ML20138H000
| ML20138H000 | |
| Person / Time | |
|---|---|
| Site: | Braidwood  |
| Issue date: | 04/28/1997 |
| From: | COMMONWEALTH EDISON CO. |
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| Shared Package | |
| ML20138G051 | List: |
| References | |
| NUDOCS 9705070036 | |
| Download: ML20138H000 (34) | |
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{{#Wiki_filter:- - -. -. - _ .. . .- -. . .. Braidwood Unit i Locked Tube TTS Circumferential ludication Root Cause Determination Evaluation April 28,1997 Executive Summary ! A root cause determination evaluation has been performed which examines the discrepancy between the frequency of TTS circumferential indications identified in locked tubes relative to the in-ser6 population of tubes. The issues examined in this root cause are those which lead to the initiation, growth and detection of stress corrosion cracking (SCC) in mill annealed inconel 600 steam generator tubing, and include: materials, stress, environment, fabrication and growth. i The root cause was determined to be as follows: i Only a smallincrease in stress is required to cause initiation and growth of the undetected flaws over the detection threshold due to the presence of existing residual stresses from the l tube expansion process. The tube lock, independent of configuration, provides enough of an additional axial stress to result in a shorter time to crack initiation than for in-service tubes. l The most compelling evidence for this conclusion is that tubes with locks at the lowest J elevation (3H) have a higher axial stress than other tube lock configurations. This increased stress causes larger crac% u. tubes with locks at 3H. The affects of reduced tocaperature due to the tubes being taken out of service does not offset the increased affects of the additional axial stress. 1.0 Introduction This evaluation is being performed in order to determine the root cause of top-of-the-tube-sheet (TTS) circumferential degradation detected in Braidwood Unit 1 Model D4 steam generator locked tubes. Of the 85 locked tubes inspected with the plus point probe,49 have TTS circumferentialindications or 57.6% of the population. In comparison, the population of approximately 16,000 active tubes (18,312 tubes - 2354 plugged) has had approximately 1700 tubes repaired due to TTS circumferentialindications or 10.6% of the total population of tubes have indications. This root cause examines the discrepancy between the frequency of TTS circumferential indications identified in locked tubes relative to the in-service population of i tubes. The issues examined in this root cause are those which lead to the initiation, growth and detection of stress corrosion cracking (SCC) in mill annealed inconel 600 steam generator tubing, and include: l 1. Materials i 2. Stress l
- 3. Environment
- 4. Fabrication
- 5. Growth 9705070036 970430 PDR ADOCK 05000456 P PDR Uagrp rpucam'cpndw doc \1 l
6 4 2.0 Objective Development and understanding of the root cause will be used to identify necessary actions to ensure that the integrity of Braidwood Unit 1 steam generators is maintained through the end of the next operating cycle. 3.0 Background As part of the licensing of 3.0 Volt IPC during A1R05, selected tube intersections were expanded (i.e. locked) and the tubes subsequently plugged making the tubos function as tie rods. Steam generators shall be determined operable by the performance of an augmented in-service inspection program and application of the acceptance criteria as described in the Technical Specification 4.4.5.4. Technical Specification Section 4.4.5.4.a.11 specifically addresses the use of the 3.0 Volt interim Plugging Criteria. This criteria as described in the WCAP-14273 " Technical Support of an Alternate Plugging Cnteria with Tube Expansion at Tube Support Plate Intersections for Braidwood 1 and Byron 1 Model D-4 Steam Generators" (Reference 6) discusses limiting deflection of the tube support plate to obtain negligible tube burst probabilities along with minimizing the leakage from a tube support plate ODSCC indication. Limiting deflection of a TSP is accomplished by hydraulically expanding select steam generator tubes at various support plate intersections. The expanded locked tubes would limit the vertical movement of the TSP's under postulated accident conditions, thereby
- restricting the potential free span exposure of an ODSCC flaw to less than 0.10 inches. The WCAP also contains the axialload required to be carried by the locked tube from support plate
; loads during an MSLB for each tube location. The maximum amount has been determined to be less than 500 lbs per locked tube (Reference 6 Table 8-13). Confirmatory load calculations, using RELAPS pressure drops, were also run anc the maximum load was shown to remain less than 500 lbs.
The original locked tube selection criteria applied was that the locked tubes be free from defects at the TTS area. The TTS area was inspected with the plus point probe prior to TSP expansion during the Braidwood Unit 1 Fall 1995 refuel outage (A1R05). No degradation was detected. Braidwood Unit 1 operated for a period of approximately 412 days >500 F from A1R05, when the locked tubes were installed and baseline inspected, to A1R06 when TTS indications were detected in the locked tubes. The rate of degradation of the locked tubes in comparison to the general population of tubes is the focus of this root cause anal,..is. 4.0 Causes of Circumferential Cracks (Reference 1) Cracking of steam generator tubes is mainly due to stress corrosion cracking (SCC). SCC of steam generator tubes occurs when the combination of material susceptibility, stress, and environment exceeds a threshold for cracking for the exposure time being considered. Experience and tests have shown that the mill annealed alloy 600 used for steam generator tubes is susceptible to SCC on the primary side (i ., to PWSCC) when stresses on the ID surface exceed about 35 to 40 ksi, with the time and stress required for SCC varying somewhat depending on temperature and material susceptibility. Tests and experience indicate that secondary side stress corrosion cracking (ODSCC) can occur with OD curface Lsggqvteamenaw ac2
. a I
stresses as low as about 10 ksi, apparently because very aggressive electrochemical I 4 environments can develop in crevice and sludge pile arcas on the secondary side. The higher frequency of axial cracks that is typically observed in steam generators is j considered to be the result of stresses in the hoop direction typically being larger than stresses ) in the axial direction. This has been confirmed by stress analyses and residual stress measurements for many situations. However, the difference in the magnitude between hoop i and axial stresses is often small, and relatively minor changes in residual or operating stresses l can result in the axial stresses approaching or exceeding the hoop stresses. For this reason. the recent detection of increasing numbers of circumferential cracks throughout the industry is not un-expected. i 1 A few situations have been found that often result in axial stresses being larger than hoop i stresses. To date, these are primarily the OD TTS area of full depth expanded tubes, dents at I the TTS and at TSP's, and the parent tube at sleeved joints. An important case is for stresses at the OD of hard roll transitions. Because of the higher axial stresses, if cracking develops at I the OD of standard hard roll transitions, circumferential cracks are the expected orientation. l This type of cracking has been observed at many plants (most pre-heater units with hard rolled mill annealed alloy 600 tubing and an increasing number of feedring units with higher hot leg temperatures and hard rolled mill annealed alloy 600 tubing). l l Finite element analyses generally indicate that axial stresses in expansion transitions are 1 higher than hoop stresses, for both the inner diameter (ID) and outer diameter (OD) surfaces. I However, for hard rolled transitions axial cracks predominate on ID surfaces, which indicates I that hoop stresses are in fact larger than axial stresses. The reason for the predominantly
- axial orientation of ID cracks in hard rolled transitions is not fully understood; it is suspected i that the stress distribution is affected by the wall thinning involved in hard rol'ing. On the OD, l circumferential cracks predominate in standard hard roll transitions, as discussed in the l preceding paragraph. These OD circumferential cracks occur just above the contact with the tube sheet, at about the same elevation at which a@l cracks tend to initiate on the ID. l l
Operating stresses, as opposed to residual stresses from tube fabrication and installation and stresses induced by denting, are generally low and rarely are the main factor in circumferential , cracking. In addition, pressure induced operating stresses are generally higher in the hoop l direction than the axial direction, thus tending to cause axial cracks rather than circumferential ! cracks. However, there are some s,ituations where operating stresses tend to cause l circumferential cracks. For example, tube bending at the TTS, cuch as that caused by a combination of tube sheet deflection and thermal expansion and tube interaction with flow distribution baffles, causes axial stresses that have, in some cases, been postulated as being a factor in circumferertia' cracking. 1 In summary, with regard to causes of circumferential cracks, mill annealed alloy 600 is l susceptible to SCC in primary and secondary environments in steam generators, stresses are I frequently over the thresholds needed for SCC to occur and, in some situations, axial stresses exceed the hoop stresses, causing the cracks to take a circumferential orientation. LagrpspvteamcpridxdaeG
l l > l 4.1 Observed influence of Temperature, Time, and Stress PWSCC and ODSCC are the degradation mechanisms that lead to circumferential cracking at ! the TTS. SCC varies as a function of temperature, time, stress, and matenal susceptibility. An I empirical model has been found to adjust for these variations reasonably well for use in the i prediction of PWSCC by comparing stress temperature and material parameters as fullows: l tr = A' o-" e" ' where, l te = time to failure at some defined fraction of a tube population with similar properties experiencing PWSCC, where experiencing PWSCC means developing a detectable PWSCC crack. A' = constant mainly reflecting effect of material properties on time-to-PWSCC; this constant is also affected by the size of the crack at the ; time of detection. I o = total stress at the material surface, ksi ! n = stress exponent, often taken as 2 - 4
)
O= apparent activation energy, kcal/racle l R= gas constant,0.001103 kcal/ mole R l T = absolute temperature, ( R) ; The time to PWSCC of a case being analyzed can be compared to a reference case, as follows: 1, = A x t,g x (c/c,g)* x e "" *d) where the "ref" values are for the reference case, and A is a constant similar to A'. A similar relationship is believed to also apply to ODSCC, except that differences in chemistry make the constant A' or A more variable. In addition, tests indicate that the stress exponent is lower in more aggressive secondary side conditions (e.g.,2 to 2.5). Evaluations of plant data indicate that the best estimate of the activation energy is about 54 kcal/mola for ODSCC and 50 kcal/ mole for PWSCC. 5.0 Material Susceptibility (Reference 1) Plant experience and laboratory tests have shown that HTMA material has lower susceptibility to PWSCC than LTMA material. Higher mill anneal temperatures were used for the Combustion Engineering (CE) steam generator tubing mainly because CE imposed a 55 ksi upper limit on yield strength, as opposed to the 65 ksi limit imposed on LTMA tubing.
. Achieving the lower yield strength required higher mill annealing temperatures, of cver 1800T.
The decrease in susceptibility of HTMA material is attributed to the change in microstructure associated with the higher temperature, characterized by larger grains, fewer intragranular carbides, and more intergranular carbides. In addition, the lower starting hardness of the HTMA material probably results in lower peak stresses in areas of cold work, such as expansion transitions. Comparative tests between LTMA and HTMA indicates that PWSCC in HTMA tubing occurs about 4 times slower than in LTMA tubing, and also somewhat slower for secondary side SCC. l Lagrpspvteamgirids doc.4
4 o r f . l 5.1 Braidwood Unit 1 Experience Braidwood Unit i nas LTMA material which has experienced a significant level of ODSCC at tube support plates and at the roll transition at the TTS. Therefore it can be concluded that , Braidwood Unit 1 tubes have material which is highly susceptible to SCC. l l 5.2 Material Heat Assessment l l An assessment of the susceptibi!ity of the tubing material of in-service tubes to the susceptibi'ity of locked tubes was performed. The rate of TTS circumferentialindications for tubes in-service and for locked tubes as a function of the tubing heat was assessed. Tubes in- , service meaning unlocked and not plugged. The results of this assessment are provided in (. Attachment 1, Inputs into the assessment are described below; , I l 1. The heats of locked tubes were identified. l 2. The total number of in-service tubos with these heats were determined. > , 3. The number of TTS circumferentialindications in in-service and locked tubes of these heats were identified. l 4. The rate of indications (number of indications / number of tubes of the heat) for each heat and a total were determined for locked tubes and for the in-service tubes. The results from the assessment are:
- 1. The number of TTS circumferential indications in in-service tubes for heats which have locked tubes is 8.5% (128 flaws in 1523 tubes). l
- 2. The rate of degradation in locked tubes for which heat data is available is 52% (34 out of 1 66).
- 3. For heats in in-service tubes which also have locked tube flaws the rate is 12% (out of 36 !
heats 785 tubes).
- 4. Many heats with locked tube flaws have not had TTS circumferential indications identified in in-setvice tubes (16 heats representing 95 tubes without defects).
There appears to be some correlation between the rate of degradation in heats of locked tubes and heats of in-service tubes as determined by the results of 1 and 3 above.. However, the 1 rate of indications in these heats in in-service tubes is not of the magnitude as in locked tubes as determined by comparison of 1 and 2 above. 5.3 Flow Stress Comparison A comparison of the flow stress for heats of locked tubes with TTS flaws and heats of locked tubes without flaws was performed. The average flow stress is approximately the same for i both groups of locked tubes and therefore there is no correlation between the tubes physical properties and the rate of degradation. l l 5.4 Tube Puli Analysis (Reference 2) l l One tube in Braidwood Unit 1 CG's has been pulled of the same heat as in a locked tube with a flaw (SG B R22C73). The tube pull analysis showed that circumferentially oriented ? l LWapucarnepri<hedW I
- b. . __e .
t
- degradation at the top-of-the-tube-sheet was OD initiated intergranular stress corrosion cracks and intergranular attack in the tubesheet expansion transitions. The degradation was present
, as narrow bands tnat extended for 360 degrees around the circumference of tna tubes. A base metal assessment of the tube concluded that the carbide dis:ribution was discontinuous. Chemical analysis of the tube indicated that it was consistent with tha ,
; requirements of SB 163.
A sensitization assessrnent was perfonned on these tubes to assess the degree of ! sensitization (i.e. the leve! of chromium depletion in areas adjacent to the grein boundaries). i The assessment results indicate that the microstructure present in the alloy 600 tubing was not
- sensitized.
l Cracking of the Byron and Braidwood pulled tubes occurred in a neutral or near neutral environment. There has been evidence in tube examinations that neutral or near neutral conditions were present in crevices where OD initiated degradation occurred. Material fabiicated with low final anneal temperatures (LTMA), with its resultant grain structure high strength and few grain bourdary carbides has a high susceptibility to SCC and IGA as demonstrated by numerous laboratory tests and field experience. The Braidwood Unit 1 tubes i thus are expected to be susceptible to SCC and/or lGA in environments such ss pure water, ! caustic solutions, and concentrated near neutral solutions. Therefom, metallurgical evaluation confirms the tubing material at Braidwood Uni? 1 is
- - susceptible to SCC in both in-service tubes and locked tubes.
The stresses which initiated and propagated the TTS circ.umferential defects in tube SG B. R22C73, were primarily residual stresses from the hard rolling tube installation process. The pre.sence of circumferential cracks requires axial stresses significantly higher than hoop l stresses. In a thin wall cylinder, such as steam generator tubes, noop stresses from intemal pressures are twice the axial stresses. Thus cracks resulting solely frorn operationct pressure ! will be atial in orientation This was not the case for the Braidwood Unit 1 tubes where the { defects were only circumferentially oriented. Rolling does produce high residual stresses, l nigher than any other tube installation techniques, and the highest stresses may be axial cr ! ::ircumferential in direction. Another indication that ro!!ing induced residual stresses initiated the defect was that the bands of defects were relatively narrow and were concentrated at or , near the transition zones between rolled and unrolled sections of tubes There were not any defects observed even a short distance above the roll transitions, indicating significantly reduced stresses at this location. l The environment that caused the Braidwood Unit 1 tube cracking probably developed as a result of the concentration of contaminants in the bulk waterin the sludge pile at the top of the tubesheet. Thick deposits on tubes and tubesheet sludge piles have demonstrated capability of concentrating chemicals to levels where corrosion can occur. Thus, chemical species , present in the ppb range in bulk water can become concentrated to the several weight percent
- level as a result of this concentrating mechanism. An indication that sludge is present in the area of degradation was seen during the post insitu test conducted in the Braidwood Unit 1 fall
- 1996 mid-cycle inspection in which sludge was seen oozing through cracks into the ID of tube i R22C73. This sludge may have concentrated chemical species present in the secondary side j bulk water to produce an environment deleterious to Alloy 600.
l f & YUlW & W Wh a
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5.5 Conclusion Tubing material for locked tubes is no more susceptible to SCC than tubing rnaterial of irF l service tubes. 6.0 Stress 6.1 Sources of Stresses (Reference 1) The operating, residual, and total stresses that are present at the ID and OD of expansion transitions have been estimated for each of the main tube sheet expansion methods. The operational circumferential and axial stresses at typical expansion transitions are relatively small on both ID and OD surfaces. However, the residual fabrication stresses at the ID are re'atively high, and the residual stresses at the OD are moderately high, as a result of hard
-ing. The total stresses at both the ID and OD tend to be dominated by the residual stresses
,w in-service tubes.
P Estimated stresses are provided laterin this section. It should be recognized that thers are man / uncertainties in the listed values, for a number of reasons. These include:
. There are many variables that can affect the residual stresses at expansion transitions, including both material and fabrication variables, such as yield strength varia'. ions, surface damage done during expansico, size and shape of tube hole, etc. For this reason, rather wida spreads are expected in residual stresses. This is especial!y the case for mechanical
, roli;ng processes.
. The effects of sludge, denting and tube bundle distortion on expansion transition stresses have not been well quantified. However, analyses performed by EdF show that some of these effects can lead to yield level stresses, especially for peripheral tubes.
6.2 Operating and Residual Stresses (Reference 1) The stresses acting at the inner and outer surfaces of expansion trancitions that tend to cause circumferential cracking are axial stresses from several sources. These stresses are generally separated into operating stresses and residual stresses. However, some stresses, such as those caused by denting, while caused by long term chan,;es that occur during operation, are residual in nature in that they persist after operating pressures and temperatures are removed in this evaluation, centing and similar stresses are treated as residual stresses. Operating and residual stresses tnat can contribute to the total ID and OD axial stresses at transitions include the following. 6.2.1 Operating Stresses e Local pressure stresses due to differentiaijressure across the tube wall. For tubes in . service at a normal operating pressure differential of 1300 psi the axial stress is
, approximately 4800 psi and the hoop stress is approximately 9600 psi. Tne stresses k:%grpttpuenn grukuke7
.- - . . . . -- . - . - . . . . - . ~ --. . . - - .--
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1 l result in a smali axial extension of the operating tubes. The extemal pressure acting cn a locked tube tends to cause a somewhat larger extension (it has zero pressure on the ID). The net cxial operating stress on a locked tube taken out of service is 402 psi. The tube ;
- is in compression since both the locked tube and the in service tubes are rigidly connected l to the TSP (assuming all tubes are locked at the TSP). Thus, the locked tube strain will be -
forced to be the same as the operating tube strain This requires that the locked tube be l l compressed by the difference in the strains of the locked tube and the operating tubes l (Reference 3). -) 1 ' l
- Global pressure stresses caused by tube sheet deflection and tube bend _ing due to ]
pressure differential across th_e_ tube sheet. The magnitude of this stress is expected to be I i the greatest tcNards the center of the tubesheet where displacement of the tube sheet i would be at a maximum. There does not appear to be a dependence on location for the J initiation of TTS circumferential cracks (see Figures 1 - 4), therefore the influence of this ) stress is determined to be minimal and is not considered further in this analysis. j
. Local neat transfer stress - Operating tubes have hot primary coolant inside them at about ]
612 F. Above the TTS these tubes have boiling secondary water on the OD surface. The , boiling secondary wateris at saturation temperature of about 536 F. Because of boiling ! film losses, the OD metal temperature is about 7 F above the saturation temperature i.e., about 543"F. In freespan areas, the temperature gradient across the wall of the tube generates a compressive stress of about 6 ksi on the ID surface and a corresponding 6 ksi tensile stress on the CD surface. An additional source of stress at the roll transition is the fact that the average tube wall temperature is lower in the freespan area immediately above the tube sheet (about 581'F using data in EPRI report TR-104030). while it will be close to 612 F in the roll transition, especially if the roll transition is recessed in the tube sheet and/or is covered by deposits. This rapid transition from higher temperature to l lower temperature is expected to result in the cooler and hence smaller diameter ragion above the tube sheet tending to pull in the tube at the roll transition, putting the OD in tension and the ID in compression. In summary, both freespan through-wall heat transfer and the sharp temperature change from below the TTS to the freespan area imroediately above the TTS tend to ptrt the OD surface of operating tubes at the roll transition in tension, and the ID surface in compression. This stress distribution tends to enhanco ! crack growth at the OD surface, but to inhibit crack growth through the wall. l For locked (non Operating) tubes the fuli thickness of the tube wall in freespan areas will be essentially at the saturation temperature since there is no primary coolant on the inside and the tube is surrounded by saturated water. At the TTS, the tube wit; be heated by conduct;on through the tube sheet from adjacent operational tubes, and some boiling is likely to occur immediately above the roll transition. Because of the long heat transfer path from other tubes, the amount of heat transfer will be limred, and the axial temperature gradient will be relatively small i.e., the tube will not recch full hot leg temperature for some distance dewn into the tube sheet. In summary, the temperature distribution at the TTS of non operational tubes is characterized by no through wall l gradient ana by a gentle axial gradient at the roll transition. This distribution is not l expected to Generate significant thermal stresses as compared to those generated in this j area in operational tubes. l l l L:vgrpqMeam'ep akedMS
)
The stress distribution present in operating tubes, with tensile stress on the OD and compressive stress on the ID, is expected to enhance the growth of shallow cracks and inhibit the growth of deep cracks. The thermal stresses in non-operational tubes are exper"ad to be low and to remove raost of accelerating and inblbitory effects of the OD i 4 tensile . :d ID compressive stresses present in operational tubes. )
. Global heat transfer stresses due to thermal expansions of tubes. The in-service tubes have hot primary coolant inside them and cooler boiling secondary water on the outside, j and thus have average metal temperatures between the temperatures of these two fluids. )
The locked tubes wili b9 at the caturation temperature of the secondary side. Since the tubes are rigidly connected at the tube sheet and at the TSP this temperature difference ; will generate compensating elastic strains. Since there are many operating tubes per j tocked tube, the mismatch will all be accommodated by tension of the locked tubes. The average metal temperature of an operating tube between the tube sheet and tha first TSP is calculated to be 579.5 degrees (Reference 3). The temperature of a locked tube l between the tube sheet and the first TSP is at the saturation temperature of 536 de0rees. With the above temperature inputs the stress induced in the locked tube by the locked tub 9 to in service tubes temperature difference is calculated to be 9.76 ksi (Reference 3). i e Flow induced stressesmare expected to be similar for locked tubes as for in-service tubes and therefore tney are not quantified. Additionally, because the distributions of TTS circumferentialindications at Byron and Braidwood are evenly distributed throughout the tube sheet there does not appear to be a significant influence of flow induced stressed on the level of degradation in SG tubes. 6.2.2 Residual Stresses
. Residual stresses resultina from tube fabrication. These stresses would be expected to be the same for in-sen/ ice tube.: and the locked tubes removed from service. Additionally, ECT inspection data was analyzed to determine if there were any apparent fabrication flaws i e., scratches, gouges, over-expansion, etc. in the locked tubes at the TTS which could have led to accelerated degradation of the tubes. No fabrication flaws were detected.
. Residual stresses resultina from tube insertion. These residua! stresses would be expected to be similar for in-service tubes and locked tubes or related to the location in the SG. No pattem of TTS circumferentialindications locations in the SG's has been observed.
. Residual stressns, resultino from tube exp_ansicn into the tube sheet. These stresses are estimated to be 20 kai on the tube OD (Reference 3). These stresses should be consistent for locked tubes and in-service tubes. The location of TTS circumferential indications in the locked tubes were consistent with those in in-scrvice tubes from the TTS to approximately 07 below the TTS. Evaluation of ECT data at the roll transition suggested that the location and profile of the roll transition is consistent between the locked tubes and in-service tubes. Therefore the effect of this stress should ba consistent for locked tubes and in-service tubes.
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- . - - - . . - - . . . .- . -. - ~ . . - - - . . _ _ - - .. -
- 1 1 i i !
! Shot peening of the primary side of the steam generator tubes was performed on the hot and cold legs to remove the residual stresses at the ID surface associated with the tube expansion process at the TTS. The shot peening puts the ID of the tubes in compression
- and the OD in tension. This would tend to reduce the occurrence of PWSCC on the ID. ,
^ The effect of shot penning on ODSCC has not been determined but is expected to be I small since the change in stress at the OD is small. The shot pcening process at l
- Braidwood Unit 1 was performed prior to unit start-up. ;
1 l- + Stresses induced by dentina due to corrosion of the tube sheet. tube su.ppofs, or material ) deposited on the tube sheet. Denting at the TTS has not been identified in any of the j rubes with TTS circumferential indications in in-service tubes or locked tubes. Additionally, , an evaluation of tubes with dents at the TSP's was performed to identify if tubes with , j dents would increase the axial stress in the tube and accelerate the rate of TTS j circumferential degradation. Since all Braidwood 1 dents are related to initial fat:rication, ! ] TTS circumferential indications are not due to denting. i j . Stresses induced by exoansion of tubes for the purpose of mechanically lackina tubes in l place at tube support prates (TSP). When the tube is hydraulically expanded, its diameter i j and thus its circumference is increased by plastic deformation. This requires that the wall l i thickness and/or length of the tube decrease as its circumference is increased _ by the ! hydraulic expansion. Assume that the fractional mhange in thickness and axiallength are the same. Then, the amount of axial shortening strain / stress in the locked tube can be
- calculated analytically. Because of the large number of in-service tubes which are expected to be naturally locked at the TSP's the locking process is not expected to l l significantly affect the stress in the in-service tubes (the affect would be to add a l 1- compressive load on the in-service tubes). The axial stress in the locked tube at the TTS i j from the tube locking process was determined to be 31 ksi (Reference 3).
In addition to calculating the axial strain / stress a mock-up of the locking process was ! performed and the strain in the locked tube was measured. The combined effects of i l locking the tube at six TSP locations resulted in a total maximum stress at the TTS of i 32.5 ksi(Reference 5). The mock-up did not take into acccunt the highly probaole l condition of the tubes being locked due to deposits at the TSP which would be expected l to signibcantly reduce the strain at the TTS by transmdting some of the strain to the a TSP's. 4 ! In order to further evaluate the impact of the tube locking process on the initiation and 4 growth of ODSCC at the TTS roil transition the different configurations of locks were
- examined. Parameters which would affect the total stress in the tube at the TTS, and thus
[ the rate of degradation, include:
- 1. The distance of the expansion from the TTS (strain is related to AS/L, especially important if tubes are naturally locked at TSP's)
- 2. Number of expansions (impacts AS)
- 3. Number of lock over expansions (impacts AS )
Because these factors directly affect the predicted axial stress in the tube at the TTS it was expected tnat there would be a correlation between the number and size of indications and the 3 above parameters. The locked tubes were sorted by the above 3 bgrp rpvtcam cpridw:.aW10 m
i parameters so that any trends relating the parameters to degradation could be identified. ) Attachment 2 provides a summary of the data and a summary of the evaluation is:
- 1. The distance of 'he expansion from the TTS (higher TTS strain at closest lock 3H, lower strain at 10H farthest away) does not in itself affect the number of indications in locked tubes as is evidenced by the fraction of locked tubes with indications regardless of the location of the lowest expansion. l
- 2. The size of the indications in the tubes does appear to be affected by the distance l of the lowest expansion from the TTS as is evidenced by the fact that the 5 largest
+ point vert max voltage indications are locked at the lowest TSP (3H) which leads to the highest TTS strains. I
- 3. The number of expansions (increases M), and number of lock over expansions (increases M ) do not increass 'ne number of indications in locked tubes as determined by the fraction of locked tubes with indications regardless of the number of expansions and/or overexpansions.
Only a smallincrease in stress is required to cause initiation and growth of the ) undetected flaws to over the detection threshold due to the presence of existing l residual stresses from the tube expansion process. The tube lock, independent of configuration, provides enough of an additional axial stress to cause a shorter time to j crack detection than for in-service tubes. Tube locks at the lowest elevation (SH) provide a higher axial stress than other configurations which causes growth of deeper cracks. i 1 6.2.3 Total Stresses I Estimated total stresses, i.e., including both operating and residual stre.sses, that act at expansion transitions are listed in Table 1 for locked and in-service tubes. Table 1: 4 Estimated Total Operational and Residual Axial Stresses at Expansion Transitions for L.ocked and in-Service Tubes from Previous Sections 6.2.1 and 6.2.2 Locked Tube 60 ksi in-Sersice Tube 25 ksi The affect of this increased stress at the TTS for locked tubes on crack initiation and propagation to the level of detection was determined by the methods discussed earlier using a stress exponent of n=3 to be a factor of 20 (Reference 4). Additionally, for laboratory grown TTS circumferential cracks an axial load of typically 1/2 to 2/3 of the yield strength (approximately 26 ksi to 35 ksi) is applied to initiate crack growth, The total axial load in the locked tube is above the stress where ODSCC is expected to initiate. L 'spp rpumn epndoc dov M
i . . l l l l l Addition of an axial stras da to the tube expansion (locking) process to existing residual l stresses for the expanwn process has increased the stresses in the tubes at the TTS region l well beyond where ODSCC is expected to initiate. l Insitu pressure testing of the tut'e with the most severe degradation (2.34 + point vert max l voltage)in a tocked tube (SG C R44C84) was performed to 4900 psi reduced by 13% to account for ti? ntiffness of the locked tubes (Reference 8) or 4340 psi. The axial stress imposed on a tune with a 50% throughwall crack during in situ pressure testing is approximately 32 ksi. With this test induced stress in addition to the total stresses discussed above, yielding and tube leakage at the TTS would be expected No leakage was encountered during the test indicating that the estimated stresses are conservative. 7.0 Environment 7.1 Temperature The surface temperature at the OD of the roll transition (the location of the circumferential cracks) for in-service tubes is variable, depending on whether the roll transition is recessed in the tube sheet and on the amount of sludge on the tube sheet surface. It is assumed that, for operating tubes with detected cracks the temperature was close to the maximum possible, which is the primary hot leg temperature of 612 degrees as a result of the roll transition being recessed and/or there being a significant amount of sludge present. The metal temperature at roll transitions in locked tubes is expected to be much lower than in operating tubes since there is no primary coolant in the tube. The lowest the temperature could be is the saturation temperature of 536 degrees. However, some heating will occur via the tube sheet from adjacent i tubes. It is estimated that the temperature at the locked tubes roll transitions is about 10 ) degrees above the saturation temperature or about 546 degrees. Using a temperature difference of 612 degrees to 546 degrees, and an activation energy for crack growth of 24 kcal/ mole, the deceleration factor for temperature between cracks in locked tubes and in-service tubes using the methodology discussed earlier (Arrhenius equation) is 0.26. Additionally, the primary side temperature at Braidwood Unit 1 was raised after start-up from the mid-cycle inspection conducted in 2/95 (A1M05) from 609 to 612 F. Because temperature is known to have an effect on ODSCC, the rate of degradation was accelerated after start-up from A1M05 however this affect would be expected to be similar for tubes in-service and locked tubes. 7.2 Cold Leg Indications The average metal temperature of tubes at the TTS on the cold leg is more than 50 degrees cooler than tubes at the TTS on the hot leg. Because temperature has a significant influence on the inRiation and growth of ODSCC, it would be expected that tubes with TTS circumferential ODSCC on the cold leg woulri also have a flaw on the hot leg since the material susceptibility and residual stress should be similar. Tubes on the cold leg were not expanded for the 3.0 Volt IPC. Inspection of the cold leg TTS at Braidwood Unit 1 during A1R06 has identified 36 circumferential flaws (ODSCC). Examination of inspection records indicates that there is no corresponding TTS circumferential flaw on the hot leg. This indicates that the influence of temperature and material susceptibility may not be as severe as the influence of wmmmp un
the environment and stress since these are the only influences which are expected to be different between the het leg and cold leg. The residual stress from the tube sheet expansion
- process is similar on both the hot leg and cold leg because the same expansion process was used and both ends were shot peened from the ID to minimize the susceptibility to PWSCC.
However, stresses due to thermal expansion will be different between the hot and cold leg due to temperature differences. 1 I This indicates that the influences of temperature and material susceptibility may not be as )
. severe as the influence of the environment and stress. The hot leg TTS temperature of locked tubes is estimated to be 7 degrees above the TTS cold leg temperature and since degradation has been experienced at the cold leg TTS, if all other factors were the same, it would also be expected to occur in hot leg of locked tubes.
! 7.3 Degradation of Plugged Tubes l During A1R06 three tubes which had been previvusly plugged due to ODSCC at the TSP (10/93, 9/92 and 2/95) without locks at the TSP's were inspected using the plus point prcbe. ; No TTS circumferentialindications were detected in these tubes. At the 58% rate of d l degradation in the locked tubes taken out of service it would be expected that at least one of the three tubes would have had a flaw if the ougradation was due solely to removing the tube from service and plugging the tube. 7.4 Presence of Sludge Pile (Reference 1) 1 Circumferential cracking at the TTS has shown some correlation with the presence ; , of sludge piles, as follows: j
. Circumferential PWSCC at the TTS in the central portions of sludge piles has been observed in French units. About twelve (12) units with Kiss rolled LTMA tubing have l
detected this type of circumferential cracking in sludge piles. Only a kmited number of such cracks have been observed outside the sludge pile. i t
. Several French units with Alloy 600TT tubing expenenced circumferential PWSCC at the TTS associated with denting caused by corrosion of iron shot in the sludge pile, and a Korean unit experienced both circumferential PWSCC and ODSCC at the TTS due to this type of denting.
. The circumferential cracking observed in Model E design SGs was detected initially in the sludge pile regions, although it has subsequently been detected outside of these regions (across the whole tube sheet at Doel 4 and Tihar.ge 3).
I i . Experience with explosively expanded units, with both LTMA and HTMA Alloy 600 tubing, has shown higher concentrations of circumferential cracks in the sludge pile region than i outside (the cracks are sometimes located mainly in periphera! region of the sludge pile and sometimes mainly in the center of the sludge piM). However, some cracking has also been observed outside the sludge pile regions. I (hM([ Y
3 i I- The presence of a sludge pile can have a pronounced effect on both the temperature and axial
- stresses at the TTS expansion transition locations. The possible reasons for higher j occurrence of circumferential cracking in the sludge pile regions include:
. Evaluation of stresses by EdF show that ID axial stresses are increased by the sludge, while OD axial stresses are decreased by the sludge.
. The sludge increases temperature, thereby accelerating both PWSCC and ODSCC. This ,
- is primarily due in an insulating effect, which can raise the temperature to as high as the j hot leg temperature, depending on the depth of the sludge piie.
1 i . The sludge can act to concentrate impurities which can aggravate ODSCC. However, . tests and experience indicate that ODSCC can occur without a sludge pile; this possibly is l due to increased impurity concentrations at sludge filled crevices and deposit build-up at i the tube to TS interface.
- As described above, the presence of a sludge pile is a contributing cause to circumferential l cracking at the TTS due to increased ID axial stresses, higher temperatures, and
- increased impurity concentrations.
I i 7.4.1 Braidwood Sludge Pile Experience l l Sludge removalis performed on the secondary side of Braidwood Unit 1 steam generators ; l each refueling outage. Typically 150 lbs of sludge is removed from the steam generators. The
- Comed level !!! reviewed the A1R06 ECT results from the locked tubes and determined that j the sludge pile was not significant at the TTS of the locked tubes (some sign of sludge, possibly a coating up to 3/4" on tube). Based upon the quantity of sludge removed (approximately 500 lbs A1R06) it is believed that there is sludge covering the TTS at the locked tubes and the in-service tubes consistently and therefore the issues discussed above
- i. are applicable to both the locked tubes and the in-service tubes.
i , t i 7.5 Effects of Chemical and Electrochemical Environment (Reference 4) , i As discussed in Reference 7 cracking can be influenced by many chemical and electrochemical factors. The most important of these factors that should be considered when i evaluating possible causes of accelerated crack growth rates in the Braidwood Unit 1 locked 1 j tubes include: j ~
- 1. Many tests show that pH has a strong effect on crack growth rates, with acidic pH and strongly alkaline pH increasing crack growth rates by several orders of magnitude over i those at near neutral pH.
i 2. Increases in electrochemical potential above the hydrogen line can have strong effects on crack growth rate. I 3. The presence of specific aggressive species, e.g. lead and partially reduced sulfur, can
- lead to accelerated attack.
- 4. Increased concentrations of impurities, even if near neutral, may allow corrosion processes involved in crack growth to occur more rapidly as the result of higher electrical conductivity Usgrpqvtcam cpridoc.dov i4 5
s . . 4 e i The difference in the hcat flux between operating tubes and locked tubes is expected to result l in more benign chemical conditions at TTS crevices of locked tubes than at TTS crevices of i operating tubes. This is because the lower superheat at the locked tube will result in lower
- concentrations of impurities, thus reducing the likelihood of developing adverse pH, high j conductivity and aggressive concentrations of deleterious species. However, it does not
, appear practical to determine what the difference in electrochemical potential between : operating tube and locked tube TTS crevices is likely to be, nor whether such changes would accelerate or decelerate crack growth rate. It is expected that the environment at a locked 4
- tube TTS crevice is generall9 tess aggressive than at these crevices in operating tubes.
8.0 Denting i . 4
- The typical yield strength for Alloy 600 at room temperature is about 50 - 60 ksi for LTMA l l tubing and 40-50 ksi for HTMA tubing. Thus, strains well below the yield strain induced by i denting and/or bending could easily develop stresses significant enough to cause a circumferential cracks on the OD or ID. As demonstrated in Appendix B of Reference 1, OD 1 j axial stress on the order of 20 ksi is induced when a 0.001 inch radial dent is created just above the TTS. Even higher stress.es are induced on the ID. Lift-off signals from ECT data at j Maine Yankee suggest that the TTS may have experienced some minor (e.g., <2 mils) of denting, which likely contributed to the large increase in cracked tubes detected in 1995.
3 Similar denting was observed at Millstone 2, which experienced significant circumferential - ODSCC at the TTS. Denting has also been associated with many cases of ID circumferential i SCC just above the TTS in French units, and with ID and ODSCC in Alloy- 600TT tubing at , Kori 2. ! The axial stresses associated with denting are important for the following reasons: e 1 l . High stresses can be generated by even small amounts of denting. l i . The magnitude of axial stress can change over time due to the effects of denting, possibly , j causing a sharp incresse in the fraction of tubes cracked from one inspection interval to another. 4 Slow strain rate" deformations of the type associated with denting are likely to enhance the j initiation of SCC. a 8.1 Braidwood Dent Experience
- An evaluation of Braidwood Unit 1 tubes with dents was performed to assess if tubes with dents had an increased rate of TTS circumferentialindications due to the increased axial stress from dents. The results indicate that of 208 dents with voltages greater than 2.5 volts there were 3 tubes with TTS circumferential indications. This is a lower rate of cracking than for tubes without dents. This result indicates that the presence of dents does not increase the axial stress in the tubes at a level which would accelerate the rate of TTS circumferential cracking.
LNppesteam eridoe doel5
9.0 Fabrication Analysis of A1R06 ECT data was performed to assess whether there was a difference in the fabrication of the steam generator tubes at the TTS. The Comed level ill reviewed the locked tube ECT data for
- 1. Evidence of tube damage from the locking process or tube fabrication (e.g. marks from the rolling process)
- 2. Inconsistent location of the roll transition
- 3. Over-expansion of the tube beyond the TTS
- 4. Under-expansion of the tube leaving an excessive crevice The conclusions of the review are that the locked tubes and in-service tubes have consistent roll transition profiles, are located similarly relative to the TTS and are free from manufacturing defects and flaws.
Additionally, results of the tube locking process were reviewed to evaluate the extent of over-expansion which could lead to additional axial stress in the tube due to the locking process. Sixty-six of the locked tubes at Braidwood had at least one over-expansion. An evaluation of the impact of the over-expansions on the size and frequency of TTS circumferentialindications j was performed The results are presented in Attachment 2. In summary, no correlation was ! identified between the number of over-expansions and the rate of TTS circumferential degradation. l l No issues relevant to the fabrication process had an impact on the increased rate of TTS l circumferential degradation identified in locked tubes compared to in-service tubes. l t l 10.0 Growth 1 In order to assess the locked tube rate of degradation compared to in-service tubes at Braidwood Unit 1 an assessment of growth rates was performed. The locked tubes were installad and inspected in A1R05 (10/95) using the plus point probe. At . that time the tubes had no detectable degradation after being scrutinized for selection by the analysis primary / secondary / resolution process and assessment by the Westinghouse Level til and Westinghouse Engineering Level ill. In order to calculate the locked tube growth rates normalized to 1 EFPY the following inputs were used: I
- 1. Indication size in 3/97 using +pt,0.080" RPC maximum and average voltage l
- 2. indication size in 10/95 was 0
- 3. period of operation is 412 days > 500 degrees in order to assess the in-service tube population growth rates normalized to 1 EFPY the i i
following inputs were used: k:bgrp\rpvteam\cpridoc. doc \l6
, . - l l 1 i 1. only tubes which were NDD in 10/95 were selected (1996 + point indication look-back) l 2. indication size in 10/96 using 0.080" RPC maximum and average voltage l indication size in 10/95 was 0 3.
- 4. period of operation is 293.76 days > 500 degrees
)
The locked tube and in-service tubes growth rate frequency vs. maximum and average voltage l were then plotted together and tha results compared. This data is presented in Figures 1 - 8 for SG A, B, C and D for maximum and average 0.080" RPC voltage. l The ECT signal of the indications in locked tubes and in-services tubes are consistent indicating that the flaw morphology is similar. 10.1 Conclusion i The results indicate that there is not a significant difference between the growth of locked tube and in-service tubes which were NDD in 10/95. The locked tubes were interrogatt.d with the
+ point probe in 10/95 where as the in-service tubes were inspected with the standa.rd 3 coil RPC probe. This difference would result in a slightly lower growth rate of the in-service tubes due to the higher detection threshold using the RPC probe on the in-service tubes in 10/95.
The results should not change significantly due to the 10/95 NDD call being based upon look-back of indications having knowledge of the actuallocation of the defect from its 1996 indication. 11.0 Defect Location i The location of locked tubes with indications and without indications was marked on tube sneet maps in order to evaluate whether the location of the locked tubes had an impact on the frequency of degradation of locked tubes relative to in-service tubes. The tube sheet maps are included as Figures 9 - 13. Results of the evaluation indicate that there is no trend to the number of locked tube indications (i.e. the locked tube indications are not located in areas in the SG where there is a high fraction of tubes with ind' cations). 12.0 Summary of Causes (Reference 1) The likely cause of ODSCC in Alloy 600 tubing at the TTS of locked tubes are summarized below:
. Susceptibility of Alloy 600 to SCC in many possible secondary environments; this is aggravated by higher plant temperatures. It is known that water chemistry variables can affect this susceptibility, especially on the secondary side. Both the locked tubes and in-service tubes are susceptible to similar levels of SCC from a material susceptibility perspective (both LTMA alloy 600)
. Axial residual stresses and cold work at the ID and OD of expansion transitions. The level of stress required at the OD for SCC to occur is <20 ksi, though this will vary depending on the specific local chemistry, electrochemical potential (ECP), and material condition.
L ?r.grp rpsteam epn&c AV 17
1 i i { Additional axial stresses and strains at the TTS due to locked tubes. J
. High temperatures and lack of subcooling at the TTS for locked tubes and in-service tubes in SG designs with preheaters.
i i 13.0 Results ! The results of the evaluations reported in this root cause analysis are presented below addressing the issues believed to contribute to the rate of degradation of Inconel 600.
- 1. No significant increase in the material susceptibility for heats of locked tubes based upon ,
the degradation rate of the same heats in in-service tubes. i 2. Braidwood Unit 1 tube materialis highly susceptible to SCC.
- 3. Total axial stresses at the TTS for locked tubes are greater than the axial stresses in in-
. service tubes due to the tube expansion and locking process. [ 4. The effects of reduced temperature at the TTS of locked tubes is not sufficient to l overcome the increased rate of degradation due to the increased axial stresses.
- 5. The temperature of locked tubes does not preclude them from SCC based upon the frequency of cold leg degradation.
! l
- 6. The tube plugging process and operational conditions associated with a plugged tube does
. not lead to tube degradation. 8
- 7. Based upon a comparison of the growth rates of the locked tubes and in-service tubes there is no increase in the growth rate of tube degradation for locked tubes. ,
i 14.0 Conclusions
- The root cause of the increased number of TTS circumferentialindications detected in locked tubes compared to in-service tubes has been determined to be
- The increase in stress required to cause initiation and growth of the undetected flaws to over the detection threshold due to the presence of existing residual stresses from the tube
$ expansion process is smal!. The tube lock, independent of configuration, provides enough } additional axial stress to reduce the time to crack initiation. Tube locks at the lowest elevation { (3H) provide a higher axial stress which causes deeper crack growth. ) The affects of reduced temperature due to the tubes being taken out of service does not offset j the increased affects of the additional axial stress. 2
)
l LNgrpspvtcam'epridos doe'l8
4 15.0 Reference ! 1. Causes and occurrences of Circumferential Cracking in PWR Steam Generator Alloy 600 Tubing, Draft EPRI Report, Dominion Engineering, February 1996 i j 2. Examination of Steam Generator Tubes Removed From Braidwood Unit 1 in 1996 Draft Final Report April 1997, ABB
],
1 3. Estimated Axial Stresses in Braidwood Unit 1 Tie Rod Tubes, Dominion Engineering ! j Inc., Calculation No. 50-46-10-01 Rev.1, Dated 4/18/97. l 1 l
- 4. Estimated Relative Crack Growth Rates - TTS of Braidwood Unit 1 Tie Rod Tubes,50- l 46-10-02, Rev.1 Dated 4/21/97 l- 5. Draft Westinghouse Letter Dated 4/18/97 i
- 6. Westinghouse WCAP 14273, Technical Support for Altemate Plugging Criteria with !
! Tube Expansion at the Tube Support Plate intersections for Braidwood and Byron 1 l
- Model D4 Steam Generators, February 1995 l 2
1 2
- 7. Section 2, " Technical Basis for Water Chemistry Control, "PWR Secondary Water l Chemistry Guidelines" - Rev. 4, EPRI TR-102134-R4, November 1996 i
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1_1 1 1 1 1 1 1 m. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 D D DD D DD DD D D D D D # D T e D D Tto D D D D D D D D D D D D D D D D D D D D W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W W R R R R R R R R R R R R R N R : R R R R R R R R R R R R R R R R R R R R R R 4, B B B B B B B BB B E B B6 B B B B B B B B B B B B B B B B B B B B B B B S I
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p.-.-. _. ___.. _ .- _ ..._ _ _ __ _ ___._ _ _ _ _ ___ _ _ ._ _.-._ E rr4 cum m t sL i i
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l 19 of 29 @ 8H w/cire (2 above 0.8V +pt max) 4 17 of 32 @ 3H w/cire (7 above 0.8V +pt max)* l 12 of 24 @ 10H w/ circ (0 above 0.8V +pt max) i l . Sort by # Locks I i i I Lock: 1 of I w/ circ (0.48V +pt max)
- 2 Locks
- 17 of 32 w/ circ (2 above 0.8V +pt max) l 3 Locks: 5 of 8 w/ circ (0 above 0.80 V +pt max) i 4 Locks: 17 of 28 w/ circ (5 above 0.80 V +pt max)
- 6 Locks: 8 of 16 w/cire (2 above 0.80 V +pt max) j Sort by # Over Expansion of Locks j, # Over Expanded I 9 of 17 w/ circ (1 above 0.80 V +pt max) 2 15 of 27 w/cire (4 above 0.80 V +pt max) 3 10 of 13 w/cire (2 above 0.80 V +pt max) l 4 1 of 5 w/ circ (1 above 0.80 V +pt max)
- 5 2 of 2 w/ circ (0 above 0.80 V +pt max) 6 0 of 2 w/ circ None 11 of 19 w/cire (1 above 0.80 V +pt max)
Total 37 of 85 Locks had over expand w/ circ 11 of 85 Lock had cire w/no over expand Sort by Size 5 largest +pt max IND @ 3H 7 of 10 largest +pt max IND @ 3H 2 of 10 largest @ 8H I of10 largest @ 10H Largest l'ND @ 3H w/4 Locks & 4 over expanded
-+ 2 @ 3H w/4 Lock & 4 over expanded -+ NDD ;
% 4 w/6 Locks & 4-6 over expanded -+ NDD '!
-+ 2 w/6 Locks & 4-6 over expanded a NDD I
- Contains the Largest +Pt Max Volts Indication L:Vgrp;misetlockdo.
- i
". t Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG A Ind's NDD in 10/95, RPC AVG Volts /EFPY l
i 0.35 E110/95 to 10/96 Growth / EFPY Freq.
@ 95 to 57 Locked lobe Growth / EFPY Freq. ,
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Growth Rate, RPC Average Voltage, volts /EFPY ' Fisugg 1
- l 4"21/97 4.30 PM 10-95NDD.XLS c5 RPC avg vcJts grth freq I
Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG A Ind's NDD in 10/95, RPC Max Volts /EFPY l 0.35 510/96 Ind NDD in 10/95 Growth, Freq. E 97 Locked Tube Growth Freq. 0.3 l 0.25 , x 0.2 - i - F5 i E ! :
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- . _ . . . ~ . _ . . . . _ _ _ . _ _ _ _ . _ . _ . - - . . _ . . _ - - . _ - _ . _ . _ _ . . . . - . - - _ -
Q 4 Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG B Ind's NDD in 10/95, RPC Avg. Volts /EFPY , 0.35 510/96 Ind's NDD in 10/95 Growth Freq. , E 97 Locked Tube Growth Freq. 0.3 0.25 i g 0.2 , 5 g , S 0.15 - 0.1 - - - - 0.05 - - - - -
-{
l !? O ' - - i i ' i ' M i 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 Growth Rate, RPC Average Voltage, volts /EFPY FIGwle } 4/21/97 5:09 PM 1095BNDO.XLS ch RPC avg volt grwth freq.
Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG B Ind's NDD in 10/95, RPC Max Volts /EFPY O.35 B 10/95 to 10/96 Growth Freq. E 95 to 97 Locked Tube Growth Freq. 0.3 0.25 g 0.2 c ci E 0.15 - -
~
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! h Nk 4/21/97 5:09 PM 1095BNDO.XLS ch RPC max volt gewth freq -
j
Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG C ind's NDD in 10/95, RPC Avg. Volts /EFPY 0.4 D 10/96 Ind's NDD in 10/95 Growth Freq. I E 97 Locked Tube Growth Freq. 0.35 - 0.3 - 0.25 - c 5 0.2 - E ll 0.15 - --- 0.1 - - - 0.05 - - - 1 -- m 0 + '
! i E ' E i i 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 Growth Rate, RPC Avg. Voltage, volts /EFPY FIGugg 5 4/21/97 8 59 PM OUTPUTC.XLS ch RPC avg volts gr freq i
s , Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG C Ind's NDD in 10/95, RPC Max Volts /EFPY . 0.45 I E 10/95 to 10/96 Growth Freq. E 95 to 97 Locked Tube Growth Freq. 0.4 y l?
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Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG D Ind's NDD in 10/95, RPC Avg. Volts /EFPY 0.35 E 10/96 Ind's NDD in 10/95 Growth Freq. E 97 Locked Tube Growth Freq. O.3 0.25 Ei! ;
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Growth Rate, RPC Average Voltage, volts /EFPY FI6 wee 7 4/21/97 5:25 PM 1095DNDO.XLS ch RPC avg volt grvith freq. j
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t Comparison of Growth Rates: 1997 Locked Tubes vs.10/96 SG D Ind's NDD in 10/95, RPC Max Volts /EFPY 0.35 _. 1510/95 to 10/96 Growth Freq. t R 95 to 97 Locked Tube Growth Freq. 0.3 -- f 0.25 - Il ;
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