ML20211N463
| ML20211N463 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 10/07/1997 |
| From: | ENTERGY OPERATIONS, INC. |
| To: | |
| Shared Package | |
| ML20211N461 | List: |
| References | |
| NUDOCS 9710160145 | |
| Download: ML20211N463 (36) | |
Text
- -_________ _ _ _ _ _ _ _ - _ _ _ _ - _ _ _ -
ANO-2 Steam Generator Tube Integrity Evaluation October 7,1997
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Executive Summary
'!he purpose of this report is to provide results from the ANO 2 2R12 Steam Generator (SO) inspection and to provide results fiom evaluations performed that address SO tube integrity for cycle 13, Like previous assessments', control room and off site dose consequences are addressed in the evaluation.
Enhancements made to the eddy current program and improvements made to the probabilistic models allowed an improved evahwtion to be performed. Following is a listing of the program enhancenents,
- - Lowering the flaw detection threshold and increasing the overall probability of detection (POD) through the following program changes:
- Use of a larger diameter bobbin probe increasing the fill factor
- Establishment of an Enurgy Review Team which reviewed resolved oddy current data
- Computer Data Screening (CDS) as a third party review of production data
- Development of a detailed training mannat by Entergy personnel
-Training for Resolution Analysts
- Development of Examination Technique Specification Sheets (ETSS)
-Independent resolution analysis
- Requirement of Qualified Data Analysts (QDAs)
- Implemented individual production analysts performance tracking
- Audits of remote eddy current analysis e Expanded in-situ pressure test program
. Conducted diagnostic testing of bobbin indications that were accessible
. Conservatively length sized axial flaws at the eggerates and circumferential
- cracks at the top of tubenhaet rITS) expansion transition
. Increased accuracy of growth rate data for eggerate flaws by consistently comparing eddy current signals from 2R12,2F% and 2R11
. - Increased accuracy of growth rate data for circumferential cracks by comparing the eddy current data from 2R12 to that data from 2F96
- Plugged confirmed crack-like indications e Enhanced the probabilistic models for leakage and burst The previous evaluations used both a deterministic and probabilistic approach that built upon significant work pieviously conducted", Likewise, this report uses and builds upon the previous evaluations performed on tube degradation. The evaluation follows guidance prosided by the Nuclear Regulatory Commission (NRC) for performing condition monitoring and operational assessments of steam generator tubing degradation found in the draft version of the Steam Generator Tube Integrity Regulatory Guide *.
This report describes the above mentioned items in further detail and addresses the ANO 2 SG integrity program and the success in identifying tube degradation. Other program actions previously implemeated include preventive measures such as primary temperature reduction,--
installation and use of Na radiation monitors, comprehensive inspections, and a conservative operating philosophy. Strict controls are maintained on primary-to-secondary leakage limits, with procedures and rigorous operator training providing heightened awareness and guidance to assure safe shutdown in the event ofleakage.
I
4 ne in-situ pressure testing conducted during the 2R12 outage demonstrated that the degradation found in the ANO-2 steam generators nwt the 3 delta-P structural requirements with negligible leakage. Probabilistic evaluations using guidance from the draA regulator / guide were performed to ensure the integrity of the AFO 2 steam generators will be maintained until the next planned outage. A run time of 0.87 effective full power years (EFPY) was evaluated and the results demonstrate that ANO-2 can safely operate until March 1998 with margin, 2
i .
I. BACKGROUND and INSPECTION RESULTS 1.0 Introducti9n The purpose of this report is to describe the condition monitoring "as found" and the operational assessment " forward looking" work performed to evaluate the integrity of the S0 tubing during the current operating interval, scheduled for approximately 10 months.
Included in this assessment is degradation due to axial cracking at the support structures and in the freespan as well as circumferential cracking at the expansion transition.
A detailed description of ANO 2 operational history was prosided in Reference 2, and updated by Reference 3, Currently, an equivalent of 14.18% of the tubes in the "A" SG and 12.88% in "B" are plugged. Included in this are 920 tubes repaired with tubesheet sleeves.
2.0 ANO 2 Steam Generator Description
'Ihe ANO-2 steam generators are of the U-tube design manufactured by Combustion Egineering (Model 2815). Each steam generator contains 8411 tubes constructed of high unperature mill annealed (IfrMA) Inconel alloy 600 material with an outside diameter of 3/4 inches and a wall thickness of 0.048 inches.11e tubes are explosively expanded the full depth of the tube shect. There are seven full eggerate tube support plates (TSP), two partial eggerate TSPs, and two partial drilled TSPs. The SG layout is shown in Figure 2-
- 1. Commercial operation began March 1980, and all volatile treatment (AVT) chemistry has been used since that time. Secondary side boric acid addition wm initiated in 1983 to arrest denting at the partial drilled TSPs. The hot leg operating temperature was initially 607' F, but was reduced to ~600" F following the ninth refueling outage in the fall of 1992.
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Figure 2.1 ANO-2 Steam Geerators STEAM OUTLET NOZZLE n
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NOZZLE X /
BAT WING ~
j TUBE SUPPORT /' l u
I E n EGG CRATE E - ) TUBE SUPPORT g ASSEMBLIES e_ 7 _, r .
HOT LEG COLD LEG 4
3.0 Regulatory Basis it is the current industry practice with guidance from the Nuclear Rpd=y Commission (NRC) staff that utilities develop a program which prmides reasonable assurance that the steam generator tubes are capable of performing their intended safety function 'Ihis includes establishing performance criteria commensurate with adequate tube integrity, programmatic considerations for prosiding reasonabic assurance that the performance criteria will be mct during plant operation, and the guidelines for monitonng the condition of the SG tubing to confirm that the performance criteria are in fact met, Additionally, the program framework should preside that measures be maintained to mitigate the consequences of occurrences invohing gross rupture of the tubing and/or abnormal leakage.
The program strategy begins with an NDE inspection followmg plant shutdown. The in=pa daa is intended to provide informaton concerning the active degradation mechanisms present in the SGs, the identity of the tubes containing flaws, and the size of these flaws for each active damage mechanism. This information is to be used as part of other r , ram elements to assess tube integrity performance, to detenmne the appropriate time intewal to the next inspection, to determine the tube repair limits, and to determine which tubes fail to satisfy these repair criteria.
The tube inspections are followed by assessments of tube integrity performance relative to NRC accepted performance based criteria that are currently provided in the draft version of the Steam Generator Tube Integrity Regulatory Guide'. NRC Regulatory Guide (RG) 1.121, " Basis for Plugging Degraded PWR Steam Generator Tubes", provides the guidelines for determining the currest depth based tube repair criteria and operational leakage limits. These criteria are specified in the ANO-2 technical specifications. The draft regulatory guide supplements the guidance of RG 1.121. The draft regulatory guide addresses alternate types of repair criteria and it addresses the treatment of uncertainties in tube geometry, material properties, NDE flaw measurements, and flaw growth rates.
Tube integrity performance is subject to two different types of assessments in the proposed regulatory guide: a condition monitoring assessment and an operational assessment. 'Ihe -
condition monitonng assessment is " backward lookmg" and its purpose is to confimi that
_ adequate tube integrity has been maintained s'mce the previous inspection Condition monitonng involves an assessment of the "as found" condition of the tubing relative to the tube integrity performance criteria. The operational assessment difncs from the condition monitoring assessment in that it is " forward looking". Its purpose is to demonstrate -
reasonable assurance that the tube integrity performance criteria will be met throughout the period prior to the next scheduled tube inspection. This projection is based on the inservice la==~ naa results, the tube repair criteria to be implemented for each degradation mechanism, and the time interval to the next reheduled inspection.
Both the condition monitonng and operatior I assessments have been completed utilizing
= the current inservice inspection results. The results from 2R12 and the projected time interval until 2P98, the midcycle outage scheduled for March 1998, have been compared to the performance criteria identified in the draft Steam Generator Tube Integrity Regulatory Guide
- and are discussed in detail in this report 5
_ . _ _ _ _ _ _ - - ~ . _ . _ -
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9 I
L 4.0 Eddy Current inspection i
, ne 2R12 eddy current inspection was conducted with several enhaaMents made to the 3-
. program as a result of reviews of the 2RI 1 program performance ne enh;c.;m.e .ts, i which are addressed below, increase the defect pmbability of detection (POD) and helped
- achieve a higher quality inspection of the SO tubing and sleeving.
l 4.1 Program Improvesments
- In order to increase the POD or reduce the possibility of human error in performing data
- acquisition as well as prMaa and resolution eddy current analysis, several changes
- were implemented prior to and during to the 2R12 eddy current inspection 4
i Trainian and Performance Demonstration Testian
, Prior to 2R12, the eddy current vendor was relied upon to conduct the site specific training I
and performance da.a-ti tion testing. As part of Entergy's increase in the oversight and L control of the eddy current process, Engineering Programs personnel developed the training l and testing packages that were implemented during 2R12". EPRI site shell soRware, an j ECT training and testing riataha- deve'.eped for utility use, was used and prsved to bc .
effective. By ANO personnel becoming more involved with the training and testing it allowed emphasis to be put on specific topics such as " missed indications". Additionally, I
it allowed a selection of tubes for training /testmg that contamed ambiguous indications,
[ that are at risk of being improperly characterized.
- i. Using the EPRI site shell software allowed the testing to be set up on a degradation
[ specific basis. His was the approach taken when cWing the testmg Two specific
- . tests were given, one for the TTS circumferential cracking which addressed flaw detection through the use of Motorized Rotating Pancake Coil (MRPC), the other performance demonstration test was the bobbin results which included mostly eggerate flaws with several freespan and batwing wear indications All of the data used in the exam werc j taken specifically from ANO-2 steam generators. In order to ensure the bobbin indications
!. utilized in the exam were legitimate flaws a rigorous selection process was implemented.
i - All of the bobbin indications that were used had correspondmg MRPC data that confirmed
! and characterized the flaw.
He bobbin data associated with the ANO-2 SG eggerates is difficult to analyze due to the
- influence of the deposits and support straps, This was confirmed when several analysts j - failed to call ambiguous eggerate axial flawi during performance testing ANO required all indications that were not called during the test to be reviewed by the anal >it. The ANO
{ program requires a 80% score prior to allowing any analysis to be conducted. De
[ nebulous flaw signals associated with predominately small axial defects at the eggerates
- caused several of the analysts to be retrained and retested By sensitizing the analysts it
- created a heightened awareness for detecting the smaller axial flaws mainly at the l eggerates, which increased the POD at the production analyst level. This wm evident by j the increased number of calls that required resolution and further diagnostic testing.
i l To further emphasize the importance of detecting the ambiguous flawi and proside information on previous " missed indications" special resolution training wm conducted 1
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l with the resolution analysis staff. Covered in this training was the flow of data through the resolution process, emphasizing the importance of performing a diagnostic exam on any qwiaanble signal. He effect of the training was evident by the number of diagnostic exams required. Additionally, a review of specific examples of signals previously not called during earlier itages was performed. His provided a heightened awareness for the signals which an analyst may have a propensity to overlook.
Probe Imorovements and Computer Data Scra-ai==
With the exception of the low rows and tubes with a restriction, a 0.600" diameter bobbin coil probe was used to perform the 100% inspection. His probe improves the fill factor over the previous 0.580" diameter probe and resulted in a better signal response Bobbin probes are qualified as small as 0.540"" .
Computer Data Screening (CDS) was used as a third party resiew of the bobbin data. De CDS sorts and setups were calibrated to allow indications with a certain amplitude and phase threshold to be flagged for review. This ensured that larger indications did not go umb*~ '~J. %c CDS results were compared to the resolved data by the senior analyst.
CDS was also used to identify and profile sludge and dents.
Enterav Review and Oversieht Team
.. most significant program improvement resulted from addmg the Entergy Review team (ERT). De team wm made up of two Entergy Level III eddy current analysts, a Palo Verde Level III, and a Level Ill from Zetec.. The team's responsibilities included review of all bobbin indications called by the production analy sts and dispositioned as no detectable degradation (NDD) by the resolution staff. A review of approximately 10% of tubes classified as NDD by the production analysts was also enad *~i Feedback to the resolution and production analysts by the team provided an excellent vehicle to improve iaW ani quality at the production and resolution level. Additionally, the ERT provided an indanandaat review of all pluggable indications and helped to identify appropriate in-situ candidates Individual analyst performance tracking (APTS) was performed to proside feedback to the production analysts. De intent was to ensure that the anal >3ts were making calls consistent with the guidelines. It also provided a tool to identify if a analyst was overcalling.
Resolution Process. ODA Reauirements. and channes to the ETSS Due to the increased number of calls made from the bobbin analysis by the production analysts an increase in the number of resolution analysts was required. A more conservative approach was taken during the resolution process requiring a large number of
~
special interest exams Resolution of bobbin data required an independent resiew prior to any indication being dispositioned as NDD. nis resiew and the subsequent ERT review helped strengthen the quality of the resolution process. An extensive resolution analysis review of Possible Loose Parts (PLP's) and Ligament Cracking Indications (LCI's) at support plates was also accomplished during 2R12.
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, i To ensure the quality of data analysis was not compromised, all analysts were required to be Qualified Da'.a Analysts (QDA's). Additionally, ANO personnel created the Examination Technique Specifications Sheets (ETSS) with review from the vendors to ensure compliance with the EPRI Guidelines'. Prior to 2R12, the vendor had the task to complete the ETSS. By Entergy
- creating the ETSS it prosided several advantages, one of which was the ability to control revisions during the outage from an acquisition and analysis standpoint.
l l Acauisition Dats Review A i improvement that helped the POD along with increasing the inspection efficiency was the use of a " Data Cop". A Data Cop is a QDA who is specifically assigned to review data as it is acquired to assure a maximum quality level. His improved the quality of the data analyzed, allowing the production analysts to focus on data analysis.
Diarnostic Testine Previously, indications below the repair threshold were left in senice without characterization of flaw morphology with a MRPC. To reduce the probability ofleasing cracks in senice, during 2R12 all bobbin indications were diagnostic tested with MRPC.
If the bobbin signal was confirmed to be a crack-like indication the flaw was repaired regardless of depth. This approach took a considerable amount of time, but provided a high level of confidence that all crack-like indications were repaired. Additionally, by performing diagnostic testing it provided a way to quantify the degradation from a morphology and size standpoint.
4.2 Program Observations he improvements made to the inspection program were esident when a comparison was made to the 1996 Forced Outage (2F%) and the Eleventh Refueling Outage (2RI1) results. A review of the past raw data revealed a relative change in signal response. He majority of the flaws were extremely ambiguous and without the enhanced training and testing along with the other changes to the program provided during the 2R12 inspection, an analyst could not have been expected to detect the flawi.
The enhancements outlined in the above paragraphs helped increase the bobbin POD. The POD is an important factor in both condition monitoring and developing the operational assessment. An increased POD for condition monitoring gives a higher level of confidence that significant defects are detected, ensuring flaws that may possibly challenge structural integrity are adequately addressed. An in-situ pressure test was used to addresses the structural and leakage integrity of the SG tubing and is discussed later in this report.
Bobbin POD is a required input for the probabilistic model used in the operational assessment and is desirable to be as representative of the field detection capability as possible. The run time models that use these described inputs are discussed later in this report.
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. i 4.3 Inspection Scope and Results Table 4.1 shows the inspection scope of 2R12 and Table 4.2 reveals the indications detected that were confirmed to be degradation. Detailed inspection results are provided in reference 5.
Table 4.1 t 2R12 ECT Inspection ECT Examination Troe inspections Conducted % Scope Expansion Reauired SG "A" Bobbin 7594 100 No Bobbin Below Sleeves 153 20 No RPC ET HL 6802 100 No RPC ET CL 1742 20 No B&W Sleeve 354 100 No TIG Sleeve 131* 20 No U-bends (row 1) 28 40 No DTSP 728 20 No SG "B" Bobbin 7700 100 No Bobbin Below Sleeves 71 20 No RPC ET HL 7476 100 No RPC ET CL 1742 20 No B&W Sleeve 49 100 No TIG Sleeve 44* 20 Yes
< TIG Expansion 130 100 No U-bends (row 1) 25 40 No Drilled Support Plate 698 20 Yes Drilled Support Plate Exp. 722 20 No
- Includes 20% plus the previous indications.
Table 4.2 2R12 ECTInspection Results Location SG " A" SG "B" Hot Leg ET Region (circumferential) 66 53 Sludge Pile (axial, cire, and vol) 31 21-Cold Leg ET Region (circumferential) 0 0 EC Support Plate 166 246 BW Support 4 24 Drilled Support Plate 4 4 Sleeved Tubes (Kinetic) 9 0 (TIG) 9 10 Free-Span 38 4 Tube Sheet Below Sleeves 0 0 Miscellaneous 23 4 Total 350 366 9
t As previously mentioned, bobbin indications were diagnostically tested wlth MRPC for
- confirmation and morphology characterization. Dere were 1,550 diagnostic exams pei-fu...d at the eggerates and drilled supports revealing 449 confirmed flaws (CMR) and 1,001 dispositioned indications (DMR). His low confirmation percentage demonstrates the conservatism inherent with upgraded 2R12 bobbin analysis.
A review of all indications that were dispositioned by the resolution analysts was conducted by the ERT. There were more than 2000 diagnostic exams performed on freespan bobbin indications with less than 50 indications being removcd from senice due to a change in signal or what was interpreted as flaw growth. Inspections were performed to confirm a flaw existed and to assure no crack-like indications were left in senice.
Additionally, the diagnostic testing allowed the size and morphology of the flaws to be quantified.
4.4 Axial Maw Sizing and Growthrate Evaluation The probabilistic models for leakage and burst require several inputs. In addition to the POD, other inputs required are size (length) distribution and growth rate estimates.
Previous evaluations have been performed using conservative estimates from other plaats.
A study was conducted using ANO-2 data' that allowed a more accurate growth rate value to be obtained. He ANO-2 specific growth rate values obtained increase the validity of the model used to predict the end of cycle (EOC) conditions.
he previous practice at ANO-2 was to utilize a qualified depth sizing technique to leave axial flaws insenice that were below the 40% through w311 (TW) limit specified in the ANO-2 technical specifications (T.S.). The axial length distributions used to assess EOC
- cracking came from industry experience Prior to 2R12 the axial indications in the ANO-2 steam generators were not tested with MRPC for confirmation, therefore, the axial length
- could not be obtained. By using the qt.alified sizing technique, and leaving the flaws in senice over several cycles an extensive site specific data base exists to develop a statistically robust growth correlation.
The eggerate growth rate study conducted during 2R12 used several Level Ill/QDA's. A total of 449 indications were identified for comparison,175 in SG "A" and 274 in SG "B".
De approach consisted of a re-review of the presious bobbin data from 2F% and 2Rll, then comparing the signature of the signals to the indications detected during 2R12.- The same analyst was required to compare the data from all three outages to ensure consistency in analysis, calibration, and set-up.
A comparison was made using the 400/100 kHz mix channel, the 400 kHz channel, and the 200 kHz ciannel. De distributions from all three bobbin channels were comparable, the 400/100 mix and the 400 channels were almost identical, which is expected since the signals from each should reflect the other. He 400/100 mix channel growth rates were chosen as the input to the Monte Carlo model since it is our prime detection frequency and closely resembles the 400 kHz channel.
The growth distributions are provided in Figure 4.1 for the growth from 2R11 (October 95') to 2F96 (November 96'). The distributions from 2F% to 2R12 (May 97') are illustrated in Figure 4.2.
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Figure 4.1 l 2Ril - 2F96 400/100 Mis Bobbin Relative Signal Change
. x0 250 i
< 2x 150 g 100 I SO O
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%Twme.e Angle ne depth sizes taken form the last three inspections were paired together to compare change in relative signals. The global average was taken from the longest interval (2R11-2R12) establishing a standard deviation from the 95% largest growth rates. In order to obtain an accurate length distribution for input to the probabilistic model, the confirmed eggerate axial flaws were length sized with MRPC. De length values were obtained directly from the raw MRPC data. This is a conservative method of determining extent due to the eddy cunent field look ahead and fall behind, which is defined as the influence of the flaw seen prior to the coil reaching the metal loss and after the coil passes the flaw. Therefore, the MRPC values used are, on average, longer than what actually exists as confirmed by previous ANO-2 tube pull data. Of the five axial fhws pulled from ANO-2, the MRPC axial extent oversized the flaws by an average of 15% indicating that a conservative distribution is being used for axial lengths. A comparison between the field MRPC length and the length determined by the 11 l
I
destructive analysis is given in Table 4.3. Length distributions for axial eggerate flaws in both the "A" and "B" SGs are provided in Figure 4.3.
Table 4.3 Axial Crack Lengths from ANO-2 Pulled Tubes Year SIG Tube Field MRPC Length linches) DE Length (inches) 1992 *B" 19 55 0.72 0.66 1992 *B" 19 55 0.57 0.31 1992 "B" 96 116 0.51 0.65 1996 "B" 16 56 1.20 1.03 1996 "B" 70-98 1.40 0.96 Figure 4.3 RPC Axial length Distribution for the Eggerates 35 30 -
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RPC Irngth (inches) 4.5 Circumferential Cracking Sizing and Crowthrate Evaluation As with the axial cracking model, the circumferential cracking model requires the same inputs. Eddy current examinations provide the data to determine the growth rates and length distributions.
Due to the inability of the bobbin coil to adequately detect a circumferential oriented flaw in a expansion transition, a 100% MRPC cxam was p:rformed at the TTS. Flaws detected during 2R12 were length sized to determine it.-situ candidates and to validate the percent degraded area (PDA) distribction utilized in the Monte Carlo simulation. He Monte Carlo model utilizes a PDA distribution from the EPRI/ANO Circumferential Crack Program" as an input. The EPRI/ANO Circumferential Crack Program" 12 l
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- 11. CONDITION MONITORING 5.0 In-Situ Pressure Testing in order to demonstrate the flaws detected during 2R12 meet the "as found" condition monitoring requirements, several in-situ pressure tests were performed, in-situ tests were performed on two tubes containing two circumferential cracks at the TfS and six tubes containing ten axial flaws at the eggerates. An example of two flaws in the same tube can be observed in Figure 5.3. A list of I
the tubes with the associated pressures achieved are provided in Table 5.1. The pressures listed in the table are designated in the 2R12 ABB/CE in-situ traveler and were chosen to be target levels representing the normal operating pressure difference, accident pressure differential, and three times normal operating pressure difference. Further details on in-situ results can be found in reference 5.
The corresponding Regulatory Guide 1.121 limit for ATO-2 is 4600 psi which includes a 13%
temperature compensation. All tubes tested exceeded this bt rst pressure.
Table 5.1 In-situ Pressure Tests SG Tube R/L Pressure (psig) Flaw Type Leakage Leak rate (gpm)
A R82Cl18 1600 OlH AXIAL NO 2850 NO
( 4700 5000 YES BURST 0.15 B R9Cl17 1600 OlH AXIAL NO 2900 NO 4900 NO B RI1Cl29 1650 Olli AXIAL NO 2900 NO 4850 NO B R34C70 1650 OlH AXIAL NO 2850 NO 4850 NO B R23C65 1650 02H AXIAL NO 2850 NO 4850 NO B R37C67 1650 OlH AXIAL NO 2850 NO 4650 BURST A Rl5C25 1650 'ITS CIRC NO 2850 NO 4850 NO A R52C50 1650 TTS CIRC NO 2850 NO 4850 NO 14
1 i
5.1 In-situ selection process Several parameters were considered when choosing the " largest" or most structurally challenging flaws for testing in order to bound all remaining flaws. Not only were the ECT parameters considered, the morphology of the flaw was also considered. Stmetural characteristics such as depth, overall length, length of through wall extent, and presence of ligaments were also considered.
Attal Cracks Due to the difTerences in flaw morphology and the potential presence ofligaments it is difficult to quantitatively determine which axial flaw provides the greatest challenge to structural integrity; therefore, several combinations of ECT parameters were considered.
Figure 5.1 shows 2R12 bobbin %TW and RPC length. This is a good independent comparison due to the different inspection methods and more importantly the relationship each parameter has to structural integrity. The red triangular points show the flaws that were in-situ pressure tested. Additionally, since amplitude is considered to be a good indicator of structural integrity, bobbin %BV was plotted against bobbin amplitude as illustrated in Figure 5.2. Again, the triangular points show the flaws in situ pressure tested.
Figure $.1 Bobbin %TW vs. RPC Length for Flaws In-situ Pressure Tested 9 .,++..
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 RPC Isngth 15
Figure 5.2 Bobbin %TW vs. Bobbin Amplitude for Maws In-situ Pressure Tested
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The previous two plots provide a good starting point for the in-situ selection; however, due to the concems over morphology differenees, the ERT reviewed all potential candidates and provided recommendations based on a thorough resiew of the bobbin and RPC data.
Some examples of axial flaws that were in-situ pressure tested are prosided in Figures 5.3 through 5.5. As illustrated in these figures there are other less apparent factors, such as the presence ofligaments or multiple flaws in the same plane that may affect the bobbin amplitude and phase signal response. Along with the sorted parameters illustrated on the previous figures this information was considered when selecting the in-situ pressure test candidates.
Figure 5.3
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Circumferential Crackj As previously stated, circumferential length is a valid parameter for the selection ofin-situ pressure test candidates. De 2R12 distribution for RPC measured circumferential length is provided in Figure 4.4. He tubes selected for the in-situ pressure test are tubes 15/25 and 52/50 both in the "A" steam generator; they are the two longest points shown in Figure 4.4, S.2 Results ne results of the pressure tests are shown in Table 5.1. As indicated, all of the flaws tested exceeded the temperature corrected 3AP limit with additional margin. The rigorous selection process and thorough review of the axial flaws ensures that the most challenging flaws were pressure tested and bound all other detected flaws. De longest two -
circumferential cracks are considered to bound all other cire cracks. De 2R12 tests along with numerous in-situ pressure tests previously performed on larger cracks using pressures up to 6800 psi' ensure that the "as found" structural integrity meets the required 3 AP limit with margin.
I 18
111. OPERATIONAL ASSESSMENT 6,0 Probabilistic Run Time Assessment A detailed probabilistic analysis of the e% cts of axial cracking and circumferential cracking in the ANO-2 steam generator tubing was performed to address allowable operating intervals Two intervals with varying parameters were addressed and are
' provided within this report. Each of the cracking mechanisms are addressed in accordance with the draft regulatory guide
- requirements and are demonstrated to meet the 0.01 cumulative probability of burst (POB) for a given operating interval. Additionally, the probability ofleakage under an accident scenario is also addressed. 'the information from the leakage model is then used to address off site and control room dose consequences.
'Ihis section provides a detailed look at how the values are obtained.
'Ihe processes of crack initiation, crack growth, and detection via eddy current inspection are simulated using a Monte Carlo model. The model follows these processes over multiple cycles of operation allowing benchmarking of the results. The simulation model tracks both detected and undetected populations of cracks. End of cycle (EOC) conditions in terms of total number of cracks, crack depths, and lengths are calculated as a function of cycle length using a fully probabilistic approach. Hence, the POB under a postulated MSLB condition can be computed. Projected leak rates during a MSLB scenario sie also computed.
6,1 Approach As mentioned the model uses several inputs to obtain a single probabilistic output. In order to reduce the error band from v. hat has historically been the case, due to the uncertainties associated with the oddy current POD and the growth rate input, ANO implemented the enhancements to the ECT program as mentioned previously in section 1.4.
Both the axial and circumferential cracks are modeled using the same methodology. A flaw is initiated from an' adjusted Weibull function determined from several parameters.
Once the crack is initiated it is assigned a length from a given length distribution then the model assigns a material characteristic or flow stress, which is dermed as the average of the yield and ultimate strengths. For the leakage model, a form factor is assigned at
- initiation to determine EOC leakage values. Once these input parameters are selected they remam constant through out the life of the crack. A simulated oddy current exam is performed using a variable POD curve to determine if the flaw is detected and subsequently plugged. If the flaw is not detected it is assigned a growth rate from a defined growth rate distribution. The sequence is then repeated until the flaw is detected 6.2 Axial Flaw Probability of Burst In order to enhance the accuracy of the probabilistic output values the input values must be as consistent and accurate as possible. The process begins with the eddy current POD.
When a flaw is detected and repaired it provides no subsequent threat to challenge structural or leakage integrity. Hence, the reason for the numerous enhancements to the eddy current program as described in section 1.4. The variable POD curve for axial I9
p.
cracking used in the Monte Carlo simulation is provided in Figure 6.1. T,wo curves are shown. The curve to the IcA is based on data compiled by St. Lucie in a resiew of an Appendix H qualification and is considered to represent best practice. ANO 2 tube pull data was used in the development of this curve. The dotted curve is an estimate of the worst case POD and is applied to earlier inspections at ANO-2. The enhancements made to the program in 2R12 will shiA the curve further ich of the solid cunt increasing the POD of smaller flaws; however, it is dinicult to quantify the improvement that wu made during the 2R12 inspection. During the assessment a consenative approach is used, taking no credit for the program deu.u.ts and using the POD curve established by the Appendix H qualification data. A sensitivity check was conducted utilir.ing the more conservative POD curve for all inspections including 2R12, without taking credit for the enhancements to the 2R12 ECT program. The resulting probability of burst value for 0.87 EFPY was less than the figure of merit of 0.01.
Figure 6.1 notun poo curves 1
POD Cwve kom *
./.
Apperdu H Date oa. ,/
es. l l
! Previous j POD Curve e4 [
l l
l l
03-l l
l
,/
e e 20 40 80 to 100 eversee depeh MTw) 20
As with POD, the lengths of the flaws are a critical input. The use of MRPC to determine the axial extent and then establishing a distribution to sample allows this input to be l obtained. Figure 4.3 provides the distribution for the axial lengths, By MRPC sizing the axial flaws during the 2R12 outage as explained in section 1.4, it is believed that an accurate distribution is obtained. Once the distributions are obtained they are sampled during the model simulation. Figure 6.2 shows the function obtained from the axial length distributions.
Figure 6.2 Distribution of Axial Crack lengths used in Probabilistic Model 1.0 0.9 4 t---- +-
O.8 +- b- +-
t 0.7 4-
t- +
0.6 --- +- "
t a ! ANO Unit 2 S/G B O.5 + - Eggerate Data *
--}-
d 0.4 > +- , f n-i O.3 b +- -*- +
4 I
0.2 I1
+-- +
O.1 - - - - + - - - - + - - -4 -
O.0 O.O O5 1.0 1.5 2.0 2.5 3.0 Axial Length, Inches 21
. i e
s Material property information is necessary in order to determine flow stresses. %c flow stress distributions are provided in Figure 6.3 Dese values were compiled from the original CMTR sheets provided during the fabrication of the steam generators. The mean value of the distribution used in the nxxiel is 134,000 psi with a standard desiation of 6,000 psi.
Figure 6.3 HistogrSm of Tube Flow Stresses ANO, S/G A + S/G 8, Material StrengS 1800 1600-1400-1200-k 1000 -- -
I :.
800 - - -
t 8
~
600 -
4' 400 - _
~
~
200- -
0 o o o o o R R e ,
o o $$g@ o 0oR 3
- A
- D R
- D - D M E-R
- "5 -
Sum of Adjusted Yleid and Tensile Strength, pal 22
wt If the crack goes undetected based on the sampling of the POD curve a growth rate is assigned. His value is obtained from the growth rate distributions established from the l study completed during 2R12. He bobbin 400/100 mix was selected to proside the best l estimate of growth rate for the axial cracking. When the larger amplitude signals are I
considered the measurement uncertainty associated with the eddy current exam is significantly reduced. His is based on the influence that the Imrizontal noise has on a small vertical signal, herefere, indications < 0.5 volts in amplitude were not considered in the growth rate study. Data beyond the 95th percentile was excluded due to the large ECT measuremert crror which is not representative of actual growth. Figure 6.4 prosides the log normal distnbution which was sampled from in the Monte Carlo simulation.
Figure 6.4 Distribution of Axial Crack Growth Rates 1
0.9 -
0.8 -
0.7 -
0.6 -
0.5 --
3 0.4 -
3 3
3 o 0.3 -
0.2 -
0.1 -
0 , , ,
0 10 20 30 40 AX1AL CRACK GROWTH RATE, %TW/EFPY 23
c Due to the eddy current POD variability, detected flaws more than likely were initiated during a previous cycle. De initiation function is the most complex input to the model.
A projection is made based on Weibull disiribution established from inspection data with several thousand POD and growth samples being applied. De end result is an initiation function that is used in the model.
- After the flaws are initiated and allowed to proragate through a simulated cycle, the size of the flaw must then be assessed. De goal is to determir.: if the flaw will withstand accident pressures. His physical information obtained from the model is then tested to see ifit can withstand accident pressure. The foimula used in the burst pressure determination is":
P = 0.58St'l LdIt '
R, . L + 2t.
where:
P = estimated burst pressure S = flow stress t = tube thickness R,= inner radius of tube L = characteristic degradation length d = characteristic degradation depth His formula has consistently demonstrated a conservative estimate of measured burst pressure. Figure 6.5 shows a comparison between calculated values and measured data.
]
He data in Figure 6.5 was obtained from numerous tests with a wide range of steam generator tubing.
5 24 ;
1
i e
1 .
Figure 6.5 Calculated vs Measured Burst Pressure Using the Framatome Equation 14000 i'
12000 .'
a e
10000 1 A a
- 8000 e' I
n, 4,
&a ",
'ka '
m .
- a 6000 .-4 ,
a
.W 4 :aa a 0 "
4A a
4000
'a a
2000 a a
0-0 2000 4000 6000 8000 10000 12000 14000 Measured Burst Pressure, psi Table 6.1 shows the POB value for each axial area at a run time of 0.87 EFPY. This table's data shows that the 0.01 figure of merit for a single degradation mechanism is not challenged until a run time of > 0.87 EFPY is reached. Again, this is using a consenative 25
9 e
POD. If a more accurate POD curve was used the estimated run time would be longer.
The largest contributor of the three axial degradation areas are the eggerates with a 0.0043 conditional probability at 0.87 EFPY which is well below 0.01. A cycle length of 0.87 EFPY was chosen to demonstrate that 10 months of operation prior to a bobbin exam can be achieved with added margin. Table 6.2 shows the POB values for circumferential ODSCC and axial ODSCC at the expansion transition for a run time of 1.6 EFPY, Table 6.1 Structural Margin and Projected MSLB*
Leak Rates for 0.87 EFPY Degradation Mechanism Conditional Probability of Burst 95/95 Leak Rate at at Postulated SLB t'ostulated SLB (95% Confidence Level) (GPM st 600*F)
Axial ODSCC at Eggerate Intersections 0.0043 0.0015 o Freespan Axial ODSCC 0.0006 0 Circumferential ODSCC at Expansior Transitions 0.0003 0.256 Axial ODSCC at Expansion Transitions 0.0003 0 Table 6.2 Structural Margin and Projected MSLB*
Leak Rates for 1.6 EFPY Degradation Conditional Probability of 95/95 Leak Rate at Mechanism Burst at Postulated SLB Postulated SLB (95% Confidence Level) (GPM st 600*F)
Circumferential ODSCC at Expansion Transitions 0.003 2.22 Axial ODSCC at Expansion Transitions 0.0099 0.006
- Circumferential crack POB is discussed in sedion 6.4.
26
. , i
+
l .
6.3 Atlal Flaw 95/95 Leakage A leakage assessment was r rformed usi g the htonte Carlo sampling model. During the leakage evaluation the physical properties are assigned the same v.ay as in the burst model.
He main difference is how the leakage profiles are assigned. %ey are viewed as a single ideslized planar crack. His is a conservative approach in that the strengthening and leak
- limiting effect6 of the ligaments are neglected. Additionally, the physical depth profile, U ch typicaUy varies in a non uniform farhion over the (;ngth of the crack, is modeled as a simplified ideal profile for leak calculations. The profile is allowed to grow until it is detected by cddy current, When the flaw is detected, the upper 'oound leak rate is based on a 95% probability with a 95% confidence level (95/95). for example, for 10,000 steam generator simulations, the 9537th highest computed leak rate represents the 95th percentile leak rate with 95% confidence. As expected the majority of the 95/95 leak rates are neglig41c, except for the most adverse combinations of growth rate and initiation.
Each simulated TW defect has an associated leakage value. This value is calculated using Version 3.0 of the PICEP two. phase flow algorithm". %c flow rate is calculated as a function of preasure differential (p), temperature (7), crack opening area (A), and total TW k length (L). Friction cliccts and crack surface roughness were included in the model, i line break, room tempera:ure, and normal operating condition leak rates calculated t, CEP were fitted to regimion equations %e PICEP based Icak rate regression equation for steam line break conditions is gien as:
0 = {a + b exp(c( A I L)"" + d(A Ap'"
I L) where:
a d = regression coeflicients G = gallons per minute p = pressure A = inche: 8 L = characteristic degradation length The validity of the leakage calculations are confirmed by Figure 6.6. 'he figure shows that the calculated leak rates, illustraicd by the dotted lines serve as a scod envelope of the -
, data from stress corrosion cracked samples of the same tubing dimensioni. The calculated leak rates are just below the measured data for fatigue cracked r.amples. %c total accident induced leakage calculated for the axial flaws is 0.00153 pm as shown in Table 6.l.
He predictions of the hionte Carlo simulation agrees well with past observations of corrosion degradation at ANO.2. Benchmarking of several parameters were conducted to verify this. The numbers from actual observations match well with the numbers predicted by the model as illustrated in Figure 6.7. Additionally, there has beer a good match between the predicted and measured %BV depths, using the Appendix 11 qualified sizing technique for axial ODSCC.
27
O Figure 6.6 Calculated and Measured Leak Rates for Axlal Cracks 100
= o _r
~
C 10 .
,;=
/ .-
1 .
~ ~,
gugD "
4Ni
/ *T.*
l g y..
/Z's" 01- :_ _- _ ' '
g _
- a ,
,.S.' * "
=
= \
0 01 -
p-
. .a w
,a gd =
4-c
'p - , . _
~
/ r' 0 001 - , ._
~~
3 W F ATIOUE CRACKED DATA
- * * * 'O 878" OD BY 0 060*'./ ALL
"'"
- 0 760" 00 DY 0 043* W ALL D CFOBl",C
- CE00 F ATIOUE CR ACKED D AT A 0 J001 : :
O.1 i ek:.culawerw.iucune 6.4 Circumferential Crack Probability of Burst The methodology used for the circumferential cracking probabilistic model is identical to the model used for axia! flaws. Each input to the model is similar, and the method in which the distributions are sampled using the Monte Carlo simulation is the same, llowever, since it is a different damage mechanism the distributions that are sampled from are differents The POD curves for circumferential cracking are quite a bit different since the degradation is different and the method ofinspection is also different. This is reflected in Figure 6.8 where the POD curves are illustrated for plus point and MRPC Again, there i
is no credit taken for the enhancements made to the eddy current program.
28
_ = _ _ _ -
=
s' Figure 6.7 l
Predicted Axial Cracks vs Observed Axlal Cracks l
ANO UNIT 2, s/G B 99.9 %
90,0%
l 90 %
80 %
70 %
60 %
50%
40%
, 30%
l20% 10%
l 5 s%
b u
O e
1%
M 0.6%
is ll eACTUAL o
' O PREDICTED 0.1% l l llll 1.0 10.0 100.0 RUN TIME, EFPY 29
e i
Figure 6.8 1TS Circumferential Cracking POD Curves 1.0 2R11, Plus Point 0.8 t
Other Outages, RPC 0.6 5
0.4 0.2 0.0 0 20 40 60 80 100 Percent Degreded Area lhe size distributions are obtained from the MRPC measured values and from the EPRl/AND Circumferential Crack Program. For circumferential cracking, a conservative growth rate distribution was used. In this case growth rates are expressed as changes in percent degraded area or PDA. A detailed crack rowth rate study was conducted as part of the EPRl/ANO 9
Circumferential Crack Program '. Negative growth rates were set to zero and the resulting cumulative distribution of circumferential crack growth rates is shown in Figure 6.9. This 30
_= =
en.'
e growth rate distribution is conservative in the sense that no attempt wm made to account for PDA measurement errors. Measurement errors increase the dispersion of obsen ed growth rates compared to actual growth rates.
Figure 6.9 Distribution of Circumferential Crack Growth Rates 1.0
-l l
0.9 ~~~-- L -
0.8 -------- - - - -- ~ ~ ~
0.7 ~~ ~ ~ ~ ~ ~~ "- ------
r O - - -
t --- + - - - - - - ~~-
16 0.5 + ---- N - -- ~ ~ - -
d 0.4 -
f f
0.3 -- --
0.2 ---- ----- - 1- ~ ~ ~ - ----
0.1 -- + -- - *
- } --
0.0 0 10 20 30 40 50 60 Circumferential Crack Growth Rate. PDA/EFPY Data obtained form the EPRl/ANO Cire Crack Program" shows that burst pressure of tubing with circumferential degradation is bounded by the single planar crack.
31
_ == - = _=
i.
0 The bounding equation for pressure differential is:
P = hR,' - R,8)/ R,8f1 - PDA)(S / 2) where:
PDA = percent degraded area S - flow stress P = pressure R, = outer radius Ri = inner radius As with the axial flaws, the assigned physical characteristics are carried dirough the Monte Carlo simulation and the above equation is applied to determine if the flaw will fail. Table 6.2 shows the conditional probability of burst for circumferential cracking aner 1.6 EFPY is approximately 0.003 which is well below the 0.01 figure of merit established in the draft d
version of the Steam Generator Rule Regulatory Guide . Circumferential cracking failures do not start occurring in the model until after 1.6 EFPY. This is mainly due to the low h1RPC detection threshold.
6.5 Circumferential Crack 95/95 Leakage The hionte Carlo leakage models revealed the largest leak rate of all the degradation mechanisms was attributed to circumferential cracks; however, the leakage values are still low. As noted in Table 6.1, a value of 0.256 gpm is calculated to be the 95/95 leakage value after 0.87 EFPY. 'Ihe EOC induced leakage contribution from circumferential cracking under an accident scenario after 1.6 EFPY is estimated to be 2.22 gpm as shown in Table 6.2. This estimate is bounded by the 8.7 gpm value previously addressed and subsequently determined to cause no undue risk to the Public and the Operators'.
7,0 Operational Evaluation An operational leak rate limit is established to provide reasonable assurance that flaws either missed during inspection or growing more rapidly than expected will not render the tube vulnerable to tube rupture in the event of a MSLB. 'Ihe ANO 2 T.S. limit of 0.5 GPM per SG cxists to provide adequate margin against burst, in addition, rate of change limits exist to ensure rapidly propagating cracks or damage will be addressed at the earliest possible stages.
Upon any control room alarm iridicating primary to secondary leakage, the Operations and Chemistry Departments enter abnorinal operating procedures, If the leak rate is 20.1 GPM, a plant shutdown is procedurally required. In addition, a plant shutdown is procedurally required if the leak rate is projected to be 20.1 GPM in one hour, Stable leak rates of >0.01 GPM procedurally require management awareness for continued plant operations.
'Ihe Operations Department trends the steam line radiation levels, condenser off gas activity, and activity measurements from the steam generator sample systems in determining the indication of a steam generator tube leak. Steam lines are monitored sia I
32
I 4 4 J l
4 radiation monitors and nitrogen sixtem (N 16) samma detectors, which proside the chesnists and operators with the capability of quantifying leakage. Procedures are utilized wht:1 the monitors or trend receders for the aforementioned systems exhibit increasing trends. De Operations Department enters these procedures to place the plant in a stable condition and to mitigate the consequences of a steam generator tube leak.
Extensive training for operators is performed and emphasis is placed on changes in SG primary to secondary leakage parameters. Developing an aggressive strategy to identify early signs of a potenthi steam generator tube rupture are an essential part of ANO 2 Steam Generator management program.
%e Chemistry Department routinely samples both the primary and secondary water systems, as well as condenser off-gas, and trends the san.ple results to monitor and identify possible primary-to-secondary leakage occurrences.
8.0 - Conclusions Entergy Operations has performed an extensive investigation into the axial and circumferential cracking occurring at ANO 2. %c investigation includes comprehensive inspections with the major enhancements to the eddy current rr ogram improving the POD, application of appropriate safety factors, use of statistically va id (95/95) material properties, more accurate NDE data, enhanced growth rate and tube burst test data.
A comprehensive in-situ candidate selection process assured that the most challenging flaws were tested. This demonstrated with reasonable assurance that the flaws existing at the EOC 12 met the Regulatory Guide 1.121 requirements.
The Monte Carlo simulation models were umi to project the results of different forms of degradation in the ANO 2 steam generator tubing. When modeling the degradation mechanisms, all forms of degradation are conservatively represented as a planar crack.
ne processes of crack initiation, crack growth, and detection of cracking by eddy current inspections were simulated for multipic cycles of operation. Allowing the severity of degradation to be projected, the simulation model is benchmarked by comparing simulation results with actual cddy current inspection results, notable in situ test results and operational occurrences. Excellent benchmarking results were obtained.
AAer 0.87 EFPY of operation in cycle 13, the conditional probability of burst, given a postulated steam line break event, is less than 0.006 for all four corrosion mechanisms combined. He largest contribu; ion to this total is axial ODSCC/lGA at eggerate intersections, with a value of 0.0043. Table 6.1 indicata that a bobbin examination for detection of axial cracking is not warranted until after 0.87 EFPY Additionally, the evaluation supports a 1.6 EFPY interval prior to performing a TFS examination for circumferential cracking.
He value obtained from the probabilistic evaluation demonstrates significant margin above the values required by the draft Steam Generator Tube Integrity Regulatory Guide *.
De calculated leakage values are considered negligible (0.26 gpm). Presious evaluations support EOC leakage values at 8.7 gpm. De results demonstrate that the dose criteria of 10CFR100 and GDC 19 are satisfied at the 95/95 level.
33
=
=._
- i. . .
e' '
ANO has an execlient primary 40-sec<mdary leakage detection program that is supplemented by extensive operator training in steam generator tube leakaschupture scenarios and effective depr'tmental interaction and support in the steam generator area.
Together these ensure early detection and prompt corrective action should leakage occur.
Entergy is committed to operating ANO 2 safely, and believes the inspections performed in 2R12 and the subsequent analysis ensures safe operation of the unit until the planned mid.
cycle outage i
l l
l l
34
=
i
se io
(
r a
IV. REFERENCFS 1, 2CAN079604, Evaluation of Steam Ocncrator Integrity for Cycle 12 Operation, July 30,19%.
- 2. 2CAN029505,"2P951 Steam Generator inspection Results and .
Circumferential Crack Evaluation" February 17,1995.
- 3. 2CAN089507, " Repair Limit for Circumferential Cracks in Steam Generator Tubing," August 28,1995.
- 4. Draft Reg. Guide DG 1074," Steam Generator Tube Integrity", August 5,1997
- 5. 2CAN069705, "Special Report: Steam Generator Tube Surveillance Category C 3 Results,"Junc 5,1997
- 6. EPRI PWR Steam Generator Examination Guidelines: Revision 4.
- 7. IIES 28, ANO 2 Steam Generator ECT Data Analysis Guidelines.
- 8. ANO 2 Eggerate Growth Rate Study.
- 9. ANO 2 Circ Crack Growth Rate Study.
- 10. ANO 2 OTH ESP SGMAN, ANO 2 Steam Generator Eddy Current Training Manual.
11, AES 95102556-1-6, "Probabilistic Operational Assessment of Steam Generator Tube Degradation at ANO 2",
- 13. Arkansas Nuclear One, Unit-2 Technical Specifications.
- 14. EPRl/ANO Circumferential Crack Program.
35
_. -_