ML20212D409

Safety Evaluation Supporting Amend 201 to License DPR-51

ML20212D409
Person / Time
Site:	Arkansas Nuclear
Issue date:	09/14/1999
From:	NRC (Affiliation Not Assigned)
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ML20212D406	List: ML20212D400 ML20212D409
References
NUDOCS 9909230113
Download: ML20212D409 (7)

Text

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'2 WASHINGTON, D.C. 20666-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 201 TO FACILITY OPERATING LICENSE NO. DPR-51 ENTERGY OPERATIONS. INC.

ARKANSAS NUCLEAR ONE. UNIT NO.1 DOCKET NO. 50-313 4

1.0 INTRODUCTION

By letter dated June 1,1999, as supplemented by letters dated July 29 and August 19,1999, Entergy Operations, Inc. (the licensee), submitted a request for changes to the Arkansas Nuclear One, Unit No.1 (ANO-1), Technical Specifications (TSs). The requested changes would implement an alternate repair crite;ia that would allow steam generator tubes having axial tube end crack (TEC) indications in the upper and lower tubesheet to remain in service.

The technical basis for the alternate repair criteria is contained in the Babcock & Wilcox Owners Group Topical Report, BAW-2346, " Alternate Repair Criteria for Tube End Cracking in the Tube-to-Tubesheet Roll Joint of Once Through Steam Generators," Revision 0 (proprietary).

Framatome performed analysis and testing for the alternate repair criteria and prepared the topical report for the owners group.

The July 29 and August 19,1999, letters provided clarifying information that did not change the scope of the June 1,1999, application and the initial proposed no significant hazards i

consideration determination.

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2.0 BACKGROUND

in recent years, licensees have detected eddy current indications in the steam generator tubes near the upper tube end in once through steam generators (OTSGs) at some Babcock & Wilcox nuclear plants. The indications were characterized initially as tube end I

anomalies instead of cracking at the time of discovery because they were located above the primary face of the upper tubesheet and were considered outside of the portion of the tubes j

forming the primary system pressure boundary. Subsequent inspections have shown that some tube end anomalies have grown into the cladding region and, therefore, have been j

considered as inside the primary system pressure boundary. Subsequently, the tube end I

anomalies have been referred to as TEC indications.

The steam generator inspection data from various Babcock & Wilcox nuclear units have shown that TEC indications are initiated on the inside surface of the tubes and are typically short and axially oriented. They are usually located in the rolled portion of the tube near the heat affected zone created by the tube-to-tubesheet seat weld. The degradation mechanism has been 9909230113 990914 PDR ADOCK 05000313 P

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2-identified as primary water stress corrosion cracking, which is caused by residual stresses from the rolling of the joints in the tubesheet and from the seal weld.

The topical report delineates the following requirements and limitations for application of the proposed alternate repair criteria:

1. Under the alternate repair criteria, axially oriented indications in tubo ends located adjacent to the cladding region of the tube-to-tubesheet rolled joint, or in the portion of the tube protruding from the cladding will be allowed to remain in service without repair. " Adjacent to i

the cladding" refers to the portion of the tube that was rolled into the tubesheet cladding, as 1

opposed to the portion of the tube that was rolled into the carbon steel portion of the tubesheet.

2. The alternate repair criteria does not apply to tube ends with circumferential, mixed mode, or volumetric indications.

3. The alternate repair criteria does not apply tr :ubes with any portion of an axial TEC indication that extends into the carbon steel region of the tubesheet.

4. The alternate repair criteria will only apply to those tubesheets whose cladding thickness is less than 0.625 inch. The cladding at ANO-1 has been measured to be about 0.25 inch.

5. The combined total leakage from all primary-to-secondary sources, including TEC indications left in service shall not exceed the main steamline break accident leakage limit (1 gallon per minute (gpm) for ANO-1) minus operational leakage (150 gallons per day per steam generator for ANO-1). For tubes with multiple indications, a separate leak rate for each indication must be used.

6. A plant-specific analysis for detected TEC indications must be performed to establish a plant-specific growth rate or to verify the applicability of the generic growth rate in BAW 2346P.

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7. The total number of detected TEC indications must be increased to account for the probability of detection when projecting TEC indications for the next operating cycle. This l

increase is specified to assess the population of undetected flaws.

8. Site-specific inspections of rolled joints are required to identify locations and orientation of TEC indications. The identification of the TEC indications shallinclude tube location within the bundle and crack location with respect to the clad-to-carbon steel interface.

3.0 EVALUATION To determine the acceptability of the proposed alternate repair criteria, the staff evaluated structural and leakage integrity of the tubes with TEC indications, inspection methods, the proposed operational assessment, and TS wording.

l 3.1 Structural Intearity Each end of the tube in OTSGs is hard rolled into the upper and lower tubesheets, respectively.

The rolljoint is about 1 inch long inside the tubesheet and the tube end is protruded about 0.2 inch beyond the primary face of the tubesheet. Of the 1 inch length of the rolljoint, the cladding region of the tubesheet spans about 0.25 inch of that length and the rest of that roll length is spanned by the carbon steel region of the tubesheet. The upper and lower tubesheets provide a radial restraint to the hard roll portion of tubing, which precludes the possibility of tube burst at TEC locations. The tubesheet also limits the affected tubes from bending in the vicinity i

of the TEC indications. The potential failure mode, therefore, would only be associated with the tensile (axial) loads, which may cause the tube to fail within the rolled joint. The structural integrity is therefore dependent on the axial load carrying capability of the rolled joint and/or welds.

The structuralintegrity of the tube is maintained if the weld can be demonstrated to resist the axial tube loads without assistance from any frictional forces that may be available from the hard roll joint. Framatome has used a finite element analysis to qualify the structural integrity of the original fabrication fillet weld and the flush weld used to repair tube end damage at Crystal River 3 and Oconee Unit 1. The analysis assumes that the hard roll joint does not carry any portion of the tube loads. The analysis evaluated the welds using loads from normal operating transient conditions and accident conditions. It also considered allloading mechanisms including primary and secondary pressures, tube axialloads, and weld dilations. The analysis showed that the original fillet weld and the flush repair weld satisfy the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Section Ill, for Class I components.

Based on Framatome's testing and the analysis performed, the staff finds the structuralintegrity of the TEC tubes acceptable.

3.2 Leakaae Intearity Framatome demonstrated the leakage integrity by leak testing. Framatome performed leak testing to determine leak rates for rolled joints with through-wall axial cracks of v&rious lengths 1.

for both steady-state power and bounding accident transient conditions. Leak rates at l

steady-state power conditions were determined to compare with the allowable primary-to-t secondary leakage limit. The leakage at accident conditions was determined to compare with allowable off site dose leak limits.

Before leak testing, Framatome used a finite element model to analyze the structural behavior of the tubes to determine test parameters that would produce a weaker test roll joint, which in turn, would give maximum possible leak rates in the leakage test. The analyzed conditions included heatup, normal operating conditions, main steamline break, and small break loss-of-coolant accident. Framatome found that maximizing tube thickness, yield strength of

- the test tube and tubesheet bore diameter and minimizing the yield strength of the tested tubesheet, cladding thickness and roll joint torque would give the worst case leak rates in the leakage test.

Framatome fabricated 15 mockups to perform leakage tests. Each mockup included the simulated tubesheet with actual tubes rolled into the mockup block to form roll joints. The end of the test tubes is sealed with a fillet weld. Each test tube had a through-wall notch to

. represent a TEC indication. The rnockups were subjected to dilation to simulate steam generator conditions followed by axial load cycling to simulate the 40 years of plant operation.

The leakage rates for normal power operation and accident conditions were evaluated to determine the leakage integrity of TEC tubes. Framatome established the leak rates based on radial position of the tube in the tubesheet, axial tube loading, and tubesheet hole dilation in order to compensate for the tubesheet bowing. The leak rates that will be applied to TEC indications to calculate the total leakage were established based on the statistical analysis of leak rates obtained in the leak tests. In the statistical analysis, Framatome applied a 95/50 confidence limit (95 percent confidence level for 50 percent of the leakage population) to the leak rates obtained in the leak tests. The leak rates in the tests were very small compared to the leakage limits of 150 gallons per day for normal operation and the limit under accident conditions of 1 gpm.

Based on the Framatome's leakage testing, the staff finds that tubes with TEC indications have adequate leakage integrity.

3.3

[nspection Methods The licensee stated that during each future inspection a rotating pancake coil will be used to insp9ct all hard roll expansions with known TEC indications, which were identified in inspections before the proposed amendment. The objective of inspecting previously known TEC indications is to monitor their growth from cycle to cycle. The growth is measured in terms of the distance between the tip of the TEC indication and the cladding to-carbon steel tubesheet interface. The depth of the indications will not be measured because the licensee assumes that any detected axial TEC indication is 100 percent through-wall.

In future steam generator tube inspections, the licensee will inspect, on a sampling basis, those hard roll expansions in tube ends that have no previously known TEC indications using a rotating pancake coil. The inspection of these hard roll expansions will be based on EPRI

[ Electric Power Research Institute) Steam Generator Examination Guidelines, Revision 5. EPRI specifies a 20 percent sample of the tubes to be inspected during each of the subsequent inspections. The objective of inspecting these hard roll expansions is to identify new TEC indications. New TEC indications will be included in the surveillance list and will be inspected in subsequent inspections.

An inspection technique has been developed and qualified to locate axial TEC indications relative to the cladding-to-carbon steel interface. The licensee will use existing qualified techniques for flaw detection and orientation determinations. Framatome has qualified the inspection technique consistent with the intent and protocol of the EPRI Steam Generator Examination Guidelines, Revision 5. In addition, the rotating pancake coil (e.g., plus point coil) has been qualified in accordance with Appendix H of the EPRI Steam Generator Examination Guidelines, Revision 5, for detecting axial primary water stress corrosion cracking.

The staff finds that the licensee's proposed inspection methods and scope are acceptable.

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'5 3.4 Operational,5sessment The alternate repair criteria specify that at the end of each inspection, the licensee will perform an operational assessment to ensure that the estimated leakage from projected TEC indications at the end of the next operating cycle will not exceed the leakage limit under accident conditions. To project the TEC indications at the end of next cycle, the licensee will apply a probability of detection to the identified TEC indications from the current inspection.

The application of the probability of detection willincrease the total TEC indications to account for non-detected indications. The projected number of the TEC indications is calculated by dividing the number of detected TEC indications by the probability of detection and subtracting those TEC indications that have been plugged or repaired.

To project the total tube leakage, a leak rate is applied to each TEC indication according to its radial position within the tubesheet. The leak rates from all TEC indications are summed to obtain the total TEC leak rate, which is combined with all other known sources of primary-to-secondary leakage. The total projected leakage is compared to the accident leakage limit of 1 gpm. If the projected leakage exceeds the limit, the alternate repair criteria requires the licensee to repair TEC tubes until the leakage limit is satisfied.

During its review, the staff questioned the conservatism of Framatome's use of the 95/50 confidence limit in calculating the leak rate and in calculating a probability of detection that is derived based on a 90 percent confidence level. The 95/50 approach differs from the 95/95 confidence limit used in the Generic Letter (GL) 95-05 methodology for a total steam generator leak rate. The staff requested that the licensee address the conservatism of its total leakage calculation relative to a totalleakage calculated at a probability of 0.95 to be an upper bound evaluated at the 95 percent confidence level.

The licensee stated that its leakage calculation methodology is conservative relative to the methodology in GL 95-05 because leakage tests were conducted conservatively in that (1) the testing parameters were used in the leakage tests to give the maximum leak rate, (2) the simulated flaws in the test are all 100 percent through-wall, which is conservative compared to the actual TECs in the field, and (3) in the leakage calculations, all TEC indications are assumed to be leaking, which is more conservative than actual field conditions.

Based on the staff's suggestion, the licensee did perform a total leakage calculation to obtain a 95/95 estimate using the methodology in GL 95-05. The results showed that the GL 95-05 methodology estimated a lower total leak rate of a steam generator than the total leak rate estimated by the~ methodology in the topical report. The staff believes that the methodology in GL 95-05 is a realistic bounding approach to leakage calculation. The methodology in the topical report provides even more conservative results than the GL 95-05 calculation and, therefore, is acceptable.

The licensee will evaluate and monitor the growth rate of TEC indications during each inspection to confirm that growth of TEC indications is insignificant. The length of the indication in the cladding is not of concern as long as the identified TEC indication does not grow and extend into the carbon steel region of the tubesheet. Current inspection data indicates that the growth rate of TEC is insignificant.

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V. The staff believes that the inservice TEC indications do not significantly increase the rir.k associated with steam generator tube failure because the upper and lower tubesheets provide sufficient restraint to preclude tube burst under normal and postulated accident conditions, and

. tube leakage will be limited to the limit of 1 gpm in accordance with methodology specified in

. the topical report.

The staff finds the licensee's proposed operational assessment acceptable.

. 3.5 Technical Soecification (TS) Chanoes TS 4.18.3.a.4: The licensee proposed to add the following requirements to this TS section:

" Tubes with axially-oriented tube end cracks (TEC) which have been left inservice for the previous cycle shall be inspected with a rotating coil eddy current technique in the area of the TEC and characterized in accordance with topical report BAW-2346P, Rev.0, during all subsequent SG [ steam generator] inspection intervals pursuant to [TS] 4.18.4. The results of l

this examination may be excluded from the first random sample. Tubes with axial TECs

' identified during previous inspections which meet the criteria to remain in service will not be included when calculating the inspection category of the OTSG."

TS 4.18.5.a.7: 'This section defines the plugging limit. The licensee proposed the following requirements: " Axially-oriented TEC indications in the tube that do not extend beyond the adjacent cladding portion of the tube sheet into the carbon steel portion are not included in this definition. These indications shall be assessed for continued plant operation in accordance with topical report BAW-2346P, Rev. 0."

TS 4.18.5.b: The licensee proposed to specify that through-wall TEC indications meeting the alternate repair criteria may be remain in service.

L TS 4.18.6: This section specifies the reporting requirements for results of steam generator tube inspections. The licensee proposed the following additional item to be included in the inspection report: " Summary of the condition monitoring and operational assessment results

. when applying TEC alternate repair criteria." The licensee also proposed to change the current L 45 days to 90 days for submitting steam generator inspection reports.

Bases: The following paragraph is added to the Bases section: "When the tube end cracking (TEC) alternate repair criteria is applied, axially-oriented indications found not to extend from i

the tube sheet cladding region into the carbon steel region may be left in service under the guidelines of topical report BAW-2346P, Rev. O. Following a SG inspection, an operational i

assessment is performed to ensure primary-to-secondary leak rates will be maintained within l.

l the assumptions of the accident analysis."

The staff finds that the proposed changes are acceptable based on the review of the information submitted. The staff finds the 90-day reporting period acceptable because the extended period of time would accommodate the preparation of condition monitoring and operational assessment reports describing TEC indications.

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3.6 Evaluation Conclusion The licensee demonstrated that the steam generator tubes having TEC indications that remain in service in accordance with the proposed alternate repair criteria will maintain adequate structural and leakage integrity. The staff concludes that the proposed changes to the technical -

specifications to implement the proposed alternate repair criteria are acceptable.

4.0 STATE CONSULTATION

in accordance with the Commission's regulations, the Arkansas State official was notified of the proposed issuance of the amendment. The State official had no comments.

5.0 ENVIRONMENTAL CONSIDERATION

The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes surveillance requirements. The NRC staff has 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 the amendment involves no significant hazards consideration, and there has been no 1

public comment on such finding (64 FR 35205 dated June 30,1999). Accordingly, the 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 environmentalimpact statement or environmental assessment need be prepared in connection with the issuance of the amendment.

6.0

~ CONCLUSION The Commission has 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, (2) such activities will be conducted in compliance with the Commission's regulations, and (3) the issuance of the amendment will not be inimical to the common defense and security or to the health and safety of the public.

Principal Contributor: John Tsao Date: September'14, 1999 l

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