ML20101F600

Alternate Disposition Strategy for Low Vol OTSG Eddy Current Indications

ML20101F600
Person / Time
Site:	Crystal River
Issue date:	06/30/1995
From:	FLORIDA POWER CORP.
To:
Shared Package
ML20101F584	List: 3F0396-19, Application for Amend to License DPR-72,consisting of Change Request 203,Rev 2,allowing Restart from Current Refueling Outage ML20101F590 ML20101F600 ML20101F608
References
NUDOCS 9603260079
Download: ML20101F600 (82)

Text

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i ALTERNATE DISPOSITION STRATEGY FOR LOW VOLUME OTSG EDDY CURRENT INDICATIONS i

FLORIDA POWER CORPORATION CRYSTAL RIVER UNIT 3 TECHNICAL REPORT SUPPORTING TSCRN 203 June 1995 l

l l

960326007h 960321 PDR ADOCK 05000302 P

PDR

TABLE OF CONTENTS Section Pag.t 1

INTRODUCTION 1.1 Executive Summary..........................................

1-1 1.2 Summary of the Proposed Di sposition Strategy............... 1-2 2

BACKGROUND 2.1 Signal-to-Noise Considerations.............................

2-1 2.2 OTSG Tube Degradation......................................

2-8 3

NON-DESTRUCTIVE EXAMINATION CONSIDERATIONS 3.1 Background................................................. 3-1 3.2 Assessment of Tube Integrity.............................. 3-1 3.3 Assessment of Tube Leakage Potenti al....................... 3-6 3.4 Allowance for NDE Measuremer.t Uncertainty.................. 3-15 3.5 Conclusions................................................ 3-18 4

STRUCTURAL INTEGRITY EVALUATION 4.1 Background.................................................

4-1 4.2 Development of Structural Repair Limit..................... 4-1 4.3 Assessment of Proposed Limits.............................. 4-3 4.4 Degradation Growth Rate.................................... 4-15 4.5 Lower Bobbin Coil Vol tage for MRPC Inspection.............. 4-24 4.6 Conclusions................................................ 4-26 5

LEAKAGE CONSIDERATIONS 5.1 Background.................................................

5-1 5.2 Development of Leakage Repair Limit........................ 5-2 5.3 Assessment of Proposed Limits.............................. 5-9 5.4 Comparisons to RSG Database................................ 5-12 5.5 Conclusions................................................

5-14

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6

SUMMARY

OF DISPOSITION STRATEGY 7

REFERENCES i

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TSCRN 203 Technical Report Section 1 INTRODUCTION 1.1 Executive Summary Analysis of eddy current data from the April 1990 and May 1992 Crystal River Unit 3 (CR-3) refueling outages revealed a large number of indications in the first span of the "B"

Once Through steam Generator (OTSG).

These indications, primarily low amplitude signals with low signalato-noise (S/N) ratios (<5:1 from Reference 1)l, were dispositioned as 'S/N' and allowed to remain in service. In I

addition to the indications in the first span of the "B" steam generator, eddy current inspection also identified S/N indications at other locations in both CR-3 steam generators. To investigate S/N indications further, seven tubes with representative indications were pulled from the "B" steam generator during the May 1992 refueling outage.

Based on the 1992 pulled tube examination results, Florida Power Corporation (FPC) confirmed a depth-based acceptance criterion was inappropriate for S/N indications. This conclusion was based on two primary considerations: (1) burst testing demonstrated S/N indications, including those which exceeded the technical specification depth-based limit, had minimal effect on tubing structural margins, and (2) bobbin coil eddy current was shown to be inaccurate l

with respect to estimating the degree of S/N throughwall penetration. Alternate repair criteria which addressed these considerations as well as others were developed for the population of CR-3 S/N indications based upon examination of the pulled tubes, past CR-3 eddy current inspection history, and other technical resources.

l Following extensive interaction with the NRC Staff, the NRC Staff agreed the l

current technical specifications do not address the discontinuities producing low I

signal-to-noise indications. Accordingly, Confirmatory Action Letter (CAL) No.

2-94-004, dated April 26, 1994 was issued to allow FPC interim use of an alternate criteria for dispositioning S/N indications during Refuel Outage 9.

Tte alternate criteria used during 9R along with supporting technical documentation was submitted to the NRC in Reference'1. The CAL also confirmed FPC's intent to pull an additional four tubes fr9m the "B" OTSG during Refuel 9 and to submit a license amendment request to incorporate updated alternate repair criteria within the plant technical specifications.

This report provides the technical basis for the proposed license amendment required by the CAL.

The S/N ratio of 5:1 is intended to limit the error associated with through wall sizing of indications to 10 per cent or less.

1-1 l

l

i 1

TSCRN 203 Technical Report 1.2 Sunnary of the Proposed Disposition Strategy

' he major elements of the proposed strategy for This section summarizes t

i dispositioning low volume (S/N) indications in the CR-3 OTSGs. The section is provided in order to generally familiarize the reader with the proposed strategy prior to presenting the detailed discussion of the approach.

1.2.1 Anolicability FPC proposes to limit this approach solely to eddy current indications in those portions of the tubes which are located outside of either tubesheet and which l

have less than a 5:1 signal-to-noise ratio.

All S/N indications outside the tubesheets are evaluated against a low voltage cutoff (0.9V); however, only those confirmed by MRPC inspection to have a volumetric morphology (i.e., not crack-like, or tapered wear) are evaluated further against the proposed structural and leakage repair limits. All other eddy current indications (i.e., tapered wear, crack-like indications, all indications (including S/Ns) within either tubesheet, i

and all indications with a 5:1 or greater signal-to-noise ratio) will be l

dispositioned in accordance with current Technical Specification cr'iteria.

1.2.2 Structural Repair Limit i

MRPC is utilized to measure the axial and circumferential extent of applicable S/N indications (as defined in Section 1.2.1). Tubes with indications exhibiting an axial extent greater than or equal to 0.33 inches, or a circumferential extent greater than or equal to 0.6 inches, will be considered defective and repaired.

The proposed maximum allowable dimensions are intended to ensure the minimum burst pressure required by NRC Regulatory Guide 1.121 is maintained for i

degradation that is conservatively assumed to be 100% through wall.

MRPC is 6nly required to be performed bn new S/N indications which exhibit bobbin coil signal amplitudes greater than 0.9 volt. Subsequent MRPC is only required on previously-identified applicable S/N indications which have exhibited signs of growth as defined in Section 4.4.2.

1.2.3 Leakaae Repair Limit The proposed approach does not allow through wall S/N indications to remain in service. Applicable S/N indications (as defined in Section 1.2.1) which exhibit a bobbin coil signal amplitude of 2.5 volts or greater will be considered defective and repaired. The proposed maximum allowable bobbin coil voltage is intended to ensure a minimum tube wall ligament is maintained, such that, primary-to-secondary leakage during a worst case accident scenario is precluded.

1.2.4 Conservatism included Within the Proposed Stratecy Considerable conservatism is maintai'ned with the proposed approach to dispositioning S/N indications. The structural analysis, for example, was based on modeling the degradation as a through wall crack.

None of the indications 1-2 e

TSCRN 203 Technical Report observed in the CR-3 pulled tubes contained any through wall degradation. '

Further, with the exception of wear, none of the observed degradation in the pulled tubes exhibited any wall loss in their as received condition. Therefore, the calculated burst pressures were generally lower than those measured empirically in the laboratory.

\\

Added conservatism is provided by the leakage repair limit to preclude p'rimary-to-secondary leakage through very deep or through wall S/N indications. Both NRC Regulatory Guide 1.121 and the current Crystal River-3 Technical Specifications allow leakage, but limit it to very small amounts. The approach described in this document, conservatively does not allow indications that are deeper than 87%

through wall as determined by bobbin coil signal amplitude, to remain in-service.

When compared to other industry pulled tube data bases, i.e., the one for outside diameter stress corrosion cracking at the tube support plates within recirculating steam generators, the data obtained from the CR-3 pulled tubes support a simple voltage repair limit of 2.5 volts for the Crystal River-3 steam generators. However, inasmuch as the proposed approach also requires measurement of the indication axial and circumferential dimensions, additional conservatism and increased confidence are provided that structural and leakage margins will not be exceeded.

e e

4 1-3

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l TSCRN 203 Technical Report Section 2 BACKGROUND 2.1 Signal-to-Noise Considerations j

2.1.1 History Since beginning commercial operation in March of 1977, the Crystal River-3 once-through steam generators (OTSGs) have consistently exhibited a minimal amount of tube degradation due to corrosion and/ or mechanical damage. Six (6) tubes were plugged pre-service and another twenty-seven (27) tubes were plugged between 1978 i

J and 1981 due to UTS tube end/ tube-to-tubesheet joint damage caused by a sizeable loose part. The first tubes to be plugged as a result of in-service degradation were not removed from service until 1987.

At that time, three (3) tubes were plugged in the "A" OTSG due to wear.

In 1989, FPC began to pro-actively inspect more steam generator tubes than the minimum number required by plant technical specifications.

This increase in j

inspection scope was based both on industry guidelines, (i.e., EPRI Report NP-6201, PWR Steam Generator Examination Guidelines: Revision 2 was issued in December of 1988 and recommended an inspection sample of 20% of the active tubes), and on the implementation of a more pro-active steam generator management philosophy. By the end of 1989, a total of only 36 tubes had been plugged in the Crystal River-3 OTSGs.

From 1989 to 1992, as the total number of tubes inspected increased, the population of tubes inspected also increased to include more tubes that had not been inspected since the pre-service insp'ection. As a result of the 1989, 1990, 1992, and 1994 OTSG eddy current (EC) inspections, all active tubes in both OTSGs wer.e inspected at least once during the period from 1987 to 1994, i.e.,100% of the tubes were inspected in seven (7) years (three fuel cycles). Currently,182 (0.59%) of the total number of OTSG tubes at Crystal River-3 (31,062) are plugged. The increase in the number of plugged tubes between the 1987 and 1994 inspections is attributed to the increased number of tubes undergoing EC inspection.

Many tubes underwent EC inspection for the first time during the period from 1987 to 1994. As the inspection scope increased, the number of EC indications also increased. The majority of the bobbin coil EC indications were in the "B" OTSG, were located in tubes that had no previous in-service EC inspection history, and exhibited a signal-to-noise ratio (S/N) less than 5:1.

With the 1990 inspection results, concentrations of low S/N indications became obvious in certain regions of the "B" 0TSG. The regions with the highest number of S/N indications were the first span, 6" - 18" above the lower tubesheet secondary face.(LTSF); the 7th tube support plate (TSP); and the 9th TSP (see Figures 2-1, 2-2, and 2-3). More than 90% of these indications have bobbin coil amplitude signals of less than 1 volt (see Figure 2-4).

2-1 O

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l TSCRN 203 Technical Report VOLTAGE DISTRIBUTION - OVERALL i

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2-5 i

TSCRN 203 Technical Report Prior to 1992, FPC did not attempt to assign a through wall depth estimate to any of the low S/N indications present in the CR-3 OTSGs. This was consistent with industry practice at the time and was based on two primary considerations.

First, it was not conclusively known whether S/N indications represented real tube degradation or if they were caused by tube deposits; and secondly, because of the low signal-to-noise ratio, any indications corresponding to actual tube degradation could not be accurately sized using the bobbin coil phase angle technique. Due to their small volume, the indications were not considered to be structurally significant even if they did have appreciable wall penetration (appreciable wall loss was agi expected). However, all inservice tubes with EC indications, including those classified as tow S/N, were re-inspected in subsequent outages to ensure they did not indicate an active degradation mechanism.

Leaving these indications in service was not considered a safety concern for the reasons stated above.

l 2.1.2 Crystal River-3 Tube Pull Proaram l

1992 Pulled Tubes In 1992, the industry practice for dispositioning low S/N indications began to change, and more focus was placed on trying to size S/Ns and disposition them j

according to the depth-based plugging criterion specified in plant technical specifications.

FPC decided to pull tubes from the "B" OTSG during the Refuel 8 outage (in 1992) to determine if degradation was actually present where low S/N indications were being observed, to determine the probability of detection and accuracy in sizing of S/Ns by the available non-destructive examination (NDE) techniques, and to determine the structural significance of any degradation that might be present in the tubes.

i The first span of the "B" OTSG had the largest number of S/N indications, and afforded the highest potential for a successful tube pull due to the short length of tube that would require removal. Therefore, six (6) tubes with firs,t span S/N indications were removed in June 1992. These tubes were cut just below the 2nd TSP and pulled from the bottom of the OTSG. A seventh tube with an EC indication at the 7th TSP was cut below the 8th TSP and its removal was attempted. However, it_became stuck in the LTS before the area of interest could be removed.

This l

tube was stabilized in place, and the tubesheet holes plugged.

These tubes were destructively examined in the laboratory by B&W Nuclear Technologies (BWNT) in collaboration with the Electric Power Research Institute (EPRI). The results of these examinations have been reported previpusly to the staff in Reference 1. A detailed description of the examinations and the results are documented in Reference 2.

These examinations revealed that actual degradation was present in the tubes and the S/Ns Were not caused by tube deposits. Small patches of pit-like intergranular attack (IGA) were present on the tubes.

Burst testing showed that the pit-like IGA did not significantly lower the burst pressure of the pulled tubes.

Analysis of EC dr.ta sh' owed that bobbin coil and rotating pancake coil inspection techniques could not reliably detect this degradation type when the depth was i

2-6 l

TSCRN 203 Technical Report less than 20% through-wall. It was further found that, once detected, the pit-like IGA could not be accurately through wall-sized using the bobbin coil phase angle technique.

Evaluation of the available historical EC data for the 1992 pulled tubes indicated that the pit-like IGA had been present as early as 1980.

No change in EC voltage has been observed since that time. Historical EC data of the pit-like IGA inferred to be in the first span of the other tubes in the "B" 0TSG was reviewed and determined to have been dormant since at least 1989.

1994 Pulled Tubes Having determined the cause of the EC indications in the first span of the "B" OTSG, attention shifted to determining the cause of the EC indications at the 7th and 9th TSPs, the other large concentration of S/N indications in the "B" 0TSG.

To accomplish this objective, FPC decided to pull tubes with S/N indications at these locations during the 1994 Refuel 9 outage.

This tube pull effort was closely coordinated with, and technically supported by the resources of the B&W Owners Group as discussed below.

Other OTSGs also have varying numbers of low S/N indications. In 1993, the Steam Generator Committee of the B&W Owners Group (B&WOG), at FPC's urging, began development of a pro-active tube pull program.

The purpose of the program was to pull tubes with similar EC indications at similar elevations within the OTSGs from different plants in order to determine the cause of the indications, to determine their effect on tube structural integrity, and to determine the detection and sizing capabilities of several NDE techniques. This program was also intended to aid in both the early identification of any degradation mechanism that might be active in OTSGs in general,. and the development of preventative measures.

Based on a comparison of the quantity and location (elevation) of EC indications within each OTSG, the B&WOG Steam Generator Committee recommended pulling at least four (4) tubes wth EC indications in the 7th through loth TSP region from the Crystal River-3 "B" OTSG. Additional tube pulls were planned for Oconee-1 in 1994 and Oconee-3 in 1995.

The Crystal River-3 tube pull was thus incorporated into the B&WOG program.

Based on the results of this first phase of tube pulls, a determination was to be made by the Steam Generator Comittee on the need to pull additional tubes. EPRI technical involvement was solicited by the Steam Generator Committee, and the tube pull program became a joint B&WOG/EPRI collaborative program. The combin'ed technical expertise of all five B&WOG member utilities (1. e.,

Duke Power Company, Entergy Operations, GPU Nuclear, Toledo Edison, and Florida Power Corp.), B&W Nuclear Technologies, and EPRI was thus obtained for the tube pull program.

In May 1994, four (4) tubes were pulled from the Crystal River-3 "B" OTSG. The tubes were cut below the 11th TSP, and pulled from the bottom of the OTSG. The tubes were sent to BWNT for examination.

A summary report which described all of the examinations performed and the results available as of November 1994 was submitted to the NRC as Reference 3.

As was the case with the 1992 pulled tubes s l

real tube degradation was found to be associated with the low S/N indications.

l Mechanical wear with a volumetric morphology (i.e., circular, oval, D-shaped) was 2-7 l

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TSCRN 203 Technical Report present at the 7th TSP on two tubes, and at the 3rd TSP on one tube. Mechanical wear with a tapered morphology was found on two tubes at the 9th TSP. Rotating pancake coil (RPC) inspection of 413 S/N indications during Refuel 9 (1994) renaled the presence of 64 tapered wear indications in both OTSGs in the 6th -

i lith TSP region. Pit-like IGA, like that observed in the 1992 pulled tubes, was l

found in the lower tubesheet sections of two tubes, and in the first span about l

12 - 13 inches above the LTSF of one tube.

Additionally, an archived tube from the 1992 tube pull (41-44) was examined by BWNT as part of the 1994 examination. This tube contained S/N indications in the i

first span, approximately 6" - 18" above the LTSF.

Numerous small patches of pit-like IGA were found in this tube also.

Consistent with the findings from the 1992 pulled tubes, burst testing showed that none of the defects posed a threat to tube structural margins. The EC j

probability of detection and sizing accuracies were also found to be comparable to that for the 1992 pulled tubes.

j 2.2 0TSG Tube Degradation j

2.2.1 0TSG Desian and Operation l

Crystal River-3 has two Babcock & Wilcox once-through steam generators (07SGs).

j Each OTSG has 15,531 Alloy 600 tubes with nominal dimensions of 5/8 inch outside i

diameter, and 0.034 inch wall thickness. The tube bundle has a triangular pitch.

Each steam generator has 15 broached tube support. plates fabricated of carbon -

steel (see Figure 2-5 and 2-6). The upper and lower tubesheets are each 24 inches thick. The tubes are rolled 1 - 2 inches into each tubesheet. The tubing has a i

'sensitizedmicrostructure(chromiumcarbideprecipitatedecorationinthegrain boundaries with associated chromium depletion) due to a post fabrication full-l vessel stress relief performed in which each steam generator was heated up to j

1100 - 1150' F for about 15 hours1.736111e-4 days <br />0.00417 hours <br />2.480159e-5 weeks <br />5.7075e-6 months <br />.

i The OTSG is a counter-current flow heat exchanger with feedwater preheating l

occurring in the..downcomer annulus (see Figure 2-7). Hot pressurized primary fluid from the reactor enters at the top of the OTSG at about 603' F, and 2220 i

psia at full power, then exits counter-current to the secondary fluid out the i

bottom of the OTSG at about 554' F.

The secondary fluid (i.e., feedwater) is I

injected via 32 feedwater spray

• nozzles into the downcomer annulus (or feedwater j

heating chamber) at about 455 F at full power. High quality or slightly i

superheated steam enters the downcomer annulus through the aspirating ports to heat the subcooled feedwater to saturation temperature (about 535' F) within a j

few feet of the feedwater spray nozzles. A saturated mixture of steam and liquid j

water results in the downcomer annulus.

i Saturated nucleate boiling begins immediately as the fluid first contacts the hot tubes at the lower tubesheet.

Steam quality increases as the secondary fluid ascends the tube bundle.

By the t'ime the working fluid reaches the 10th TSP, 100% quality is achieved. Steam superheating occurs in the upper portion of the' I

2-8 i

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bLRN 203 Technical Report OTSG DESIGN FEATURES l

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TSCRN 203 Technical Report tube bundle. At full at a superheat of 60* power, approximately 5.4 million pounds per hour of steam F is produced from each OTSG.

)

Unlike other steam generator designs, the greatest heat flux and highest differential temperature across the tube (dT) occurs at the mid-bundle region in the OTSG (around the 10th TSP).

The upper tubesheet (hot leg) and the lower tubesheet (cold leg) are both regions of low heat flux and low dT.

All OTSGs, including CR-3, began operation with all-volatile chemistry (i.e.

ammonia and hydrazine), and all have full-flow condensate polishing. Water chemistry controls have typically been good to protect both the OTSG and the turbine due to the steam generators once-through characteristics.

2.2.2 OTSG Dearadation Experience OTSG degradation experience in general has been quite good when compared to other i

steam generator models (Reference 5). As of December 1992, only 2 % of all OTSG l

tubes (7 units) had been plugged. Only 1.3 % of all OTSG tubes had been plugged j

due to secondary side causes. By comparison, 2.4 to 3.2 % of the tubes in other steam generator models had been plugged (Reference 6).

1 To date, 36 tubes have been pulled from 8 of the 14 operating OTSGs (References 5,7). Seventeen (17) tubes were pulled during the period of 1976 to 1982.

j Seventieen (17) tubes were pulled between 1992 and 1994.

Only two tubes were l

pulled in the interim period from 1983 to 1991. Laboratory analysis of the first J

17 tubes pulled (1976 - 1982) identified sulfur-related IGA as the only form of corrosion degradation present (References 5,7,8).

The remainder of the degradation observed was mechanically induced, i.e. high-cycle fatigue along the untubed lane, particle impingement, erosion, and fretting wear (References 5,7,9).

The 17 tubes pulled during the past 3 years have not shown any new or 1

different mod, es of degradation in OTSGs than those observed from the tubes pulled from 1976 to 1982. Sulfur related IGA was once again the only form of corrosion identified (References 2,4,5). Fretting wear, particle impingement, and erosion were the only forms of mechanical degradation observed.

From this experience, FPC concludes no new modes of degradation have appeared in OTSGs over the 18 year period in which these tubes were pulled.

i

)

Reduced sulfur related IGA on the secondary side of the tubes has been found at the lower tubesheet secondary face (LTSF) and in the LTS and 1st span (References 2,3,8,10); in the lane and wedge region at the 14th and 15th TSPs, and at the upper tubesheet secondary face (UTSF) (Reference 8); and in the freespan of the l

upper bundle. The ' sensitized microstructure of the OTSG tubing makes it more resistant to caustic attack, but makes it susceptible to low temperature, intergranular attack by sulphur oxyanions under acidic, oxidizing conditions (Reference ll).

Under these conditions, nickel is preferentially dissolved at j

the grain boundaries due to the lower chromium content present there.

These conditions are not achievable at power operation, thus isolated events during shutdown are believed to be responsible for the initiation of '.he observed IGA in OTSGs.

2-12 l

l l

TSCRN 203 Technical Report High cycle fatigue has been confined to the lane region'at.the 15th TSP or UTSF due to the high steam cross-flow present in the region (References 5,7). With high cycle fatigue, typically a circumferential crack forms and rapidly goes through wall resulting in primary-to-secondary leakage. Preventative sleeving of this region has been undertaken by the OTSG owners to address this form of degradation.

Particle impingement and erosion has been observed at only one plant site and is restricted to the upper portion of the tube bundle (References 5,7).

Fretting wear has been seen at various elevations and locations across the width of the tube bundle. Most OTSGs have some tubes plugged due to fretting wear (References 5,7).

1 With the exception of high cycle fatigue, all of the above forms of degradation have a volumetric morphology. Outside diameter stress corrosfon cracking has not been observed in any pulled tubes from OTSGs.

The microstructure of OTSG tubing makes it more resistant to the caust'ic corrosion which other steam generator models have experienced. The low heat flux at the lower tubesheet also makes concentrating impurities underneath sludge piles and in the LTS crevice more difficult than other steam generator models.

Accordingly, OTSGs have not experienced difficulty in these regions. The broached TSP structure is more open than the drilled-hole design. Therefore, concentrating impurities in tight TSP crevices which can lead to widespread denting has also e

not been encountered in OTSGs.

2.2.2.1 Crystal River-3 Decradation Experience The Crystal River-3 degradation experience is consistent with that observed in OTSGs in general. The tubes pulled from the "B" 0TSG in 1992 revealed the presence of small pit-like patches of IGA. IGA with this morphology was observed in a tube pulled previously at another OTSG (Reference 8).

In addition to pit-like IGA, circular and tapered wear was observed in the tubes pulled from the "B" 0TSG in 1994.

These findings are not outside of the range of OTSG experience based on all 36 pulled tubes.

The 1994 Crystal River-3 pulled tubes was the first time that circular wear had been' observed in OTSG tubes. However, circular wear has been observed in other steam generator models, and is not unique to Crystal River-3. For example, circular wear has been observed on a tube removed from Almaraz-1 (Reference 3).

The wear scar shape is strongly controlled by the inclination of the tube relative to the adjacent support structure; therefore, various wear scar shapes are possible depending on the inclination of the tube relative to the TSP, and the presence of any rough spots or raised areas on the TSP.

While no tubes have been pulled from the Crystal River-3 "A" OTSG, there is no evidence to indicate that the "A" OTSG would experience any different modes of.

degradation than that identified by pulled tubes from other OTSGs over the past 18 years. The operation and feedwater chemistry of the "A" 0TSG has been similar 2 13

i i

TSCRN 203 Technical Report -

j to the "B" OTSG, and to that of other OTSGs. Eddy current inspections have shown the "A" 0TSG to contain far fewer indications than the "B" 0TSG, but the voltage distribution of S/N indications (Figure 2-4) is very similar.

RPC inspections have shown the morphologies of the indications to be volumetric like these in "B" 0TSG, and consistent with the morphology of the modes of degradation observed in the OTSG pulled tubes, excluding those with high cycle fatigue.

Further, the fact that the modes of observed degradation in.0TSG pulled tubes have remained unchanged over the past 18 years in tube pulls that involved 8 of 14 operating OTSGs, indicates that no new modes, of degradation have occurred among the population of OTSG tubes. Therefore, there is no reason to believe that the "A" 0TSG experience would not be bounded by the total OTSG experience as determined by tube pulls.

i l

l O

l l

6 2-14 l

e l

=

TSCRN 203 Technical Report Section 3 Non-Destructive Examination Considerations

{

3.1 Background

J i

Non-destructive examination (NDE) is an important element in implementing any l

steam generator tube repair criteria.

This is especially true for the CR-3 i

approach to dispositioning low S/N indications since the proposed strategy j

utilizes several NDE parameters and two distinct detection techniques.

In the CR-3 approach, the axial and circumferential dimensions (i.e. length and width) of volumetric S/N indications are used to assess tube structural integrity, i

Implicit in this approach is the capability to reliably determine degradation 1

morphology as well as conservatively measure length and width of the indication.

i The motorized rotating pancake coil. (MRPC) probe is used to provide this 1

capability. Bobbin coil signal amplitude (i.e., voltage) is used to assess tube leakage potential and monitor indications for signs of growth.

Bobb'in voltage is also employed to make inspection' workload more manageable by minimizing, to the extent possible, unnecessary MRPC. Other inspection equipment and techniques could be used in the' future with this approach provided they are similarly evaluated.

The probability of detection (POO) for low S/N indications remains unchanged by the proposed approach since the primary means of detecting these indications (bobbin coil eddy current) remains the same.

Thus, P00 is unaffected by the proposed repair criteria and' is therefore not discussed further within this report.

This section discusses CR-3 and industry experience in determining indication morphology and measuring axial and circumferential dimensions of detected degraded areas in tubes using MRPC.

Use of bobbin coil signal amplitude (voltage) to assess degradation depth is also evaluated. Measurement uncertainty, an important consideration in establishing repair limits (maximum allowable axial and circumferential dimensions, and bobbin coil voltage.of volumetric S/N indications), is also discussed.

3.2 Assessment of Tube Integrity 3.2.1 Accuracy of Mornholoey Determination The ability to reliably determine the structure and form of a S/N indication is fundamental to the application of a morphology-specific tube repair criteria. The proposed CR-3 approach has been developed for volumetric indications (i.e., IGA, wear, etc.) which have a bobbin coil signal-to-noise ratio of less than 5:1. The approach excludes cracks and other non-volumetric morphologies (e.g., tapered wear).

While the approach of excluding crack-like indications bounds the degradation known to be present in the CR-3 OTSGs, the conservative analytical approach taken during development of the proposed structural repair criteria 3-1

TSCRN 203 Technical Report bounded a crack-like morphology.

Thus, the ability to determine morphology is largely an issue of preserving the conservatism in the analysis as opposed to maintaining baseline technical adequacy.

In other words, should the proposed criteria be applied to other than volumetric indications, there would be minimal safety impact or consequence.

Nevertheless, MRPC has been shown to be very effective at determining the morphology of low S/N indications.

The ability of MRPC to accurately identify the morphology of tube indications was assessed using tubes pulled from the "B" CR-3 steam generator in 1992 and 1994.

Data employed in this assessment is presented in Table 3-1.

Table 3-1 CR-3 Pulled Tube Morphology Identification TUBE SECTION AXIAL POSITION ID NUMBER EC ANALYSIS CODE 52-51-2 LTSF + 9.25" 0

VOL LTSF + 12.75" 12 VOL I

LTSF + 12.75" VOL 90-28-2 L TSF + 13.6" C

VOL l

LTSF + 13.6" 8

YOL L TSF + 15.3" E

VOL LTSF + 17.7" VOL LTSF + 17.7" H

VOL LTSF + 17.7" G

VOL 97-91-2 LTSF + 19.0" U

VOL l

LTSF + 19.0" i

VOL l

LTSF + 19.0" S

VOL LTSF + 21.6" W

VOL 106-32-2 LTSF + 13.8" X2 VOL LTSF + 13.8" Y

VOL LTSF + 13.8" X1 VOL LTSF + 16.3" AG2 VOL l

LTSF + 16.3" AH VOL LTSF + 21.0" AT VOL LTSF + 21.0" AU VOL 68-46 14 075 - 0.19" 148 VOL 12-49-13 075 - 0.87" 138 VOL 109-71-7 035

_0.00" 78X VOL 109 71-14 075 - 0.68" 148 VOL 136-26-15 075 - 0.68" 158 VOL All of the indications examined by field MRPC and reported as exhibiting a volumetric morphology were subsequently confirmed as volumatric by metallographic examinations of the pulled tubes. In no case did MRPC incorrectly report the morphology of the indication. The indications in Table 3-1 include both pit-like IGA found in,the first span, and circular or oval wear found at the 3rd and 7th TSPs. Although not included in the table, subsequent laboratory. examination of 3-2

TSCRN 203 Technical Report the 1994 pulled tubes also found MRPC to have correctly reported the indications at the 9th TSP that exhibited a tapered wear morphology.

Although no crack-like indications were present in the CR-3 pulled tubes, circumferential crack-like (CCL) indications were identified by MRPC in six tubes along the untubed lane during the 1992 Refuel 8 inspection. All CCLs were at the secondary face of the upper tubesheet, a region OTSG field experience has shown to be prone to high-cycle fatigue.

While no tubes have been pulled from CR-3 with indications at this location, the presence of CCLs here is consistent with the current understanding of the mechanism of high-cycle fatigue initiation and propagation. Therefore, it is concluded that CR-3 experience demonstrates MRPC can identify crack-like morphologies in OTSG tubing where present.

Further, MRPC has been qualified for detection of primary water stress corrosion cracking (PWSCC) at the roll transition in recirculating steam generators in accordance with Appendix H of the EPRI "PWR Steam Generator Examination Guidelines (Reference 12). Thus, recirculating steam generator experience also shows that MRPC can reliably identify crack-like morphologies.

The 1992 and 1994 CR-3 pulled tube data, as well as CR-3 and other industry experience, thus demonstrate the high reliability of MRPC to correctly identify the morphology of eddy current indications in the CR-3 tubes. Further, the CR-3 pulled tube and industry data demonstrates that MRPC can be used confidently to discriminate indication types (1.e. volumetric, tapered wear, crack-like) for the purposes of applying a morphology specific repair criteria.

3.2.2 Dimensional Measurement Accuracy The ability to accurately measure S/N axial and circumferential extent is necessary to justify the application of a dimension-based (length and width) repair criteria.

Reference 1 demonstrated the inability of bobbin coil eddy current to accurately determine the depth of S/N indications based on a comparison of field eddy current data to pulled tube metallurgical examination results. Figure 3-1 (Reference 2) illustrates both this inaccuracy in sizing of bobbin coil technology and the inappropriateness of the current technical specifications for S/N indications.

Accordingly, FPC developed an approach utilizing MRPC to measure the axial and circumferential dimensions of S/N indications for. the 1992 Refuel 9 outage.

Use of this interim approach was allowed by the NRC staff (References 1 and 3). Tubes containing S/N indications with lengths or widths greater than or equal to specified maximum values were plugged.

1 The ability of MRPC to reliably measure the axial and.circumferential dimensions of S/N indications was assessed using tubes removed from the CR-3 "B"

steam generator in 1992, and reported to the staff in Reference 1. This assessment has been updated to incorporate data from the 1994 pulled tubes. Table 3-2 provides a listing of the data employed in this assessment.

3-3 n

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TSCRN '203 Technical Report h

COMPARITIVE ANALYSIS 5/14/92 100 h9 u

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3-4 e

i TSCRN 203 Technical Report Table 3-2 CR-3 Pulled Tube Dimensional Sizing TU8E SECTION AX!AL 10 NUMBER MET. AXIAL MET. CIRC.

MRPC AXIAL MRPC CIRC.

POSITION EXTENT, IN.

EXTENT, IN.

EXTENT, IN.

EXTENT, IN.

52-51-2 LTSF + 9.25 0

0.061 0.041 0.15 0.20 LT5F + 12.75 12 0.048 0.037 0.19 0.17 LTSF + 12.75 0.053 0.037 0.19 0.17 90-28-2 LTSF + 13.C C

0.053 0.010 0.14 0.19 j

LTSF + 13.6 8

0.063 0.020 0.11 0.14 LTSF + 15.3 E

0.071 0.038 0 16 0.20 LTSF + 17.7 I

0.063 0.023 0.19 0.19 j

LTSF + 17.7 H

0.059 0.022 0.15 0.19 LTSF + 17.7 G

0.079 0.039 0.15 0.19 91-91-2 LTSF + 15.3 P

0.073 0.074 0.15 0.17 LTSF + 15.3 0

0.076 0.062 0.15 0.19 LTSF + 19.0 0

0.054 0.047 0.15 0.25 LTSF +*19.0 T

0.061 0.066 0.19 0.18 LTSF + 19.0

'O.011 0.0005 0.08 0.10 LTSF + 21.6 W

0.061 0.047 0.16 0.18 106-32-2 LTSF + 13.8 X2 0.071 0.053 0.11 Q.14 j

LTSF + 13.8 Y

0.015 0.008 0.13 0.11 LTSF + 13.8 XI 0.016,

0.038 0.11 0.19 LTSF + 16.3 AG2 0.062 0.035 0.11 0.13 LTSF + 16.3 AH 0.056 0.028 0.12 0.15 LTSF + 21.0 AT 0.060 0.050 0.15 0.19 LTSF + 21.0 AU 0.047 0.054 0.11 0.25 68-46-14 075 - 0.79 148

0. 09 0 0.119 0.18 0.20 72-49-13 071 - 0.87 138 0.094 0.134 0.11 0.22 109-71-7 035 - 0.00 78X 0.086 0.97 0.08 0.10 109-71-14 075 - 0.68 148 0.112 0.101 0.20 0.19 136-26 15 015 - 0.68 158 0.112 0.170 0.17 0.23 The actual dimensions as determined by metallographic examinations and the dimensions predicted by the field inspection of the 1992 and 1994 pulled tubes are listed in the table. In addition, to assess the dimensional sizing accuiccy of MRPC for much larger areas. of degradation than that found in the CR-3 pulled l

tubes, similar dimensional sizing data for laboratory grown sulfur-related IGA i

specimens was performed (see Table 3-3).

3-5 l

9 6

b TSCRN 203 Technical Report Table 3-3 B&W0G NDE Connittee IGA Samples MRPC Sizing Data Sample No.

Length, in.

Width, in..

MRPC MRPC Length, in.

Width, in.

1217423-8 1.508 0.263 1.62 0.524 1217423-C 0.772 0.255 0.88 0.524 1217423-0 1.5 0.236 1.56 0.436 1217424-B 1.49 0.245 1.59 0.436 i

1217424-C 0.751 0.243 0.85 0.480 12174243 1.51 0.246 1.65 0.480 1217425-B 1.506 0.253 1.56 0.393 1217425-C 0.746 0.246 0.99 0.436 l

The laboratory IGA specirens were produced for the NDE, Committee of the B&W Owners Group.

The IGA samples were prepared by exposing sensitized Alloy 600 tubing to acidified sodium tetrathionate at near ambient temperature.

This process has been found to successfully produce in the laboratory the reduced sulfur IGA found in the field in OTSGs.

1 Figure 3-2 compares the actual S/N dimensions as determined by metallographic examinations versus the dimension predicted by MRPC for the pulled' tubes (pit-j 2

like IGA and volumetric wear) and laboratory produced IGA. The axial and circumferential dimensions are treated similarly since they are both measures of extent. The two are plotted together in the scatter plot for a combined data set comprised of 62 data points. Linear regression analysis shows that the data is highly correlated with a correlation coefficient (R) of 0.99.

4 The dimension measurement error, as evidenced by the regression line, is strongly systematic with a parallel offset from the true line by approximately 0.1 inch.

The data clustered at the bottom of Figure 3-2 consists entirely of the 54 data points from the Crystal River-3 "B" steam generator pulled tubes, whereas the remaining data are from the B&W Owners Group IGA samples. The measurement error, defined as predicted dimension minus actual, is shown as a histogram in Figure 3-3.

A Gaussian fit to the histogram gives a mean error of + 0.103 inch, and standard deviation of 0.045 inch.

Figures 3-2 and 3-3 thus show that the dimension measurement process by MRPC is systematically conservative in that the measured dimensions are consistently predicted to be greater than the actual.

3.3 Assessment of Tube Leakage Potential The ability to develop an NDE parameter indicative of S/N depth is important to the assessment of tube leakage potential.

In Reference 1 it was shown that

• bobbin coil phase angle cannot be used to accurately determine the depth of S/N i

indications.

Howaver, a general relationship can be shown to exi.st between through wall penetration and several other NDE measured parameters. Eddy current 3-6 E

1 TSCRN 203 Tschnical Rep rt

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3-7

TSCRN 203 Technical Repcrt l

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TSCRW 203 Technical Report signal amplitude is proportional to the removed or affected metal volume for three-dimensional discontinuities. This proportionality can be used to provide a statistical basis for sizing discontinuities that exhibit some regularity in their shape such as pit-like IGA or circular wear. This approach is very well suited to a morphology-specific disposition strategy such as the one proposed by i

CR-3.

To address primary-to-secondary leakage potential, bobbin coil signal amplitude was correlated to discontinuity volume and depth, and a maximum bobbin coil voltage was defined to preclude operation with through wall S/N indications. This apprcach was reviewed by the NRC staff and allowed on an interim basis for dispcsitioning S/N indications in the 1992 Refuel 9 outage (see References 1 and 3). Tabes with indications that exceeded the maximum voltage criterion were removed from service.

The ability of bobbin coil signal amplitude to reliably estimate the depth of S/N indications was assessed using tubes removed from the CR-3 "B" steam generator in 1992, and reported to the NRC in Reference 1. This assessment has been updated to incorporate data from the 1994 pulled tubes and its conclusions revised to reflect new data.

Detailed dimensional data of the degradation examined in the 1992 and 1994 Crystal River-3 pulled tubes show that the dimensions for free-span pit-like IGA and circular wear ara correlated with volume. This correlation is illustrated in Figures 3-4 through 3-6 which are scatter plots of volume versus depth, axial length, and circumferential extent, respectively. For each dimension, the volume is highly correlated as evidenced by the large' values of correlation coefficients. The relationship

• between discontinuity volume and bobbin coil signal amplitude is established using data from the Crystal River-31992 and 1994 tube pulls.

Table 3-4 provides.a listing of this data.

Table 3-4 r,obbin Coil Voltage / Volume Correlation Tube Numbe:-

Location Volume,E-6 in' Voltage' 90-28 LTSF + 6.38" 37.5 0.74 LTSF + 7.88" 62.3 1.19 LTSF + 10.35" 28.8 0.72 97-91 LTSF + 8.28" 68.8 0.74 LTSF + 14.28" 70.9 0.52 109-30 LTSF + 5.66" 26.9' O.47 LTSF + 7.99" 39.2' O.58 s

68-46 075 - 0.56" 68 1.1 72-49 07S - 0.69" 35 0.42 109-71 03S - 0.67" 17 0.25 109-71 07S - 0.70" 66 0.91 136-26 07S - 0.70" 68 0.87 CR-3 voltage normalization using Channel P1 Total volwne of multiple discontinuities 3-9 m

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TSCRN 203 Technicil Reptrt

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S 3-12

TSCRN 203 Technical Repcrt

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3-13

TSCRN 203 Technical Report Voltage values were obtained by re-analyzing selected Crystal River-3 1992 and 1994 eddy current field data. Single eddy current indications that could be uniquely associated with discontinuities found by metallographic examinations were included in Table 3-4. Figure 3-7 shows the relationship between bobbin coil voltage and discontinuity volume using the data in Table 3-4.

Using Figure 3-7, 1

the discontinuity volume associated with a particular bobbin coil voltage can be derived. Figures 3-4 through 3-6 can then be used to bound the dimensions of a discontinuity of that volume.

These dimensions can then be compared to structural and leakage limits to develop or evaluate appropriate NDE limits.

To illustrate the approach, the lower bobbin voltage cutoff value (0.9V) proposed as part of the disposition strategy is evaluated using the data in Figures 3-4, 3-5, 3-6, and 3-7.

A bobbin coil signal amp a discontinuity volume of about 55 x 10 in.,litude of 0.9 volts corresponds to from Figure 3-7.

From Figures 3-5 and 3-6, this volume equates to an axial length of 0.07 inches and a.

circumferential extent of 0.08 inches, respectively. From Figure 3-4, a volume of 55 x 104 in.' corresponds to a depth of approximately 45 % through wall. The discontinuity through wall is much less than the calculated maximum allowable of 87% TW, therefore it would not be expected to contribute to leakage. The predicted length and width are much less than the maximum values calculated based i

on burst considerations in Section 4, i.e. 0.33 inch and 0.6 inch, respectively.

Thus, these dimensions are not critical from either a burst or leakage 1

standpoint.

Because the majority of the field S/N indications exhibit bobbin coil signal l

amplitudes of less than one (1) volt, laboratory samples were also evaluated to verify the relationship of discontinuity volume to voltage for much higher signal amplitudes. Data from the laboratory samples are given in Table 3-5.

The bold-type values listed in the table are the values used in Reference 1.

The other values a.re new data that has resulted subsequently. The raw eddy current data from the laboratory bobbin coil inspection of these specimens was reanalyzed using Crystal River-3 procedures and analysis guidelines to obtain the signal amplitudes listed in Table 3-5.

q 3-14 4

TSCRN 203 Technical Report l

Table 3-5 l

B&W0G NDE Coemittee IGA Samples Voltage vs. Volume Data Sample Maximum

Axial, Circum.,

Bobbin Approx.

2 Number

%TW i n. i.2 i n. i.2

. Coil

Volume, l

Voltage in.*

1217423-A 55 0.75 0.245 2.7 0.0034 1217423-8 72 1.5 0.245 7.5 0.0083 1217423-0 67 1.5 0.245 6.1 0.0082 l

1217423-E 55 0.75 0.245 2.9 0.0034 1217424-A 56 0.75 0.245 3.4 0.0035 1217424-8 79 1.5 0.245 9.8 0.0092 1217424-0 80 1.5 0.245 6.9 0.0102 1217424-E 71 0.75 0.245 7.7 0.0044 1217425-A 22 0.75 0.245 0.5 0.0013 1217425-B 42 1.5 0.245 2.25 0.0046 1217425-E 41 0.75 0.245 1.1 0.0025 Nominal dimensions Olmensions not used in calculation of volwie Figure 3-8 shows a scatter plot of the bobbin coil voltage versus discontinuity volume from Table 3-5.

A linear regression analysis shows that the data is highly correlated, indicating a strong relationship between voltage and degraded

(

metal volume. The laboratory IGA samples thus show that the strong voltage to volume correlation exists over a wide range of defect volumes: 10' in' for the Crystal River-3 pulled tubes to 10'8 in' for the laboratory samples.

The use of a bobbin coil signal amplitude repair limit is not unique to the CR-3

)

application for S/N indications, but is also ustd to disposition 00 SCC in the tube support plates of recirculating steam generators (Reference 13).

Projections of through wall penetration for the population of low S/N indications inservice at CR-3 have been made based upon pulled tube metallography results (Refer to Section 5.0 of this Report). These projections indicate there is a low probability of any S/N exceeding the maximum allowable %TW.

CR-3 operational history of essentially no primary to secondary leakage provides confirmation of these projections.

These factors speak to the conservatism in developing an additional NDE limit specific to leakage.

3.4 Allowance for NDE Measurement Uncertainty The general repair limit equation can be written as follows:

l RL - AL - (NDE + GR)

(Equation 3-1) where RL is the NDE measured repair limit parameter, AL is the maximum allowable

)

analytical limit, NDE is the measurement error, and GR is the growth rate. Based i

upon the results of the growth rate studies discussed in Section 4.0 of this 3-15 j

TSCRN 203 Tcchnicil R:ptrt 1

l

• a g.

...r..

m.

l Y = 0.82

• X + 0.32 l E.E R e 0.91 SD e 0.12 weii e= = =

5

....E..

M e

f 8

l 3

3. 4 3

3 0.1 ai 1

9 Signal amplitude, wits Figure 3-8. BWOG NDE Committee IGA Samples - Voltage / volume correlation.

3-16

l i

TSCRN 203 Technical Report report, a value of zero is assumed for 'GR' throughout. The dimension-based repair criteria described in this document uses MRPC to assess tube structural integrity by measuring the axial and circumferential extents of the S/N indication. Bobbin coil signal amplitude is used in this approach to assess the potential for tube leakage by inferring the depth of the S/N indication.

An analysis of the NDE error associated with each of the two techniques is described below.

3.4.1 Uncertainty in MRPC Dimensional Measurements The MRPC measurement accuracy data is shown in Figures 3-2 and 3-3. From Figure 3-3, the mean measurement error is + 0.103 inches with a standard deviation of 0.045 inch. The general repair limit equation (Equation 3-1) can be re-written l

in terms of the mean and standard deviation as follows:

RLu

- Sla

+ mean error - (N

• standard deviation) (Equation 3-2)'

1 i

where N is a multiplier determined by the desired confidence limit. For example, N - 1 for a 1-sigma value; N = 2 for a 2-sigma value; etc. The growth rate term is neglected based on the 1994 S/N growth rate study (Reference 14). Choosing N

- 2, the net correction is positive with a value of 0.013 inches. The repair limit can thus be larger than the structural limit by 0.013 inches because of the conservatism in the MRPC predicted dimension being larger than the actual dimension. No additional error margin need be included. Therefore, in practice, the repair limit can be taken to equal the structural limit, i.e. RL - SL.

Determination of the dimensional repair limit values are discussed further in

• Section 4.0 of this report.

4 3.4.2 Uncertainty in Bobbin Coil Voltace Measurements The two principal sources of error in voltage measurements are hardware and analysis var,iation (Reference 13). An estimate of analysis variation has been determined using multiple probing of tubes and analysis of indications removed from CR-3 in 1994.

Six indications, exhibiting mean voltage values ranging from approximately 0.4 to 1.4 volts, were each examined ten times.

The average percentage error is defined as the ratio of the standard deviation for bobbin voltage to the mean value. An average percentage arror of 13.05% was calculated.

The error allowance assumed for acquisition (i.e., hardware) variability is 7%

(Reference 13).

This value is consistent with industry accepted standards.

The bobbin coil voltage errors for acquisition and analysis are independent; this allows the two errors to be combined in a root-sum-square manner. Additionally, since one is only concerned about non-conservative random error components, the one-side percentage error is calculated by multiplying the standard error by a factor of 1.645. This yields a final NDE measurement percentage error of 24.2%.

Determination of the voltage-based repair limit value is discussed further in Section 5.0 of this report.

1 3-17

TSCRN 203 Technical Report 3.5 Conclusions Regression analysis of volumetric degradation (pit-like IGA and circular wear) dimensional data (i.e., length, width, and depth) has shown that these dimensions l

correlate well with volume. An amplitude / volume correlation was demonstrated l

using both the 1992 and 1994 Crystal ' River-3 pulled tubes, and the B&W Owners l

Group laboratory IGA samples. This correlation implied that discontinuity l

dimensions can be bounded using bobbin coil voltage since signal amplitude and l

volume are proportional. It is thus possible to develop empirical bobbin coil alternate repair criteria in a manner similar to 00 SCC at the tube supports plates of recirculating steam generators.

Comparison of Crystal River-3 rotating probe diagnostics showed that all in-generator indications were reported as having a volumetric morphology, consistent-with the metallographic examination results in which all discontinuities were identified as being either pit-like IGA or wear.

Error analysis of MRPC dimensional sizing data showed that the measurement process is strongly conservative; therefore, no additional margin needs to be provided in implementing a dimension-based (length and width) repair limit. The structural limit (derived in Section 4) can be used as the repair limit.

Finally, an estimate of NOE measurement error for bobbin coil voltage was provided in the context of error analyses conducted for other industry alternate tube repair criteria.

e G

e 3-18 4

TSCRN 203 Technical Report l

l Section 4 l

Structural Integrity Eval' ation u

4.1 Background

This section develops the relationship between burst pressure and axial and i

circumferential extent, conservatively modeling the three-dimensional volumetric i

degradation present in the CR-3 OTSGs as 100% through wall cracks. The allowable degradation axial and circumferential extents are the largest discontinuity i

dimensions which can be left in service.

On the basis of the allowable degradation dimension limits, axial and circumferential extent repair criteria i

are developed for volumetric S/N indications outside the tube sheet regions.

These dimensions include margin on load, and consideration of variable material properties, degradation growth between inspections, and degradation dimencion i

measurement uncertainty.

This dimension-based, morphology-specific approach j

is proposed as an alternate to the existing technical specification depth-based criteria, for dispositioning low S/N indications.

The decision to pursue this approach was based on the inaccuracies associated with bobbin coil phase angle determinations of through wall sizing.

There is precedence for use of MRPC to measure a dimension other than depth in order to assess the pressure retaining capability of steam generator tubing. A similar approach is currently used in Europe to measure the length of primary water stress corrosion-induced cracks in the tubesheet roll transition of certain recirculating steam generators (Reference 15).

4.2 Development of Structural Repair Limiti 4.2.1 Structural Analysis Reference 1 included a structural evaluation of maximum allowable OTSG tubing degradation. The calculation was performed by MPR Associates, Inc in order to establish a tube repair criteria to-address Regulatory Guide 1.121 (Reference 16).

structdral requirements. The calculation determined th,e maximum allowable axial and circumferential extents for S/N indications using probable tubing properties (with a 95% probability of occurrence at a 95% confidence level) and included a point-by-point discussion of the Regulatory Guide requirements.

To determine allowable axial extent, an axial slot-type defect was analyzed.

A number of different combinations of circumferential and axial extent were analyzed in the analysis to determine allowable circumferential extent.

A review of the original structural analysis was performed by Packer Engineering, Inc. during preparation of this report and a revision initiated to incorporate the latest calculational techniques. A summary of the results of the original analysis as well as a discussion of the revision is provided below.

i j

4-1 i

l

TSCRN 203 Technical Report 4.2.1.1 Allowable Circumferential Extent The structural analysis of Reference 1 calculated the maximum allowable I

circumferential extent for a through-wall defect in OTSG tubing considering the structural limits specified in NRC Regulatory Guide 1.121.

The analyses were performed using both code minimum and probable CR-3 tubing properties. Further, the analysis used the axial tube load corresponding to a full-area guillotine break of the reactor coolant system at the OTSG inlet or outlet (i.e., 2641 pounds) as the limiting case to calculate the maximum allowable circumferential extent for a through wall defect. For probable CR-3 tube groperties, the maximum allowable circumferential extent was determined to be 122 arc length or 0.6 inch for a through wall defect. For defects which did not extend through wall, larger I,

allowable circumferential extents were calculated.

i It should be noted that there is still significant conservatism in the Reference 1 structural analysis for circumferential extent. All defects are considered as planar, taking no credit for ligaments between microcracks.

Furthermore, j

application of the General Design Criteria 4 leak-before-break exclusion to the guillotine break of the reactor coolaht system would eliminate this as a credible l

transient, and would result in a significantly reduced limiting axial load for the tube structural analysis. The maximum steam line break load of interest for this analysis is calculated to be 1402 pounds tensile. Incorporation of this lower tube load into the structural analysis would result in about a factor of 2 increase in the maximum allowable circumferential extent for through wall defects. Since the Reference 1 analysis for circumferential extent is not being revised to incorporate a lower accident tube load, the proposed circumferential structural limit of 0.6 inch remains very conservative.

4.2.1.2 Allowable Axial Extent Investigators have developed a number of burst behavior relationships for axial cracks, and correlated them with experimental results (i.e., burst ~ tests of tubes containing EDM slots, or cracks grown in the laboratory, in model boilers, or in the field in operating steam generators). These relationships are in terms of tube parameters that control burst behavior and can be used to predict tube burst performance. Parameters used in the correlations are tube and crack dimensions, material properties and load. One such empirical relationship that has been developed is the EDM slot equation of NUREG/CR-5117, " Steam Generator Tube Integrity Program / Steam Generator Group Project", May 1990 (Reference 17).

Reference 1 calculated the maximum allowable axial length foi a through wall defect in OTSG tubing by modeling three dimensional degradation as a crack using the EDM slot equation. Calculations were performed using both code minimum and CR-3 probable tubing properties.

For a through wall axial crack, a maximum allowable axial length of 0.25 inches was calculated.

This axial limit was applied in the interim repair criteria used during Refuel 9 (1994) to disposition S/N indications.

l 4-2 i

i

TSCRN 203 Technical Report i

Recently, based on industry wide data, EPRI has revised its burst equation to te essentially in agreement with the Hernalsteen equation (Reference 18).

Using lower bound tensile properties, an allowable axial crack length of 0.33 inch was calculated by Packer Engineering, Inc. using the new equation.

This higher allowable axial length is employed in the proposed S/N repair criteria for CR-3.

4.2.2 Development of Structural Repair Limit i

In Section 3.2.2 it was shavn that MRPC measurement of axial and circumferential extent is systematically conservative in that measured dimensions are consistently predicted to be greater than actual. As a result, the uncertainty analysis presented in Section 3.4.1 showed that the structural repair limit (RL) could be justified to be greater than the analytical structural limit (SL) by' 0.013 inches. For simplicity and as an added measure of conservatism, the RL is assumed to be the same as the SL.

Therefore, the limits on axial and i

circumferential extent proposed for incorporation into technical specifications are 0.33 and 0.6 inches, respectively.

4.3 Assessment of the Proposed Structural Repair Li;aits 4.3.1 Review of Field Data In response to Reference 19, Confirmatory Action # 4, a large number of CR-3 S/N field indications underwent MRPC inspection during the 9R Outage (May 1994). One of the purposes of this examination was to assess axial and circumferential extent of the larger indications (as determined by bobbin coil voltage and previous MRPC inspection experience) against the then-current dimensional repair criteria.

Figures 4-1 and 4-2 provide the results of this examination for volumetric indications.

A total of 413 locations were included in the supplemental MRPC inspection with 4

approximately 260 of these being indications exhibiting a volumetric morphology.

Of the 260 volumetric S/N indications, six exhibited axial extents > 0.25 inches with two of the six having an axial extent greater than the proposed RG 1.121 structural limit (0.33 inches).

All tubes containing indications > 0.25 inch axial extent were removed from service during 9R.

None of the indications examined during 9R had measured circumfer.ential extents greater than the proposed limit of 0.6 inches.

The indications included within the MRPC examination exhibited the largest bobbin coil amplitudes from the population of inservice S/N indications.

Given the direct relationship between bobbin voltage and dimensional extent (See Figures 3-4, 3-5, 3-6, and 3-7), the population of indications included within Figures 4-1 and 4-2 are expected to be representative of the largest indications (in terms of axial and circumferential extent) in service at CR-3. The conservatism inherent to the MRPC technique of sizing volumetric indications has also been documented (See Section 3.2.2). Based upon these considerations, it is concluded that the structural integrity of the remaining inservice S/N indications is adequate.

4-3

l i

i

'TSCRN 203 Technical Report i

1 i

4 i

j q

i l

Figure 4-1 9R MRPC Axial Extent Distribution of Volumetric S/N Indications i

120 --

i I

100 --

f RC 1.121 Limit of 0.33 inches l

e I

8 m-E h

o 60 --

l

[

n s

40 --

l z

20 --

i 4

0 1

I I

I i

1--

-I f

0-0.05 0.06-0.1 0.11-0.15 0.16-0.2 0.21-0.25 0.26-0.3 0.31-0.35 t

i.

Axial Extent (loches) i l

4-4 l

... - _ ~ _

I l

-~

TSCRN 203 Technical Report i

l l

i l

t i

l Figure 4-2 j

9R MRPC Circumferential Extent Distribution of Volumetric S/N Indications i

l 120 -

i l

~

RG 1.121 Limit of 0.60 inches l

e 80 --

E s 80 N

\\

3

.o 40 --

y\\

z x

t

[

N 20 E

~

t O

1 I

I I

I

-t 1-1 l

t 0-0.05 0.06-0.1 0.11-0.15 0.16-0.2 O.21-0.25 0.26-0.3 0.31-0.35 0.36-0.40 Circumferential Extent (Inches) i t

4-5 1

J TSCRN 203 Technical Report 4.3.2 Review of Pertinent Laboratory Examinations 4.3.2.1 Suissary of Pulled Tube Defects Pulled tube metalographic examination information was reviewed to assess the i

range of defect sizes, including maximum defect size', contained within the population of CR 3 pulled tube defects.

For volumetric defects, the largest dimensional extents identified during the 1992 and 1994 CR-3 pulled tube examinations were 0.228 inches axial (IGA patch) and 0.170 inches circumferential (circular wear; estimated).

Therefore, the actual size of the largest l

degradation observed in the pulled tubes is much smaller than that determined by calculation to be structurally significant.

l 4.3.2.2 Burst Testino Results Burst testing was performed on sections of seven tubes pulled from CR-3 in 1992 and 1994.

The testing included both defect-free samples and sample sections containirig wear and IGA defects. Results of the burst testing from the two tube pulls are discussed below.

4.3.2.2.1 1992 Pulled Tube Results Two tube samples were burst tested in 1992: the first span section (6-21 inches above the lower tube sheet secondary face (LTSF)) of CR-3 pulled, tubes 97-91 and 106-32.

These tube sections contained a combined total of more than 80 IGA 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 0.0829 inches. Tube 106-32 contained defects with axial extents up to 0.0979 inches, through wall depths up to 51%, and circumferential extents up to 0.0529 inches.

A first span section from 1992 CR-3 pulled tube 4i-44 was burst tested during the 1994 examination.

Stereovisual inspection following burst testing revealed a total of 33 small IGA patches from 5 to 18 inches above the LTSF.

This tube section contained indications with axial extents up to 0.080 inches, through wall depths up te 54%, and circumferential extents up to 0.106 inches. Burst pressure test results for the tubes pulled in 1992 are sumarized in Table 4-1. Tube burst pressures wera well above (> 2.4 times) the Regulatory Guide 1.121 limit of three times operating differential pressure (4050 psi).

Table 4-1 1992 CR-3 Burst Test Results Tube Number Number of Burst Pressure Defect Depth at Indications (psi)

Burst (%TW) 41-44' 33 9,800 54%

97-91 17 10,300' 54%

106-32 65 10,900' 40%

Pullb in 1992. Surst in 1994.

Burst pressure presented in Reference I was adjusted per discussion in Reference 3. Appendix A.

4-6

^

f i

TSC.'IN 203 Technical Report The results also demonstrated that, despite the large number of defects present, i

there was no significant reduction in burst pressure.

This was an important l

finding relative to addressing the possibility of multiple defects " combining" to adversely affect burst pressure. This finding.is consistent with previous studies of the effect of multiple axial cracks on steam generator tube burst pressure (Reference 15).

i Defect-free sections of pulled tubes 97-91, 106-32, and 41-44 were also burst.

Results are shown in Table 4-2.

Calculated burst pressures are based upon measured tensile properties and show good agreement with measured values.

In each case, measured burst pressure exceeded calculated.

Table 4-2 Burst Pressures of Defect-Free 1992 Pulled Tube sections Tube Number Measured Burst Calculated Burst Pressure (psi)

Pressure (psi) 41-44 10,100 9,582 97-91 10,450 9,961 106-32 11,250 10,141 A comparison of Table 4-1 and 4-2 results shows that the reduction in burst pressure due to the IGA present in the pulled tubes is insignificant; amounting to only a 1 to 3% reduction in measured virgin tube burst pressure.

In each case, the measured burst pressure for the samp' es containing IGA was greater than that calculated for defect-free tubing based upon measured material tensile properties.

4.3.2.2.2 B&W Owners Grous ISA Samoles Section 3.2.2 of this report presented a discussion of a BW Owners Group Non-Destructive Examination (NDE) Committee project to grow IGA in the laboratory.

The original intent of the project was to gather performance data' on NDE techniques and IGA', but it was subsequently decided to burst test six sections of tubing as well. The IGA degradation on the tubing was present as patches each approximately 1.5 inches long and 0.25 inches wide. Through-vall extent of the patches was variable from 41.5 to 80.4 %TW. The results of tne burst testing is presented in Table 4-3.

IGA defect volumes present in the Owners Group tubing samples are significantly larger than those exhibited in the CR-3 pulled tubes.

Tne laraest volume IGA defect observed in CR-3 pulled tubes was calculated to be 138 x 10' cubic inche while the smallest calculated volume of the Owners Group defects was 4600 x 10'g cubic inches (a factor of 33). In as much as tube burst pressure varies directly 4-7

TSCRN 203 Technk:al Report i

i Table 4-3 Laboratory IGA Samples (Calculations Based on Maximum Degradation Depth) i i

i Tube Degradation Degradation Maximum Measured Calculated Calculated Calculated Calculated Sechon Width Length Degradation Burst Burst Burst Burst Burst (in.)

(in.)

Depth Pressure Pressure Pressure Pressure Pressure

(% TW)

(psi)

(psi)

(psi)

(psi)

(psi) l (Framatome (BHK (Uniform (PNL Slot l

Equation)

Equation)

Thinness Equation) i Equation) 23-8 0.263 1.508 73.4 6850 4363 6210 5378 5530 23-D 0.236 1.500 66.6 6300 5056 6847 6502 6233 24-B 0.245 1.490 78.7 5800 4019 6128 4727 5401 24-0 0.246 1.510 80.4 5250 3874 6027 4419 5285 25-B O.253 1.506 41.5 9750 7044 8006 8465 7623 25-D 0.239 1.534 68.8 8400 4861 6704 6215 6070 4-8

TSCRN 203 Technical Report in proportion to the inverse of defect length and' volume, the Owners Group test results provide an upper-bounds indication of the impact of IGA on the structural integrity of CR-3 OTSG tubing, Figure 4-3 shows the margin above the Regulatory Guide 1.121 limit of 3AP for the large-volume Owners Group samples, and the additional margin available for the range of degradation observed in the CR-3 OTSGs.

4.3.2.2.3 1994 Pulled Tube Results Sections of all four tubes pulled during 9R were burst tested. As discussed in the Background section of this report, the focus of the 1994 tube pull shifted somewhat to indications located at mid-bundle (7th - 9th) tube support plates (TSP). These indications were subsequently determined to be attributable to a wear mechanism. As a result, the 1994 examination results include burst pressure data relative to both wear and IGA.

The two degradation types are addressed separately in order to facilitate comparison with other examination results.

4.3.2.2.3.1 Burst Test Results for IGA.

Sections of two tubes containing IGA were burst tested during the 1994 examination. The tube samples were from the lower tube sheet /first span region of tubes 68-46 and 72-49.

Tube 68 46 contained indications with axial extents up to 0.228 inches, through wall depths up to 75%, and circumferential extents

~ up to 0.089 inches. Tube 72-49 contained defects with axial extents up to 0.041 inches, through wall depths up to 19%, and circumferential extents up to 0.029 inches.

Burst pressure test results are shown in Table 4-4.

Table 4-4 1994 CA-3 Burst Test Results for IGA l

Tube Number Number of Burst Pressure, Defect Depth at Indications (psi)

Burst (%TW) 68-46-3 4

7,000 75%

1 72-49-2 8

10,650 19%

1 sunt pre ur has b n adjusted for the presence of a brass sni.

The burst data for section 68-46-3 is significant in that it represents the largest pit-like IGA patch observed within all CR-3 pulled tubes and provides the upper bound data point for CR-3 ' pulled tube degradation from a burst perspective.

I The burst initiated at a defect exhibiting an axial extent of 0.228 inches and 75% TW.

Burst pressure as noted in Table 4-4 is 7000 psi which is still substantially greater than the Regulatory Guide 1.121 limit of 4050 psi..

j 4-9 e

TSCRN 203 Technical Report i

\\

l l

l PULLED TUBES WITH Pfr LIKE GA 10000 o o i

A L

a acco -

4 I

I a

PULLED TU8ES WTH WEAR DEGRADATION 4

m.

a 8-1

• me.

LA80RATORY IGA PATCHES 2000-0 0

2000

• log 8000 0000 10000 12000 edSL8 N GLAST M S SW E Pel FIGURE 4-3 CALCULATED VERSUS MEASURED BURST PRESSURES FOR 0.625" OD TUBING 4-10

1 TSCRN 203 Technical Report i

Defect-free sections of tubes 68-46 and 72-49 were also burst. Results are shown in Table 4-5.

Calculated burst pressures are based upon measured tensile properties and show good agreement with measured values. In each case, measured burr' pressure exceeded that calculated.

Table 4-5 Burst Pressures of Defect-Free 1994 Pulled Tube Sections Tube Number Measured Burst Calculated Burst Pressure (psi)

Pressure (psi) 68-46 10,850 10,500 72-49 10,550 10,120 109-71 11,100 10,493 i

136-26 10,750 10,272 l

A comparison of Table 4-4 and the pertinent portions of Table 4-5 shows that no reduction in burst pressure occurred due to the IGA present in tube 72-49. The measured burst pressure for the section of 72-49 which contained IGA is greater than that measured for a defect-free section of the tubing based upon measured material tensile properties.

Tables 4-4 and 4-5 also show that the limiting IGA defect in 68-46-3 resulted in a 35% reduction in burst pressure relative to a defect free section. However, the 7000 psi burst press,ure is still well above the Regulatory Guide 1.121 limit of 4050 psi.,

4.3.2.2.3.2 Sununary of IGA Burst Testina Results Based upon the results of burst tests performed on pulled tube IGA patches, it i

is concluded there is significant margin to burst for the balance of inservice i

S/N indications relative to the proposed structural limits. This conclusion is based upon dimensional measurement and burst test data. None of the 127 pit-like IGA patches removed from the CR-3 "B" OTSG and burst tested exceeded either of the proposed maximum allowable extents calculated in Section 4.2. Only 2 of 413 indications MtPC-inspected duHng 9R exceeded the proposed limits (and these were plugged). None of the burst pressures of the five tubes were below Regulatory Guide 1.121 allowable (i.e., 4050 psi). Four of the five tube sections had burst pressures almost equal to that of defect-free tubing.

Only one tube, with a 0.228 inch long IGA patch saw a significant decrease in burst pressure compared

)

to defect-free tubing, bursting at 7000 psi. However, this value was still well above 4050 psi.

4-11 9

TSCRN 203 Technical Report The laboratory grown IGA, despite the much larger dimensions (1.5 inches long and up to 80% TW) and volumes of the degradation, still burst at pressures well above 4050 psi.

1

{

This data shows that over a wide range of degradation dimensions, minimum tube structural margins are maintained.

]

4.3.2.2.3.3 Connarison of IGA Burst Test Results to Ennirical Correlations As illustrated in Tables 4-3 and 4-6, there are a number of approaches which may be followed in calculating the burst pressure of tubes with local regions of IGA.

One extreme bounding model would be to assume uniform thinning around the circumference of the tube and apply the burst equation of NUREG/CR-0718 (Reference 20).

The axial burst mode of the IGA suggests the use of partial through wall crack models.

Several have been applied.

These include the Framatome equation (Reference 21) and two similar.models (References 22, 23) where bur.st pressure is a linear function of the depth of degradation between undegraded burst pressure and that of a through wall crack. These are designated the PNL Slot equation and the Begley-Houtman-Keating, BHK, equation.

The BHK equation for partial through wall cracks when applied to the length and maximum depth of the pit-like IGA gives a good prediction to the observed burst pressures.

Figure 4-4 illustrates a comparison of the burst pressure predictions from these equations for a degradation length of 0.228 inches and through wall penetration of 75% (largest IGA patch observed in CR-3 pulled tubes). The degradation depth is expressed as a function of the tube wall thickness. For conservatism, lower 95/95 material tensile properties are assumed. The BHK and PNL Slot equations for partial through wall cracks or slots are shown to agree reasonably well in Figure 4-4.

Further, the BHK equation is shown to be slightly conservative for the case of the 0.228 inch long, 75% TW IGA patch.. Predicted burst pressure for a 0.228 inch long,100% TW crack is still predicted to be greater than 4050 psi using the BHK equation.

The best experimental burst pressure determination for through wall axial cracks in steam generator tubes has been performed by Hernalsteen (Reference 24). In this work, very high flow capacity tests eliminated much of the uncertainty on burst pressure measurements. Thus, the Hernalsteen equation is included within the correlations evaluated. As previously discussed, this correlation is the basis for the proposed CR-3 axial size limit.

4 e

4-12

--,e

TSCRN 203 Technical Report Table 4-6

~

Burst Pressure of Tubes with IGA Dearadation Tube Degradation Degradation Maxunum Measured Calculated Calculated Calculated Calculated Section Width Length Degradatum Burst Burst Burst Burst Burst (in.)

(in.)

Depth Pressure Pressure Pressure Pressure Pressure

(% TW)

(psi)

(psi)

(psi)

(psi)

(psi)

(Framatome (BHK (Uniform (PNL Slot Equation)

Equation)

Thinness Equation)

Equation) 41-44-2' O.106 0.069 54 9800 7155 8746 8705 8410

.68-46-3 0.089 0.228 75 7000 4548 6705 5654 5996 97-91-2' O.098 0.076 54

.10300*

7305 9022 8968 8636 106-32-2' O.070 0.062 40 10900' 8417 9568 9643 9305 72-49-2 0.029 0.041 19 10650 9638 9945 9998 9848 Notes:

1. Tube pulled in 1992.

2.1992 burst pressure artpdart per Appendix A of Reference 3.

4-13

TSCRN 303 Tcchnical Report i

8000 - -

BHK PARTIAL THROUGH WALL AMAL CRACK 7000-HERNALSTEEN THROUGH WALL 6000-I PNL SLOT t!

5000-5 s

.(4050 PSI)

REQUIRED STRUCTURAL MARGW

\\

i c.

.l 4

2000 -

UNFORM THINNNG 1000-0 0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

RELATIVE DEGRADATION DEPTH,'d/t FIGURE 4-4' CALCULATED BURST PRESSURE VERSUS RELATIVE DEGRADATION DEPTH CRACK LENGTH OF 0.228 INCHES 4-14 I

TSCRN 203 Technical Report 4.3.2.2.3.2 1994 Burst Pressure Test Results for Wear Four sections of tubing containing volumetric wear defects were burst tested as part of the 1994 examination. Results are presented in Table 4-7.

Table 4-7 1994 CR-3 Burst Test Results for Wear Defects Tube Number Length of Defect Burst Pressure Defect Depth at at Burst (in.)

(psi)

Burst (%TW) 68-46-14 0.090 10,850 30%

68-46-18 0.425 10,700 22%

I 1

72-49-7

.NDO 10,650 N00 72-4Q-13 0.094 10,550 18%

i j

l Tapered wear' visually observed on 00 deposit A comparison of Table 4-5 and 4-7 shows that burst pressures for th'ose sections i

of tubing containing wear defects are essentially 100% of that exhibited by defect free tubing.

4 4.3.2.2.3.2.1 Comoarison of Wear Burst Test Results to Emoirical Correlations Table 4-8 provides a comparison of measured tube burst pressure test results to predicted values, of burst pressure.

The table contains calculated burst pressures for a variety of idealized geometries. As was the case with IGA, the various correlations, in particular the BHK, provide good prediction of measured burst test results for wear, with the conservatism of the measured results evident.

4.4 Degradation Growth Rate 4.4.1 Growth Rate Studies Reference 1 presented two growth studies of CR-3 eddy current indications; one performed by the EPRI NDE. Center and the other by BWNS. These studies relied on data available as of Refuel 8 (1992) and each one concluded there was little or no growth of the CR-3 indications from 1989 to 1992.

Packer Engineering, Inc.

performed an additional growth rate study considering data available through Refuel Outage 9 (1994). This study (Reference 14) also concluded there was no growth of eddy current indications in the CR-3 OTSGs at either the free-span or tube support plates from 1992 to 1994. A sunnary of the previous growth studies 4-15 e

m TSCRN 203 Technical Report TABLE 4-8 l

Burst Pressure of Tubes with Wear Dearadation i

Tube Degradahon Degradahon Maximum Measured Calculated Calculated Calculated Calculated

[

Sechon Wusth Length Degradation Burst Burst Burst Burst Burst l

(in.)-

(in.)

Depth Pressure Pressure Pressure Pressure Pressure

(% TW)'

(psi)

(psi)

(psi)

(psi)

(psi)

(Framatome (BHK (Uniforn)

(PNL Slot Equation)

Equation)

Thinness Equation)

Equation) 68-46-14 0.119 0.090 32 10850 8806 9802 C979 9544 i

68-46-18 0.141 0.425 19 10700 9005 9106 9534 8915 72-49-7 0

0 0

10650 10380 10120 10120 10120 72-49-13 0.134 0.094 16 10550 9416 9769 9903 6442

1. Based on tube wall thickness of 0.037 inches.

G G

1 4-16

\\

TSCRN 203 Technical Report i

is provided below along with a detailed discussion of the latest study.

More detailed discussions of the EPRI NDE Center and BWNS studies can be found in i

Reference 1.

j I

The EPRI NDE Center growth study (Reference 25) focussed on free-span IGA.

The study consisted of a review of eddy current data for selected tubes examined i

during three (3) previous inspections to assess the growth of IGA indications observed in the first span of the CR-3 "B" steam generator.

The EPRI review i

showed that overall, eddy current signal amplitude was observed to decrease, with only a slight increase in percent wall loss. The increase in percent wall loss 4

was well within the sizing error band of the analysis; such that, EPRI concluded that the first span IGA patches had not grown since they were first detected.

l The BWNS growth study (Reference 26) was a more global study, considering both freespan and tube support plate indications. The review consisted of a side by side comparison of normalized eddy current signals spanning the previous three inservice inspections (1989,1990, and 1992). All freespan indications (not just the first span) from both steam generators were conr.idered together in the study due to the limited data from the "A" OTSG. The study found that for the total population of all freespan indications (in both OTSGs), the average change in signal amplitude was +0.01 volts, with a standard deviation of 0.11 volts. The study also considered all TSP indications from both OTSGs together due to the limited amount of data from the "A steam generator.

The average change in amplitude for all tube support plate indications (in both OTSGs) was -0.19 volts, with a standard deviation of 0.30 volts.

The larger standard deviation associated with the tube support plate signal comparison was attributed to variations in contribution of the support plate. signal to the total signal response. Based upon"these findings, BWNS concluded little or no growth occurred from 1989 to 1992. Table 4-9 provides a summary of the average voltage change and standard deviation for each OTSG on a TSP and freespan basis.

Table 4-9 1992 CR-3 Growth Rate Study Results OTSG Region Number of Average Voltage S.tandard Indications Change Deviation A

Freespan 14

-0.05 0.16 A

TSP 14

-0.05 0.16 B

Freespan

'51

+0.02 0.08 8

TSP 48

-0.23 0.32 Packer Engineering calculated voltage growth rates and developed scatter plots and distributions for the "A" and "B" steam generators using bobbin coil eddy current data acquired during 1992 (8R) and 1994.(9R). The availability of more 4-17

TSCRN 203 Technical Report data from the 1992 and 1994 inspections made it possible to study each S/G separately. Separate growth rates were, determined for freespan indication; and indir.ations at tube support structures.

Table 4-10 provides a summary of the average change and standard deviation for each steam generator and region of interest.

The average voltage changes and standard deviations are essentially the same for all cases considered, with mean values and standard deviations of approximately zero (0) and 0.15, respectively.

The data in Table 4-10 shows there was no growth of eddy current indications in either steam generator at either the freespan or tube support plates from 1992 to 1994.

This finding is consistent with findings of the two previous growth rate studies and indicates no growth has been observed in these indications since 1989.

A limited growth rate study of tubes inspected in the 1976 preservice inspection and the 1980,1983, and 1989 outages likewise concluded no growth j

(Reference 27).

j Table 4-10 1994.CR-3 Growth Rate Study Results OTSG Region Number of Average Voltage Standard Indications Change Deviation A

Freespan 62

+0.060 0.15

}

A TSP 73

+0.042 0.18 l

8 Freespan 232

+0.002' O.11 l

8 TSP 234

+0.002 0.16 j

i l

Figures 4-5, 4-6, 4-7, and 4-8 are scatter plots of the data used to develop i

Table 4-10. These Figures also present results of a linear regression analysis of the data.

The regression analysis :, hows no strong dependency of voltage l

change to voltage amplitude.

In each case, the dependency which is calculated exhibits a slight negative bias with increasing voltage. This is a conservative l

trend.

4 1

l 1

l l

4-18 e

TSCRN 203 Tcchnicci Rcpert i

15 10 Y = 0.84

• X + 0.01 R = 0.47 N = 62 deis peitts 15-gn 1.5 n.'.

.t.

7

.. e 1'0 g,1,;. =

4 0.5 qu.

m osia

• Regreenien a0 a0 a5 1.0 1.5 10 15 10 15 1992 V%4 Figure 4-5. OTSG "A" Voltage Scatter Plot - Free-Span Indications.

4-19 e

TSCRN 203 Technical Repcrt 35 30-- *G A N Y =0M*X +0M n =0M a-25-- " " " " ' ' * * *

,. ~

$ 20

>s a

m:

15 a

..n to QS

=

Y...-

8

...... nm Q0 00 Q5 to 15 20 25 10 35 1992 Voitage F1gure 44. OTSG "A" Voltage Scatter Plot - Tube Support Plate Indications.

4-20

TSCRN 203 Technleri Repsrt 15 ao soan n

v.os2 x.om n.ou N o 232 he We 25 20 h

s' 15 s

se, to s-

'a t.

,o, QS-

..e 00 00 QS to 15 20 25 10 35 1992 Veitage Figure 4 7. OTSG "B" Voltage Scatter Plot Free-Span Indications.

4-21

TSCRN 203 Technical Report i

1

-l 15 I

g g gy 10-- 'n

• 8 8 * * ! *

• eo es

,a**,,.-

234eseposes 4

j 25 n-l15 n

a5-

? U-N l

00 00 d5 1.0 1.5 20 25 10 35 1982 Vdta0s 4

ii-l 1

i Figure 44. OTSG "B" Voltage Scatter Plot - Tube Support Plate Indications.

1 1

)

i 1

I I

i i

4 22 i

l TSCRN 203 Technical Report Table 4-11 presents the results of the 1992 and 1994 growth rate studies together in order to facilitate a direct comparison.

1

)

Table 4-11 Comparison of 1992 and 1994 Growth Study Results (All Units are Differential Voltage) 1 OTSG/ Region 1992 Mean 1994 Mean 1992 1994 4

Standard Standard Deviation Deviation i

A Freespan

-0.05

+0.06 0.16 0.15 A TSP

-0.05

+0.042 0.16 0.18 B Freespan

+0.02

+0.002 0.08 0.11 B TSP

-0.23

+0.002 0.32 0.16 a

)

The results of the 1992 and 1994 studies agree quite well, indicating the trend of no growth to be continuing. Differences in mean values between 1992 and 1994 '

are well within the calculated standard deviations.

Note also that as sample populations become larger, as was the case in 1994, differences in the standard deviation between generators becomes much less. This is further evidence of the similarity in behavior of the "A" and "B" steam generators.

i In conclusion, FPC considers the growth studies discussed above to provide compelling evidence there is little or no growth of indications occurring in the CR-3 steam generators.

Three independent studies each arrived at the same l

conclusion; that is, the CR-3 eddy current indications exhibit essentially no growth. FPC has confirmed these results with internally prepared growth studies.

The various studies have also been diverse in the use of different analytical techniques to evaluate growth.

Regardless of the technique used, each study concluded no growth was apparent.

The conclusion of no growth rate allows the CR-3. approach to developing an alternate repair criteria to be fundamentally different than that taken for an actively growing degradation mechanism. This difference is reflected in several areas of this report, including the structural area. Based upon the conclusion of no growth from the various growth rate studies, it is reasonable to conclude s

beginning of cycle (BOC) and end of cycle (EOC) burst pressures will be essentially the same, and the probability of burst does not change appreciably over the operating cycle.

4-23 4

e e

l l

TSCRN 203 Technical Report 4.4.2 Growth Rate Triceer In Reference 1, it was proposed that a bobbin voltage threshold of 0.75 volt be established to define when a new S/N is identified, or when growth in the size of an S/N has occurred. A S/N identified in subsequent outages that was not previously indentified in any previous outage since 1987, would have to have a bobbin coil signal amplitude of greater than 0.75 volt in order to be considered a new S/N indication. As stated in Reference 1, the purpose for this voltage threshold was to screen out very low voltage indications which are just above the noise level of the tubes which tend to " fade in and out" from outage to outage.

The lower voltage threshold was allowed by the staff for the interim repair criteria for S/N indications that was employed during the Refuel 9 outage.

The MRPC campaign performed during Refuel 9 (Section 4.5) demonstrates that a lower voltage threshold of 0.75 volt does not diminish the structural limits of the tubes. Any indication less than 0.75 volt would, based on the 1994 field MRPC inspection, be expected to have axial and circumferential extents less than the maximum allowable values of 0.33 inch a'nd 0.6 inch, respectively. As discussed in Section 4.5, a lower voltage threshold of 0.9 volt is established for S/N indications. It is proposed in Section 4.5 that no MRPC dimensional sizing be required for S/N indications with bobbin signal amplitudes of 0.9 volt or less.

It is proposed that this voltage (i.e.,0.9 volt) also be the lower threshold for defining new S/N indications rather than 0.75 volt as orginally proposed in Reference 1. Therefore, for an S/N indication to be considered a new S/N, it would have to meet both of the following two criteria: (1) have a bobbin signal amplitude of greater than 0.9 volt, and (2) have not been identified in any previous outage since 1987.

Reference'I also proposed that a S/N would have to have a bobbin signal amplitude

.of greater than 0.75 volt, and exhibit a voltage increase of greater than 0.5 volt since the last time that it was inspected for it to be considered to have grown. This definition of growth was included in the approach for dispositioning S/N indications that was allowed by the staff to be used during the 1994 Refuel 9 outage. It is proposed that the same voltage increase be employed in this approach to define a S/N as having grown since the last inspection, that is, the bobbin coil signal amplitude must have increased by more than 0.5 volt. Further, it is proposed that this definition of growth bronly applied to those S/N indications which exceed the lower bobbin voltage threshold which has been established as 0.9 volt. Therefore, for a S/N to be considered to have grown, it must meet both of the following criteria: (1) have a bobbin coil signal amplitude of greater than 0.9 volt; and (2) the bobbin coil signal amplitude must have increased by more than 0.5 volt since the last inspection.

4.5 Lower Bobbin Coil Voltage for NRPC Inspection Section 4.3.1 discusses the supplemental MRPC inspection conducted during the 9R outage and its use in evaluating the largest S/N indications against the then-current dimensional repair criteria. A secondary purpose of this' examination was to establish, by test, a lower bobbin coil voltage (i.e., a " cutoff") below which 4-24

- - ~. _. _.

TSCRN 203 Technical Report indications would not be expected to exceed the dimensional criteria.

To meet this objective, an initial 20% MRPC sample inspection was conducted from the population of indications exhibiting bobbin voltages greater than or equal to 0.5 volts.

Locations selected for the initial 20% sample included all locations where previous MRPC inspection revealed indications exceeding one-half of the proposed MRPC sizing criteria. The balance of the indications selected for the This section presents the results of this evaluation.

The median bobbin voltage for all indications included within the initial 20%

sample was used as the starting point for the evaluation of a lower bobbin voltage " cutoff". For locations with bobbin voltages less than this mean voltage value, if 1% of these locations exhibited axial or circumferential extents equal to or exceeding the dimensional repair criteria, additional MRPC samples (10% of the remaining locations with S/N ratios less than 5:1) consisting of locations with the largest available bobbin voltages were to be performed until fewer than 1% of the locations in a.given sample were found to exceed the MRPC size criteria.

Results of the MRPC inspection are discussed below.

FPC performed a 20% non-random sample of S/N indications inspected by M'PC with R

3 additional 10% expansions.

The initial MRPC sample was composed of 191 S/N indications, 50 in the "A" OTSG and 141 indications in.the "B" 0TSG. The sample consisted of indications exceeding one-half of the MRPC dimensional criteria of 0.25" axial dimension and 0.60" circumferential dimension (30 indications). The balance of this sample consisted of indications with the largest voltages. The median voltage of all indications in that sample was calculated as 1.06V. The first expansion consisted of 78 indications with voltages greater than 0.8V, 24 indications from the "A" OTSG and 54 from the "B" OTSG.

The second expansion consisted of 81 indications with voltages greater than 0.73V, 23 indications from the "A" OTSG and 58 from the "B" OTSG. The third and last expansion consisted of 63 indications with voltages greater than 0.7V, 17 indications from the "A" OTSG and 46 indications from the "B"

0TSG.

There were a total of 413 S/N indications analyzed (all indications greater than 0.7V) from the S/N population of both OTSGs. A total of six indications from the 9R MRPC inspection exceeded the 0.25 inch axial size limit in effect at the time. The results of the initial and supplemental MRPC inspections are presented in Table 4-12.

Table 4-12 Supplemental 9R MRPC Inspection Results for Both OT5Gs Sample Sample size Voltage Range Failures' Initial 20%'

191

> 0.9 4

lst Expansion 78

> 0.8 < 0.9 1

2nd Expansion 81

> 0.73 < 0.8

'1 3rd Expansion 63

> 0.7 < 0.73 0

Failures determined based on an MRPC axial string limit of 0.25 inches 2 [n addition to all $/M indications > 0.9 volts,. this sample also included indications which based upon 8R MRPC inspection data, exhibited extents which exceeded one-half the 0.25 inch limit. Some of these had bcbbtn coil voltages less than 0.9 volts.

4-25

TSCRN 203 Technical Report A review of the 9R inspection results was performed in light of the revised axial MRPC sizing limit of 0.33 inches. The review concluded that only two (2) of the 413 indications MRPC inspected during 9R would have exceeded the revised criteria.

Both of these indications were in the 'B' OTSG initial 20% sample,-

possessing bobbin voltages of 1.13 and 0.98 volts respectively. Based upon this information, the 9R supplemental MRPC examination would have been terminated following the,1st expansion sample if the higher allowable axial extent had been in effect. Per the Confirmatory Action Letter sampling plan, this value is the voltage below which indications would not be statistically expected to exceed the dimensional repair criteria. Based upon the field data presented in Table 4-12, a lower bobbin voltage cutoff of 0.9 volts is adequate.

This voltage value is proposed as the lower bobbin coil voltage for the proposed disposition strategy.

Indications which exhibit a bobbin coil signal amplitude below this value will receive no MRPC to assess structural integrity.

4.6 Conclusions o

CR-3 pulled tube burst testing results demonstrate significant margin above the limiting Regulatory Guide 1.121 structural acceptance criteria (3AP, 4050 psi),

i both for the IGA and wear degradation mechanisms.

Equations used'to predict burst pressure of tubing containing IGA defects agree well with measured values.

These same equations are not as accurate in predicting the burst pressures for wear defects, but the difference is in the conservative direction.,

+

Burst test results for pulled tubes and B&WOG NDE Committee IGA samples demonstrate the conservatism in the proposed Technical Specification axial and circumferential extent limits.

The majority of the conservatism arises as a t

result of the assumption in the structural analysis that the defect extends 100%

through-wall.

Field and laboratory experience show this to be extremely conservative.

Refuel 9 MRPC inspection results confirmed the conservatism in the axial and circumferential extent limits compared to measured dimensions.

The axial and circumferential extent population distribution of indications within the CR-3 OTSGs indicates > 95% are less than 75% of the proposed limits. The results of the 9R MRPC inspection also indicate the acceptability of allowing all indications exhibiting a bobbin coil amplitude less than 0.9 volts to remain in service from a structural perspective without further NDE.

Growth rate studies show essentially no growth of the indications within the CR-3 i

OTSGs.

Physical dimensions of the degradation, and thus burst pressure, are expected to vary a minimal amount over time. As such, beginning of cycle and end of cycle burst probabilities are considered equivalent.

Indications adjudged acceptable to remain in-service following inservice inspection of the OTSG are expected to retain the required structural integrity until the next inspection.

The conclusions on growth rate are verified each outage to ensure continued validity of this assumption.

4-26

l

\\

TSCRN 203 Technical Report Section 5 Leakage Considerations

5.1 Background

Most Technical Specifications, including those in use at CR-3, allow a certain amount of primary-to-secondary leakage through steam generator tubing. For CR-3, this leakage is restricted to less than 1.0 gallon per minute (gpm). Regulatory Guide 1.121 also allows leakage, but specifies primary-to-secondary leakage rate through the steam generators, under normal operating pressure, should be maintained less than that determined for the largest single permissible longitudinal crack. Accordingly, other defect-specific alternate repair criteria (e.g., PWSCC in the roll transition zone, 00 SCC at the TSPs) have developed I

repair criteria which sought to allow through wall cracks to remain in service.

These approaches included more restrictive limits on allowable primary-to-secondary leakage as a defense in depth measure.

While this approach is appropriate for the defect morphologies mentioned, there are several fundamental differences (discussed throughout this report) between l

the CR-3 experience and that addressed by the other criteria. Firstly, no crack-like defects or corrosion mechanism known to produce crack-like defects have been observed in the CR-3 pulled tubes. The proposed disposition strategy is limited l

in application to volumetric indications and specifically excludes crack-like i

indications.

Secondly, all pulled tube examination results and operational history argue the CR-3 degradation has not in the past, nor is jt likely in the future (based on growth rate study results), to progress through wall.

FPC has chosen to approach leakage by developing a non-destructive examination l

(NDE) limit which provides assurance that a minimum tube wall thickness is I

maintained.

Providing this minimum tube wall crecludes primary to secondary leakage undcr cc.ditions typical of worst-case-accident differential pressure.

l This approach simplifies the disposition strategy and is made possible as a result of the CR-3 historical experience with operational leakage and growth rate. Experience indicates. essentially zero primary to secondary leakage through the steam generator tubing and no growth of inservice S/N indications over time.

i By taking this approach, FPC is not proposing a decr. ease in the CR-3 Technical l

Specification allowable primary to secondary leak rate as part of the disposition strategy. Furthermore, since no through wall indications are permitted by the proposed strategy, increased reliance on installed leak detection capability need not be evaluated. A discussion of both of these considerations has been provided to the NRC Staff in previous correspondence (References 28, 29). Neither one is

. discussed further within this report.

Bobbin coil signal amplitude, i.e., voltage is utilized as the HDE limit. The use of a bobbin coil signal amplitude repair limit is not unique to the CR-3 application for S/N indications, but is also used to disposition 00 SCC in the tube support plates of recirculating steam generators (Reference 13).

This 5-1

,~

~

TSCRN 203 Technical Report section discusses the technical basis for the proposed limit as well as several other relevant plant-specific leakage considerations.

5.2 Development of the Leakage Repair Limit 5.2.1 Minimum Licament Analysis Reference 1 presented a discussion of the minimum tube ligament width which must be present during a main steam line break to ensure that existing outside diameter tube degradation will not result in primary to secondary leakage. For a differential pressure equivalent to 2250 psi (normal operating Reactor Coolant System pressure is 2155 psi), the minimum ligament required is 0.0029 inches, which equates to a degradation depth of approximately 91.5% of the tube wall.

For primary to secondary pressures up to 2600 psi, the minimum ligament required is 0.0043 inches, which equates to a degradation depth of about 87% of the tube wall.

For conservatism, the 87% TW value is selected as the minimum ligament width which must be ensured by the proposed S/N disposition strategy.

The conservatism in this approach lies in the assumed calculational values of differential pressure and modeling the degradation as a crack.

• Further, calculating a minimum ligament requirement to ensure no leakage under accident conditions goes well beyond (conservatively) the requirement given in Regulatory

. Guide 1.121, Position C.3.d(3), which allows leakage.

5.2.2 Develonment of ISE Leakaee Limit Based upon the results of the minicum ligament analysis described in Section 5.2.1, an NDE parameter which conservatively corresponds to an 87% TW penetration was developed.

NDE examination results from both Refuel 8 and Refuel 9 CR-3 pulled tubes shows a direct relationship between discontinuity volume and bobbin coil signal amplitude (See Section 3.0).

For ease of reference, Figures 3-4, 3-5, 3-6, 3-7 and 3-8 are included within this section as Figures 5-1 through 5-5, respectivel.y. Volume is a function of the axial extent, circumferential extent, and depth of the indication.

The relationship between each of these dimensional attributes and volume, based upon pulled tube results, is given in Figures 5-1, 5-2, and 5-3.

Figure 5-4 shows that as voTume of the indication increases, the corresponding bobbin voltage also increases. B&WOG data for large volume indications, presented in Figure 5-5, also confirms this relationship.

The volume corresponding to 87% TW (from Figure 5-1) is used as input to Figure 5-4 in order to determine the bobbin voltage equating to this size wall penetration.

Reading a value for volun yields a volume of approximately 550x10'g which corresponjs to an 87% TW flaw cubic inches (in. ).

From Figure 5-4, this volume corresponds to a bobbin voltage in excess of 10.0 volts.

l The 550x10-s in.3 value for volume lies outside the range of degradation volumes observed in the pulled tubes and is an extrapolation from data weighted towards small volume discontinuities.

Inherent to the extrapolated data is the assumption (which holds true for the range of discontinuities examined in the laboratory) that defect geometry, and the relationship between length, width, and i

depth remain fairly constant as defect depth varies. Both the pit-like IGA and 5-2 l

S

TSCRN 203 Technical Rep:rt o

100 i

m

-jf n

)

a m a

a u

u a

G m

g 88 aa "g

a g t

n a

W Y = 0.29

• X + 1.14 10 6

Il R = 0.77 SD = 0.14 E

E N = 67 data points 8

a n

S II 8

1 0.1 1

10 100 Volume, E 06 cubic in Figure 5-1. Scatter plot showing correlation between discontinuity volume and depth as measured by metallography e

i 5-3

TSCRN 203 Tcchnical Rep:n 100,

p;. *,,

==

n a

m s

E 10,

p k

Y = 0.30

• X + 1.31 R = 0.85 SD = 0.14 N = 50 data points 1

0.1 1

10 100 Volume, E 06 cubicin Figure 5-2. Scatter plot showing correlation between discontinuity volume and axial length as measured by metallography 5-4

TSCRN 203 Technical Repert 1000

.cr:

1.:-

+-

Y e a3

• X + 12 i

Rea75 30e02

• - g.*'

N e 9 ese pares g

g W

],,,

=,a

= = =,, :.z..*

.c

=

=

B

. xii s

,e :._,

j

,..a...._....

a_.

.a,..

'y a;q

.~

.... I.. : a E.._i. e ?" -

5

. =:

=.- ::-

g

.;}.

.. !...;.l

.i

. 14 1

.m at i

10 1CD Volume, E-06 ctbic in Figure 5-3. Scatter plot showing correlation between discontinuity volume and circumferential extent as measured by metallography.

1 1

5-5

TSCRN 203 Tcchnical Repcrt

,/

Y = 0.80

• X + 179 R = 0.77 SO = 0.0

,e N = 12 data poirte e

100

,e-

.g

. s..,;, p.

..E.

...y

....g-

s?

,..g..J...,

t.u s.

,e..: -

.i.

...'s..)..;.. m..

/.

e - p....

,?' a 10 ai 1

Signal Amplitude, dts Figure 5-4. Scatter plot showing correlation betw6en discontinuity volume and eddy current bobbin coil voltage. CR-3 pulled tube data points.

5-6 e

i

. TSCRN 203 Technic:1 Rescrt

/

D *

.......'7.-

g,

• '8-Y = 0.82

• X + 0.32 l

R = 0.91 30 = 0.12 N = 11 esas powee i

C

~

~

as s 5

4 f

l 1

. i.. s..

.y..;.

.s.

1 3..1 3

o.1 a1 1

e Signalamplitude,Wts Figure 5-5. BWOG NDE Committee IGA Samples -Voltage / volume correlation.

e 5-7

l 1

TSCRN 203 Technical Report circular wear found in the CR-3 pulled tubes follow this trend, generally exhibiting a half ellipsoid geometry. Tapered wear was the only other geometry found in the pulled tubes, but tapered wear is excluded from the proposed j

disposition strategy. Therefore, a half-ellipsoid geometry is assumed to exist j

for all S/N indications subject to this disposition strategy.

j 1

t To test the validity of the assumption that a half-ellipsoid geometry exists for larger S/N indications (i.e.,

volume of approximately 500x10',a through wall S/N indication would possess a in.3), a geometric analysis was performed using the dimensional data obtained from metallographic examinations of the CR-3 pulled i

tubes.

Axial length, circumferential extent, and depth information on each

~

volumetric S/N indication destructively examined in 1992 and 1994 was assembled j

(86 indications). Metallographic information on tapered wear was not included 1

since this mechanism / morphology is not included within the scope of the proposed I

disposition strategy. Relationships between axial length and depth, and

{

circumferential extent and depth were developed.

The evaluation examined both the total population of indications (86) as well as a smaller subset (23) comprised of those exhibiting the largest percent through wall penetration (> 40%). For the total population, the average depth is 30% TW, i

Calculating an average length to depth ratio and circumferential extent to depth ratio which corresponds to 87% TW yields an average axial length of 0.187 inches and an average circumferential extent of ellipsoid with these dimensions is 558x10',0.17J. inches.

The volume of a half-in.

The larger discontinuities (in terms of depth) were also evaluated separately given their closer approximation to the %TW of interest.

Again, an average axial length to depth ratio and an average circumferential extent to depth ratio was calculated. From these ratios, an average length of 0.113 inches and an average circumferential extent of 0.0827 inches was calculated to correspond to a degradation' dept The volume of a half-ellipsoid with these dimensions is 210x10',h of g7% TW.

in.. This volume is reasonable and agrees within reason with 'the volume of the largest volumetric indication found in the CR-3 pulled tubes. This indication was present in tube section 68-46-3 and exhibited an axial extent of 0.228 inches, a cir extent of 0.089 inches, a depth of 75% TW, and a volume of 138x10',cumfgrential in..

From Figure 5-4, a volume of 210x10 in.3 corresponds to a bobbin coil signal amplitude of 3.6 volts. This signal amplitude is proposed as the TIDE limit for leakage. It is selected because of the added conservatism as compared to the 10 volt value, and due to its' development from larger %TW indications.

5-8 D

TSCRN 203 Technical Report 5.2.3 Develoosent of Leakace Repair Limit As discussed in Section 3.4, Equation 3-1 of this report can be modified with appropriate nomenclature changes for the case of leakage. The leakage repair limit equation is presented below as Equation 5-1:

RLa, = LLg,- (NDE + GR)

(Equation 5-1) where RL is the NOE measured repair limit for leakage, expressed in units of voltage, LL is the NDE measured parameter corresponding to the 87% TW minimum ligament value, NDE is the measurement error, and GR is the degradation growth i

rate. Based upon the results of the various CR-3 growth rate studies (References 14, 25, and 26), no error allowance is included for growth.

Determination of NDE uncertainty is discussed in Section 3.4.2 of this report.

Considering bobbin coil voltage errors for acquisition and analysis yielded a final NDE measurement percentage error of 24.2%. Conservatively applying this error to the proposed analytical-based leakage limit of 3.6 volts, results in a repair limit of 2.73 volts. This value is rounded off (conservatively) to 2.5 volts for simplicity of presentation.

5.3 Assessment of Proposed Limits 5.3.1 Review of Field Data During Refuel Outage 9 (May 1994), there were a total of approximately 2200 indications with signal-to-noise ratios less than 5:1 observed in the CR-3 OTSGs.

Of the total population of volumetric S/N indications inspected during the outage, only three tubes ~(0.14%) contained indications which exhibited a bobbin coil voltage greater than 2.00 volts.

All three of these tubes were plugged.

Figure 2-4 of this report presents the distribution of S/N indications from Refuel 9, based upon bobbin coil voltage. From Figure 2-4, there is greater than a 99%. probability there are no volumetric S/N indications inservice which exceed the proposed voltage-based repair limit of 2.5 volts.

I CR-3 operating experience supports the conclusion of the adequacy of the current inservice S/N population from a leakage perspective.

CR-3 has historically operated with assentially zero primary to secondary leakage through the steam generator tubing and in 15 years of commercial operation has yet to experience a tube leak outage.

5.3.2 Review of Pertinent Laboratory Examinations

~

Laboratory examinations of 1992 and 1994 pulled tubes included several tests which relate to the leakage-based limit. Pulled tube metallographic information was reviewed to assess the range of through wall sizes, including maximum percent through wall, contained within the population of CR-3 pulled tube defects. For volumetric defects, the largest penetration was determined to be 75% for tube 5-9

TSCRN 203 Technical Report section 68 46-3. This was largest defect found in the laboratory. Themajo'rity of laboratory defects are considerably smaller than this as shown in Figure 5-6.

The five volumetric wear defects contained in the pulled tubes were likewise much smaller, exhibiting an average depth of 20% TW (range from 8 to 32 %TW).

Helium leak testing was performed on seven (7) sections of the 1994 pulled tubes and one (1) section from the archived 1992 pulled tube (41-44-2). No leaks were found in the eight (8) sections that were tested.

Refuel Outage 9 field data was also reviewed against the laboratory results and the voltage versus volume, volume versus depth scatter plots presented in Figures 5-1 and Figure 5-4.

Figure 2-4 of this report indicates a median bobbin voltage of 0.4 volts for the Refuel 9 outage inspection.

Figure 5-4.was utilized to determine a corresponding volume for thi,s bogbin voltage based on pulled tube metallographic data. A value of 30 x 10' in. is calculated. From Figure 5-1, this volume would correspond to 37 %TW.

This evaluation indicates the median depth for CR-3 inservice S/N indications to be approximately 37 %TW.

These results compare favorably with the data presented in Figure 5-6, with the projection occurring slightly above the median percent through wall determined from the first span pulled tube results. This is conservative, since the various correlations to volume were evaluated as limits and they predict higher percent through wall penetrations than actual.

Comparing Figure 5-6 to Figure 2-4 is considered reasonable since 50 percent of all CR-3 S/N indications occur in the first span of the OTSGs and exhibit similar morphology.

Section 4.3.1 contained a discussion on the MRPC inspection conducted during Refuel Outage 9.

The initial 20% sample from this inspection was conducted from the population of indications exhibiting the largest bobbin voltages greater than 0.5 volts. Locations selected for the initial 20% sample included all locations where previous MRPC inspection revealed indications exceeding one-half of the proposed MRPC sizing criteria at the time, i.e., 0.25 inches axial and 0.6 inches circumferential.

The balance of the indications selected for the initial 20%

sample included those indications with the largest bobbin voltages. Given the relationships demonstrated in Figures 5-1, 5-2, 5-3, and 5-4, it can be concluded that this inspection sample was comprised of the largest S/N indications known to exist in the CR-3 OTSGs. Thus, applying the approach used in the previous paragraph, an upper bound median defect depth can be calculated. Given a median bobbin voltage of 1.06 volts,(from Section 4.3.1), a percent through wall of 46%

is calculated.

Based upon the results of Section 5.3.1 and 5.3.2, FPC concludes use of a bobbin coil voltage limit of 2.5 volts will maintain the potential for operational leakage, due to an inservice S/N subject to this proposed repair, criteria, to a value which is acceptably small.

I 5-10 G

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l TSCRN 203 Technical Report l

Figure 5-6 l

First Span IGA Depth Distribution l

25 t

4 20 --

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Minimum Required Ligament Width O

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O-10 011-20 21-30 31-40 41-50 51-60 61-70 71-80 81-90 91-100 Range (%TW)

I 5-11

TSCRN 203 Technical Report 5.4 Comparisons To RSG Database i

To investigate use of a strictly voltage-based repair criteria, Packer l

Engineering compared recirculating steam generator (RSG) burst testing experience l

, against CR-3 pulled tube results.

While the RSG alternate plugging criteria (APC) database is focussed on Outer-Diameter Stress Corrosion Cracking (00 SCC)

I as opposed to defects with a volumetric morphology, it is useful for a l

quantitative assessment of degree of structural adequacy provided by the proposed l

NDE leakage limit. The conservatism inherent in this evaluation is reflected in the following.

1)

The results are representative of a crack-like defect rather than the volumetric morphology present in CR-3 tubing.

However, the volumetric defect produces bobbin voltages much larger than cracks, making the crack geometry an exaggerated limiting case.

2)

CR-3 burst pressure test results for larger eddy-current indications (bobbin voltage > 2.0V) are significantly in excess of Regulatory Guide i

1.121 limits and 00 SCC burst pressure values given similar bobbin amplitudes.

I The evaluation utilized a ratioing or scaling technique to relate the RSG APC I

correlation for burst pressure versus bobbin voltage to OTSG tubing. It presumed geometric scaling of eddy current indications was possible if all linear dimensions scale for both tubing and bobbin probes and coil excitation frequency scales with the inverse of the square of the wall thickness of the tubing. For both tubing and probe dimensions, scaling of RSG design and practice values to l

OTSGs is close, but not exact.

Based upon these comparisons, bobbin probes calibrated to the same voltage for 207, deep flat bottom holes should behave similarly for the various tubing sizes.

Scaling is not expected to be exact.,but large differences are not expected. To test the validity of this conclusion, the scaling technique was applied to the 0.875 inch 00 APC database in order to predict an APC for 0.750 inch 00 tubing.

As can be seen in Figure 5-7, the prediction correlates very well with the APC developed from actual destructive testing of 0.750 inch tubing. The fact that measured and calculated lower limit burst pressure versus bobbin voltage curves for 0.750 inch tubing agree so well, supports the validity of calculations done for the 0.625 inch tubing.

The lower curve in Figure 5-7 is the calculated lower limit APC curve for cracks in 0.625 inch tubing, using the same scaling technique discussed above.

For cracks, a bobbin voltage of less than or equal to 2.72 volts would ensure that the burst pressure requirement of 4050 psi is met for tubes with minimum tensile properties at operating temperature. The OTSG structural limit voltage.of 2.72 volts is estimated be 3.5 volts in the Crystal River-3 steam generator tubes using voltage normalization techniques. Therefore, although the basis for the proposed 2.5 volt repair limit is to preclude operational leakage over the course of the operating cycle, the results of the Packer evaluation show the 2.5 volt limit provides a defense in depth measure from a structural perspective as well.

5-12

TSCRN 203 Technical, Report auRsT PRESSURE VERSUs aceslN VOLTAGE 10000 as. burst pressure)

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5-13

l

\\

TSCRN 203 Technical Report The results also provide additional confidence in the validity of the proposed lower bobbin voltage cutoff value of 0.9 volts.

5.5 Conclusions The proposed S/N disposition strategy addresses primary to secondary leakage with a defense-in-depth approach that includes an NDE repair limit for leakage designed to ensure a minimum tube wall thickness is maintained for all tubes.

This approach is structured to preclude primary to secondary leakage through the i

steam generators.

Such an approach is possible given the CR-3 operating experience with leakage.

The repair limit is based upon bobbin coil voltage which laboratory metallographic examinations show to correlate well to discontinuity volume and extent, specifically, depth. Given the conservatism in the proposed limit and the observed growth rate of S/N indications, it is concluded the probability of a S/N discontinuity contributing to operational leakage as a result of the proposed disposition strategy, is acceptably low.

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TSCRN 203 Technical Report Section 6 1

Sunmary of Disposition Strategy 6.1 Summary of Approach This section summarizes the major aspects of the proposed disposition strategy i

for low volume (i.e., low signal-to-noise) indications present in the Crystal River Unit 3 OTSGs. Cross-reference to the applicable sections of this report l

are provided, as appropriate.

l The proposed disposition strategy utilizes three distinct parameters to assess indications subject to the scope of this TSCRN (i.e., inoications exhibiting a volumetric morphology and which are not located in the tube sheet region of the j

l OTSG). The three parameters utilized are bobbin coil signal amplitude (voltage),

axial extent, and circumferential extent. This is a very conservative approach to assessing the acceptability of indications to remain in service.

Each indication detected with the bobbin coil ECT technique undergoes a determination of signal amplitude to noise (S/N) ratio.

Those indications l

exhibiting a S/N ratio greater than 5:1 are assigned a through wall depth estimate using the bobbin coil phase angle technique and are dispositioned in j

accordance with the current Technical Specification limit of 40 %TW.

Those i

indications which exhibit a S/N ratio less than 5:1 undergo additional

(

evaluation.

Low S/N indications are assessed against a bobbin coil voltage criteria to address leakage considerations.

A conservative acceptance criteria is established, to limit the potential for the low S/N indications to contribute to operational leakage (Section 5.2.3). Tubes which contain indications exhibiting a bobbin coil voltage >'2.5 volts are considered defective and are repaired.

Bobbin coil voltage is also utilized as a screening criteria below which no further action (beyond inspection in future outages) is required for any low S/N 2

indication.

A value of 0.9 volts has been shown to be acceptable from both a structural and leakage perspective (Sections 4.5 and 5.'2.3).

For those S/N indications exhibiting a bobbin voltage less than 2.5 volts but greater than 0.9 volts, additional evaluation is necessary.

The additional evaluation consists of motorized rotating pancake coil (NRPC) inspection to determine morphology. Volumetric indications are further assessed for axial and circumferential extent.

The measured extents are assessed against the proposed limits of 0.33 inches axial and 0.6 inches circumferential (Section 4.2.2).

J Tubes. containing indications which exceed either one of these criteria are considered defective and removed from service.

1 6-1 i

}

TSCRN 203 Technical Report Once an initial MRPC inspection of the indication is performed, subsequent reinspection using MRPC is not required unless the indication exhibits signs of growth. An increase in bobbin voltage of 0.5 volts from one inspection to the next indicates growth may have occurred and re-confirmation of previous MRPC axial and circumferential extent is prudent (Section 4.4.2).

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TSCRN 203 Technical Report Section 7 References 1.

FPC letter to NRC, 3F0494 09, dated April 19, 1994.

2.

EPRI TR-103756, " Examination of Crystal River Unit 3 Steam Generator Tube Sections," Final Report, April 1994.

3.

FPC letter to NRC, 3F1194-10, dated November 30, 1994.

4.

" Eddy Current Examination Report for Florida Power Corporation's Crystal River Unit 3 Refueling Outage 9", BWNT Report, June 1994.

5.

"0TSG Trending Report", 7th Edition, BWNT Report 51-1229259-00, July 1994.

6.

"EPRI Steam Generator Progress Report", Revision 9, October 1993.

l 7.

"0TSG Pulled Tube Catalog", Revision 1, BWNT Report 1190991, August 1994.

8.

EPRI NP-4504-LD, " Evaluation of Alloy 600 Tube A77-34 from Steam Generator A of Arkansas Nuclear One, Unit 1," March 1986.

9.

EPRI NP-1794, " Evaluation of Steam Generator Tube 85-127 from Oconee-1B",

April 1981.

10.'

EPRI NP-3026-LD, " Evaluation of the Lower Tubesheet Region of an Oconee-1 Steam Generator Tube", July 1983.

11.

"L'iterature Review - OTSG Secondary-Side Tube IGA / SCC Evaluation," B&W l

Nuclear Technologies, Report No. 51-1177882-00, January 1990.

12.

EPRI "PWR Steam Generator Examination Guidelines: Revision 3",

dated November 1992 (Reference letter dated February 23, 1994 from Mohamad Behravesh, EPRI, to the Technical Advisory Group.

13.

EPRI TR-100407, Rev.

1, "PWR Steam Generator Tube Repair Limits Technical Support Document for Outside Diameter Stress Corrosion Cracking at Tube Support Plates", August 1993, 14.

Packer Engineering Report B51956-R1-Rev. O, " Crystal River 3 8R/9R Bobbin Voltage (S/N) Growth Rate Calculations", dated March 1995.

1 15.

EPRI NP-6864-L, "PWR Steam Generator Tube Repair Limits - Technical Support Document for Expansion Zone PWSCC in Roll Transitions (Rev.1),

December 1991.

16.

NRC Regulatory Guide 1.121, " Bases for Plugging Degraded PWR Steam Generator Tubes," August 1976, i

7-1 4

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e a

1 y

TSCRN 203 Technical Report i

17.

NUREG/CR 5117

" Steam Generator Tube Integrity Program / Steam Generator Group Project," dated May 1990.

18.

Keating, R.

F.,

Hernalsteen, P.,

and Begley, J. A., " Burst Pressure Correlation for Steam Generator Tubes with Through Wall Axial Cracks,"

Draft EPRI report December 1993.

t 19.

Letter NRC to FPC, " Crystal River Nuclear Generating Plant Unit 3-Confirmatory Action letter (CAL) - Regarding Once-Through-Steam-Generator (OTSG) Tube Inspection During Refuel 9", dated April 26, 1994.

20.

Alzheimer, J.

M.,

Clark, R.

A.,

Morris, C.

J.,

Vagins, M.,

" Steam Generator Tube Integrity Program Phase I report," NUREG/CR-0718, dated September 1979.

21.

Framatome 22.

Kurtz, R. J., Bickford, R. L., Clark, R. A., Morris, C. J., Simonen, F.

A., Wheeler, K. R., " Steam Generator Tube Integrity Program," Phase II Final Report, NUREG/CR-2336, August 1989.

i 23.

Begley, J. A., Keating, R. F., and Houtman, J.

L., to be published.

24.

Hernalsteen, P., "The Influence of Testing Conditions on Burst Pressure Assessment for Inconel TvMng," laternational Journal of Pressure Vessels and Pioina, Vol. 52, 1991, pp. 41-57.

25.

Letter from K.

Krzywosz (EPRI) to P.

Sherburne (BWNS), " Review of Successive Eddy Current Data on Pulled Steam Generator Tubes From Crystal River Unit 3",

dated March 24, 1993.

26.

BWNT Document 51-1229575-00,

" Repair Criteria for Small Volume Indications, CR-3," Draft dated February 27, 1994.

27.

B&W Drawing No. 1208687, " Crystal River 3 Eddy Current Data Review on Selected Tubes," dated March 8,1991.

2 28.

FPC to NRC letter, 3F0294-01, dated February 4, 1994.

29.

FPC to NRC letter, 3F0993-22, dated September 30, 1993.

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i STATISTICAL EVALUATION SUPPORTING THE LEAKAGE INTEGRITY DISPOSITION CRITERIA i

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