ML20064E993

SG Regulatory Guide 1.121 Evaluation

ML20064E993
Person / Time
Site:	Crystal River
Issue date:	03/04/1994
From:	FLORIDA POWER CORP.
To:
Shared Package
ML20064E908	List: 3F0394-02, Forwards TS Change Request 198,requesting Amend to License DPR-72,proposing Interim Rev to Repair Criteria Contained in OTSG Tube Surveillance Program in TS 5.6.2.10.4.a ML20064E903 ML20064E976 ML20064E993
References
RTR-REGGD-01.121, RTR-REGGD-1.121 NUDOCS 9403150115
Download: ML20064E993 (110)

Text

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Florida Power

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. O CRYSTAL RIVER UNIT #3 STEAM GENERATOR REGULATORY GUIDE 1.121 EVALUATION i

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i TABLE OF CONTENTS '

SECTION PAGE  ;

1.O INTRODUCTION 1 '

2.0 PURPOSE 3

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3.0

SUMMARY

OF RESULTS 4 4.0 OTSG DESIGN 7 5.0 STRUCTURAL ANALYSIS 13 6.0 GROWTH STUDIES 20 7.O EDDY CUR. RENT CORRELATIONS '26 8.0 REVISED REPAIR LIMITS FOR S/N INDICATIONS 40 9.0 LEAKAGE CONSIDERATIONS 52 i 10.0 ANALYST GUIDELINES FOR S/N INDICATIONS 53 -

t 11.0 BWOG TUBE INTEGRITY PROGRAM 57 12.O CONCLUSIONS 58' 13,0 REFERENCES 59 APPENDIX'A: MPR STRUCTURAL ANALYSIS i

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List-of Figures

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1. OTSG longitudinal section

2. OTSG Tube sheet map

3. OTSG broached hole tube support plate

4. Historical Burst Test Data Burst Pressure Parameter Curves i

5. Comparison of Indications on pulled tubes to inservice S/Ns

6. EPRI Voltage to Indication through wall correlation ,

7. Palisades Voltage to Indication through wall correlation

8. Voltage to Volume Correlation for Various IGA indications

9. Accuracy of Axial sizing using RPC

10. Accuracy of Circumferential sizing using RPC

11. Allowable tube wall penetration for axial slot type defects

() 12. Maximum allowable tube wall penetration versus circumferential extent of defects

13. IGA voltages compared to allowable axial indication size

14. IGA voltages compared to allowable circumferential size

15. Estimated size of sample inservice S/N indications

16. Revised Analyst Guidelines

17. Analyst Guidelines for Non Quantifiable Indications f

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List of Tables

1. MPR Calculated Dimensional Limits

2. CR#3 Burst Test Result.s r

3. Projected Burst Pressures for MPR Calculated Defect Dimensions

4. S/N Growth Study 1

5. B&W Owners Group NDE Committee IGA Work

6. Palisades IGA Samples >

7. Axial and Circumferential Sizing Accuracy Evaluation 9

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1.O INTRODUCTION Analysis of eddy current data for the April 1990 and May 1992 i refueling outages (RFO) at Crystal River Unit 3 revealed a  !

significant number of indications in the first span of the "B" Once Through Steam Generator (OTSG). These indications are primarily low amplitude signals with low signal-to-noise (S/N) ratios (<5:1)1, that under plant-specific eddy current analysis guidelines were dispositioned as S/N and left in service.

In addition to the indications in the first span of the "B" steam generator, eddy current inspections have also identified a number of S/N indications at other locations in the Crystal River Unit 3 steam generators. A list of all indications documented as S/Ns was submitted to the NRC via Reference 1.

To investigate S/N indications further, seven (7) tubes with representative indications were pulled from the CR-3 "B" OTSG during the 1992 RFO. .These tubes have been subjected to detailed examination under EPRI Project RPS 413-06. The laboratory examination included eddy current testing, tube burst testing, and destructive examination to determine actual defect locations .

and depths. The objectives of the laboratory examination were:

1. Physically characterize any tube degradation, particularly damage associated with low S/N indications in the i O boiling / free span regions, for correlation with field non-destructive examination.

2. Obtain burst pressure data to determine the effect of defects on the structural integrity of the tubing.

3. Attempt to establish the damage mechanism responsible for 3 the eddy current indications.

4. Establish correlations between plant' chemistry trends and the degradation observed.

The laboratory examination identified small patches on the ,

outside diameter of the pulled tubes which were classified as very small " pit-like" intergranular attack (IGA). Burst pressure testing of tubes-demonstrated that tubes with pit-like intergranular attack have significant margin above Regulatory Guide 1.121 requirements.

I Selection of 5:1 as the S/N ratio below which noise is considered ,

too great to make accurate sizing estimates is intended to limit the error associated with - through wall sizing of indications to 10 percent or less. 10 percent is generally accepted as the allcaable eddy current technology sizing error band. 3 O 1

. FPC' met with members of the NRC staff on September 9, 1993 in Rockville, Maryland to discuss the results of the pulled tube analysis. A draft report summarizing the results of the examination was submitted to the Nuclear Regulatory Commission at that' time. The final draft of this report is being submitted to the NRC under separate cover.

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2.0 PURPOSE On December 20, 1993, FPC received a request for additional [

information (RAI) . as a followup to the September 9, 1993 meeting ,

regarding CR#3 Steam Generator Tube Examinations. The RAI .;

requested that FPC submit a Regulatory Guide 1.121 evaluation that justifies indications similar to the pit-like indications >

found during Refuel 8 are acceptable. This report documents the evaluation done to determine the maximum allowable tube wall degradation for OTSG tubing in accordance with (draft) Regulatory Guide 1.121 and provides a comparative discussion relative to the S/N indications observed at Crystal River Unit 3. This report also addresses the non-destructive examination methods which will be used to identify tubes with limiting defects and provides the basis for a revision to the existing Technical Specification '

plugging limit.

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3.0

SUMMARY

OF RESULTS The existing Technical Specification repair limit is solely based ,

on the percent through wall penetration of an indication.

Determination of percent through wall penetration using standard bobbin coil phase angle methodology can be inaccurate for very small volume indications (S/Ns). Therefore, development of an alternative method for dispositioning S/N indications is necessary. This alternative repair limit should be conservative such that it not only ensures structurally significant indications are removed from service, but also ensures that indications which could potentially result in leakage are-identified.

CR#3 has performed an extensive investigation of S/N type indications. This investigation has thus far included removal of portions of seven tubes from the "B" steam generator, burst testing of tube samples with multiple S/N indications, and review of eddy current historical data. Examination of pulled tubes revealed the source of the S/N indications to be very small volume pit-like IGA with no associated cracking. Burst testing demonstrated a margin of safety above Regulatory Guide 1.121 limits for the tubes. Review of eddy current bobbin and Rotating Pancake Coil (RPC) historical data for S/N indications resulted in the following findings:

1. S/N type indications in the CR#3 steam generators can be characterized as small, volumetric indications.

Conservative estimation of the axial and circumferential size of S/Ns from available RPC clip plots indicates that  ;

most are similar in size to the-very small pit-like IGA which was identified in the first span of the "B" steam l generator. No crack-like indications were identified during review of RPC records-for S/Ns. -This is consistent .

with OTSG operating experience. No cracking has been observed in any OTSG outside the open lane / wedge region.2 l

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Preventive upper tube sheet sleeves will be installed in the lane / wedge region of the CR#3 steam generators during Refuel 9. ,

This region is the only region which has been shown to be t susceptible to cracking by 0TSG. field operating experience.

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( 2. Review of eddy current bobbin examination historical data indicates that the S/N type indications in the Cr#3 steam generators are not growing between inspections. Review of ,

data for "B" steam generator first span indications was  !

performed by both BWNS and the EPRI NDE center. The single conclusion of both independent reviews was that there is no evidence of growth. BWNS subsequently performed a side by side comparison of historical eddy current signals for S/N .

indications outside the "B" first span. The conclusion of l this review was that no significant growth rate was observed.

A structural analysis has been performed by MPR Associates, Inc.

to determine the maximum allowable indication size which can be present on an OTSG tube while maintaining the margin of safety established by Regulatory Guide 1.121. This analysis is very conservative for all potential defect types and locations. Based ,

on the MPR analysis, the axial and circumferential extent of ,

indications must be limited to 0.25 inch and 120 degrees for 100 percent through wall penetration. Operating experience with no primary to secondary leakage over several cycles since S/Ns were ,

first observed indicates a low probability that 100 percent through wall indications are present. Therefore, establishment of a 0.25 inch axial and 120 degree circumferential limit on indication size is conservative. Axial and circumferential size of indications can be conservatively estimated using RPC clip '

O^ plots.

Examination of indications using RPC requires a great deal of i time due to the low pull speed of the RPC probe. Since RPC clip plots are required to estimate the axial and circumferential size  ;

of indications, it is desirable to establish a screening criteria which would be as conservative as the dimensional limit criteria but would not require performance of RPC prior to its application. Signal amplitude (i.e. voltage) has been shown by ,

experience in recirculating steam generators to be a function of indication size, which can then be directly related to. tube structural integrity. Therefore, application of an eddy current bobbin coil voltage limit below which the requirement for RPC can be waived is a reasonable approach. Eddy current correlations for IGA grown by both Consumers Power Corporation and the B&W:

Owners Group NDE Committee indicates that bobbin coil voltages of up to'6 volts can be observed during examination of indications having dimensions deemed acceptable on OTSG tubing per MPR's'OTSG tubing structural analysis. In consideration of NDE i uncertainties, data scatter, the potential for growth between inspections, and the need to minimize the-possibility of leakage, the. recommended voltage limit is reduced to 2 volts. S/N indications which have been identified previously and have bobbin coil. voltages less than or equal to 2 volts will be considered imperfections and documented as non quantifiable signals (NQS). ,

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J S/N indications dispositioned as NQS will be reinspected each outage for purposes of tracking and trending. S/N indications O. less than or equal to 2 volts which have not been identified in previous inspections will still receive RPC inspection to. ensure I they are volumetric in nature and to provide assurance that application of.the voltage limit will not screen any new type of damage mechanism which might be present. S/N indications above 2 volts will be examined with RPC and the dimensional limit applied to disposition the indication.

Eddy current analyst guidelines will be revised prior to refuel 9 to provide guidance to analysts as to how the new repair limits are to be applied to S/N type indications. Indications with- ,

signals greater than five times their background noise will be l dispositioned by applying the existing 40% through wall criteria.

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4 4.0 OTSG DESIGN T.he Once Through Steam Generator (OTSG) is a straight-tube, str&ight-shell heat exchanger as shown in Figure 1. Heated react 7r coolant enters each OTSG through a single 36 inch ID verticsl nozzle in the. upper head, flows downward through more than 15,000 tubes, transfers heat to the secondary water, and '

exits through two 28 inch ID outlet nozzles in the lower. head.

On the secondary side, feedwater enters a 14 inch id toroidal header and is distributed through 32 nozzles, which spray the water downward into an annular feedwater heating chamber between.

the lower shroud and shell. The feedwater is heated to saturation by direct contact condensation of steam aspirated from the tube bundle through bleed ports in the shroud. The saturated feedwater flows downward in the annulus, entering the tube bundle through ports in the lower shroud. The water begins to boil '

immediately as it flows upward, becoming saturated steam and then superheated steam before exiting the tube bundle in the upper tube span, which is located between the 15th tube support plate and the upper tube sheet. The superheated steam is routed downward in the annulus between the upper shroud'and the shell, finally exiting through two 24 inch steam outlet nozzles.

O Each OTSG contains 15,531 Inconel 600 tubes, 0.034 inch wall, 56 ft. 2-3/8 inches long, rolled and sealed-0.625 inch OD, a

welded into 24 inch thick carbon steel tube sheets at the top and bottom. Following fabrication, the CR#3 OTSGs were subjected to a full-furnace stress relief at 600 to 620 C to reduce residual stresses. This step in the manufacturing process resulted in a sensitized tubing microstructure. Tubes are arranged in a tri-angular pattern distributed over the entire cross section of the OTSG. Half of row 76 was left untubed and is referred to as the "open tube lane" (see Figure 2). Tubes are stabilized by fift.een support plates with tri-lobed holes which allow upward flow.

(Figure 3).

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4.1 SUSCEPTIBILITY OF OTSG TUBING TO DEGRADATION There are several design features of the OTSG which makes the OTSG less susceptible than recirculating steam generators to certain types of degradation:

1) Sensitization of Inconel 600 tubing in OTSGs makes the tubing less susceptible to caustic forms of corrosion.

2) The broached hole design of the OTSG tube support plates allow more flow than a drilled tube support plate.

Therefore, there is less small crevice area where contaminants can collect and initiate corrosion.

3) OTSGs also have lower heat fluxes in areas where deposits, which can initiate corrosion, are most predominant (Fourth and Fifth Tube Support Plate region). The lower temperature differential in this region provides less concentrating mechanism to initiate under deposit corrosicn; and

4) No large, hard sludge piles which can lead to corrosion have been observed at CR#3. Visual inspection of the CR#3 lower tube sheet secondary face indicates the presence of a very loose flake pile.

O. Corrosion-assisted high-cycle fatigue cracking is the primary cause of leaker outages in 0TSGs. This type of damage mechanism was first confirmed on tubes removed from Oconee 1 and has been identified at other once-through units. .The cracking is limited to the lane and wedge regions of the OTSG at elevations between the 14th TSP and upper tube sheet. CR#3 has never experienced a tube leak due to corrosion assisted high cycle fatigue. The area of the OTSG considered most susceptible to this damage mechanism is routinely inspected using RPC probes, with any crack-like indication removed from service. During Refuel 9, CR#3 will be installing upper tube sheet sleeves in tubes considered most susceptible to corrosion-assisted high-cycle fatigue. This will further eliminate the possibility for problems at CR#3 related to this damage mechanism.

Wear or tube fretting is flow related and has occurred at tube support plate intersections in tubes near the periphery of the tube bundle. Wear may be flat or tapered, and may occur at one or more of the broached tube support plate contact areas. It is generally detected at the 9th through 15th tube support plates.

Wear indications have been observed in CR#3's "A" and "B" steam generators.

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~ p. Impingement Erosion is an erosion / corrosion type. phenomenon- 1 LN_/ occurring from the flow of secondary side' water with contaminants-or debrisfentrapped in the secondary side. water fluid steam. j This type of damage generally occurs at the 10th tube support plate and above near the outer periphery.

Per reference 7, minor pitting has also been identified on' tubes removed from OTSGs.

t Intergranular Attack has been observed on tubes removed from four  ;

different OTSG plants. As stated in section 1 above, intergranular attack was determined to be the cause of S/N indications detected in the first span of the CR#3 "B" steam generator. The sensitized Alloy 600 microstructure of'OTSG tubing is susceptible to IGA in acidic solutions containing -

reduced culfur oxyanions, even when the sulfur is present in small quantities.

ECT indications observed at CR#3 predominantly indicate secondary  :

side damage, typically characteristic of wear, pitting, or. IGA.  !

It is important to note that no problems with outside diameter '

stress corrosion cracking (ODSCC) have been identified at CR#3 or any other operating OTSG plant.

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5.0 STRUCTURAL ANALYSIS

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5.1 MPR EVALUATION A structural evaluation of maximum allowable OTSG tubing degradation was performed by MPR Associates, Inc. The report of-this work is contained in Appendix A. MPR also provides a point by point discussion of the Regulatory Guide-requirements as well as a conservative ASME Code Stress analyses.

Table 2-1 and Figures 1 and 2 of Appendix A provide the results of the structural analysis, which is conservative and bounding for any damage mechanism and defect geometry. Based on these results, it can be concluded that defects with dimensions less than or equal to the following are allowable per Regulatory Guide 1.121.

TABLE 1: MPR Calculated Dimensional Limits Axial Size Through-Wall Circumferential Depth Size 0.25 inch 100% 122 degrees 0.5 inch 65.6% 187 degrees

() 0.75 inch 1.5 inch 62%

60.3%

198 degrees 204 degrees The largest dimensional extents identified by metallographic '

examination of tubes removed from the CR#3 "B" steam generator in 1992 were 0.097 inch axial, 62% through wall, and 15.2 degrees circumferential. These dimensions were documented on three -

separate indications. Therefore, the actual size of degradation which resulted in eddy current signal to noise indications on pulled tubes examined following Refuel 8 is significantly smaller than those determined by MPR to be structurally significant.

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5.2. BURST TESTS-OF CR-3 PULLED TUBES ,

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Two of the CR-3 tube samples were subject to burst testing prior to metallographic examination. The tube samples were 106-32 and 97-91. These tube sections contained a combined total of approximately 80 indications. Tube 97-91. contained indications.

with axial. extents up to 0.0756 inches, through wall depths up to 54%, and circumferential extents up to 15.2 degrees. Tube 106-32 contained defects with axial extents up to 0.0979 inches, through-wall depths up to 51%, and circumferential extents up to 9.7 1 degreee. The burst test pressures and location of failure are summarized below, t

TABLE 2: CR#3 Burst Test Results Tube No. Number of Burst Pressure Defect Depth Indications at Burst-97-91 17 12,400 psi 54% .

106-32 65 11,400 psi '40%

Both burst pressures are well above the Regulatory Guide 1.121

• limit of 3 times operating differential pressure (3 x 1350 psid'-

4050 psid). The burst pressure of unflawed OTSG tubing has been determined by testing to be approximately 13,000 psi, so the ,

burst pressure of both defected tubes were above 85%fof the virgin. tube burst pressure.

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l 5.3 HISTORICAL BURST TEST DATA COMPARISON Tube burst pressure testing was performed as part of the Steam Generator Tube Integrity Program (SGTIP) / Steam Generator Group Project (SGGP). This three phase project was sponsored by the NRC and conducted by Pacific Northwest Laboratory, and is documented in Reference 2. The goal of the project was to provide the NRC with validated information on the reliability of nondestructive examination techniques to detect and size flaws in generator tubing and to determine the remaining integrity of service-degraded tubing.

The types of defects simulated during the SGTIP included wastage / pitting, uniform thinning, and axial cracks. These ,

defects were both mechanically produced (Phase I) and chemically induced (Phase II). Data from burst pressure tests were used to develop empirical relationships between burst pressure, defect length, and defect depth. In addition, tubes pulled from the Surry 2A steam generator were used to verify the empirical models (Phase III).

This testing resulted in the development of burst pressure parameter curves similar to the one shown in Figure 4. The normalized burst pressure on the y-axis is the ratio of the burst pressure of the defected tube to the burst pressure of a virgin-Ox tube. The normalized defect length on the x-axis.is a ratio of-defect length to the square root of the radius of the tube times the tube thickness. Each curve represents a specific defect depth, expressed as the ratio of depth to the tube thickness.

Additional data of interest is also shown on Figure 4 for comparison to the empirical relationships. The burst test data from pulled Surry tubes reported in Reference 2 is shown. The flaw lengths at the failure location ranged from 0.06" to 0.53" with a percent through wall range ot 24% to 80%. Burst pressures ranged from 62% to 97% of the virgin tube pressure. Also represented on the figure is the Regulatory guide 1.121 limit of 3 times operating differential pressure for OTSG tubing.

Based on the maximum defect length of 0.097 inches reported from the destructive examination of CR-3 pulled tubes, the normalized length for the CR-3 tubes is shown on Figure 4. Since these indications are very small, it would be expected that the burst' pressure is a high percentage of the virgin tube burst pressure.

This is supported by the burst pressure testing of the two CR-3 samples summarized in the previous section.

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() It-is also apparent that the CR-3 tubes fall into the area of the.

curve where burst pressure is not strongly related to the depth of the defect. For this axial size indication, even a'100%

through wall defect would be expected to burst at a pressure ,

above 70% of the virgin tube pressure (more than 7 times  !

operating differential pressure). It is therefore concluded that ,

the~ degraded tubes at CR-3 satisfy the Regulatory Guide 1.121 limits for burst pressure.

Figure 4 can also be'used to project burst pressures for defects determined by MPR (section 5.2) to be allowable on OTSG tubing.

TABLE 3: Projected Burst Pressures for MPR Calculated Defect Dimensions Axial Size Through-Wall Projected Depth Burst Pressure _

0.25 in 100% 4,940 psi  ;

0.5 in 654 6% '6,370 psi 0.75 in 62% 5,720 psi 1.5 in 60.3% 5,200 psi ,

O A's shewn on Table 3, the projected burst pressures for defects ,

determined by MPR to be allowable on OTSG tubing are well above the Regulatory Guide 1.121-limit of three times operating differential pressure, 4050 psi. ,

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Figure 4: Historical Burst Test Data Burst Pressure flarameter Curve

O 5.4 COMPARISON OF PULLED TUBES WITH REMAINING TUBES WITH S/N h INDICATIONS Figure 5 shows a comparison of eddy current signals from indications on pulled tubes with the remaining S/N eddy current signals on CR#3 inservice tubes. This figure plots the amplitude versus phase relationship for indications. It can be seen from this plot that indications on the pulled tubes exhibit a phase / amplitude relationship typical of the majority of the remaining indications on inservice tubes.

Review of RPC for S/N indications outside the first span shows similar volumetric characteristics with no crack-like indications observed. This is consistent with OTSG operating experience.

OTSGs have not experienced a problem with any type of cracking outside the lane / wedge region which is examined using RPC to detect cracking. (NOTE: The CR#3 lane / wedge region considered most susceptible to corrosion assisted high cycle fatigue cracking will be sleeved during Refuel 9.)

Estimation by RPC clip plot of the axial and circumferential size of pit-like indications on tubes removed from the CR#3 "B" steam generator resulted in an average size range from 0.11 to 0.19 inch axial and from 0.14 to 0.25 inch circumferential.3 Estimation of size for inservice S/N indications for which RPC data is available resulted in an average size range from 0.0625 to 0.20 inch axial and from 0.1 to 0.22 inch circumferential.

Therefore, reasonable assurance exists that the remaining S/N indications in the "A" and "B" steam generators are approximately the same size as the very small volume pit-like IGA indications found in pulled tubes which have been determined to be structurally insignificant.

3 As noted in section 7.5, use of MRPC clip plots to estimate axial and circumferential size of indications is conservative. When comparing the estimated axial and circumferential size of indications on pulled tubes to the actual sizes determined by metallographic examination of the indications, the estimated size is conservatively over estimated. The actual' average size range of the ,

indications on pulled tubes ranged from 0.01 to 0.075 inch axial and from 0.01 to 0.07 inch circumferential.

18

O O O AMPLITUDE VS. PHASE RELATIONSHIP CR-3 PULLED TUBES VS. REMAINING w

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m .; . .  :- : ; n.. -

o o o o o do lo s'o o'o ibo iko do do too 200 PHASE ANGLE, DEG.

EJ ALL S/N INDIC. M PULLED TUBES Figure 5: Comparison of S/N indications on pulled tubes to Inservice S/Ns

6.O GROWTH STUDIES 6.1 EPRI NDE CENTER REVIEW The EPRI NDE Center performed an independent review of eddy .

current data for selected tubes examined during three previous I inspections to assess growth of IGA indications observed in the first span of the "B" steam generator. Per Reference 3, any growth or active damage form typically results in higher eddy current signal amplitude, accompanied by reduction in the phaie angle. The net result is the overall increase in percent wall loss. Evaluation of data for CR#3 first span indications showed just the opposite effect. Decreased signal amplitude was observed, with only a slight increase in. percent wall loss. The increase in percent wall loss was considered well within the sizing error band. Based on this result, EPRI concluded that the first span IGA patches have not grown since they were first detected.

6.2 BWNS REVIEW In. order to assess the growth rate of the tube flaws at CR#3, a O growth study was performed on indications from the past three eddy current inspections -(1989, 1990, 1992). This study is documented in Reference 10. A side by side comparison of eddy current signals was performed.

For the comparison, all data was normalized to 4 volts on the four 20% through wall flat bottom holes in the ASME calibration -

standard, using the 400 kHz frequency channel. The results are reproduced in Table 4 of this report for freespan and tube support plate indications. From Table 4 it can be seen that when all freespan indications are considered together, the average change in signal amplitude is +0.01 volts, with a standard deviation of 0.11 volts. The freespan indications therefore do not show evidence of growth, which is consistent with the conclusion reached for first span indications, documented in Reference 3.

The results from the growth study performed on support plate indications is also shown on Table 4. The average change in amplitude for these indications is -0.19 volts, with a standard deviation of 0.30 volts. The largest positive growth over one inspection cycle was 0.20 volts. The larger standard deviation associated with the support plate signal comparison is attributed to the variation in contribution of the support plate signal to the total signal response.

1

. ... . - _ .- . .. . ._ - .. _ = _ - - . .

l Based on growth studies which have been performed by.BWNS, it can be said that neither the freespan nor the support plate indications'have significant growth rates in the CR#3 steam generators. However, based on the relatively'large standard' deviation from the tube support plate indication study, an allowance for growth is recommended in the. revised plugging criteria. .Using statistical rules, a 99% confidence level can be-assumed when a factor of three is-applied to the' standard  ;

deviation. Therefore, the recommended growth allowance'for these ~I CR#3 indications is'3 x 0.30 volts, or 0.90 volts. ,

_. h a

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y TABLE 4: S/N Growth Study (Sheet 1 of a)

CR-3 GROWTH EVALUATION OF FREESPAN INDICATIONS 1989 1990 1992 DELTA RATIO ROW COL LOCABON VOLTS VOLTS VOLTS VOLTS VOLTS S/GB 83 50 001 + 31.45 0.60 ' O.71 0.11 1.18 S/G B 73 26 001 + 34.79 0.59 0.62 0.03 1.05 S/G A 48 111 002 + 24.73 0.47 0.39 i -0.08 0.83 S/G B 74 25 002 + 25.96 0.31 4 0.36 0.05 1.16 S/G B 74 25 002 + 29.55 0.35 0.36 0.01 1.03 S/G B 93 22 002 + 35.20 0.61 0.68 0.07 1.11 S/G B 74 25 002 + 36.25 0.49- 0.45 -0.04 0.92 S/G A 4 18 003 + 9.86 1.19 0.95 : 0.64 4 55 0.54 S/G A 29 73 OCX3 + 11.90 0.87, 1.03 0.16 1.18 S/G A 61 88 004 + 9.63 0.54 0.42' -0.12 0.78 S/G A 25 81 000 + 1.46 0.83 0.75 -0.08 0.90 S/G A 27 93 008 + 3.34 0.15 0.14' -0.01 0.93 S/G A 27 93 008 + 12.40 0.71 0.64; 4 07 0.90 S/G A 27 93 008 + 22.54 0.51 0.52- 0.01 1.02 8/G A 27 93 008 + 31.29 0.35 0.35! O.00 1.00 S/G A 61 88 010 + 8.79 0.72 0.65: -0.07 0.90 S/G A 16 41 011 + 16.07 0.57 0.70 0.13 1.23 S/G A 67 73 012 + 23.62 0.54; 0.54 0.00 1.00 S/G B 121 1 013 + 16.97 0.42 0.44 0.02 1.05 S/G B 34 70 013 + 18.89 0.37- 0.49 0.12 1.32 S/G B 80 41 014 + 11.88 0.65 0.61 -0.04 0.94 S/G B 62 7 015 + 1.67 0.68 0.79; 0.54 -0.14 0.79 S/G B 63 5 015 + 6.99 0.82 1.01 0.19 1.23 S/G B 62 7 015 + 9.89 , 0.57 0.52, 0.59 0.02 1.04 S/G B C2 7 015 + 21.19 0.51 0.56 0.70 0.19 1.37 S/G B 27 92 015 + 22.41 0.58 0.61 0.03 1.05 S/G B 27 92 015 + 22.70 0.81' O.78 4 03 0.96 S/G B 62 7 015 + 24.75 0.73 0.81 0.8G 0.13 1.18 S/G A 27 93 015 + 43.29 0.36 0.38 0.02 1.06 S/G A 27 93 015 + 43.65 0.85 0.81 4 04 0.96 S/G B 89 43 LTS + 5.37 0.34 0.30 0.04 1.12 S/G B 46 44 LTS + 6.03 0.64 0.63 0.01 0.98

, S/G B 90 43 LTS + 6.42 0.82 0.74 -0.08 0.90 S/G B 58 83 LTS + 6.52 0.39 0.50 0.11 1.28 S/G B 64 39 LTS + 6.90 0.37 0.33 4 04 0.89 S/G B 89 43 LTS + 7.00 0.68 0.78 0.10 1.15 22

t TABLE 4: S/N Growth Study (Sheet 2 of 4)

CR-3 GROWTH EVALUATION OF FREESPAN INDICATIONS 1989 1990! 1992 DELTA RATIO ROW COL LOCATION VOLTS VOLTS VOLTS VOLTS VOLTS 48 47 LTS + 7.28 i 0.62 0.09 1.17 S/G B 0.53l 49 35 LTS + 7.40 0.81, 0.97 0.16 1.20 S/G B i 46 44 LTS + 7.42 0.54; 0.64 0.10 1.19 S/G B S/G B 117 44 LTS + 7.47 0.48! 0.46 -0.02 0.96 48 47 LTS + 7.90 '

O.60j 0.64 0.04 1.07 S/G B 90 44 LTS + 8.22 0.54; 0.59 0.05 1.09 S/G B ,

63 29 LTS + 8.29 O 50 0.59 0.09 1.18 S/GB S/G B 46 48 LTS + 8.71 0.64' O.68 0.04 1.06 S/G B 61 38 LTS + 9.36 0.60, 0.71 0.11 1.18 S/G B 104 51 LTS + 9.56 0.72 0.76 0.04 1.06 S/G B 104 31 LTS + 9.97 0.81' O.81 0.00 1.00 S/G B 105 36 L13 + 10.11  ; 0.65, 0.56 -0.09 0.86 S/G B 69 99 LTS + 11.09 0.43 -0.09 0.83 l 0.52 S/G B 110 45 LTS + 11.22 l 0.62 0.51 0.11 0.82 S/G B 63 29 LTS + 11.49 1 0.60 0.62 0.02 1.03 S/G B 97 27 LTS + 11.55  ; 0.83 0.78 -0.05 0.94 O S/G B 103 44 LTS + 11.66 i 0.71 0.60 -0.11 0.85 S/G B 64 39 LTS + 12.33 0.38 0.39 0.01 1.03 103 44 LTS + 12.36 j 0.67, 0.54 -0.13 0.81 S/G B S/G B 63 29 LTS + 12.37 j 0.39 0.46 0.07 1.18 S/G B 98 43 LTS + 12.54 0.57, 0.44 -0.13 0.77 l

S/G B 52 81 LTS + 12.85 0.42 0.47 0.07 0.87 l 0.54 S/G B 46 44 LTS + 13.25 l 0.49 0.59 0.10 1.20 S/G B 49 49 LTS + 13.61 0.42 0.55 0.13 1.31 l

S/G B 67 43 LTS + 14.64 O.55 0.57 0.02 1.04 S/G B 70 42 LTS + 14.71 O.76 0.76 0.00 1.00 S/GB 63 39 LTS + 15.42 i 0.37 0.46 0.09 1.24 S/G B 70 42 LTS + 15.67 0.52 0.55 0.03 1.00 S/G B 46 44 L13 + 24.60 l 0.52 0.51 -4.01 0.98 AVG.DEV.  ! 0.01 1.03 STD.DEV. '. Q.11 0.16 C

b) 23

a l

l O

V TABLE 4: S/N Growth Study (Sheet 3 of 4)

CR-3 GROWTH EVALUATION OF SUPPORT Pl. ATE INDICATION I

1989 1990 1992 DELTA RA110 ROW COL LOCATION VOLTS VOLTS VOLTS VOLTS VOLTS S/G A 28 93 007 + 0.00 1.27 0.69 -0.16 0.81 l 0.85 S/GB 59 113 007 - 0.86 0.77 -0.06 0.93 l 0.83 S/G B 67 52 007-0.69  ! 0.90 0.84 0.93 0.G3 1.03 S/G B 66 58 007-0.72 0.72 0.70 -0.02 0.97 S/G B 88 12 007-0.72  ! 0.91 0.74 -0.17 0.81 S/G B 92 36 007-0.73 I 0.80 0.68 -0.12 0.85 S/G B 142 11 007 -0.73 > 0.38 0.35 -0.03 0.92 S/G A 114 100 007-0.74 0.39 0.28 -0.11 0.72 119 63 007 -0.75 , 0.68 0.69 0.81 0.13 1.19 S/G B S/G B 142 12 007-0.75 i 0.35 0.28 -0.07 0.80 S/G B 130 23 007-0.76  ; 0.82 0.58 -0.24 0.71 S/G B 136 32 007 0.76 1.24 0.62 -0.62 0.50 S/G B 17 74 007- 0.77 l' 1.43 0.69 -0.74 0.48 S/G B 109 52 007 -0.78 O.80 0.53 -0.27 0.66 S/G B 145 34 007-0.78 O.51 0.61 0.10 1.20 S/G B 148 14 007-0.78 1.05 0.64 -0.41 0.61 S/G A 14 8 007- 0.79 0.41 0.52 0.11 127 S/G B 132 30 007-0.79 1.06 0.24 -0.82 0.23 l

S/G B 132 36 007-0.79 i 0.90 0.31 4 65 0.32 S/G B 133 35 007 -0.79 1.41 0.26 -1.15 0.18 l

141 29 007-0.79 [ 0.41 0.16 0.25 0.39 S/G B S/G B 128 43 007-0.80 0.51 0.34 4 17 0.67 S/G B 144 15 007-0.80 0.76 0.71 -0.05 0.93 144 22 007 -0.81 0.41 0.31 -0.10 0.76 S/G B 144 24 007-0.81 0.37 0.14 -0.23 0.38 S/G B 117 73 007 -0.84 I 0.51 0.47 -0.04 0.92 S/G B 144 12 007-0.84 0.67 0.58 -0.09 0.87  ;

S/G B  ;

S/G B 57 39 007 -0.88 O.71 0.52 -0.19 0.73 147 24 007 -0.86 l 0.53 0.39 -0.14 0.74 S/G B 15 007 - 0.87 ' O.64 0.65 0.01 1.02 S/G B 150 144 56 007 -0.90 0.35 0.40 0.05 1.14 S/G B 71 007 - 0.95 0.39 0.33 -0.06 0.85 S/G B 117 l 73 128 008 + 0.00 j 0.51 0.53 0.02 1.04 S/G A S/G A 28 93 008 + 0.64 i 0.67 0.44 0.61 0.06 0.91 l 125 008-0.73 0.58 0.14 1.32 l S/G B 58 [ 0.44

= 1 24 I

r~.

%J $/N Growth Study (Sheet 4 of 4)

TABLE 4:

CR-3 GROWTH EVALUATION OF SUPPOR i 1989 1990 ! 1992 DELTA RATIO VOLTS VOLTS l VOLTS 0.10, VOLTS 0.67 VOLTS ROW COL LOCATION 0.75 0.65 94 129 008 0.74 0.55 l 0.46 0.06 1.13 S/G A 0.40 ,

82 128 000- 0.77 0.50 -0.94 0.35 S/G A 1.44 l 6 46 008-0.77 0.14 4 68 0.17 S/G B  ????? 0.82 146 26 008 -0.81 0.20 1.36 S/G B 0.56 0.76 59 120 008-0.83 0.14 1.33 0.57 S/G B 7 008-0.84 0.43f -0.47 0.59 S/G B 31 0.83 l 0.68 1.15 1 009 + 0.59 -0.06 0.90 S/G A 61 0.50 l 0.45 4 19 009 + 0.66 -0.08 0.87 S/G B 0.60 0.52' 88 53 009 + 0.75 -0.10 0.83 S/G A 0.60 0.561 0.50 86 6 009 + 0.78 0.23 -0.10 0.70 S/G B 0.33 ,

54 124 009 -0.68 0.62 0.04 1.07 S/G B 0.58 l 82 6 000-0.72 0.69l 0.42 0.27 0.61 S/G B 82 38 000-0.72 -0.80 0.37 S/G B 0.46 1.26 1.22l 0.44 6 49 009-0.78 0.40 0.51 S/G B 0.91 'l -

14 7 009-0.78 -0.81 0.36 S/G B 1.10j 0.46

\ 1.27 0.51 4 24 009- 0.81 1.10 0.44 -0.42 S/G B 0.86 3 10 12 000 -0.83 -0.60 0.15 S/G B 0.00 0.59l 0.67 146 26 009-0.85 0.46l 0.31 0.16 S/G B 22 59 010 + 0.06 -0.13 0.75 S/G A 0.53; 0.40 149 30 010 + 0.66 0.16 1.36 S/G B 0.44: 0.60 56 3 010 + 0.73 0.14 0.67 S/G A 0.42! 0.28 151 3 010-0.68 0.56 0.07 1.14 S/G B 0.49 f 127 96 010-0.73 -0.13 0.83 S/G B 0.78' O.63 151 13 010-0.75 0.06 1.14 S/G B 0.44 0.50 149 20 010-0.77 0.05 1.12 S/G A 0.42 0.47 148 36 010-0.78 -0.03 0.94 S/G A 0.54 0.51 146 50 010-0.79 S/GA 0.79 0.19 0.30 0.31 AVG. DEV.

STD.DEV.

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O 7.0 EDDY CURRENT CORRELATIONS Metallographic data from indications found on sections'of four tubes removed from the CR#3 steam generators were used to evaluate the ability of eddy current examination to detect and size degradation. Indications on the subject tubes were physically characterized as pit-like intergranular attack (IGA) with wall metal in place. IGA occurs as the result of grain boundary degradation. Physically, there is no metal loss. As a result, IGA causes only a slight decrease in conductivity over an area and is more difficult to detect and size than other damage mechanisms where wall metal loss occurs. Per Reference 7, experience at recirculating units confirms that detection and sizing of IGA using conventional eddy current techniques is less reliable than for other volumetric forms of degradation where metal loss occurs. Therefore, detection rates and sizing techniques based on correlation of eddy current data to actual metallographic data for CR#3 IGA indications are considered conservative when applied to other volumetric type indications.

7.1 EPRI CORRELATION OF CR#3 PULLED TUBE DATA Following destructive examination of CR#3 pulled tubes, the EPRI NDE center was requested to perform an evaluation to determine the accuracy of detectability and sizing for CR#3 pit-like IGA.

Reference 3 provided the results of this evaluation.

CR#3 pit-like IGA was found by EPRI to be detectable 80% of the time for 45% and greater through' wall penetration with eddy current bobbin coils. This detection rate is consistent with industry experience.

To evaluate the accuracy of bobbin coils to size IGA. patches, eddy current depth estimates were compared with metallurgical test results. Sizing accuracy was evaluated using a linear regression analysis by comparing eddy current estimates made using bobbin coil phase angle to metallurgical test results.

In general, flaw sizing accuracy increases as the value of best-fit slope and correlation coefficient increases. However, sizing precision decreases with increasing RMS error values. For optimal flaw sizing, a correlation coefficient of greater than 80% with RMS error of less than 20% is desirable. Under this-condition, the slope of the best-fit line generally falls between 0.8 and 1.2.

O 26

-~ - .. . . . .. . . . -

i For CR#3 pulled tube data, a high degree'of scatter was observed 2

A - in' comparing eddy current estimates made using bobbin coil phase angle to metallurgical test results. The best statistical values obtained were a 25% correlation coefficient and 27% RMS error ,

with a slope of 0.646. . As a result, it was determined that the bobbin coil phase angle method of sizing could not be used for CR#3 small volume pit-like IGA. This condition is not unique to-CR#3 but generally applicable to the whole industry.

Since bobbin coil phase angle methodology was found by EPRI to be unreliable for use in estimating IGA through wall depths, EPRI was asked to evaluate sizing accuracy using signal amplitude ,

(voltage) in lieu of bobbin coil phase angle. y EPRI reevaluated the CR#3 pulled Lube eddy current data to determine if a correlation could be developed using signal amplitude. As a result of this evaluation, a calibration curve. 1 was developed which could be used to estimate through wall size of pit-like IGA using signal amplitude. The calibration curve.

. was used by EPRI to estimate IGA depths for comparison to actual i metallurgical. data. Best results were obtained using a 600 kHz i dif f erential VMax amplitude curve. A correlation coefficient of 74% with an RMS error of 8%'and slope of 0.83 was achieved.  !

These values represent significant improvements over the statistical values obtained using bobbin coil phase angle to estimate IGA size.

Figure 6 shows the calibration curve developed by EPRI. Based on this curve, CR#3 freespan IGA patches are. projected to be 100%

through-wall at a 600 kHz bobbin coil voltage of approximately 3.4 volts. Since this calibration curve was developed using 600 kHz. data from freespan indications,.it.cannot be directly applied to indications located at tube support plates, Tube support plate indications are typically examined using a mix frequency as the prime frequency to suppress the tube support plate signal. Analyses to be performed on tube samples with tube support plate indications (see section 11.0) will be available in the future for use in determining applicability of Figure 6 to volumetric indications located within the tube support plate regions.

It should be noted for later discussion that the largest voltage CR#3 indication used to develop this curve was 1.4 volts, with a corresponding defect volume of approximately 2.04E-5 in 3.

27

O .-

O O Plot of Canned IGA to TRANS/ ROTATED ASME Field Data for Smallest Voltage's 100 90 80

, //

1l" //

u 50

//

30

/

0 0.5 1

1.5 2 2.5 3 3.5 Voltage

-1 1 fioure 6: EPRI Voltane to Percent Throuoh wall correlation

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l 7.2 B&W OWNERS GROUP NON-DESTRUCTIVE EXAMINATION COMMITTEE IGA 'I WORK The data in Reference 4 were used to correlate a known defect I size with an ECT voltage parameter. In the Reference 4 study, l IGA samples were fabricated in the laboratory and inspected with i various eddy current techniques to compare sizing and detection I capabilities. Bobbin coil eddy current data were acquired with i three different probes: a 0.510 M/ULC, a 0.510 M/ULC/HF, and a '

O.510 ULC. Six of the samples were then destructively examined to determine the actual flaw depth of the' IGA patches. All of the destructively examined flaws were approximately 0.75" long and extended approximately 45 degrees around the tube ,

circumference.

The 0.510 M/ULC/HF (high frequency) probe is the identical probe design used at CR#3, so the results from this probe were used in the evaluation. An ASME calibration standard was acquired with l the bobbin data, and was used to normalize the signal amplitudes  !

from the IGA study to eddy current data from CR#3. CR#3 establishes the bobbin coil signal amplitude at 4.0 volts on the  !

four 20% through wall holes using the 600 kHz channel. The data from the IGA study was re-analyzed using the same setup, and all calls were made off the 600 kHz channel to be consistent with '

current practice at CR#3.

The through-wall extent and corresponding bobbin coil voltage O from re-analysis of the eddy current data are tabulated below.

TABLE 5: BWOG NDE Committee IGA Samples Sample # Maximum % Bobbin Coil Approximate Through-Wall Signal Volume Amplitude, '

Volts 1217423-A 55 2.7 .0034 in 3 1217423-E 55 2.9 .0034 in 3 1217424-A 56 3.4 .0035 in 3 1217424-E 71 7.7 .0044 in3 1217425-A 22 0.5 .0013 in 3 1217425-E 41 1.1 .0025 in 3 The 7.7 volt signal is associated with an unacceptable flaw per the MPR analysis (71% through wall and 0.75 inches axial length).

The 3.4 volt signal, however, is associated with an acceptable i flaw size (56% through wall and 0.75 inches axial length).

29

7.3 PALISADES MIZ-18 QUALIFICATION PROGRAM N Reference 5 presents data generated during an eddy current qualification program for IGA performed by the Consumers Power-Palisades Plant. The samples used in this study had IGA patches of 0.2 inch axial length and 0.588 inch circumferential extent  ;

(equivalent to 90 degrees on the tube OD) with varying through-wall depths.

Per Reference 5 the Palisades data was obtained using a 0.580 high frequency probe, using 400 kHz as the prime reporting frequency. The Palisades tubing is 0.750 inch OD by 0.048 inch wall, which yields a fill factor of 79% using the .580 probe.

CR#3 uses a .510 probe in the 0.625 OD tubing, which has a fill factor of 83%. The lower fill factor in the Palisades tubing ,

would tend to depress the signal amplitude for the same size flaw. The Palisades data was not corrected for the fill factor difference for purposes of this evaluation since it is in the conservative direction for this evaluation. In addition, the calibration for the Palisades data established the voltage setting at 5.0 volts on the ASME 20% calibration standard. CR#3 calibrates at 4.0 volts on the ASME 20% calibration standard.  ;

The Palisades data was therefore normalized by multiplying the reported voltages by 4/5 to arrive at an equivalent voltage for the CR#3 setup.

Since the Palisades tubing is thicker than OTSG tubing (0.048

() wall vs 0.034 wall), the volume of the ASME 20% holes is larger.

Assuming that bobbin coil voltage is proportional to flaw volume, the Palisades data was further normalized by multiplying by the ratio of wall thicknesses, so that a given percent through wall flaw would produce the same voltage in both size tubes.

Results from the Palisades IGA correlation with bobbin coil j voltage normalized to CR#3 are presented in Figure 8 for those samples which did not have dents associated with the flaw. Data from a total of twelve samples are included, each having two (2) patches of IGA. In the qualification, each sample was run four times with four different probes, which gives eight (16) data points for each IGA patch. ,

30

t 4

The following tabulation is representative of information shown

('g,j/ graphically on Figure 7, with the addition of approximate defect -

volumes.

Table 6: Palisades IGA Samples Average % Bobbin Coil Approximate

-Through Wall Signal Amplitude, Volume-Volts 18% 0 to 2 .0007 in 3 25% .5 to 3 .0009 in 3 35% 2 to 4 .0013 in3 3

45% 4 to 7 .0017 in 55% 6 to 8 .0021 in3 .;

79% 22 to 27 .0031 in3 t

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i s VOLTAGE VS. ACTUAL %TW CORRELATION ~

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Figure 7: Voltaae: Correlation for Palisades data m__ _ _ _ . _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _________.___ ___ _ _ _ _ ._ _ ___ , ._

l i

\ 7.4 ECT VOLTAGE TO DEFECT VOLUME CORRELATION Using information obtained from the CR#3 pulled tube data, the BWOG NDE Committee Work and the Palisades Miz-18 qualification study, a voltage to volume correlation may be performed for IGA.

This correlation is shown on Figure 8. The one data point shown for CR#3 data represents the largest voltage indication included i in EPRI's evaluation of CR#3 data. Palisades Data has been normalized for purposes of comparison to OTSG eddy current data.

All points shown under the horizontal line on Figure 8 represent indications which are acceptable on OTSG tubing per Appendix A.

O 33

O O 0 4

EDDY CURRENT CORRELATION Intergranular Attack 30 t

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- 22 r-1 3 20 L o . KEY:

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Def ect Volume; Cubic Inches Figure 8: Voltage to Volume' Correlation for Various IGA Indications 4

[ 7.5 CORRELATION OF RPC CLIP PLOTS TO DEFECT AXIAL AND b CIRCUMFERENTIAL SIZE A review of CR#3 pulled tube data has been performed to assess the accuracy of using clip plots generated from RPC examinations to estinate the axial and circumferential size of an eddy current indication. This method is discussed in reference 10. In general, the method allows the defect to be sized based on when the RPC probe first detects the indication and when the indication is no longer detected after the probe passes. Sizing is based on known axial pull and rotational speeds.

In order to assess the accuracy of this technique, clip plots were produced from the RPC examination conducted on the first span indications pulled during the May 1992 Refuel 8 outage. The eddy current measurements were then compared to the actual flaw sizes reported from metallographic examination of indications.

The results of this comparison are provided in Table 7 and Figures 9 and 10. The clip plot technique over-estimated the actual flaw size for every case examined. From table 7, the average ratio of measured to actual flaw size for axial extent is 3.2, with a minimum of 1.5, Measurement of circumferential extent was even more conservative, with an average measured to actual ratio of 5.6 and a minimum of 2.3. in general the amount of conservatism decreases with increasing flaw size for both O axial and circumferential flaws.

The last two columns of table 7 show the error in the clip plot measurements as the difference between the actual dimension and the clip plot estimation. The errors reported in this way are more consistent than the errors reported as a ratio, which leads to the conclusion that the clip plot method consistently overestimates by the same amount. Once again, the circumferential measurement is shown to be more conservative than the axial measurement. For axial measurements the average difference is 0.09 inches, compared to 0.14 inches for the circumferential dimension.

To verify that the same conservatism would be present when estimating the axial and circumferential size of indications resulting from a damage mechanism other than pit-like IGA, a similar comparison was made using RPC data from an ASME calibration standard 40% through wall flat bottom hole. The RPC clip plot estimated an axial size of 0.19 inch and a circumferential size of 0.25 inch. The actual diameter of the hole was 0.094 inch. Therefore, the RPC clip plot axial estimate was conservative with a difference of 0.096 inch and the clip plot circumferential estimate was even more conservative with a difference of 0.156 inch, which closely matches the results achieved for pit-like IGA.

35

These results indicate that the RPC probe does not have to be directly over the indication to detect it. Therefore this technique will tend to overestimate the actual flaw size since the probe will detect the indication before it actually reaches it, and will still detect it for some distance after it has passed. It is therefore concluded that no additional adjustment for RPC measurement needs to be made. Using sizing estimates direct from clip plots without further adjustment for NDE uncertainty is appropriate and conservative.

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Table 7: COMPARISON OF DESTRUCTNE EXAM RESULTS TO RPC CLIP PLOT MEASUREMENTS STEROV.SUAL RESULTS RPC SIZING RATIO. MEAS / ACTUAL ERROR. ACT - MEAS TUBE AX1AL FLAW D A)0AL ORC. CRC. AX1AL ORC.

SECTION NO POSITION NUMBER EXTENT EXTENT EXTENT EXTENT EXTENT AX1AL CIRC AXIAL CIRC ON.) (DEG) (IN.) (N) (IN.)

52-51-2 LTSF + 9.25* D 0.061 8.4 0.041 0.15 0.20 2.5 4.9 0.09 0.16 LTSF + 12.75* 12 0.046 7.6 0.037 0.19 0.17 4.0 4.6 0.14 0.13 11 0.053 7.6 0.037 0.19 0.17 3.6 4.6 0.14 0.13 90-28 2 LTSF + 13.60* C 0.053 2.0 0.010 0.14 0.19 2.6 19.5 0.09 0.18 8 0.063 4.1 0.020 0.11 0.14 1.7 7.0 0.05 0.12 LT5F + 15.30* E 0.071 7.9 0.036 0.16 0.20 23 5.2 0.09 0.16 LTSF + 17.70 1 0.063 4.7 0.023 0.19 0.19 3.0 S.3 0.13 0.17 H 0.059 43 0.022 0.15 0.19 2.5 8.7 0.00 0.17 G 0.079 6.1 0.039 0.15 0.19 1.9 4.6 0.07 0.15 j 97912 LTSF + 15.30* P 0.073 15.2 0.074 0.15 0.17 2.1 2.3 0.06 0.10 0 0.076 12.8 0 062 0.15 0.19 2.0 3.1 0.07 0.13 LTSF + 19.00* U 0.054 9.6 0.047 0.t 5 0.25 2.8 5A 0.10 0.20 T O.061 13.6 0.066 0.19 0.16 3.1 2.7 0.13 0.11 S 0.011 0.1 0.0005 0.06 0.10 73 205.7 0.07 0.10 LTSF + 2180* W 0.061 9.6 0.047 0.16 0.16 2A 3.9 0.10 0.13 10642-2 LTSF + 13.60* X2 0.071 11.0 0.053 0.11 0.14 15 2.6 0.04 0.00 Y 0.015 1.6 0.006 0.13 0.11 8.7 14.1 0.12 0.10 X 0.016 7.6 0.036 0.11 0.19 6.9 5.0 0.09 0.15 LTEF + 16.30" AG2 0.062 73 0.035 0.11 0.13 1.6 3.7 0.05 0.09 AH 0.056 5.7 0.028 0.12 0.15 2.1 5.4 0.06 0.12 LTSF + 21.00" AT 0.060 10.3 0.050 0.15 0.19 25 3.8 0.09 0.14 AU 0.047 11.2 0.054 0.11 0.25 2.3 4.6 0.06 0.20 AV 0.040 9.6 0.047 AVG = 3.2 5.6 0.09 0.14 STD = 1.9 4.1 0.03 0.03 MIN = 1E 23 0.04 0.09 NOTE: STATISTICS FOR CIRC. FACTOR DO NOT INCLUDE DATA FOR SAMPLE 97-91-24.

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8.0 REVISED REPAIR LIMITS l

l 8.1 EXISTING REPAIR LIMITS l The CR#3 Technical Specifications currently define che steam .l generator tube repair limit as "the imperfection depth at or i beyond which the tube shall be restored to serviceability by the i installation of a sleeve or removed from service because it may become unserviceable prior to the next inspection." The existing repair limit is equal to 40% of the nominal tube wall j thickness. The basis for the 40% through wall " depth-based" repair limit is:

(1) to conservatively account for all potential flaw types which include uniform thinning of the tube wall; and l (2) to satisfy the criteria of regulatory guide 1.121 which includes:

margin of safety consistent with NB-3225 of ASME Code,Section III under postulated accident conditions (i.e.;

LOCA, MSLB),

assure no burst at 3 times normal operating pressure, an allowance for NDE measurement error, and an allowance for incremental flaw growth between

()

inspections.

l 4

40 i

8.2 DIMENSIONAL BASED REPAIR LIMIT Figures 11 and 12 provide the results of the MPR structural ,

analysis. These figures can be used to select dimensional parameters which provide a conservative limit for defects which may remain in service on once through steam generator tubes.

As stated in section 7.1, the bobbin coil phase angle method of percent through wall estimation has been determined to be inaccurate for the very small volume pit-like IGA found on CR#3 steam generator tubes. Although a voltage to percent through wall. correlation was found to be more accurate than phase angle, it cannot be applied to indications located at tube support plates where mix frequencies must be used to suppress interference. Therefore, there is no existing methodology which can be used to estimate percent through wall for the very small indications observed at CR#3 with an acceptable degree of accuracy.

Since accurate percent through wall estimates cannot be made, the conservative approach to establishing a dimensional based repair limit is to select the allowable axial and circumferential dimensional limits which correspond to 100% through wall depth.  ;

Axial and circumferential defect size can be conservatively estimated using RPC clip plots as discussed in section .

() As shown on figures 11 and 12 and discussed in section 5.1, the calculated allowable axial and circumferential dimensions for a 100% through wall indication are 0.25 inch axial and 120 degrees circumferential (0.6 inch). Projected burst pressure-for a 0.25 inch axial and 100% through wall indication is 4,940 psi. This burst pressure is above the regulatory guide 1.121 limit of three times the operating differential pressure (4050 psi).

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Figure 12: Maximum Allowable Penetration Versus Arc Length for 34.1 % Maximum Allowable Area of Degradation WMPR

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43

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() 8.2 VOLTAGE BASED REPAIR LIMIT Examination of indications using the motorized rotating pancake coil requires a significant amount of outage time due to the very low pull speed of the RPC probe. Therefore, it is desirable to establish a repair criteria which may be applied to S/N indications to minimize the amount of unnecessary RPC examinations which must be performed while ensuring that indications which could potentially approach the dimensional based repair limit receive proper evaluation.

As shown on Figures 13 and 14, defects with volumes which correspond to Signal Amplitudes as high as 6 Volts on Figure 8, fall into the area of the graphs where defect size is considered structurally acceptable for OTSG tubing. Additionally, reference 12 discusses recent work performed by BWNT which has concluded that OTSG eddy current signals which are below a bobbin signal voltage of 4 volts are not structurally significant .

In order to account for data scatter, uncertainty, and the limited amount of data available, a more conservative approach should be caken in establishing a voltage based repair criteria for OTSG tubing.

In comparing the voltage correlation performed by EPRI using CR#3 pulled tube data to the voltage correlations based on BWOG NDE committee work and Palisades Miz-18 qualification program information, it is apparent that in addition to defect volume, defect geometry must also be considered. The IGA patches on CR#3 pulled tubes were pit-like in geometry, with a relatively high penetration for their size compared to the IGA patches grown by the B&W Owners Group and Palisades. CR#3 pit-like IGA yielded relatively low voltages in comparison'to the BWOG NDE committee and Palisades IGA patches which had significantly larger axial and circumferential extent, covering more surface area of the tube and resulting in greater volume for a given penetration.

The larger volumes result in a larger bobbin signal response.

Although the volumes of CR#3 pit-like IGA are as much as 100 times smaller than MPRs calculated maximum allowable. defect' dimensions, it is desirable to establish a conservative limit. "

This limit should not only ensure that potentially significant defects are examined using RPC, but should also. ensure that structurally insignificant defects with 100% through wall penetrations which have the potential for leakage are identified.

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) Correlations based on low signal amplitudes from CR#3 pit-like IGA, shown on Figure 6, result in the projection that 100%

through wall penetration may be present in indications with voltages as low as 3.4 volts. Therefore, establishment of 3 volts as the CR#3 voltage based repair limit represents not only l a conservative limit from the standpoint that it-ensures ,

potentially significant defects will be identified for further l R

evaluation, but it also provides reasonable assurance that no 100% through wall defects will remain in service without l additional examination. -;

1 Based on growth studies that have been performed for signal to- )

noise type indications, incorporation of a 1 volt margin of a safety to account for potential growth between inspections is l conservative. This further reduces the proposed voltage based j repair limit to a bobbin coil signal amplitude _of 2 Volts. ,

Existing RPC data for 96 S/N indications examined during the Refuel 8 outage has been reviewed to evaluate the acceptability of the proposed 2 volt repair limit as compared to the 1 dimensional based repair limit. Figure 15 provides the results I of this review. All 96 indications reviewed would be considered 1 acceptable to remain in service based on the'2 volt repair' limit.

Ninety-four out of 96 indications reviewed would be considered ,

acceptable to remain in service based on the dimensional based 1 O repair limit. Therefore, application of the 2 volt repair limit as a-screening criteria by which the amount of RPC to be performed may be reduced is acceptable with a high confidence I level that no potentially structurally significant indications will remain in service.

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O O 0 CRYSTAL RIVER 3 B-OTSG, 5/92 OUTAGE BOBBIN AMPLITUDE VS.. EXTENT, S/Ns o.75 i5 z

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o AXIAL + CIRCUMFERENTIAL Figure 15: ' Estimated Sizes of Sample Inservice S/N in'dications

8.4 BASIS FOR REVISED REPAIR LIMITS The bases for the proposed signal amplitude based Repair limit

('-) of 2 Volts and for the Dimensional based Repair limit are summarized below:

Criteria (1) to conservatively account for all potential flaw types which include uniform thinning of the tube wall The proposed structural limit of 2 Volts was based on information obtained during various studies of Intergranular Attack. Basing the voltage correlation on IGA flaw types is conservative compared to other volumetric flaws. Since the material is still present in an IGA flaw, the bobbin coil signal will be depressed when compared with a wear or wastage type flaw of the same size where more material is missing. The structural strength of the two flaws is comparable, therefore, basing the structural limit on smaller IGA signal amplitudes is conservative. This voltage based structural limit will not be applied to indications in the lane / wedge region which are observed to be crack-like by RPC.

The dimensional based structural limit is based on a calculation performed by MPR which conservatively determined the maximum allowable flaw size on OTSG tubing for any type of damage mechanism.

Criteria (2) to satisfy the criteria of regulatory guide 1.121 O which includes:

margin of safety consistent with NB-3225 of ASME Code,Section III under postulated accident conditions (i.e.; LOCA, MSLB)

Postulated accident conditions have been addressed in the calculations performed by MPR Associates, Inc, and provided as attachment A to this report. The maximum allowable defect sizes calculated by MPR take into account the requirement that tube burst pressure must be greater than the pressure difference across the tube wall for accident conditions and the tube stress intensity should be less than the lesser of 2.4 times the design stress intensity (Sm) or 0.7 times the Ultimate Stress. The ,

proposed dimensional repair limits of 0.25 inch axial and 120 degrees circumferential are based on these calculations. .The proposed 2 volt voltage based limit was arrived at by conservatively _ reducing the upper voltages observed to correspond with defect volumes and dimensions found to be smaller than the maximum allowable defect sizes calculated by MPR Associates, Inc.

A 2 volt limit is believed to be adequate to ensure defects are removed from service well before they approach such size that they would be subject to failure under postulated accident conditions.

O 49 U

assure no burst at 3 times normal operating

()

pressure Burst tests performed on two tube sections removed from the CR#3 "B" steam generator, containing over 80 S/N indications, resulted in a burst pressure margin of more than 2.5 times the Regulatory Guide 1.121 limit of 3 times operating differential pressure.

Based on review of RPC data for similar indications located in various regions of the "B" steam generator, it can be determined that the majority of inservice S/Ns are similar in size to the indications present on burst tubes. Additionally, historical burst test data discussed in section 5.3 can be used to project burst pressure for a 0.25 inch axial, 100% through wall indication. As stated in section 5.3, projected burst would occur above the regulatory guide 1.121 limit. Both the voltage based and dimensional based plugging limits proposed contain adequate margin to ensure the Regulatory Guide 1.121 burst pressure requirement can be met by defects which will remain in service under the revised limits.

includes an allowance for NDE measurement error The proposed 2 volt Repair limit was arrived at following.

conservative reduction to account for NDE measurement error.

Operating experience with no primary to secondary leakage over several cycles since first detecting S/Ns indicates a low i probability of inservice 100% through wall indications. The maximum penetration observed on pulled tubes removed from the "B" steam generator during Refuel 8 was 62%. The axial and circumferential dimensional based plugging limit was conservatively selected to account for potential 100% through wall indications due to identified inaccuracies in depth estimation. Since the allowable axial size increases as the percent through' wall of an indication decreases and the maximum penetration observed on pulled tubes has not exceeded 62%,

application of a 0.25 inch axial and 120 degree circumferential repair limit is conservative. Additionally, as discussed in section 7.5, there is a built in conservatism in estimation of axial and circumferential size of indications using RPC clip plots. Therefore, there is no need to further reduce the axial and circumferential limits selected to account for NDE uncertainty.

1 1

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includes an allowance for incremental flaw growth  !

between inspections.

Review of eddy current historical data indicates an insignificant f growth rate for S/N type indications in the CR#3. steam j generators. Additionally, per the structural analysis performed  ;

by MPR Associates, Inc., even a postulated axial crack would grow l only about 0.006% due to fatigue from cyclic loads per- .

startup/ shutdown cycle. Since the number of startup/ shutdown cycles between eddy current inspections is not large, growth due to fatigue between inspections would not be significant.

Although growth due to either continued degradation or fatigue is '

not significant, a one volt allowance for flaw growth was incorporated into the proposed voltage limit.

O 51

- . . - - .. . - _ - . . .. =

( 9.0 LEAKAGE CONSIDERATIONS  ;

Crystal River Unit 3 has a primary to secondary administrative ,

leakage limit of 0.3 gpm. This leakage limit-provides reasonable  !

assurance of tubing structural adequacy as well as being l i

practical in terms of detectability, Defect opening displacements and resulting leakages have been analyzed in a number of cases, including for OTSGs (Reference 11). However, industry operating experience has shown that sometimes even substantial tube defects do not exhibit much leakage. (This is apparently due to tight cracks and-the presence of deposits on the tube.) Accordingly, CR#3 uses an l administrative leakage acceptance criteria based on engineering judgment and satisfactory-past operating experience.

l 52

l

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10.0 ANALYST GUIDELINES FOR S/N INDICATIONS

~

Figures 16 and 17 show the proposed methodology for dispositioning Signal to Noise (S/N) Indications during fature eddy current inspections using the new repair limits.

For ECT indications which have been observed in previous outages and documented as S/Ns, a repair limit of 2 volts will be -

applied as a screening criteria. If the indication does not exceed 2 volts it will be considered a tube imperfection and will .

be documented as a Non Quantifiable Signal (NQS). Tubes with NQS indications will remain in service and will be reinspected during each subsequent eddy current examination. If the indication exceeds 2 volts it must receive further evaluation. For > 2 volt indications for which RPC has been performed previously, existing RPC data may be used to assess the indication size and disposition it without repeating the RPC examination if signal amplitude does not indicate growth since its previous inspection.

Indications > 2 volts which have not been examined using RPC will be classified as Non Quantifiable Indications (NQI) and will '

receive further evaluation.

ECT indications which have not been documented in previous outages will be documented as NQIs for further evaluation. As O stated in section 5.4, there is a high degree of confidence based on RPC performed in previous inspections that the S/N signals which will be dispositioned using this methodology are small, volumetric defects with similar characteristics, exhibiting little to no growth between inspections. It is desirable, however, to incorporate a factor of safety into the logic used to disposition S/Ns for purposes of ensuring that any new S/N observed receives adequate evaluation prior to disposition.

Therefore, instead of applying the 2 volt plugging limit initially, new indications will be classified as NQIs for further evaluation.

Indications classified as NQIs must receive a second level examination using a RPC probe. The RPC probe provides improved examination capabilities in the prese.ce of either circumferentially oriented tube wall degradation or variations in tube wall geometry. RPC probes have been'shown to provide better characterization of certain types of tube wall degradation than standard bobbin coil-technology. . The.RPC probe will be used on NQIs to obtain a three dimensional view of the S/N for purposes of characterizing the indication and estimating size.

53

Any new crack-like indication identified by RPC in the lane / wedge O- region will be repaired.' Axial and Circumferential estimates will be made for indications similar to those observed during the Refuel 8 tube pull using RPC clip plots as discussed in section 7.5. If either the axial or circumferential size estimated exceeds the allowables, the tube must be repaired. If the estimated size is within all allowable dimensions, the tube containing the indication will be classified as either an imperfection or degradation (i.e. degraded tubes must be included in the Technical Specification calculation with respect to escalation). Indications characterized as wear from RPC examination will default to the current 40% through wall plugging limit since ASME wear standards can then be used to estimate $

percent through wall penetration. Indications identified as manufacturers burnish marks (MBM) must receive extensive historical review and verification must be made that no change in signal outside standard eddy current signal inaccuracies has occurred since the indication was first detected.

Written guidelines will be provided to eddy current analysts prior to analysis of eddy current data during Refuel 9.

O 4

OTSG operating experience and previous pulled tube data has shown no evidence of cracking in regions outside the lane / wedge region. The proposed analyst guidelines for use during Refuel 9 require .that tubes with crack-like indications be repaired. Since the lane / wedge region will be sleeved during Refuel 9, the number of new crack-like indications which must be dispositioned using these guidelines is .

expected to be very low. In recognizing that steam generator ,

degradation in general is a dynamic mechanism influenced by many factors, it is - not unreasonable to assume that crack-like indications may eventually be-found outside the lane / wedge region in OTSGs. For this reason, the planer defect evaluated in the MPR calculation was modelled to bound crack-like indications. Future revisions to analyst guidelines will include a means by which to apply the voltage based repair criteria 'to 'any potentially new -

damage mechanism which produces crack-like indications.

54

, _ . -. .-.-_ . - - ~ ._. - -.. . .

l Is S/N s 5:1?

YES NO Is Indication in Existing Dbase?- Size Based on Phase 'I J ,

<20% >2bt >4'0% i Imperfection Degraded Repair .

1 -l YES NO ,

I  ;

Is Voltage s 2 Volts? l NQI

.YES NO  ;

i NQS Is RPC data available for this indication? ,

(Imperfection) l 1 I YES NO ,

I i Is bobbin coil voltage NQI within +/- 0.25 Volts of last recorded examination?

1 I ,

i 1 7 YES NO I

NQI Use existing RPC to characterize k l ,

IGA /Pitlike Wear MBM I I I Is axial size s 0.25 TW Size based Historical data inch AND circumferential on Wear Standard Review must be size s 0.60 inch? performed to-confirm its l l presence in all .

< i <20% >20% >40% previous exams. j YES NO l l l No change- .

Imperfection Degraded Repair observed in signal.

Degraded Repair O ,

Figure 16: Revised Analyst Guidelines ,

55

t f

NQI Perform RPC examination

_l Crack-like Volumetric ,

Use RPC to Characterize Repair IGA / Pit-like Wear MBM Is Axial Size s 0.25 inch TW size based Historical data AND Circumferential size on wear standard review must be s 0.60 inch? performed to verify its  ;

i i l presence in. i

<20% >20% >40% all previous

} } exams. No Imperfec: ion Degraded Repair change observed  ;

in signal.

I i 1

YES NO 1 N l i

\s Repair s2 Volts >2 Vpits Imperfection Degraded .

1

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Figure 17: Analyst Guidelines for Non Quantifiable Indications 56 l 1

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4 l

11.O BWOG TUBE INTEGRITY PROGRAM 11.1 TUBE PULLS The BWOG Steam Generator Committee has recently initiated a tube pull program to identify and characterize degradation currently affecting steam generator tubing. As part of this program, co-funded tube pulls are planned at both Crystal River Unit 3 and the Oconee Nuclear Station during 1994.

The CR#3 tube pull will focus on the 7th to 9th tube support plate regions of the "B" steam generator. Four tubes will be removed for chemical and metallurgical analysis. Burst testing will also be performed as part of the analysis plan. Analysis of tube samples from both Crystal River and Oconee should be ,

completed in 1995. Information obtained from the Refuel 9 tube pull analysis will be compared with the results of the Refuel 8 tube pull analysis and the information contained within'this document. Should any of the conclusions reached within this document change as a result of the Refuel 9 tube pull analysis or the Oconee Tube Pull Analysis, it will be revised and resubmitted' as appropriate.

11.2 BURST TESTING Burst testing will be performed as part of the BWOG tube integrity program to determine the structural significance of various defect types. The testing will include tube samples with wear, pitting, IGA, axial cracks, circumferential cracks, and dings. The samples will be non destructively examined prior to burst testing in order to correlate a measured NDE parameter with burst pressure for various flaw sizes. Leak testing of specific flaws will also be performed as part of this program. This work will begin in 1994 and is scheduled for completion prior to Crystal River's next scheduled Refueling Outage (RFO 10) in'1996.

Information obtained from the BWOG burst test program will be used to verify the conclusions reached within this document.

Should any of the conclusions reached within this document change  :

as a result of the BWOG burst test program, it will be revised and resubmitted as appropriate.

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12.0 CONCLUSION

S Based on evaluations which have been performed it can be concluded that application of a revised repair criteria to '

volumetric indications with signal to noise ratios less than or equal to 5:1 is acceptable. The following recommended Technical Specification change may be implemented to allow use of-the new criteria:

Revise section 5.6.2.10 definition 4.a.2 to read:

2. Imperfection means an exception to the dimensions, finish or- .

contour of a tube from that required by fabrication drawings or specifications. Eddy current testing-indications with signal to noise ratios > 5:1 and <20% of the nominal tube wall thickness, if detectable, may be considered as ,

imperfections. Indications with signal to noise ratios s5:1 and voltage amplitude s 2 volts may be considered imperfections.

Revise section 5 6.2.10 definition 4.a.4 to read:

4. Degraded tube means a tube containing imperfections 2 20% of the nominal wall thickness if signal to noise ratio is >

5:1; or indications with signal to noise ratios s 5:1 if voltage amplitude is greater than 2 volts, and < 0.25 inch axial, < 120 degree circumferential extent, caused by degradation except where all such degradation has been spanned by the installation of a sleeve.

{

Revise section 5.6.2.10 definition 4.a.7 to read:

7. Repair limit means the imperfection at or beyond which the tube shall be restored to serviceability by the installation of a sleeve or removed from service because it may become unserviceable piior to the next inspection. The repair limit for eddy current indications-with signal to noise ratios >5:1 is equal to 40% of the nominal tube wall thickness. The repair limit for eddy current indications with signal to noise ratios s 5:1 is a bobbin coil signal-amplitude of > 2 volts, AND axial extent >0.25 inches OR circumferential extent > 120 degrees. No more than five thousand sleeves may be installed in each OTSG, Impleme-tation of these changes will provide a more reasonable methodology by which to disposition S/N type indications while maintaining the margin of safety established by Regulatory Guide 1.121.

58

1 l

l 13.O REFERENCES

1. Florida Power Corporation submittal to the Nuclear  ;

Regulatory Commission dated July 31, 1993. l

2. R.J. Kurtz, et al, " Steam Generator Tube Integrity Program / Steam Generator Group Project - Final Project Summary Report", NUREG/CR-5117, 5/90.

I Letter frem K. Krzywosz (EPRI) to P.

Sherburne (BWNS),

3.

" Review of Successive Eddy Current Data on Pulled Steam Generator Tubes from Crystal River Unit 3", 3/24/93.

The B&W Owners Group NDE Committee Report on Intergranular  !'

4.

Attack (IGA) Detection and Sizing Capabilities of Various ECT and UT NDE Examination Methods in OTSG Tubing, Document

1. 47-1228838-00, 11/11/93.

5. Palisades Nuclear Plant 1989 Miz-18 Eddy Current System Qualification, Project Number 248815222024, Consumers Power-Company NDT Services, Volume 1 of 9, 8/24/89.

6. OTSG Outage Summary Report for Crystal River Unit 3, B&W Nuclear Service Company, 5/92.

7. PWR Steam Generator Examination Guidelines, Electric Power I l Research Institute, EPRI NP-6201, 12/88.

V

8. Crystal River Unit 3 Eddy Current Data Analysis Guidelines, l~

B&W Nuclear Service Company, 5/1/92.

9. BWNT Document 51-1218868-00, Draft " Crystal River Unit-3 Refuel 8 Pulled Tube Data Evaluation".

BWNT Document 51-1229575-00.

10.

11. GPU Topical Report 008 R2, " Assessment of TMI-1 Plant Safety For Return to Service after Steam Generator Repair", March ,

1983.

12. BWNT Document 51-1229189-00, Criteria for Detemmining Structurally Significant Steam Generator Tube Flaws.

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MMPR ASSOCI AT ES INC EN GINE ERS February 24,1994 Ms. Phyllis Dixon Florida Power Corporation Cgstal River Energy Complex 15760 West Power Line Street Crystal River, Florida 34428-6708

Subject:

CR-3 Steam Generator

Dear Ms. Dixon:

/] Enclosed as requested is our report " Evaluation of Crystal River Unit 3 (CR-3) Steam L/ Generator Tube Wall Degradation," dated February 24,1994.

Please callif you have any questions or comments.

Sincerely, d , 4'h Sterlin'g J. Weems Enclosure fm m ONG STREET ALE X ANDQf A, V A M) M-3 23 6 703-$ M-02OO FAX 703 -$ M-C M 4 1

PJMPR A SSOCI A TE S INC E NGt NEE aS

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Y.Y Inrormation Contained herein may be Proprietary to Florida Power and MPR and Should be Protected Evaluation of Crystal River Unit 3 (CR-3)

Steam Generator Tube Wall Degradation February 24,1994 Prepared for Florida Power Corporation Crystal River Energy Complex 15760 West Power Line Street Crystal River, Florida 34228-6708 O

3M niNG .5 $ { { I AL(W ANCM A VA 22314 323e 703 5100200 FAa 703 6!9 0224

CONTENTS Section Page 1 INTRODUCTION 1-1 Background 11 Purpose 11 2

SUMMARY

2-1 3 DISCUSSION 3-1 NRC Regulatory Guide 1.121 Requirements 31 Allowable Tube Wall Degradation 7 4 REFERENCES 4- 1 APPENDIX A MPR Calculation 102-071-HWM2," Allowable Tube Wall Degradation for 1.5 in. Axial Length,360 Defects" APPENDIX B MPR Calculation 102-071-HWM1, " Allowable Tube Wall Degradation for Axial, Slot type Defects" O

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^ 5 99 c ' ^ " 5 ' " c onoim eeas LJ Section 1 INTRODUCTION BACKGROUND NRC Regulatory Guide 1.121 (Reference 1] describes a method for determining allowable limits for degradation of steam generator tubing. Tubes with degradation beyond these limits are required to be removed from senice by the installation of plugs at each end of the tube (or modified to be acceptable for fwther senice by the installation of suitable sleeves which meet Regulatory Guide 1.121 requirements).

As part of the technical justification for continued safe operation, structura' adequacy of the tubing can be demonstrated by showing that tube degradation will not exceed Regulatory Guide 1.121 allowables at any time during plant operation. This report calculates maximum allowable degradation. Suitable NDT conservative plugging / sleeving criteria and operating experience of CR-3 and other similar plants can then be used to ensure tube degradation will not exceed the allowable degradation determined herein.

q To further ensure tubing structural adequacy during plant operating periods between Q NDT inspections, an administrative limit is imposed at CR-3 requiring shutdown for a leak rate of 0.3 gpm per steam generator. For CR-3, which has not had major tube degradation, this leak rate limit is considered to provide reasonable assurance of tubing structural adequacy as well as being practical, e.g., in terms of detectability. CR-3 experience and other work supports this.

PURPOSFJSCOPE The purpose of this report is to address all of the structural requirements in ,

Regulatory Guide 1.121 by conservatively considering any possible defect configuration  ;

and location which could occur in the secondary side of the steam generator tubing at i CR-3 (About 3% lower defect allowables, in terms of through-wall penetration, per Figure 1 only would be calculated for defects on the primary system side of the tubing). j i

Method All possible configurations of defects are covered herein based on evaluations of the following:  ;

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1-1

i Maximum axial tube stresses (which tend to part the tube, especially for the case of a circumferential defect). The limiting axial tube load in this case is due to a LOCA and is 2641 lbs tension (per Reference 2). Notably, the steam line break load per Reference 2 has since been reduced per Reference 3, and is no longer the limiting load.

Maximum tube hoop stresses (which tend to burst the tube especially for the case of a defect with substantial axial length). The limiting load (i.e., pressure) in this case per Regulatory Guide 1.121 (which includes substantial margin a; desired) is 3 times normal operating pressure difference which results in a value of 4050 psi (per Reference 4).

The above two cases are considered separately, because of the following:

Stresses are produced in the axial direction only by axial loads and in the hoop direction only by pressure loads.

Even when the axial and pressure loads occur simultaneously, the resulting stresses are principal stresses, and there is no intermediate direction which would have a greater stress. Hence, failure would be expected to occur whenever one of these stresses exceeded the critical value.

The worst case defect location1 for the bounding analyses in this report is a defect located in a peripheral tube. A peripheral tube sees the maximum axial tube stresses, i due to axial differential expansion between the tubes and the vessel (as a result of tubesheet stiffness being greater at the periphery), during a LOCA. The elevation location of the defect has no effect on the tube burst analyses herein since no credit is taken for support from e.g., a support plate or tubesheet in resisting tube burst from ar.

axial defect.

The configuration of the defect can be either symmetrical or nonsymmetrical about the tube axis because the primary stresses of concern (axial stress due to differential expansion effects during a LOCA) are not affected by asymmetry of the defect. As indicated in Figures 3 and 4, all pertinent loads are reacted by either the tube or its supports without the need for any bending moment capability of the tube at the defect (i.e., a plastic hinge can be assumed at the defect).

1 Notably, no ECT detectable tube imperfections are considered acceptable (without plugging / stabilizing or sleeving) at the top support plate or the bottom of the upper ,

tubesheet for certain tubes adjacent to the open lane (considered susceptible to sibration/ fatigue), because of the potential for fatigue in these areas (see Reference 2).

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l To reduce the complexity of the analysis, all defects will be considered as planer defects, with no credit taken for ligaments between micro cracks (see Figure 5). Based on operating experience thus far, the extent and rate of occurrence of defects at CR-3 are sufficiently small that such a conservative and simplifying approach can be taken at this .

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Section 2

SUMMARY

The maximum allowable tube wall degradation determined herein is summarized in Tables 2-1 for " probable" material properties and 2-2 for Code minimum material properties . For the intended purpose of determining the maximum allowable tube degradation per Regulatory Guide 1.121, we consider use of the '" probable" tubing material properties (i.e., per a 95% probability of occurrence at a 95% confidence level, per Reference 2), as appropriate, rather than ASME Code minimums. Accordingly, we consider the maximum allowable degradation as shown in Table 2-1 to be appropriate and conservative.

The results in Table 2-1 are presented graphically in Figure 1 (for axial slot-type defects) and in Figure 2 (for circumferential defects) which covers the effects of arc length of the defect as well as penetration. Based on tube burst test data in Reference 5, the analysis in this report for axial slot-type defects is reasonable and conservative for other axial defects of interest as well (e.g., any substantial size uniform wastage or elliptical wastage defect). The circumferential defects analyzed herein apply for defects up to 360 of arc A length and are limited to an axial height of 1.5-in. To be acceptab:e per Regulatory V Guide 1.121, a defect must be within allowables of both Figure 1 and Figure 2, since different configurations of defects are limited by different criteria.

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Table 2-1 Allowable Steam Generator Tube Wall Degradation Ihr Various Degradation Configurations (For Probable Tubing Material Properties)

I Coidiguration Type of Degrade: ion Allowable Tube Wall Degradation

1. Circumferential (up to 1.5 in 34.1% cf ;ube cross-sectional area (see axiallength and 360 arc length) Figure 2)

2. Axial slot-type See Figure 1

a. Less than 0.25 in. long See Fig. ire 1

b. 0.50 in. long 65.6% Penetration

c. 1.5 in. long 60.3% Penetration 4

2-2

Table 2-2 O"-- Allowable Steam Generator Tube Wall Degradation for Various Degradation Configurations (For ASME Code Minimum Tubing Material Properties)

Configuration Type of Degradation Allowable Tube Walt Degradation

1. Circumferential (up to 1.5 in. 23.0% of tube cross-sectional area axiallength and 360 arc length)

2. Axial slot-type

a. 0.25 in. long 86.3% Penetration

b. 0.50 in. long 60.1% Penetration

c. 1.5 in, long 53.8% Penetration i

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Section 3 DISCUSSION NRC REGULATORY GUIDE 1.121 REQUIREMENTS Regulatory Guide 1.121 provides requirements for evaluating the allowable wall degradation of steam generator tubing, beyond which the defective tubing must be removed from service. As stated, the Regulatory Guide requires the consideration of three factors: (1) the wall thickness required to sustain the imposed loadings under normal and accident conditions; (2) an allowance for further degradation during operation until the next inservice inspection; and (3) the crack size permitted to meet the primary-to-secondary leakage limit allowed by the plant's technical specifications.

Section C of Regulatory Guide 1.121 provides the specific structural requirements which must be satisfied for degraded steam generator tubing for normal operation and accident conditions. Most of these requirements can be bound by a reduced set of requirements at the end of this section; and, others are shown to be not pertinent as follows:

For normal operation, the requirements from NRC Regulatory Guide 1.121 are:

From Section C.2., " Minimum Acceptable Wall Thickness,"

' Tubes with detected part through-wall cracks should not be stressed during the full range of normal reactor operation beyond the elastic range of the tube material" (C.2.a.(1)).

' Tubes with part through-wall cracks, wastage, or combinations of these should have a factor of safety against failure by bursting under normal operating conditions of not less than three at any tube location" (C.2.a(2)).

'The margin of safety against tube rupture under normal operating conditions should be not less than three at any tube location where defects have been detected" (C.2.a(4)).

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- "Any increase in the primary-to-secondary leakage rate should be gradual to provide time for corrective action to be taken"(C.2.a(5)).

CR-3-Experience at CR-3 and at other similar plants has demonstrated this requirement to be met; accordingly, this requirement is not included in the reduced set of requirements at the end of this section.

• "An additional thickness degradation allowance should be added to the minimum acceptable tube wall thickness to establish the operational tub'e thickness acceptable for continued service. An imperfection that reduces the remaining tube wall thickness to less than the sum of the minimum acceptable wall thickness plus the operational degradation allowance is designated as an unacceptable defect. A tube containing this imperfection has exceeded the tube wall thickness limit for continued sersice and should be plugged before operation of the steam generator is resumed"(C.2.b).

CR-3-This requirement is addressed by the current practice at CR-3 of sufficient NDT examinations and sleeving or plugging (and stabilizing) for any actualindicated degradation (irrespective of tube wall penetration) for tube locations where experience (at CR-3 and others) indicates sufficiently rapid degradation should be expected (e.g., for certain tubes adjacent the open lane with degradation at the top support plate or at the bottom of the top tubesheet). Also, experience (at CR-3 and others) is n;ed to ensure degradation between NDT examinations will not exceed structural allowables.

Finally, an evaluation of fatigue is discussed in the CR-3 comments for Regulatory Guide 1.121 Section C.3 as follows.

From Section C.3," Analytical and Loading Criteria Applicable to Tubes with either Part Thru-wall or Thru-wall Cracks and Wastage,"

+ " Loadings associated with normal plant conditions, including start up, operation in power range, hot standby, and cooldown, as well as all anticipated transients (e.g., loss of electricalload, loss of offsite power) that are included in the design specifications for the plant, should not produce a primary membrane stress in excess of the yield stress of the tube m'aterial at operating temperature" (C.3.a.(1)).

- "The margin between the maximum internal pressure to be contained.by the tubes during normal plant conditions and the pressure that would be required to burst the tubes should remain consistent with the margin incorporated in the design rules of Section III of the ASME Code" (C.3.a.(2)).

3-2

.p. - 'The fatigue effects of cyclic loading forces should be considered in V determining the minimum tube wall thickness. The transients considered in the original design of the steam generator tubes should be included in the +

fatigue analysis of degraded tubes corresponding to the minimum tube wall thickness established. The magnitude and frequency of the temperature and pressure transients should be based on the estimated number of cycles anticipated during normal operation for the maximum service interval expected between tube inspection periods. Notch effects resulting from tube thinning should be taken into account in the fatigue evaluation" (C.3.b(2)).

CR-3-This requirement is addressed by the current practice at CR-3 of sufficient NDT examinations and sleeving or plugging (and stabilizing) for any actual indicated degradation (irrespective of tube wall penetration) for tube locations where experie'nce (at CR-3 and others) indicates sufficiently rapid degradation should be expected (e.g., for certain tubes adjacent the open lane with degradation at the top support plate or at the bottom of the top tubesheet). Also, experience (at CR-3 and others)is used to ensure degradation due to fatigue between NDT examinations will not exceed structural allowables.

p In essence, crack growth between tube inspections due to either d corrosion or fatigue has not been a problem at CR-3 or other once-through steam generators ( OTSGs) with the exceptions of fatigue defects in certain tubes adjacent to the open tube lane near the upper tubesheet and a small number of other incidents where the cause of the degradation has been found and resolved. Since the appropriate " lane" tubes at CR-3 are plugged or sleeved irrespective of size of degradation and since operating experience indicates no significant growth of other degradation, crack growth between inspections is not significant for CR-3. For completeness; however, . ,

a fatigue evaluation has been performed for this report.

Specifically, crack growth due to fatigue is evaluated herein for a worst-case circumferential crack based on a 100% through-wall' defect per Figure 2. For this evaluation, the controlling cyclic loads are due to startup/ shutdown / operation with little effect from tube vibration (see Reference 6). Based on calculations used for Reference 6 and assuming design basis loads, a crack growth rate of only about .9' of are length per startup/ shutdown / operation load cycle (up to 1107-lb tube tension) is predicted even for the above worst-case crack size per Figure 2. We understand.that subsequent to Reference 6, the above design basis load has been increased -

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slightly; therefore, a somewhat greater than (.9 / cycle) would be

O calculated. However, the actual expected tube load cycle (more like half the design basis load cycle) would result in essentially no crack growth. Accordingly, such crack growth due to fatigue is not significant since the number of cycles between inspections is not large; and, only small growth rates if any are indicated.

Crack growth due to fatigue between inspections for axial cracks (per Figure 1) can also be evaluated in a similar manner as for circumferential cracks discussed above. Specifically, a worst-case axial crack (maximum length of 1.5 in, per Figure 1) would grow only about .006% through wall per startup/ shutdown / operation cycle.

Accordingly, such crack growth due to fatigue is not significant since the number of cycles between inspections is not large; and, only small growth rates if any are indicated.

Accordingly, and in overall summary, degradation and crack growth due to corrosion or fatigue between inspections is small (if any) and not significant for CR-3 (based on evaluations herein and operating -

experience thus far).

"The maximum permissible length of the largest single crack should be such that the internal pressure required to cause crack propagation and tube O restere is ettee ttaree tim es 8reeter 18ee twe eerm ei evereti#8 vree ere.

The length and geometry of the largest permissible crack size should be determined analytically either by tests or by refined finite element or fracture mechanics techniques. The material stress-strain characteristics at temperature, fracture toughness, stress intensity factors, and material flow properties should be considered in making this determination" (C.3.d(1)).

+

"The primary-to-secondary leakage rate limit under normal operating pressure is set forth in the plant technical specifications and should be less than the leakage rate determined theoretically or experimentally from the largest single permissible longitudinal crack. This would ensure orderly plant shutdown and allow sufficient time for remedial action if the crack size increases beyond the permissible limits during service" (C.3.d(3)).

CR-3-This requirement is addressed by an administrative limit requiring shutdown for a leak rate of 0.3 gpm per steam generator.

For CR-3, which has not had major tube degradation, this leak rate limit is considered to provide reasonable assurance of tubing structural adequacy as well as being practical, e.g., in terms of detectability. CR-3 experience and other work supports this.

3-4

Crack opening displacements and resulting leakages have been

. analyzed in a number of cases, including for OTSGs such as at CR-3 (see Reference 6). However, experience has shown that sometimes even substantial tube defects do not exhibit much leakage (apparently due to tight cracks being stopped up with magnetite).

Accordingly, CR-3 uses an administrative leakage acceptance criteria ,

mentioned above based on engineering judgment and satisfactory past operating experience.

- " Conservative analytical models should be used to establish the minimum acceptable tube wall thickness generally applicable to those areas of tube length where tube degradation is most likely to occur in sersice due to cracking, wastage, intergranular attack, and the mechanisms of fatigue, vibration, and flow-induced loadings. The wall thickness should be such ,

that sufficient tube wall will remain to meet the design limits specified by Section III of the ASME Boiler and' Pressure Vessel Code for Class 1 components, as well as the following criteria and loading conditions" (C.3.a.).

CR-3-This requirement is interpreted as being covered by other requirements in Regulatory Guide 1.121 as discussed herein. The only conflict is per requirement C.3.a(1) which limits to yield stress versus a lower limit per Section III of the ASME Code. In this case -

f we consider the stated Regulatory Guide limit per C.3.a.(1) of yield stress to be appropriate and note that others have done the same.

For accident conditions, the requirements from NRC Regulatory Guide 1.121 are:

From Section C.2, " Minimum Acceptable Wall Thickness,"

"If through-wall cracks with a specified leakage limit occur either on a tube wall with normal thickness or in regions previously thinned by wastage, they should not propagate and result in tube rupture under postulated accident conditions" (C.2.a(3)).

"The margin of safety against tube failure under postulated accidents, such as a LOCA, steam line break, or feedwater line break concurrent with the SSE, should be consistent with the margin of safety determined by the stress limits specified in NB-3225 of Section III of the ASME Boiler and Pressure Vessel Code" (C.2.a(6)). i From Section C. 3, " Analytical loading criteria applicable to tubes with either part through-wall or through-wall cracks and wastage,"

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" Loadings associated with a LOCA or a steam line break, either inside or O

v outside the containment and concurrent with the SSE, should be accommodated with the margin determined by the stress limits specified in NB-3225 of Section III of the ASME Code and by the ultimate tube burst strength determined experimentally at the operating temperature" (C.3.a.(3)).

"The stress calculations of the thinned tubes should consider all the stresses . '

and tube deformations imposed on the tube bundle during the most adverse loadings of the postulated accident conditions. The dynamic loads should be obtained from the modal analysis of the steam generator and its support structure. All major hydrodynamic and flow-induced forces should be considered in this analysis" (C.3.b.(1)).

"The combination ofloading conditions for the postulated accident ,

conditions should include, but not be limited to, the following sources:

Impulse loads due to rarefaction waves during blowdown, 1

Loads due to fluid friction from mass fluid accelerations, Loads due to the centrifugal force on U-bend and other bend regions caused by high velocity fluid motion, Seismic loads, Transient pressure load differentials" (C.3.c).

" Adequate margin should be provided between the loadings associated with a large steam line break or a LOCA concurrent with an SSE and the loading required to initiate propagation of the largest permissible longitudinal crack resulting in' tube rupture. The loadings associated with the postulated accident conditions should include the transient hydraulic and dynamic loads listed in C.3.c." (C.3.d.(2)).

The pertinent NRC Regulatory Guide 1.121 tube structural requirements as stated above can be reduced to the following set of requirements:

For Normal Operation:

The tube stress intensity should be less than the tube material yield stress.

The tube burst pressure should be greater than three times the normal operating pressure difference across the tube wall. This is the limiting 3-6

requirement for normal operating conditions (see calculation in p

Appendix B); the results of this limit are shown in Figure 1.

x_)-

For Accident Conditions:

The tube burst stress should be greater than the pressure difference across the tube wall.

• The tube stress intensity should be less than the lesser of 2.4 times the design stress intensity (S m ) or 0.7 times the ultimate stress. The 0.7 ultimate stress requirement is the limiting requirement for accident conditions (see calculation in Appendix A); the results of this limit are shown in Figure 2.

ALLOWABLE TUBE WALL DEGRADATION Based on the evaluations and calculations herein, the allowable tube wall degradation for various types of degradation of the CR-3 steam generator tubing was determined. The results of the evaluations are summarized in Tables 2-1 and 2-2 and in Figures 1 and 2, based on the calculations presented in Appendices A and B.

O P

3-7

FAMPR ASSOCI ATE S INC

/ E tJ G N. EN S V

Section 4 REFERENCES

1. US Nuclear Regulatory Commission Regulatory Guide 1.121," Bases for Plugging Degraded PWR Steam Generator Tubes," August,1976.

2. " Determination of Minimum Required Tube Wall Thickness for 177-FA Once-Thro: gh Steam Generators," Babcock & Wilcox, No.10146. April,1980.

3. " Review and Update of OTSG Tube Loads, Task 1 Summary," Babcock & Wilcox No. 51-1202303-00, February 28,19')1.

4. " Crystal River Unit 3 Tube Pull Project Summary Report," Attachment 1 to Florida Power Corporation letter to U.S. Nuclear Regulatory Commission-Document Control Desk, dated July 29,1993.

5. PNL-2684 (NUREG/CR-0277), " Steam Generator Tube Integrity Program -

Annual Progress Report for January 1 - December 31,1977," Battelle Pacific (v; Northwest Laboratory, August,1978.

6. " Assessment of TMI-1 Plant Safety for Return to Service after Steam Generator Repair Topical Report 008, Rev.3, August 19,1983.

l l

1 l

l 4-1

PAMPR A .S S.O C.I.A T E S .I. N C.

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Appendix A MPR Calculation 102-071-HWM2,

" Allowable Tube Wall Degradation for 1.5 in. Axial Length,360 Defects" O

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