LIC-98-0141, SG Eddy Current Test Rept for 1998 Refueling Outage. with

SG Eddy Current Test Rept for 1998 Refueling Outage. with

ML20155D599
Person / Time
Site:	Fort Calhoun
Issue date:	10/27/1998
From:	Gambhir S OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LIC-98-0141, LIC-98-141, NUDOCS 9811030254
Download: ML20155D599 (12)

Text

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October 27,1998 i LIC-98-0141 U. S. Nuclear Regulatory Commission ATTN: Document Control Desk Mail Station P1-137 Washington, DC 20555

Reference:

Docket No. 50-285

SUBJECT:

Fort Calhoun Station (FCS) Steam Generator Eddy Current Test Report -

1998 Refueling Outage Attached is the FCS Steam Generator Eddy Current Test Report which summarizes testing performed during the Spring 1998 Refueling Outage. This submittal fulfi!ls the reporting requirements of FCS Technical Specification 3.17(5)(ii).

Please contact me if you have any questions.

Sincerely,

& . Gambhir S.

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Division Manager jj ,

Nuclear Operations  !

j JMC/ tem Attachment C: E. W. Merschoff, NRC Regional Administrator, Region IV L. R. Wharton, NRC Project Manager W. C. Walker, NRC Senior Resident inspector Winston & Strawn (w/o Attachment)

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LIC-98-0141 Attachment Page1 FORT CALHOUN STATION STEAM GENERATOR EDDY CURRENT TEST REPORT 1998 REFUELING OUTAGE INTRODUCTION This report summarizes steam generator eddy current results obtained during the Fort Calhoun Station (FCS) 1998 Refueling Outage. Omaha Public Power District (OPPD) submitted 4

summaries of results of the two previous eddy current inspections to the NRC in the following documents:

• Fort Calhoun Station Steam Generator Eddy Current Test Report - 1995 Refueling Outage, dated July 18,1995 (LIC-95-0138) e Fort Calhoun Station Steam Generator Eddy Current Test Report - 1996 Refueling Outage, dated May 12,1997 (LIC-97-082)

SCOPE OF EXAMINATION ABB/ Combustion Engineering conducted an inservice eddy current examination of the steam generator tube.s at FCS in May 1998. The examination program was conducted to meet the requirements of FCS Technical Specification Section 3.17.

The test program included:

1. Full length bobbin coil testing of 100% of the tubes in steam generators RC-2A and RC-

28. This 100% testing encompassed 4913 tubes in RC-2A and 4929 tubes in RC-28.

2. Rotating Pancake Coil (RPC) testing of the top of the hot leg tubesheet, including 26% of the tubes in steam generators RC-2A and RC-2B,

3. RPC testing in the tight radius U-bends of 48 tubes in each steam generator (20% of the tubes in rows 1 through 4).

4. RPC testing of a minimum of 20% of the dents, which measured greater than 5 Volts during the 1996 exam, at tube supports H1, H2, H8, and V1. This exam consisted of 569 test locations.

5. RPC program expansion to 100% of the dents greater than 5 volts at H8,20% of the remaining tubes at H8, and 20% of the dents greater than 5 volts at the drilled tube support plate at H7. This expansion encompassed 1053 exams, and was performed as a result of two circumferential cracks being found at dented drilled support intersections in each steam generator at H8.

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LIC-98-0141 Attachment Page 2

6. RPC exams of 216 locations at the top of the cold leg tubosheet in steam generator RC-2B to assess whether circumferential volumetric indications (CVis), tirst identified in 1996, represented an active damage mechanism.

7. RFC exams to characterize approximately 255 indications detected with the bobbin probe.

8. RPC exams of 20% of mechanical rolled tube plugs (47 exams).

BOBBIN COIL EXAMINATIONS AND RESULTS The tubes in the bobbin coilinspection plan were exa. mined fulllength. The data were independently analyzed by two groups of certified Level llA or Level ill data analysts.

Discrepancies between the two sets of evaluation results were reviewed and resolved by a Lead Level ll1 Eddy Current Examiner.

The examination was conducted with a Zetec MlZ-30' digital eddy current acquisition system and analyzed utilizing the Eddynet 95' digital analysis system. The frequencies utilized during the bobbin coil examination were as follows:

400 kHz differential and absolute 100 kHz differential and absolute 600 kHz differential and absolute 20 kHz differential and absolute 400/100 kHz differential support ring mix 600/100 kHz differential copper mix 400/600/100 kHz differential transition mix 400/100 kHz absolute support ring mix The primary frequency of 400 kHz satisfied the requirements of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code for the examination of non-ferromagnetic steam generator tubing. Dent analysis was performed using the differential support mix and a voltage base of 2.75 volts on the 20% OD ASME signal, which is consistent with current industry practices.

The iOO kHz frequency was provided to assist in the confirmation of flaw indications. The 100 kHz absolute frequency detected gradual wall thickness variations. The 600 kHz frequency was provided to assist the analysts with additional mixing capabilities in the event of excessive outside diameter (OD) tubo deposits and diametric transitions. The 20 kHz frequency was provided to facilitate locating the probe position in the steam generator. The 400/100 kHz and the 600/100 kHz differential mixes were used to eliminate the tube support and OD tube deposit signals. The 400/600/100 kHz mix was used to evaluate areas of geometry changes such as top of tubesheet expansion transitions. The 400/100 kHz absolute mix was used to detect gradual wall loss.

a LIC-98-0141 Attachment Page 3 All tubes were inspected full length with the 0.540" or 0.560" bobbin probe, and indications were flagged for later characterization with qualified techniques. Tubes considered defective were identified using a " plug on detection" methodology in the following manner:

Bobbin coil indications which were confirmed by RPC to be volumetric and showed change from a review of 1984 and 1985 historical data were plugged regardless of size.

All crack-like indications were plugged, regardless of whether they showed change from history. Circumferential indications were stabilized as well. Any tube which obstructed passage of the 0.540" bobbin probe was also plugged. Indications flagged by the bobbin coil, but which did not confirm with RPC were dispositioned as requiring no further action and were left in service.

All pluggable indications are reported in Tables 1 and 2. Support notation for each steam generator is shown in Figure 1. Depth estimates were only made on pluggable indications for the purpose of providing input to assessments of overall steam generator condition. No tubes were left in service on the basis of sizing of indications. The list of indications with depth sizes is considerably smaller in this report than in the report submitted to the Nuclear Regulatory Commission after the 1996 Refueling Outage. Signals such as those reported in 1996 are not included in this report because (1) those types of signals were dispositioned as non-reportable through the use of RPC and historical reviews, (2) those bobbin signals are considered not to represent actual tube wall degradation, and (3) use of sizing techniques on those types of signals does not provide useful data. Therefore, sizing of those types of signals was not performed in 1998.

Bobbin coil dent measurements were performed using the 400/100 kHz differential mix channel.

Dent sizing is based on the 20% Flat Bottom Holes in the ASME standard set at 2.75 volts. This method is consistent with current industry dent sizing practice, and was adopted at FCS during the 1996 Refueling Outage. A comparison of 1998 to 1996 dent data indicates that there may be a slow progression of denting occurring in the FCS Steam Generators. This progression is slight, and is only noticeable now that the industry-standard dent measurement technique has been adopted. The dent measurement technique which was used prior to the 1996 Refueling Outage (dents were measured using the 400 KHz absolute, with one volt set to equal 1 mil radial reduction) resulted in much smaller voltages than what is seen with the current technique. As a result, small changes in dent size are more evident with the current sizing technique as a result of increased sensitivity. However, it is concluded that significant dent progression is not occurring based on the fact that no tubes required plugging due to failure to allow passage of the 0.540" bobbin probe.

ROTATING PANCAKE COIL EXAMINATIONS AND RESULTS Rotating pancake coil examinations were performed at the top-of-tubesheet transitions on the hot legs, in low-row U-bends, at dented tube support intersections, and at bobbin coil suspect indications as a diagnostic tool.

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- LIC-98 0141 Attachment Page 4 l

[. The tubes in the Top of the Tubesheet RPC Program were inspected in the expansion transition ,

. regions of each steem generator. The data were analyzed for the presence of crack-like 4 Indications similar to those found in the expansion transition regions at other plants. Terrain plots were used during the analysis effort to improve the probability of detecting circumferential cracks.

The RPC exams performed at U-bends, dents, freespan sections, and tube support intersections were performed to determine if stress corrosion cracking is present in suspect areas of the steam generators, to verify bobbin coil results, and to characterize the indications j seen with the bobbin coil probe, i Plus Point' and Pancake Coil RPC probes were both used during this inspection. Various l versions of the Plus Point

• probe were used for the bulk of the RPC examination and the Single Coil Pancake probe was used on a limited basis to inspect a sample of rolled mechanical tube l a plugs.

The frequencies used for the 3 coil top-of-tubesheet examination were as follows:

l 400 kHz Pancake, Mid-Freq. Plus coil, and High Freq. Pancake coil 300 kHz Pancake and Mid-Freq. Plus coil )

100 kHz Pancake and Mid-Freq. Plus coil 700 kHz Pancake and High-Freq. Pancake coil

. 20 kHz Pancake and Mid-Freq. Plus coil The frequencies used for the single coil U bend RPC examinations were as follows:

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400 kHz Mid Freq. Plus coil 300 kHz Mid-Freq. Plus coil 200 kHz Mid-Freq. Plus coil 100 kHz Mid-Freq. Plus coil 20 kHz Mid-Freq. Plus coil The frequencies for the Flex probe RPC examinations were as follows:

400 kHz Pancake and Mid-Freq. Plus coil 300 kHz Pancake and Mid-Freq. Plus coil 100 kHz Pancake and Mid Freq. Plus coil 20 kHz Pancake and Mid-Freq. Plus coil 1 The frequencies for the Single Coil RPC tube plug examinations were as follows:

400 kHz Mid-Freq. Pancake coil 300 kHz Mid-Freq. Pancake coil 200 kHz Mid-Freq. Pancake coil 100 kHz Mid-Freq. Pancake coil

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LIC-98-0141 Attachment Page 5 There was no circumferential cracking found during any of the examinations at the top of tubesheet expansion transitions. Diagnostic exams were performed on approximately 255 bobbin coil indications, and tubes with volumetric indications in these areas which showed change from historical data were preventively plugged. The number of tubes plugged due to volumetric indications reduced significantly from 1996 to 1998, and little growth was seen when comparing indications to historical data.

A total of four circumferential crack-like indications were found in dented tube support ,

intersections at H8, two in each steam generator. All four of these indications were less than 85 i degrees in circumferential extent, and all y are at locations with dents greater than 5 volts. All .

l four of these circumferentialindications were stabilized and plugged.

There were 13 axialindications recorded at various elevations of both steam generators. All were investigated by historical review and rotating coil technology, and the tubes were plugged. I l

Circumferential Volumetric Indications (CVis), first seen during the 1996 Refueling Outage exam, were originally flagged with the bobbin coil examination as Distorted Tubesheet Indications (DTis). CVis were detected at the top of the cold leg tubesheet in both steam generators. However, historical reviews of the CVis found in 1998 indicate that the damage mechanism is not currently active, and the CVis found in 1998 are bounded in severity by those identified in 1996. All CVI indications were stabilized and plugged. ]

TUBE PULL REWLiO Sections of two tubes were pulled from steam generator RC-2B for destructive examination during the 1998 Refueling Outage. These two tubes exhibited indications which appeared linear and potentially crack-like on the terrain plots generated from Plus Point

• data, but did not exhibit the phase correlation expected of actual through-wall tube degration. These indications had been dispositioned by the analysts as Less Than Zero (LTZ) indications during the 1996 Refueling Outage examination, representing that the eddy current response fell outside the defect plane, and the indications were caused by a non-degradation related condition. A search of industry tube pull and laboratory data was performed prior to the 1998 Refueling Outage to definitively support this conclusion. None of the industry data reviewed showed eddy current responses similar to the LTZ indications. It was therefore concluded that two LTZ indications should be pulled during the 1998 Refueling Outage to verify their origin.

Both indications pulled were selected based on largest amplitude Plus Point

• signals with phase angles closest to the defect plane. One of the selections was detectable by the bobbin coil, and the other was not.

Eddy current testing performed on the tube sections immediately after the tube pull indicated that the LTZ indications disappeared during the tut:e pull process. The pulled tubes were metallographically characterized and subjected to burst test at the ABB Combustion Engineering facilities in Windsor, CT. It was determined that the indications were caused by a non-degradation related condition, and were most likely caused by deposits.

LIC-98-0141 Attachmsnt Page 6 IN-SITU PRESSURE TEST RESULTS '

Eight tubes with defects were testd with the ABB/CE In-Situ Pressure Test device. These tubes were tested to demonstrate uwt leakage would be below allowable lim.ts at normal operating differential pressure (NODP) and at 1.4 times peak accident (main steam line break (MSLB)) pressure, and that they would be able to sustain a pressure of 3 times NODP without burst in accordance with NRC Regulatory Guide 1.121.

4 The eight indications tested covered axial indications, non-oriented volumetric indications, and circumferentially oriented volumetric lndications. No single circumferential indications (SCis) were in-situ pressure tested, primarily based on their short circumferential extent. Table 3 shows the tubes tested with the in-Situ Pressure Test method and gives the results of each test.

None of the eight tubes leaked at pressures up to 3 times NODP.

The indications tested by this method bounded the severity of the other known indications in the steam generators, as well as any indicatic'1s assumed to be present but net detected.

Therefore, based on the In-Situ pressure test results and extensive historical reviews of eddy current data which show little or no growth of indications in the FCS steam generators since 1984, operation until the next sch: duled inspection is justified.

CONCLUSIONS As a result of the inspection, OPPD has reached the following conclusions:

1. Bobbin coil indications of through-wall degradation generally showed little change from the last inspections and only minor change from the 1984100% inspection.

2. After a 26% top-of-tubesheet expansion transition inspection on the hot leg side of each steam generator, there were no circumferential cracks detected at the top-of-tubesheet. Feur short circumferential crack-like indications were detected as a result of RPC examinctions performed at dented tube support plate intersections. No cracking was identified in the tight radius U-bends.

3. The most notable indications for all inspections included:

Circ:'mferential Volumetric Indications at the top-of-tubesheet in the cold leg side of both steam generators. This degradation mechanism does not appear to be active based on historical reviews performed.

Volumetric indications in both steam generators which required plugging due to minor cht.nge when compared to data from 1984 and 1985.

Axial indications detected in both steam generators, both in the freespan and just above the top of the hot leg tubesheet. These appear to be dormant or very slowly progressing based on historical reviews performed.

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LIC-98-0141 Attachment Page 7 Single circumferential indications at dented tube support intersections at hot lep H8 tube support plate in each steam generator.

5. All tubes with the above notable indications were plugged. Circumferential indications were stabilized as well. Overall,12 tubes were plugged in steam generator RC-2A and 24 tubes were plugged in steam generator RC-28. In addition, two plugs which were removed from previously plugged tubes to facilitate removal of steam generator flow orifice plates were replaced in the hot leg side of steam generator RC-2A.

6. It was confirmed through tube peil results that LTZ indications seen with the Plus Point

• probe do not represent tube wall degradation and are likely caused by deposits.

7 Eight tubes in steam generator RC-2B were examined with an in-situ pressure test tool to verify structuralintegrity. All tubes tested met the requirements of NRC Regulatory Guide 1.121, August 1976, and none showed evidence of leakage.

8. Based on the results of the extensive eddy current examination and the in-situ pressure tests performed, operation of the FCS steam generators until the next scheduled inspection during the 1999 FCS Refueling Outage is justified. l l

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LIC-98-0141-Attachment Page 8 TABLE 1 LIST OF RC-2A TUBES PLUGGED Definitions:

CVI - Circumferential Volumetric Indication SCl - Single Circumferential Indication PIT - Pit-like Indication SVI- Single Volumetric Indication SAI- Single Axial Indication -

Row /Line Flaw Type Estimated Location Comments Depth (%)

4/1 SVI 27 CTS +2.19

.16/71 SVI 15 CTS +0.00 17/88 N/A N/A N/A Previously plugged tube. HL plug reinstalled after removal of flow orifice plates 17/100 N/A N/A N/A Previously plugged tube. HL plug reinstalled after removal of flow orifice plates 20/77 PIT 54 HTS +0.77

! 24/47 CVI 66 CTS-0.02 24/51 CVI 41 CTS-0.17 24/63 SVi 46 CTS +0.10 38/63 SVi 10 HTS +0.09 40/73 SVI 63 CTS +0.21 85/96 SCI 25 H8-0.54 91/62 SAI 30 H1-0.07 94/49 SAI 2 H1-0.05 N/54 SCI 62 H8 +0.50 1~

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l LIC-98-0141 Attachment-Page 9 TABLE 2 LIST OF RC-2B TUBES PLUGGED Definitions:

CVI - Circumferential Volumetric Indication sal-Single AxialIndication LTZ - Less than Zero Signal SCI - Single Circumferential Indication MAI- Multiple Axial Indications SVI- Single Volumetric Indication PIT - Pit-like Indication J Row /Line Flaw Type Estimated Location Comments  ;

Depth (%) l 17/62 CVI 60 CTS +0.01 i i

22/89 N/A N/A N/A Pulled tube, contained LTZ signal, verified not to represent tube wall degradation 24/55 SAI 34 HTS +0.93 )

29/80 PIT 2 HTS +16.28 l

31/64 CVI 77 CTS +0.07  !

32/63 CVI 55 CTS +0.06 32/65 CVI 28 CTS +0.09 33/82 SVI 18 HTS +0.32 35/6 SVi 65 H4+1.98 35/74 N/A N/A N/A Pulled tube, contained LTZ signal, verified not to represent i tube wall degradation l 40/65 CVI 52 CTS +0.16 40/75 SVi 55 HTS +0.00 48/69 PIT 4 HTS +3.23 _

76/85 sal 35 H3-0.39 80/63 SAI 51 H6+21.25 87/66 SAI 5 H7-2.39 88/55 sal 9 HG-0.66 SAI 52 H6-1.36 89/74 - mal 29 H1 +0.05 90/73 SAI 39 H1 +0.14 90/87 sal 79 H8-0.17 91/72 sal 63 H8-0.86 93/72 sal 39 H8-0.69 94/59 SCI 18 H8+0.30 95/62 SCI 4 H8+0.29 l

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l LIC-98-0141 i Attachment j' Page 10 l '

l TABLE 3

, IN-SITU PRESSURE TESTS 1 I

' Definitions: l CVi - Circumferential Volumetric Indication NQl- Non Quantifiable Indication DSI- Distorted Support Indication sal-Single AxialIndication DTI - Distorted Tubesheet Indication SVI- Single Volumetric Indication

. NDD - No Detectable Degradation Maximum i Indication RPC Bobbin Bobbin Pressure i S/G Tube Type' Location Voltage Length Width Voltage Call (pelg) Leakage?

RC-28 R17L62 CVI CTS +0.01 0.28 0.29' 360 Dog 4.84 DTI 4900 NO I l

RC-2B R31L64 CVI CTS +0.07 0.16 0.23" 150 Deg 5.51 DTI 4900 NO RC-28 R32L63 CVI CTS +0.06 0.28 0.26* 182 Deg 2.07 DTI 4900 NO ,

RC-28 R35L6 SVI H4+1.98 3.19 0.28' O.33* 3.94 NOl 4550 NO RC-28 R40L75 SVI HTS +0.0 0.26 0.3* 0.3* N/A 1JDD 4575 NO RC-28 R90L73 sal H1+0.14 2.02 0.57' O.39' 1.23 DSI 4550 NO RC-2B R90L87 SAI H8-0.17 0.11 0.47' O.16' N/A NDD 4600 NO RC-2B R91L72 sal H8-0.86 0.19 0.33" 0.28' N/A NDD 4575 NO t

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LIC-98-0141 l Attachment I Page 11 l~

.l l FIGURE 1 FCS STEAM GENERATOR ELEVATION DRAWING ,

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