ML20215D195
| ML20215D195 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 12/05/1986 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20215D193 | List: |
| References | |
| NUDOCS 8612160251 | |
| Download: ML20215D195 (9) | |
Text
,
- pa,na rug *o UNITED STATES NUCLEAR REGULATORY COMMISSION o
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WASHINGTON, D. C. 20555
/
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR PEGULATION SUPPORTING AMENDPENT N0.106TO FACILITY OPEPATING LICENSE NO. DPR-51 ARKANSAS POWER AND LIGHT COMPANY ARKANSAS NUCLEAR ONE, UNIT NO. 1 DOCKET NO. 50-313
1.0 INTRODUCTION
By letter dated April 1,1986, Arkansas Power and Light Company (AP&L or the licensee) requested amendment to the Technical Specifications (TSs) appended to Facility Operating License No. DPR-51 for Arkansas Nuclear One, Unit No. 1 (ANO-1).
Supporting information was provided by letters dated August 22, October 14 and October 23, 1986. The proposed amendment would (1) allow the sleeving of steam generator tubes and (2) modify the designation of those areas identified as special groups in the steam generators where imperfections have been previous 1f found.
The licensee plans to conduct steam. generator tube sleeving in AN0-1 prior to startup for Cycle 8 operation which is planned for early December 1986.
2.0 BACKGROUND
As a result of degradation in the ANO-1 Once Through Steam Generators (OTSGs), a substantial number of tubes with eddy current indications in excess of the 40% through-wall plugging limit have been removed from ser-vice. The suspected mechanism affecting the tubes in the upper tube sheet (UTS) region is intergranular attack (IGA) caused by concentrated chemical contaminants which have been carried by moisture in the steam flowing up through the tube lane region.
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The lane region is cooler than the surrounding area due to reduced heat j
flux and flow resistance. Therefore, more moisture is in the steam in this region. The contaminants carried by this moisture are deposited in the upper tube sheet region.
Plugging the tubes in the lane region increases the area of reduced heat flux, thereby increasing the number l
of tubes affected by moisture in the steam. This aggravates the condi-l tion by increasing the amount of contaminants carried by the steam and deposited on the tubes in the UTS region.
To address this problem, AP&L initiated a Steam Generator Intearity Program in 1983. The goal of this program is to identify and initiate changes which will assure that the existing OTSGs can be used for the life of the facility without increased risk to the health and safety of the public or reduction in the unit's performance.
One portion of this program is a Steam Generator Sleeving Qualification g2160251 861205 ADOCK 05000313 p
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. Program which would provide sufficient justification to allow a large-scale sleeving program at AH0-1. The installation of sleeves in the affected tubes should decrease the lane region degradation rate by pre-venting additional loss of heat transfer area.
In addition, the sleeve material has better corrosion resistance than the original tubes. This Qualification Program, which consists of the tests and analyses and development of general design criteria for the Pabcock & Wilcox (B&W)
OTSG sleeves, is contained in_B&W report BAW-1823P, "Once-Through Steam Generator Mechanical Sleeve Qualification." Our evaluation of this report (Reference 2) was completed in November 1984 and included the following areas:
1.
Leak-tight integrity 2.
Pullout strength of sleeves 3.
Joint expansion tests 4.
Flow-induced vibration effects 5.
Effect of sleeve installation on adiacent sleeves 6.
Thermal / hydraulic effects of sleeving 7.
Structural and functional integrity of sleeves and compliance with ASME Code requirements Based on this review, we found the licensee's sleeve / tube qualification program acceptable for a proposed demonstration sleeving of ten tubes at ANO-1. However, additional tests were recommended prior to approval of large scale sleeving. These additional tests were reported by the licensee in the present submittal (Reference 1) and are addressed in this evaluation. The licensee has also included B&W Report BAW-1823P, reviewed earlier as stated above, with the present submittal.
3.0 EVALUATION 3.1 Effect of Corrosion on the Sleeved Joint Corrosion propagation during normal operation due to increased residual stress and during wet lay-up were investigated on a mechan-itally sleeved AN0-1 OTSG tube.
To perform this test, a specimen was fabricated from a portion of a tube pulled from the ANO-1 B-0TSG in January 1983 on which IGA was observed. The specimen was fabricated using the process developed for field installation. The specimen was exposed in an autoclave at approximately to an environment that contains the typical feedwater contaminant concentrations. A tensile load of approximately and an internal pressure of about were placed on the expanded,inint for the 2000 hour0.0231 days <br />0.556 hours <br />0.00331 weeks <br />7.61e-4 months <br /> duration of the test. Upon completion of this phase of the test, one nf the two expanded joints was removed for metallurgical examination while the other joint was replaced in the autoclave in wet lay-up conditions for 500 hours0.00579 days <br />0.139 hours <br />8.267196e-4 weeks <br />1.9025e-4 months <br />. This joint was then removed from the autoclave and metallurgically examined with a scanning electron microscope for evidence that the existing IGA had not proaressed during the testing.
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. Examination of test specimens showed that the depth of IGA in speci-mens removed from the autoclave for operational and wet lay-up simulation was the same as was found in the specimens prior to the test. This test, in conjunction with information previously reported, shows that corrosion propagation is not likely to increase signifi-cantly in the rolled joint during normal operation and wet lay-up.
Other slower corrosion mechanisms such as corrosion due to cyclic stresses are not likely to be aggravated by the sleeving process.
The ten demonstrated sleeves installed earlier will be monitored closely to provide an advance warning of possible corrosion propa-gation due to unforeseen mechanisms.
Additional data on the effects of roller expanding sleeves inside OTSG tubes affected by intergranular corrosion on the outer surface were obtained by the licensee. The AN0-1 tube samples, each were cut from a previously pulled ANO-1 tube. The samples were obtained from a portion of the tube adjacent to an area known to have IGA present on the outer tube surface.
The inner surfaces of the tube samples were chemically decontaminated to eliminate the need for rolling under hot cell conditions.
and the IGA categorized for post rolling comparisons. Upon completion of the tube decontamination, a sleeve was roll expanded into each tube sample to the maximum qualified expansion. The tube samples were examined by eddy current testing (ECT), diameter measurements, metallography and scanning electron microscopy both prior to and after roller expanding a sleeve to characterize the effects of sleeve installation on existing IGA.
The data obtained from this test served as the baseline for comparison to the cor-rosion test discussed earlier.
In addition to these B&W corrosion tests, Vestinghouse also performed tests for AP&L on actual ANO-1 tubing to detemine the effects of two different types of sleeving processes. To acccmplish this, e detailed characterization of the existing conditions was performed on tubing removed from the ANO-1 GTSGs. The characterization included non-destructive examinations (NDE), analysis of deposits, microexamination and sensitization testing.
Sleeving was then performed using both mechanical and braze processes. The mechanical process used by Westinghouse, although not exactly like that used by B&W, is very similar. After each step in the sleeving process, the outer surface of the tube was examined and photographed.
From observations, it was determined that the sleeving process did not widen any IGA areas, such as to make them detectable by eddy current, radiography, or visual inspection.
Residual stress measurements, determined by
- testinc, were performed on Westinghouse brazed sleeves and on Westinghouse mechanically expanded sleeves using AN0-1 OTSG tubing.
It was
.. verified'that the tube residual stresses from-roller expansion and brazing process are significantly low and are considered acceptable for a sleeve design.-
-3.2 Sleeves Installed on Degraded Tubes The licensee performed-tests to determine whether mechanical sleeves installed in the free span of 0TSG tubes which " ave as much as
' degradations have a reasonabk. chance of satisfying the qualification program requirements for strength and leak tightness.
These were simplified tests which used 0TSG tube specimens which
.had been machined to to e
represent tubing with_
IGA. Two hiah yield tubes were each' sleeved by rolling expansions in the free span and subjecting the assemblies'to incremental axial loads while the joint slippage and leakage under
-internal pressure were measured.
Ultimate failure of the _first of the two specimens tested occurred in the thin portion of the tube at This indicates that the~ sleeve joint is stronger than the tube and both can withstand the' maximum axial _ load of which would result from accident conditions.- The second specimen did not fail at an axial load of,
and was not pulled to failure.
The first specimen had a maximum leakage of at normal operating conditions. Ten thousand of these joints would total less than of the 1.0 gom Technical Specification limit for unidentified RCS leakage.
At accident conditions, the leakage would increase to about of the Technical Specification limit.
The leakage from the second specimen was unusually high at normal operating conditions, decreased and then increased at accident loads. The roll in this specimen was found to be defective and the data from this specimen were disregarded.
3.3 Inspectability and Plugging Limit for Sleeves Even with the state-of-the-art ECT techniques, the inspection of the rolled transition zones where the sleeve / tube joints are made and the parent tubes at the sleeve's lower end, is difficult. The ability to detect through-wall defects in all regions of the sleeve and parent tube has been demonstrated usino existing ECT techniques. The ability to detect thrcugh-wall penetrations has been demonstrated for all regions of the sleeve / tube combination with the exception of the tube at the sleeve end.
The large signal produced by the inner diameter transition at the end of the sleeve masks the signal for the through-wall tube inspection.
. The licensee performed calculations to determine the minimum acceptable wall thickness for degraded sleeves (Reference 3).
They have proposed a plugging limit of 407, for both tubes and sleeves. We have reviewed these calculations and find the 40%
limit acceptable for all defects with the exception of circum-ferential cracks. The licensee has agreed to plug all sleeved tubes with circumferential cracks in the sleeve.
The minimum acceptable wall thickness for degraded sleeves was determined in accordance with the allowable stress and pressure limits of ASME Section III and NRC Regulatory Guide 1.121.
Primary membrane stress, burst pressure, and fatigue analyses were considered for normal operation, and primary membrane stress, burst pressure, collapse pressure, and primary membrane plus bending stresses were considered for postulated accident condi-tions.
In addition, primary plus thennal stresses were evaluated.
The minimum sleeve wall thickness was calculated for these eight different acceptance criteria.
For the expected type of defects, the limiting required minimum wall was found to be 0.0131 inch.
This thickness is necessary to resist collapse under the external pressure resultina from a loss of coolant accident (LOCR) and Thus, a represents 30% of the original or greater through-wall defect would require that the sleeve be removed from service. This compared to a defect limit for the OTSG tubes.
It is to be noted that allowances for ECT uncertainty and possible tube degradation between inspections have to be incorporated into these values to obtain the plugging limit specified in the Technical Specifications.
The sleeve must be bent and straightened for installation in the outermost OTSG tubes. This results in a slightly elliptical cross section, which was evaluated for buckling pressure. The maximum expected ovality (i.e., difference in extreme ODs at any one cross section) was found to be based on sample dimensions.
The critical external pressure depends on the material yield strength.
For material with yield strength of the critical external pressure is for the sleeve and for the tube, indicating that the s. eve can sustain about times the external pressure of %e t be. Under the maximum secondary pressure of v
, no primary pressure, neither tube nor sleeve would collane.
,he event water gets trapped between the OTSG sleeve and tube, the sleeve would beccme more elliptical or distort into a cross section with each successive heatup to accommodate the increased water volume.
Cont %ued one-way leakage, although unlikely, would eventually leave the sleeve subject to collapse in the event of a sudden loss of primary pressure such as a LOCA.
However, the annular pressure increase is more likely to blow out the corrosion products which plugged the leak than to collapse the sleeve. Thus, the likeliheed of sleeve collapse is very small.
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+ Th reduced section modulus of the expanded region has only a minor impact (
the tinimum wall thickness requirement from' for the 'unexpanded sleeve to for the roller expanded portion of the sleeve for the accident loads associated with a Main Feedwater Line Break plus Safe Shutdown Earthquake). ' The analysis shows the minimum wall thicknesses necessary to resist collapse _under the external pressure resulting from a LOCA (limiting event) are for the unexpanded portion of the sleeve and' for the roller expanded portion of the sleeve.
Both of these values correspond to a through-wall defect; The sleeve wall thickness required to satisfy the primary plus thermal stresses during postulated accident conditions also
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results 1_n a sleeve wall thickness requirement of about 30% wall thickness for all defects except circumferential cracks. To account for thermal loads, defect limits were based on B&W tensile tests (Reference 4) in which tube specimens with machined defects wore pulled to. failure by tensile fracture.
NRC staff evaluation of this test data indicates that defects including axial cracks through-walt would satisfy the acceptance criteria.
We are currently evaluating the generic implications of.this axial tensile load generated under accident conditions on the various aspects of circumferential cracks and the present 40%
plugging limit in the TSs for B&W OTSG tubes. Circumferential cracks at the 15th tube support plate (TSP) region were first observed at Oconee Unit 3 and have since been identified in several other B&W OTSGs.
In fact, for many B&W plants, this is often the only predominant type of tube degradation experienced. Although, for ANO-1, circumferential cracks in lane tubes of the upper tube sheet (UTS) and the 15th TSP are not a current problem, the B&W report indicates that even ANO-1 has experienced this type of cracking in the past. B&W has recommended plugging of all tubes with detectable circumferential cracks. We are discussing this issue with the B&V Owners Group and will recommend the necessary changes, if needed, to the At'0-1 tube plugging limit after a generic resolution of this issue is finalized.
3.4 Impact of Sleeving on the ANO-1 FSAR Safety Analyses The licensee has reviewed the impact on the ANO-1 FSAR safety analyses of sleeving 5,000 tubes per steam generator. The sleeving results in a slight reduction in heat transfer due to the air gap between the sleeve and the steam generator tube. There is also a small increase in the primary side pressure drop through the steam generator due to the smaller tube diameters.in the sleeved tubes.
Analysis by the licensee has also shown that the effect of installing 5,000 sleeves in each generator would be a reduction in steam superheat temperature of approximately 7.7 F at full pcwer and a reduction in primary flow of less than 1%.
This reduction in superheat requires an additional 1% full feedwater flow in order to remove the same a.nount of primary energy.
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. For overcooling events, the FSAR analysis assumed that the feedwater flow increased during the event in order to conservatively
. increase the heat removal by the steam generator. Therefore, sleeving does not impact the safety analysis since the heat removal due to a 1% increase in feedwater flow is conservatively bounded by the heat removal rates assumed in the analysis of overcooling events.
For some overheating events such as the loss of main feedwater, the FSAR analysis assumed that the heat transfer in the steam generator is significantly reduced. The slight reduction in heat transfer coefficient along the sleeved tubes is much smaller than the
. reduction assumed in the FSAR analysis and, therefore, does not impact the analysis assumptions. Other overheating events which are initiated in the primary system, such as the control rod withdrawal event, are not affected by sleeving since the initial heat transfer rate is held constant throughout the event and overall total steady state steam generator heat transfer is unaffected by tube sleeving.
Previous generic evaluations of the effect of steam oenerator tube plugoing have shown that there is a negligible impact on LOCA results due to a 2.5% reduction in RCS flow. The licensee has indicated that a similar magnitude of tube sleeving would result in only a 1% reduction in RCS flow. Therefore, the generic LOCA analysis accounting for tube pluggina conservatively bounds that which accounts for tube sleeving. The licensee has also specifically considered the concern of steam binding in the steam generator affecting the reflood phase of the large break LOCA and has found no impact due to tube sleeving.
Plant procedures reauire the licensee to measure primary system flow at the beginning of each fuel cycle to verify that the actual flow is in excess of that assumed in the plant safety analyses. These procedures also contain surveillance requirements to monitor primary flow several times per day. Therefore, any flow degradation due to sleeving would be detected to ensure that the existing plant safety analyses remain valid and bounding.
3.5 Conclusions Review of the additional corrosion tests performed by the licensee shows that corrosion propagation is not likely to increase significantly in the rolled sleeve,ioint during normal operation and wet lay-up. Other slower corrosion mechanisms such as corrosion due to cyclic stresses are also not likely to be aggravated by the sleeving process. The tube OD residual stresses from the rolled expansion and brazing process are low and are considered acceptable for sleeve design.
The results of the licensee's analysis indicate that the minimum required sleeve wall for normal and accident conditions is inch which permits sleeve defects less than through-wall.
The licensee's analysis is in compliance with the requirements of ASME Code Section III and NRC Regulatory Guide 1.121.
Allowing an
, additional margin of 10% for continued degradation and 20% for uncertainty in eddy current measurements, the licensee's proposed 40% plugging limit for sleeves is acceptable. However, the licensee has committed to plug any sleeved tubes with detectable circumferential cracks in the sleeve.
Based on a review of the results of the supplemental corrosion tests, sleeve plugging analyses and the sleeve tube qualification program reviewed earlier, we find sufficient justification to allow a large scale sleeving program at ANO-1.
Based on the above evaluations, the thermal-hydraulic effects of sleeving up to 5,000 ANO-1 steam generator tubes with 80-inch long sleeves per generator will have a minimal and acceptable effect on plant operation, and the existing FSAP. safety analyses will continue
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to bound normal and abnormal plant conditions.
l Therefore, on the basis of the above, we have determined that the proposed changes to the ANO-1 TSs are acceptable.
4.0 ENVIRONMENTAL CONSIDERATION
This amendment involves a change in the installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20.
We have determined that the amendment involves no significant 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 this amendment involves no significant hazards consideration and there has been no public comment on such finding. Accordingly, this amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 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 this amendment.
5.0 CONCLUSION
We have concluded, based on the considerations discussed above, that:
(1) there is reasonabi'e 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:
December 5,1986 Principal Contributors:
J. Rajan, G. Vissing, L. Kopp
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REFERENCES 1.
Letter from T. Gene Campbell, AP&L, to J. F. Stolz, NRC, requestino Technical Specification change to allow steam generator tube sleeving, dated April 1,1986.
2.
Letter from John F. Stolz, NRC, transmitting Amendment No. 86 to Facility Operating License No. DPR-51 for Arkansas Nuclear One, Unit 1, to John M. Griffin, AP&L, dated November 3, 1984.
3.
B&W Report 32-1147602-02 "177 OTSG Tube / Sleeve loads," in support of the 40% plugging criteria submitted as attachment to letter from J. Ted Enos, AP&L, to J. F. Stolz, NRC, dated October 14, 1986.
4.
Letter from J. Ted Enos, AP&L, to J. F. Stolz, NRC, dated October 23, 1986, including Attachment 1 (Rupture Test Data of Damaged Alloy 600 0TSG Tubing) and Attachment 2 (Collapse Test Data on Damaged Alloy 600 OTSG Tubing).
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