged. The final evaluation method approved by this amendment is called F-STAR and is essentially the'same. The F-STAR method just uses the tube-to-tubesheet interface to determine the distance below which imperfections need not be plugged and does not include consideration of an adjacent tube. This results in a slightly greater distance of tube-to-tubesheet interface necessary to preclude plugging and results in fewer imperfections s
being addressed by the evaluation method. Therefore, this action is not being
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renoticed because the subject matter of the amendment does not significantly differ from that originally noticed.
EVALUATION OF TUBE PLUGGING CRITERION 7
Engagement Distance Detemination The licensee determined a distance below the top of the tubesheet below which tube degradation of any extent does not necessitate plugging. This criterion would be used in detemining whether or not plugging of full depth hardroll expanded steam generator tubes is necessary for degradation which has been detected in that portion of the tube which is within the tubesheet.
The proposed criterion forms the basis for obviating the need to r'emove a Y
tube from service (by plugging) due to detection of indications, e.g., by eddy current testing (ECT), in a region extending over most of the length of tubing within the tubesheet. This evaluation applies to the V. C. Summer Westinghouse s
Model D3 steam generators and assesses the integrity of the tube bundle for tube ECT indications occurring in the length of tubing within the tubesheet, relative to:
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Maintenance of tube integrity for all loadings associated with nomal plant conditions, including startup, operation in power range, hot
.s standby and cooldown, as well as all anticipated transients.
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2)
Maintenance of tube integrity under postulated limiting conditions 6f primary-to-secondary and secondary-to-primary differential pressure, e.g., steamline break (SLB) or feedwater line break (FLB).
3)
Limitation of primary-to-secondary leakage consistent with accident t
analysis assumptions.
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1 The criterion provides for sufficient engagement of the tube to tubesheet hardroll such that pullout forces that could be develcped during normal or accident operating cnnditions would be successfully resisted by the elastic preload between the tube and tubesheet even in the event of a circumferential break in the tule below the distance.
In order to evaluate the applicability of any developed criterion for indications within the tubesheet, some postulated type of degradation must necessarily be considered. For this evaluation it was postulated that a circumferential severance of a tube could occur, contrary to existing plant operating expe'ience. s liowever, implicit in assuming a circumferential r
severance to occur, is the consideration that degradation of any extent could be demonstrated to be tolerable below the location determined acceptable for the postulated condition.
~c When the tubes have been hardrolled into the tubesheet, any axial loads developed by pressure and/or mechanical forces acting on the tubes are resisted by fricticnal forces developed by the elastic preload that exists between the q ' o' tube and the tubesheet.
For some specific length of engagement of the hardroll, no significant axial forces will be transmitted further down the
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tube, and that length' of tubing, i.e., F*, will be sufficient to anchor the tube in the tubesheet.
In order to determine the value of F* for application
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in Model D3 steam generato*s, a. testing program was conducted t.o measure the elastic preload of the tubes in the tubesheet.
Tubes are installed in the ste'am generator tubesheet by a hardrolling process which expands the tube to bring the outside surface into intimate contact with the tubesheet hole. The roll process and roll torque are specified to result in a metal-to-metal interference fit between the tube and the tubesheet.
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A test program was conducted by Westinghouse to quantify the degree of '
interference fit between the tube and the tubesheet provided by the full depth mechanical hardrolling operation. The data pnerated in these tests has been analyzed to determine the length cf hardroll required to preclude axial tube
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forces from being transmitted further along the tube, i.e., to establish the F*
criterion. The amount of interference was determined by installing tube specimens in collars specifically designed to simulate the tubesheet radial N,
stiffness. The test configuration consisted of six cylindrical collars. A mill annealed, inconal 600 (ASME 58-163) tubing specimen was hardrolled into each collar using a process which simulated actual tube installation
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Once the hardrolling was completed,4the test collars were removed from the tube specimens and the springback of the tube was measured. The amount of springback was used in an analysis to 'detennine the magnitude of the interfyrence fit, which is, therefore, representative of the residual tube to tubesheet Tadial load in Westinghouse Model D steam generators.
Durfng plant operation the amount of preload will change depending on the pressure and temperature cpnditions experienced by the tube. The room temperature preload stresses, i.e, radial, circumferential and axial, are such that the material is nearly % the yield state if a comparis n is made to ASME
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Code minimum material properties. Since the coefficient of themal expansion of the tube is greater than that of the tubesheet, heatup of the plant will result in an increase in the preload and could result in some yielding of the tube.
In addition, the yield strength of the tube material decreases with temperature. Both of these effects may result in the preload being reduced upon return to ambient temperature conditions, i.e., in the cold condition.
However, based on the results obtained from the pullout tests, this is not expected to be the case as even with a very high temperature relaxation soak the results show the analysis to be conservative.
The plant operating pressure influences the preload directly based on the application of the pressure load to the inner diameter of the tube, thus increasing the amount of interface loading. The pressure also acts indirectly to decrease the amount of interface loading by causing the tubesheet to bow upward. This bow results in a dilation of the tubesheet holes, thus, reducing the amount of tube to tubesheet preload. Each of these effects was quantitatively treated.
Analytically combining the room temperature hardroll preload with the thermal, l
pressure, and tubesheet bow effects resulted in a net positive operating preload during nomal and faulted operation.
In addition to restraining the tube in the tubesheet, this preload should effectively retard leakage from indications
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in the tubesheet region of the tubes.
The applied loads to the tubes which could result in pullout from the tubesheet Wing all normal and postulated accident conditions are predominantly axial' and due to the internal to external pressure differences. For a tube which has not been degraded, the axial pressure load is given by the product of the pressure with the internal cross-sectional area. However, for a tube with internal degradation, e.g., cracks criented at an angle to the axis of the tube, the internal pressure may also act on the faces of the cracks. Thus, for a tube which is conservatively postulated to be severed at some location within the tubesheet, the total force acting to remove the tube from the tubesheet is given by the product of the pressure and the cross-sectional area of the tubesheet hole. Any other forces such as fluid drag forces in the U-bends and vertical seismic forces are negligible by comparison.
The calculation of the required engagement distance is based on detemining the length for preload frictional forces to equilibrate the applied operating loads. The axial friction force was found as the product of the radial preload force and the coefficient of friction between the tube and the tubesheet. The value assumed for the coefficient of friction was for sliding of nickel on mild steel under " greasy" conditions.
l For the maximum nomal pressure applied load with a safety factor of 3, the length of hardroll required is exceeded by the V. C. Sumer value for F* of 1.6 inches.
Similarly, the required engagement length for faulted conditions using a safety factor corresponding to a ASME Code safety factor of 1.0/0.7 for allowable stress for faulted conditions is similarly exceeded by the V. C. Sumer F*
value.
The F* value thus detemined for the required length of hardroll engagement below the BRT or the top of the tubesheet, whichever is greater relative to the top of the tubesheet, is sufficient to resist tube pullout during both nonnal and postulated accident condition loadings. Furthermore, the uncertainty in position of the ECT indication must be added to the criterion for the final calculation of F*.
A conservative allowance for uncertainty in ECT position indication is available in the F* distance of 1.6 inches in the V. C. Sumer Technical Specifications on Steam Generators Section 3/4.4.5.
Rolled Tube Pullout Tests The engagement distance determination discussed above was calculated from a derived preload force and an assumed static coefficient of friction for tube to tubesheet contact. A direct measurement of this static coefficient of friction is difficult. However, a simple pull test on a rolled tube joint provided both support for the derived preload force (less the effects of thermal expansion and internal pressure tightening) and an indirect measurement of the static coefficient of friction. The results of the testing verify the calculation as being conservative.
Pullout tests were conducted on several actual rolled joints with various
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amounts of wall thinning. As with the preload tests, the test configuration consisted of mill annealed, Inconel 600 (ASME SB-163) Model D3 tubing, hardrolled into carbon steel collars with an outside diameter to simulate tubesheet rigidity.
Inside surface roughness values of the collars were measured and recorded. The specification of surface roughness for the fabrication of the collars was the same as that used for the fabrication of the Model D.
tubesheets. After rolling, an inside circumferential cut was machined through the wall of the tube at a controlled distance from the bottom of the hardroll transition (opposite the tube weld). The machined cut simulated a severet" tube condition. To simulate any possible effect of reduced preload force due to tube yielding during manufacturing heat treatment and during reactor operation, the samples were subjected to a heat soak.
Based on the observed pullout forces, the coefficient of friction assumed in the engagement distance determination was verified to be conservative.
Rolled Tube Hydraulic Proof Tests Similar to the rolled tube pullout tests, pressure tests were conducted on rolled joints and with nominal degrees of wall thinning. As with the preload and pullout tests, the test configuration consisted of mill annealed, Inconel 600 (ASME SB-163) Model D3 tubing, hardrolled into carbon steel collars with an outside diameter to simulate tubesheet rigidity. As with the pullout test samples, a machined cut was used to simulate a severed tube condition. To simulate any possible effects of reduced preload force due to tube yielding during manufacturing heat treatment, these samples were also subjected to a heat soak. The pressure tests were perfonned at room temperature using water.
These proof tests showed that even for rolled joints considerably less than the F* distance in length at less than nominal wall thinning, pressure induced axial forces of several thousands of pounds or greater are necessary to cause
l the tube to release from the tubesheet. Thus, the preload based calculation of required engagement distance is indicated to be conservative.
Primary-To-Secondary Leakage Considerations As described above to apply the F* criterion the applicable tube must have a certain minimum length of hardroll engagement below the top of the tubsheet or the BRT, whichever is greater relative to the top of the tubesheet. For V. C. Sumer, South Carolina Electric and Gas has conservatively established an F*
distance of 1.6 inches. The presence of the elastic preload presents a significant resistance to flow of primary-to-secondary or secondary-to-primary water for degradation which has progressed fully through the thickness of the tube wall.
In effect, no leakage would be expected if a sufficient length of hardroll is present. This has been demonstrated in high pressure fossil boilers where hardrolling of tube to the tubesheet joints is the only mechanism resisting flow, and in steam generator sleeve-to-tube joints made by the Westinghouse hybrid expansion joint process. This was also confirmed by the hydraulic proof test specimens which were pressurized up to and in excess of the faulted operating conditions. Because of the difficulty in accurately sizing stress corrosion crack indications tne Technical Specifications require that no indications of cracking can be present within the F* distance in tubes to which the F*
criterion is applied. This requirement has the effect of preventing the start of a leak path.
Tube Integrity Under Postulated Limiting Conditions The final, aspect of the evaluation is to demonstrate tube integrity under the postulated loss of coolant accident (LOCA) condition of secondary-to-primary differential pressure. A review of tube collapse strength characteristics indicates that the constraint provided to the tube by the tubesheet gives a significant margin between tube collapse strength and the limiting secondary-to-primary differential pressure condition, even in the presence of circumferential or axial indications.
EVALUATION OF PROPOSED TECHNICAL SPECIFICATIONS The licensee proposed Technical Specifications to implement tube plugging criterion. Based upon discussions with SCE&G personnel, revised proposed Technical Specifications (TS) were submitted in a letter dated September 15, 1986. The following addresses the changes in the TS to implement the tube plugging criterion.
1.
The TS contain a definition of the F* distance, which is 1.6 inches, and a definition of a F* tube, which is a tube left in service by application of the F* criterion.
2.
The TS contain a specific provision for reinspection tubesheet regier, of F* tubes in addition to the nonnal TS required sampling.
3.
Special reports containing the results of inspection or reinspection of F* tubes are to be submitted to the Commission prior to restart.
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4.
The F* criterion or plugging 10mit is defined such that tubes need not be plugged because of ECT indications, equal to or greater than 40% through wall, that are below the F* distance from the top of the tubesheet (or from the top of the last herdroll whichever is lower) provided the tube is not degraded within the F* distance. The restriction on no degradation within the F* distance means that there are no indications of cracking. This restiction has been incorporated because of the difficulty in accurately sizing stress corrosion cracking.
It is recognized that stress corrosion cracking that appears by ECT to be shallow may in fact be considerably deeper. In addition the engagement distance analysis and the testing program were based upon tubes that did not contain imperfections.
5.
The application of the F* criterion is being approved until the end of the fifth fuel cycle. This time provision was included in the
' proposed TS at the request of the staff to give the staff the opportunity to review and evaluate the results of subsequent inspections before extending or revising staff approval for use of the F* criterion.
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The staff has reviewed the TS changes proposed and finds them acceptable. This TS provides acceptable implementation of the tube plugging criterion as analyzed in the Westinghouse Topical Report and evaluated in this Safety Evaluation Report.
SUMMARY
Based on a review of the licensee's submittals the staff concludes that tubes can be left inservice with eddy current indications of pluggable magnitude that are below the F* distance provided the tube is not degraded within the F*
distance. The F* distance is defined as 1.6 inches from the top of the tubesheet or from the top of the last hardroll whichever is lower.
From the results of the testing and analysis, it is concluded that following the installation of a tube by the standard hardrolling process, a residual radial preload stress exists due to the plastic deformation of the tube and tubesheet interface. This residual stress is expected to restrain the tube in the tubesheet while providing a leak limiting seal condition even if the tube is completely severed circumferentially at the F* distance below the top of the tubesheet.
The application of the F* criterion is being approved until the end of the fifth fuel cycle. The staff concludes that these proposed Technical Specifica-tion changes on Steam Generators Section 3/4.4.5 are acceptable.
ENVIRONMENTAL CONSIDERATION Pursuant to 10 CFR 51.32, the Commission has determined that issuance of the amendment will have no significant impact on the environment (51 FR 26484, dated July 23,1986).
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 Commission'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: November 5,1986 Principal Contributors:
Jon B. Hopkins, Project Directorate #2, DPLA Edmund J. Sullivan, Jr., Engineering Branch, DPLA
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