ML20112J226
| ML20112J226 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 01/11/1985 |
| From: | Giacobbe F, Janiszewski J, Von Nieda G GENERAL PUBLIC UTILITIES CORP. |
| To: | |
| Shared Package | |
| ML20112J213 | List: |
| References | |
| TDR-638, TDR-638-R, TDR-638-R00, NUDOCS 8501180210 | |
| Download: ML20112J226 (48) | |
Text
I TDR NO.
638 REVIS40N NO.
0 8UDGET TECHNICAL DATA REPORT AcTivlTY NO. 123125 PAGE 1
OF AR N
DEPARTMENT /SECTION
"
- A N 1.
TMI-1 OTSG* REPAIRS REl. EASE DATg 1/11/85 RgVl840N DATE DOCUMENT TITLE: Evaluation of Eddy Current Indications Detected Durina the 1984 Tech. Spec. Inspection
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DeSTRIOUTION ASSTRACT:
In order to identify the cause of the eddy current R. O. Barley indications detected during the TMI-1 OTSG tube examination G. R. Capodanno beginning in November 1984, Materials Engineering / Failure Analysis performed an in-depth review of the eddy current J. J. Colitz results and plant operat.ing/ chemistry history since the OTSG's were first filled af ter the kinctic expansion repairs.
D. K. Croneberger B. D. Elmu Two possibic causes for tha eddy curront indications were eisluat:d:
corrosion, aishar continuing or newly M. J. Groham initiated, an,d enhanced eddy current. detectability of N. C. Kazanas existing intergranular attack (IGA).
During unit layup, GP"N layup specifications were followed. Some out of spec-R. J. McGoey ificut. ion periods did occur; however, they were promptly T. A. Richter corrected and were not of sufficient magnitude to have caused corrosion.
Additional corrosion-preventive conditions were G. R. Taylor also maintained during layup.
R. F. Wilson During hot operations, system chemistry conditions were T. G. Broughton maintained within specifications that industry experience and TMI-1 tube testing have shown are non-corrosive.
The most likely reason for having eddy current indica -
tions at this time was enhanced detectability of pre-existing i
areas of IGA. As a result of thermally induced strains and hydraulic forces duri.ng hot functional testing, grains could fall out or grain boundaries could separate for a short distance within pre-existing IGA, resulting in greater local i
disturbance and a correspondingly larger eddy current signal.
Additional plant data from leak rate observations and the fiberscope examination of a sample of tubes also support the r.echanical damage scenario.
No leaks have been identi-fied in the tubs free span since 1983.
In the region of 1984 eddy current indications, patch-like indications sugges-tive of IGA vere seen by the fibarscope examination.
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r TDR 638 Rev. 00 Page 2 of 48 Table of Contents M
Introduction
Background
Evaluation of Eddy Current Results Post-Baseline Growth Studies In-Process Testing ISI Indications June 1984 Testing 100 Tube Sample November 1984 1984 Technical Specification Required Testing Spatial Distribution Characterization of Indications Degraded Tubes Chemistry Specifications Corrosion Experience with Inconel 600 Corrosion Test Results Long Term Corrosfor, u s t.
Short Tern Test Rr,-!t Bulk vs. Surface Effects THI-1 Chemistry Guidelines Hot Operations Layup Chemistry and Operating History Review Data Base Results of Operational / Chemistry Review Chloride and Sulfate Oxygen Other Operational Considerations
T TDR 638 Rev. O Pege 3 of 48 fa!Le In-Plant Observations Leak Testing Fiberscope Inspection of Selected Tubes Discussion General Detectability of Indications by Eddy Current Conclusions References Appendix e
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T TDR 638 RGv. O l
Page 4 of 48 j
Introduction In accordance with the requirements of Technical Specification 4.19, eddy current testing of the OTSG tubing at TMI-1 was begun in November 1984.
Initial testing with the 0.540" high gain standard differential probe method revealed previously unreported indications in the unexpanded portions of the OTSG tubes between the tube sheets.
Two possible causes for the eddy current indications were identified and evaluated; first, whether corrosion of the OTSG tubes caused either new defects or growth of existing defects and second, whether straining of existing defects caused them to become more detectable by eddy current. Since the original 100% baseline inspection of the OTSG tubes in 1982, the tubes have been subjected to mechanical loading during the kinetic expansion and thermal and hydraulic loads during the two hot functional tests.
In order to attempt to determine the cause of these indications, the Materials Engineering / Failure Analysis group revfewed 1) the historical eddy current data and 2) plant operational and chemistry data since the OTSG's were filled after the kinetic expansion repair of the tubes.
Based on the results of this review, the cause of the indications is discussed. Data supporting the conclusion are also included.
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TDR 638 Rev. O Page 5 of 48
Background
As defined by Technical Specification 4.19, GPUN conducted eddy current examinations of both steam generators at THI Unit 1.
Performance of this examination ultimately resulted in 100% of the tubes in A-0TSG and all tubes in the outer 16 tube periphery of the B-0TSG being examined.
The B-0TSG had only a limited number of indications with en indicated through-wall extent greater than 40%. Due to the limited number of B-0TSG indications, statistically-based analysis is not feasible. All these indications, however, are located near the outer periphery of the B-0TSG.
The following generalizations about the EC indications can be drawn from the A-0TSG results:
1.
They are primarily located in the upper tube sheet and 16th tube span area.
2.
They are concentrated in the outer periphery, but some indications occur across the entire OTSG.
3.
Most indication:: are less than 50% through wall.
4.
They generally exhibit voltages in the 0.5-2 v. range.
5.
By 8 x 1 absolute eddy current, the number of cofis tends to be 2 or less, indicating a small circumferential extent.
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TDR 638 Rev. O Page 6 of 48 Evaluation of Eddy Current Results Note: This section uses the eddy current data base as of Jan. 3,1985.
GPUN conducted a qualified full-length, eddy current examination program on all tubes from both generators during July to November 1982.
The purpose of this program was to screen out all relevant indications and establish a 6" qualified length in the kinetically expanded zone immediately above the new transition zone which was essentially indication free.
It was further established that, although we were using a process that was approximately 175% more sensitive than previously used at TMI in performing eddy current examinations, small defects below the threshold of detection could exist. Reference 1 identifies the maximum size of these small defects which could possibly go undetected.
Prior to the expansion, a 100-tube sample of tubes in each generator was eddy current tested periodically to check for indication changes. These tests were performed on seven occasions over a 7 month period. No growth was observed.
Post-Baseline Growth Studies In-Process Testing During and following the kinetic expansion repair, a total of 437 tubes were inspected in both the A and B generators (Ref 2, 3).
A total of 15 tubes (3.5%) with indications were found that had not been detected by our ECT inspection program prior to the repair.
An evaluation was performed on why these indications were not identified previously (Ref. 3).
It was concluded that:
1)
The recent indi aticnA vere not initiated by the kinetic expansion process nor was.there any evidence of ductile propagation of exutiny mdications.
2)
The defects were small (threshold) type indications that had been either masked by the high background noise levels in the upper tube regions or were sufficiently tight that sufficient metal removal was not present to permit detection. Kinetic expansion may have altered these areas of IGA to make them more detectable.
Confirmation on the small size of the indications was established by the visual examination using fiber-optics. Some of the indications appeared to be small pits.
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TDR 638 Rev. O rage 7 of 48 Additional confirmation was obtained that kinetic expansion would not cause ductile tearing by using test mock-ups cnd metallurgical examination (Ref. 2). Small intergranular stress j
assisted (IGSAC) cracks were examined using eddy current techniques before and after kinetic expansions. Expansion caused the cracks to become non-detectable by.540" S.D. techniques. However, the cracks remained visible to the 8 X 1 absolute technique with essentially no change in signal. These specimen tubes were subsequently removed from tfie test block and metallurgicai examination did not reveal ductile tearing or generation of new indications.
ISI Indications During OTSG repairs, a subset of tubes (28 in A-0TSG, 56 in B-0TSG) was identified as having eddy current indications that did not require plugging. That is, the indications were less than 40%
through wall, not in the lane / lane wedge area, and below the 15th tube support plate. This group of tubes (designated as "ISI" tubes by GPUN) was fully characterized and listed for eddy current inspection in the future as a distinct subset.
The "ISI" tubes were re-examined in April /liay 1983. No growth of the existing indications was detected.
As part of the eddy current campaign which started in October 1984 all 84 of the "ISI" tubes have been retested. No growth in theISIsubsetwasdetected.
(Growth is identified as a substantial increase in the through wall percentage, combined with an increase in voltage and circumferential extent.)
June 1984 Testing During June 1984, 67 tubes in B-0TSG and 3 tubes in A-0TSG were eddy current tested. This set of tubes was retested in November 1984 - no new indications were detected for the two retests performed, i
100 Tube Sample November 1984 Since discovery of the additional indications in November 1984, a second 100 tube sample with indications has been re-examined at approximate two week intervals. As of December 18, 1984, no growth and no new. indications have been detected for the two retests performed.
TDR 638 Rev. O i
Page 8 of 48 1984 Technical Specification Required Testing In November 1984, eddy current testing required by TMI-1 Technical Specification 4.19 was conducted as specified. 3% of the tubes in each generator were initially examined. This examination included tubes randomly selected across the entire generatnr plus a concentrated examination in the periphery of each generator. The more extensive examination in the periphery was performed because this was the region of highest previous (1981) damage.
As a result of this initial examination, OTSG A was classified as category "C-3" per technical specification and 0TSG B was classified as category "C-2".
Subsequently the entire A-0TSG was inspected while the B-0TSG fnspection was complete after the entire 16-tube periphery, approximately 6500 tubes, had been examined.
The number of indications is much higher in Aw0TSG than B-0TSG.
In A-0TSG, 2.0% of the tubes (299 out of approximately 14589) have indications greater than 40% through wall whfie in B-0TSG, 0.5% (33 out of approximately 6576) have such indicatio,ns.
Spatial Distribution The indications with greater than 40% through wall extent are concentrated toward the outer periphery and top of A-0TSG.
In the outer periphery, the percentage of tubes with greater than 40%
through wall indications is higher than the 2.0% average, while inside the outer support rods the percentage of indications is below 1%.
71% of the indications are located above the 15th tube support plate (TSP).
Characterization of Indications.if To understand the naturo af the defects better, we characterized the indications rccerted back in the 1981-1982 time frame and compared them to the indtcations discovered today.
The axial and radial locations of indications in A-0TSG are essentially the same in 1984 as in 1982, if one does not consider the 1982 indications in the kinetically expanded region in the 1984 evaluation.
TDR 638 Rev. O Page 9 of 48 Table 1 characterizes the 1982 and 1984 eddy current signals.
The 1984 eddy current indications exhibit a similar type of signal response as the previous test program. Details of the differences in responses are noted below:
1)
Reported voltages are essentially the same. This indicates that the 1984 indications present a similar volume for the eddy current probe to detect as the 1982 IGSAC.
2)
Both through wall penetration and number of coils is significantly lower in 1984. Thus, the 1984 indications extend a shorter distance both into and around the OTSG tube.
Statistical analysis of the eddy current data reveals that 90% of the observed indications fall between 10% and 50% through wall penetration, and between.020" and.190" long.
Degraded Tubes Per GPUN procedure, tubes with indications reported between 20 and 40% through wall were not required to be plugged if the tubes were not in the lane or lane wedge and the indication was below the 15th tube support plate. At the completion of the 1982 kinetic expansion repairs, a total of 15 A-0TSG tubes and 51 B-0TSG tubes were classified as " degraded" and were included in the ISI group.
As of January 4, 1985, 347 additional A-0TSG tubes and 98 additional B-0TSG tubes are classed as degraded.
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TDR 638 Rev. O Page 10 of 48 Table 1 Comparison of 1982 and 1984 Eddy Current Data a)
Reported Voltage - % of indications reported A-0TSG B-0TSG Voltage 1982 1984 1982 1984 41 34 40 24 27 1
44 35 30 21 2
16 20 25 29 3
4 4
10 12
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Reported through wall penetration - % of indications A-0TSG B-0TSG
% T.ll.
1982 1984 1982 1984 4, 20 41 41 12 20-40 3
61 28 75 40-60 21 25 24 18 60-80 17 10 15 5
> 80 59 4
21 2
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Number of cofis on 8 r I exar.ination - % of calls A-07;C:
B-0TSG Co f f s 1982 1984 1982 1984 1
20 90 18 80 2
26 10 24 20 3
16 41 15 41
>3 38 41 43 4.1 NOTE: 1982 data includes inspection of original tube roll transition area.
The 1984 data does not include inspection from the top of tube sheet to the bottom of the kinetically expanded region.
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r TDR 638 Rev. O Chemistry Specifications Corrosion Experience with Inconel 600 Three types of primary-side initiated attack have been identified in Inconel 600.
In recirculating steam generators using mill-annealed tubes that have not been stress-relieved after U-bending, stress corrosion cracking (SCC) has initiated from the primary side in the highly stressed bend areas. Also in mill-annealed tubes in recirculating steam generators, SCC has been found to initiate from the primary side at highly stressed transition areas in the lower tubesheet. Laboratory studies have shown that the stress relieved Inconel tubing used in OTSG's is significantly more resistant to SCC than the mf11 annealed type.
The other primary side attack of Inconel 600 that has occurred in steam generators is the intergranular stress assisted cracking (IGSAC) caused by reduced sulfur species oi. sensitized 0TSG tubing. This is the mechanism which caused the THI-1 OTSG 1eakage in 1981. This mechanism requires sensitized tubing, low temperatures, oxygen, and significant levels of reduced sulfur species.
Corrosion Test Results As part of the oserall program to evaluate the most recent eddy current testing results, we have reviewed the results of corrosion tests performed as part of the original failure analysis and OTSG requalification progrt.ms. These data provided a partial basis upon which we could evaluate the layup and test conditions to which the steam generators had been subjected.
Long Term Corrosion Test (LTCT)
The primary purpose of the long term corrosion tests was to verify that the proposed operating chemistry specifications are satisfactory to prevent corrosive attack of the OTSG tubes. To this end, chemistry conditions for the testing were established at the maximum allowable values consistent with the upgraded Tiil-1 operating specification (Ref. 4). The LTCT was conducted using actual THI-1 tubing. Temperatures, tube loads and heatup and cooldownrateswererepresentativeofactualplantoperating conditions.
In addition, as the LTCT was actually performed, specific factors which parallel actual plant layup conditions were experienced. The tubes were held in a cold, aerated condition for several days af ter the completion of each operating cycle.
Aeration was done after cooldown. Before heatups, or whfie waiting for other autoclaves in the test program to be ready for operation, the test loops were operated in a cold, deaerated, circulating mode. Because eddy current examinations were done after each test cycle, the tubes had to be removed from the autoclaves and drained. Thus, drained aerated layup conditions were also included.
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TDR 638 Rev. O Page 12 of 48 Table 2 summarizes LTCT operational times in each mode. A11 loops spent significant time under drained. cold deaerated, and aerated conditions.
Review of the chemistry history of the LTCT's revealed that the conditions were comparable to the plant's experience. The LTCT for sulfate and chlorides was 0.100 ppm +
specification (Ref 5) lysis results (Ref. 6, 7, 8) revealed thaY the
.050 ppm. Actual ana concentrations of these species were maintained at or slightly above the.150 ppm upper limit. The actual values measured in these tests bound any of the contaminant " spikes" reported in the Chemistry and Operational History Review.
C-rino tube samples from archive tubing (tubing never installed in the THI-1 OTSG's, which was included as a control sample) showed no evidence of cracking, pitting or general Corrosion.
Some intergranular attack (IGA) was noted on 4 C-rings made from a single TiiI-1 OTSG tube; this IGA was evaluated to be pre-existing damage associated with the 19G1 IGSAC incident. Of a total of 38 C-rings evaluated, 31 had no visible defects, 3 showed very shallow cracks when strained severely, and 4 had IGA as described above.
Five full tube samples were meta 11ographically examined after the LTCT.
In addition to previously reported defects, four samples exhibited scattered, shallow cracking or IGA which was not detectable by eddy current testing. This IGA was consistent in size and shape with IGA that had been seen during the failure analysis (Ref. 9). Therefore, the observed IGA on these four tubes was judged to have been present'at the start of the LTCT.
One tube sample had seere IGSAC and IGA which had progressed during the term of the LTCT and hed been detected by eddy current.
The tube sample which showed flaw growth during the LTCT was exposed in the test loop in which the sulfur species was thiosulfate, at a concentration of 0.100 ppm + 0.050 ppu (as during the LTCT was exposed to intentionally added, g flaw growth sulfate). Therefore, the only tube sample exhibitin reduced corrosive sulfur species.
The four C-ring samples showing IGA and the full tube sample showing flaw growth were removed from the same OTSG tube. This tube was recorded as having multiple eddy current indications when inspected in the OTSG. The IGA seen in the post-test examination is therefore consistent with an original tube sample which had multiple defects and, presumably, associated IGA.
Results of metallographic examination of the LTCT samples (Ref. 8) confirrmd that in the absence of intentionally added aggressive sulfur species, normal operations would not cause corrosion of THI-1 OTSG tubing.
TDR 638 Rev. O Short Term Test Results Several sets of tests were previously run on Inconel 600 tubing to establish corrosion resistance under various conditions representative of THI-1 service. Those results which apply to the period of this review are summarized below:
1)
Screening work on actual Tl!I-1 removed tubes and archive tubes (Ref. 10) identified that at oxidizing potentials, 1 ppm of thiosulfate was required to cause IGSAC.
Sulfate levels as high as 10 ppm did not cause IGSAC.
2)
Simulation of hot functional testing and cooldown (Ref.
- 11) utilizing thfosulfate contamination and actual operating temperatures and times revealed that 1 ppm of thiosulfate caused IGSAC.
These short term tests thus confirmed that in the absence of thiosulfate contamination, no short term attack of 0TSG tubes is expected.
Bulk vs. Surface Effects The above corrosion tests were performed using actual THI-1 OTSG tubing. The surface film condition was therefore representative of that in the plant. Chemistry control in both corrosion testing and actual operation is done by the measurement and control of species of interest fu the bulk fluid.
Since both surface conditions and chemistry control were identical between the laboratory tests and plant operations, the results of the corrosion tests can be directly applied to the plant environment, and, conversely, plant bulk chemistry data can be used to evaluate the propensity for corrosion.
TMI-1 Chemistry Guidelines Hot Operations After sulfur was identified as the causative agent of the 1981 IGSAC, hot operational guidelines (Ref. 4) were reviewed to ensure that adequate corrosion protection was maintained. As a result of this review, two changes were made to provide increased margins against corrosive attack.
First, a requirement was added that primary system sulfate be maintained below 0.100 ppm. Sulfate at this level does not cause corrosive attack of Inconel 600 in primary coolant, and maintaining sulfate below this level provided assurance that intermediate sulfur species could not exist at harmful concentrations.
Second, the lower limit on lithium concentration was increased to 1.0 ppm, to take advantages of 11thium's inhibiting effect on sulfur-induced IGSAC in Inconel 600 (Ref. 12).
TDR 638 Rev. O Page 14 of 48
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The net result of these changes is to ensure that total sulfur species concentrations are a factor of 10 below the level at which corrosive attack might occur. At the same time, the minimum Lf/S ratio will be 30 (or Lf/SO4 of 10), which is a factor of 3 over the recomended (Ref.12) ratio of 10 for inhibition of IGSAC initiation.
Layup For cold layup conditions, guidelines have been established to maintain as many protective conditions as feasible. The individual protective conditions that are feasible for the THI-l RCS are:
1)
Elevated pH - during layup, >H has been elevated, using ammonta, to at least 7.2.
T1e normal pH without amonia is 5.6 - 6.5.
2)
Control of contaminants - The primary water contaminants of concern are cniorices and sulfates. Chlorides have traditionally been limited to less than 0.100 ppm during operation; we have maintained this level as a general guideline during layup. The sulfate level of less than 0.100 ppm used during hot operation also applies to layup.
3)
Control of oxygen level - When the system is filled and able to be pressurized, the oxygen level is to be maintained below 0.1 ppm. For cases where the primary system is open and oxygen cannot be excluded, air saturated conditions are specified as this is more protective than some intermediate oxy 5en level.
4)
Control of 0TSG ievel - One of the contributing factors to tne 1901 uasi seident was the existence of a water line on the primary side of the OTSG tubes. For layup of the OTSG's, wherever possible, no static waterifne shall be allowed to exist in the OTSG tubes. Either the water level should be above the upper tubesheet or the OTSG primary side should be fully drained.
5)
Inventory Turnover - Periodic replenishing of the OTSG contents w111 assure that local buildup of contaminants will not occur. Layup guidelines have included provisions for periodically turning over the water inventory on the OTSG primary side to meet this objective.
TDR 638 Rsv. O Page 15 of 48 TABLE 2 Summary of Operations for Long Term Corrosion Tests Operating Days Cold Circulating Drained Loop Hot Deaerated Aerated Layup (Note 1)
Comments 1
348 52 28 132 1
2 308 69 27 157 Thiosulfate loop 3
241 42 23 58 4
242 40 22 61 Notes 1.
Does not include drained layup between completion of operational cycles and start of metallographic examination.
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TDR 638 Rev. O Page 16 of 48 Chemistry and Operating History Review Data Base The chemistry and operating history data were obtained from two sources. First, the on-site Plant Analysis group reviewed operational records to identify plant conditions during this time period (Ref.13).
Then, we retrieved the primary plant chemistry parameters of interest from the GPUN computerized chemistry data base.
The major plant activities that occurred between May 1983 and October 1984 are listed in Table 3.
Within each of these periods, we identified different plant conditions of RCS level, temperature, pressure, circulation, and pH. Then, we reviewed the chemistry data for each time period.
Chemistry data selected to be of interest with respect to corrosion were pH, cxygen, lithium, sulfate and chloride. As an additional check on the effectiveness of chemistry controls, we calculated the lithium to sulfur ratio for each operating period.
In cases where simultaneous analyses for lithium and sulfate exist, we calculated the Lf/S ratio for each data point.
The data from the operational and chemistry investigations are plotted as a function of time in Appendix A.
Results of Operational / Chemistry Review During both hot shutdown and cold layup conditions, THI-1 has maintained conditions within chemistry guidelines for about 95% of the time. For short time per,cds, sece. deviations have occurred which are discussed in the balance of t'is section.
Chloride and Sulfate There have been short time periods where chlorides and/or sulfates have exceeded specified limits.
In all instances chemistry data reflect that corrective actions were appropriately and promptly taken to return the concentrations of these species to specified levels. Collectively, these out-of-specification periods can best be described as normal chemistry " spikes".
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7 TDR 638 Rev. O Page 17 of 48 0xygen In preparation for both the September 1983 and May 1984 hot functional tests, it was necessary for the RCS to be taken from a layup to an operating mode. During this transition, oxygen levels were higher than desired for optimum protection, but other factors made it very unlikely that corrosion occurred. First, chloride and sulfate concentrations were controlled to acceptably low levels.
Second, the lithium level was maintained such that the minimum lithium to sulfur ratio was 66; the recommended minimum value for protection against IGSAC is 10 (Ref. 12). Chemistry control during these periods is summarized in Table 4.
Other Operational Considerations During the Integrated Leak Rate Test (ILRT) in April 1984, the primary side water level was maintained at about the 12th tube support plate for 8 days. This condition was both preceded and followed by drained layup with elevated pH, aerated water. Both sulfate and chloride levels remained within specification. Therefore, no OTSG tube corrosion was expected.
In August 1983 and May 1984 oxygenated water was injected into deoxygenated RCS during HPI testing. Most of these tests were conducted prior to the high temperature portion of the hot functional tests, and -
the oxygen introduced would have been consumed by hydrazine and/or hydrogen added for that purpose. One test was conducted on May 26, 1984, at the end of HFT and may be postulated to have injected 5000-6000 gallons of oxygen-saturated water. During this time period, however, the lithium to sulfur ratio was greater than 30 which was more than adequate to inhibit corrosion during this test.
TDR 630 s
Rev. O Page 18 of 48 i
TABLE 3 Major Plant Evolutions, 5/83 to 10/24 1
Event Duration Fill & Bubble Test June 1983 Peroxide Clean July 1983 Hot Functional Test Aug - Oct 1983 Circulating Wet Layup Oct - Nov 1983 DH-V1 Repair Nov 1983 Circulating Wet Layup Nov 1983 - Jan 1984 RC-PIB Repair Feb - April 1984 Integrated Leak Rate Test April 1984 Hot Functional Test May 1984 Non-Circulating Wet Layup May - June 1984
-Tube Plug Rerolling and..
June -'Oct 1984 Bubble Testing 7
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i TDR 638 Rev. O Page 19 of 48 TABLE 4 Chemistry Summary Before Hot Functional Testing
C1 Li/S 4
Period Days ppm ppm ppm ppm Ratio 8/83 29 0.3
.82-1.96
.047.079
.05.156 66-123 5/84 19
.075-2.2 1.06-2.17
.02.047
.05.110 127-240
4 TDR 638 Rev. O Page 20 of 48 In-Plant Observations Leak Testing Since completion of the kinetic expansion repairs, several leak tests have been performed to measure primary-to-secondary leakrates and identify individual leaking tubes. These tests are summarized in Table 5.
No pattern of tube leakage can be seen. After the cooldown tests included in hot functional testing some increase in leakage was seen.
Further investigation showed that this leakage was the result of leaks through a small number of tubes. These leaks were located in the expanded region within the upper tube sheet and were repaired by mechanically rolling a portion of the expanded area.
Of greatest sianificance is that since 1983 no tube which is in service has had a leak in an unexpanded portion of Ehe tube. All leaks have either been due to bypass leaks in the expanded area or leaking plugs.
Fiberscope Inspection of Selected Tubes A fiberscope inspection was performed (Ref.14) of six A-07SG tubes which exhibited typical eddy current indications. During the inspection features were observed on 4 out of 6 tubes at the same elevation as the eddy current indications.
The visual features were "patchlike" rounded areas having an outer ring which was darker than the general tube surface and slightly reflective components i-the interior. The patches were between 0.020 and 0.060" in diameter.
The patches appeared stattar to surface deposits.seen during the initial tube failure analysis.. ihese earlier deposits were found to be associated with partial through wall intergranular attack.
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s TDR 638 Rev. O Page 21 of 48 Leak Tests in OTSG's Since 5/01/83 Month / Year Test Type Reason For Test Results Repairs May 1983 Drip Test of Kinetic 2 Leaking Tubes, Plugs In. stalled / Rerolled Expansion 8 Leaking Rolled Plugs 10 Leaking Explosive Plugs June 1983 Bubble / Drip Final Test of Small Number of Slightly Repaired welded plug Kinetic Expansion Leaking Tubes and Plugs in A OTSG - 1 Leaking welded plug Sept 1983 Kr-85 Tracer Establish Baseline Baseline Leak Rate None Required Leak Rate 1 gph May 1984 Kr-85 Tracer Measure Baseline Slight Increase in None Required Leak Rate Leak Rate June 1984 Bubble / Drip Identify Leaking 4-5 Leaking Tubes in Plug 3 tubes Tube (s)
B-0TSG w/ welded plugs
}
6 Rolled Plugs Missing Reroll all W plugs Replugged tubes.
Oct. 1984 Bubble / Drip Test Rolled Small Number of Leaking Roll 8 Tubes
~
Repairs Tubes, one welded plug Reweld Plug Note: No leaks seen in final October 1904 Iwbble Test, after tube rolling.
TDR 638 Rev. O Page 22 of 48 Discussion General Removal of sodium thiosulfate from the THI-I site and tighter operational chemistry controls implemented since 1981 have made it highly unlikely that the conditions to cause sulfur-induced IGSAC could be The steam generator layup guidelines are specifically designed recreated.
to protect the steam generators from additional corrosion and are more stringent than B&W's generic recommendations, particularly in the areas of contaminant control and the use of elevated pH during cold layup.
Industry experience on B&W PWR's also does not reveal any other primary-side initiated attack mechanisms on Inconel OTSG tubing.
TMI-1 compliance with operating and layup specifications has been Transient out-of-specification conditions, which were excellent.
identified during plant operation, have been infrequent and corrected Plant conditions have always been promptly by the plant operators.
bounded by those which were evaluated during corrosion testing and found to be satisfactory.
The only period of possible vulnerability to corrosion would have i
existed during the time when the OTSG's were drained for the kinetic expansion repair. During this period sulfur would have remained in the oxide film on the tube surfaces as peroxide cleaning had not yet been During this time, however,' eddy current testing done on the performed.
100 tube surveillance sample did not reveal any growth of existing indications or any new indications. Thus, while the oxide film may have i
1 contained sulfur during this time, there is no evidence that corrosion continued.
Previcusly detected-IGA; both in the failure analysis (Ref. 9) and long term corrosion test (Ref. 8), has roughly been in the form of A pit of hemispherical pits penetrating;approximately 50% through waii.
this shape and penetrationwwid appear as a circle on the surface of diameter of approximately 0.035".
Areas of this circumferential extent i
would not be predicted to be detectable by the.540 S.D. eddy current technique (Ref. 2).
Under mechanical loadings induced by kinetic expansion or cooldown, these areas could become more detectable by eddy current through several mechanisms:
l 1) creation of a linear. grain boundary separation within the IGA 4
islands as was seen in the LTCT (Ref. 8), or 2) disconnected grains dropping out and leaving pits.
~
TDR 638 Rev. O Page 23 of 48 Two additional pieces of data from Ref.16 lend support to the mechanical scenario. First, peripheral tubes consistently see higher loads than core tubes. Therefore, in the periphery, the highest stresses would also act on this IGA. Second, the A-0TSG cooled down more quickly than the B unit. The peak load during the most rapid cooldown (Ref. 16) was 200 lb, or higher (12%), in the A-0TSG than in B-0TSG. Figure 1 is a representation of how the A-0TSG would have had significantly more tubes carrying loads high enough to cause IGA to become more detectable.
A previous study (Ref.15) on crack opening displacement of archive tubes with approximately.5" long through-wall cracks found that loads between 1500 and 2000 lbs. would induce permanent displacements in the vicinity of the cracks. Loads less than this would induce only elastic displacements with a load of 1000 lbs. producing an elastic displacement of approximately.002".
Although tubes with cracks of this size are no longer in-service with the steam generators, this study does point out that one can expect local straining in the vicinity of smaller defects, but that it would be of proportionately lesser magnitude.
During the 1983 HFT, the most rapid cooldown was calculated to have induced loads in the tubing of between 1600 and 1700 lbs. (Ref.16).
It is such loads acting on the regions of IGA which we believe leads to grain dropping or grain boundary separation.
Visual observations made during the fiberscope examination of selected 0TSG tubes support the cause of the present eddy current indications being mechanical dama At locations where eddy current indications existed,ge to existing IGA.we frequently saw rounded, darkened areas of a size consistent with IGA detected in the original failure analysis.
Detectability of Indications by Eddy Current tube integrity (i.e. tube rupture) primary defects of concern for OTSG It should be noted that the are circumferential cracks. The production of 0.540" standard differential eddy current technique is optimized and qualified for this type of defect. However, it can also be used for detecting different defect geonetries as discussed below.
The 1984 tube ID fndications as detected by eddy current and as seen during the fiberscope inspection had significantly different characteristics than the IGSAC responsible for the 1981 tube leakage.
The 1981 IGSAC consisted of tight, circumferential cracks that penetrated completely through the wall. The 1984 observed IGA is more rounded and does not completely penetrate the tube wall.
The different geonetry will have a direct effect on detectability.
The current.540" S.D. eddy current technique was optimized for the IGSAC geometry; therefore, a different gecmetry will have a different detectability. The balance of this section of this report will discuss changes in sensitivity due to changes in indication geometry.
l TDR 638 Rev. 0 Page 24 of 48 Figure 2 (Figure 2 from Reference 2) shows the measured sensitivity of the.540" S.D. technique in the range of short circumferentially oriented defects. The shaded region in Fig. 2 identifies the area in which S0% of the 1984 indications fall.
It can be seen that the eddy
. current calls span the 0.3 volt detectability limit. Thus only slight changes in indication geometry could cause a particular indication to become detectable.
In Figure 3a and 3b, we have taken the eddy current data and visual 3
observations from the fiberscope inspection (shown in Table 6) and indicated where the indications would be in relationship to the calibration curves. The tubes for fibrescope inspection were chosen to be representative of the types of indications being found in 1984.
All of the below-UTS indications (Figure 3b) are close to the 0.3 y detectability limits; the within-UTS indications (Figure 3a) do not fall into the detectable range. Therefore, it is reasonable that before mechanical loading these indications may not have been detectable.
Mechanical loading, as discussed in the previous section, can alter IGA geometry.
Because the calibration was done on a length vs. through-wall basis using Edit notches of a constant axial width of about 0.004", IGA geometry t
could produce a different signal. Patch-type indications of the same length would have a larger axial extent, and therefore a larger volume, and could be expected to give a higher voltage signal. The S.D. response would also be enhanced by increased axial extent, even at constant defect volume, since the differential coils are wound in the circumferential direction and are more sensitive to the axial extent of defects.
The large increase in the number of degraded tubes in A-0TSG and B-0TSG is alsa consistent with the scenario of pre-existing IGA becoming more detectable.
IGA islands of 20-40% through wall extent would be expected to have a Iesmth cf.about.015.030 inches; this is below the
~
300 mV sensitivity for free-span detection (Figure 2). The additional disturbances of mechanical, tnermal, and hydraulic loading could easily disturb these islands encugtrto now make them more detectable.
1 s
,.,,..---,,.-.r,.
,._,m,,,,
,,,_m..w-----..,._,..v_..-.-.-,,m---o.--,,,
+ - - - -, -.. -., - -, - -
'IDR 638 Rev. 0 Page 25 of 48 Table 6 - Conparison of Prelimmary Eddy Current Data and Fiberscope Results l
EC Results
.540 S.D.
8X1 Row Tube Elevation
% T.W.
Volta Volts Coils Visual Observations 89 124 US+5.4 98 1.6 1.6 2
US+4 Rounded indications possible 1GA US+5.8 Axial alignment of 3 rounded indications 76 119 US+2.4 97 2.1 0.8 2
US+5.5 Small dark spot when scanning w/90* head 66 129 15+27.6 70 2.8 1.3 2
'15+24.5 Rounded indications possible ICA i
61 123 15+21. 8 67 2.3 1.1 2
15+26 Small dark spot - no detail visible 15+24.7 45 1.7 0.5 1
i 57 128 US-2.6 92 1.3 0.3 1-2 i
US-1.5 Axially oriented rounded indications'
' s.
i 60 126 15-14.2/15-6.5 37/42 1/1.0 NDD Small single rounded' indication i
1
)
i
TDR 638 Rw.~ 0 Page 26 of 48 "S
.i l
Peripheral Tube Location & Load 8
I U
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TDR Rev. O Figure 2 - 1984 Eddy Current Date Page 27 of 48 Compared to Detecteability Limits 187" 4
' N' N%\\ -
1 v. Detectability
\\
Within UTS
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\\
s 100" N-
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's below UTS N\\
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TDR 538 Rev. 0 Figure 3a - Within - Tubesheet Page 28 of 48 Fiberscope Indications Compared to Detectability Limit. _....
Notch
, Length
.187 Non-Detectable Detectable l
l 1 Volt Detectability y
Limit Within UTS
.100
.060
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A 1
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Tube Identification i
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P TDR 638 R:v. O Pase 29 of 43 Figure 3b - Below - Tubesheet Fiberscope Indications Compared to Detectability Limit Natch Lsngth
- 187 Io -5etectable Detectable n
0.3 Volt Detectability Limit Below UTS
.100
.060
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TDR 638 Rev. O Page 30 of 48 Conclusions 1.
The TMI-l layup guidelines are adequate to prevent any identified mechanisms for primary side initiated corrosion of Inconel 600 OTSG tubes.
2.
The THI-1 layup guidelines have been adhered to since completion of the kinetic expansion repair. Minor deviations have been corrected promptly.
3.
Vulnerabfif ty to corrosion may have existed during the period when the OTSG's were drained for repair prior to peroxide cleaning. However, eddy current data and the absence of OTSG 1eakage during this time period do not show evidence of corrosion of OTSG tubes.
4.
Results of both GPUN-sponsored and industry corrosion test TMIgrams confirm that corrosion would not be expected during pro 1 operations since May 1983.
5.
Results of eddy current tests since 1982 do not indicate any trends of indication growth of pre-existing indications.
6.
Leak rate testing and 0TSG bubble testing do not indicate any increases in leakage or new leaks in the tube free span.
7.
The eddy current data and visual observations are consistent with a mechanism where previously existing areas of intergranular attack are made more detectable by mechanical loading during-kinetic expansion and thermal and hydraulic loading during cooldown from HFT.
N
r TDR 638 Rev. O Page 31 of 48 References 1.
R. Barley, J. A. Janiszewski, G. E. Rhedrick and M. Torborg, "Three Mile Island - Unit 1 OTSG Tubing Eddy Current Pro Qualification," GPUN Technical Data Report 423, Rev. 1, gram January 1984.
2.
G. E. Rhedrick, " Task IV Report on Eddy Current Indications Found Subsequent to Kinetic Expansion of TMI-1 Steam Generator Tubes," GPUN Technical Data Report 401, Rev. O, April 1983.
3.
T. M. Moran, " Assessment of THI-l Plant Safety for Return to Service After Steam Generator Repair," GPUN Topical Report 008, Rev. 3, August 19, 1983.
4.
" Primary Water Chemistry," GPUN Specification SP-1101-28-001, Rev. 3, July 11, 1984.
5.
"Long Term Corrosion Testing," GPUN Specification SP-1101-22-008, Rev. 2, Oct. 29, 1983.
6.
"Long Term Corrosion Test Program of Nuclear Steam Generator Tubing Samples from Three Mile Island Unit 1 - First Interim Report," Westinghouse Electric Corp. Report No.
0914c/0127c/010684:5, October 1983.
7.
"Long Term Corrosion Test Program of Nuclear Steam Generator Tubing Samples from Three Mile Island Unit 1 - Second Interim Report," Westinghouse Electric Corp., May 1984.
8.
"Long Term Corrosion Test Program of Nuclear Stean Generator Tubing Samples from Three Mile Island Unit 1 - Final Report,"
Westinghouse Electric Corp., unpublished draft.
9.
A. K. Agrawal, W. N. Stiege1meyer, and W. E. Berry, " Final Report on Failure Analysis of Inconel 600 Tubes From OTSG A and B of Three Mile Island Unit 1," Battelle Columbus Laboratories, June 30, 1982.
10.
J. V. Monter and G. J. Theus, "TMI-1 OTSG Corrosion Test Program - Final Report," Babcock & Uticox Report No.
RDD:83:5433-01-01:01, May 9, 1983.
11.
J. C. Griess and J. H. DeVan, " Behavior of Inconel 600 in Sulfur-Contaminated Boric Acid Solutions," Oak Ridge National Laboratory Report ORNL/TM-8544, March 1983.
Task 638 Rev. 0 Page 32 of 48 12.
R. Bandy and K. Kelly, " Investigation of the Sulfur and Lithium to Sulfur Ratio Threshold in Stress Corrosion Cracking of Sensitized Alloy 600 in Borated Thiosulfate Solution,"
USNRC NUREG/CR-3834, July 1984.
13.
J. R. Kasper, "TMI-1 Primary Plant Status from 5/01/83 to 11/27/84," GPUN IOM PA-TMI-84-47, Nov. 27, 1984.
14.
J. A. Janiszewski, " Observations During Fiberscope Inspection of A-0TSG Tubes, Dec. 19, 1984," GPUN IOM MT1-1550, December 28, 1984.
15.
J. A. Janiszewski, " Leakage and Crack Opening Displacement of OTSG Tubes," GPUN Technical Data Report 480, Rev. 1.
16.
G. L. Lehmann, T. M. Moran, J. R. Sipp and D. G. Slear, "THI-1 OTSG Hot Testing Results and Evaluation," GPUN Technical Data Report 488, Rev. O, October 25, 1983.
1
~
TDR 638 Rev. O Page 33 of 48 l
APPENDIX A THI-1 CHEMISTRY DATA MAY 1, 1983 to OCTOBER 26, 1984 Contents Table A1
- Chemistry Guidelines Applied to TMI-1 5/1/83 to 10/26/84 Figure A1 Al Chemistry Data for TMI-1 5/1/83 to 10/26/84
i I
TDR 638 i
Rev. O Page 34 of 48 Table Al CHEMISTRY GUIDELINES APPLIED TO THI-1 5/1/83 to 10/26/84 Operating Wet Drained Hot Shutdown Peroxide Mode Layup Layup (Hot Functional Testing)
Cleaning OTSG Primary Level Full Drained Full Full Maximum Chloride, ppm 0.1 0.1 0.1 0.2 Maximum Sulfate, ppm 0.1 0.1 0.1 Note 2 Maximum Oxygen, ppm 0.1 N/A 0.1 Note 2 pH greater than 7.2 4.6-8,5 4.6-8.5 8.0-8.5 Li. ppm 1.0-2.0 1.0-2.0 1.0-2.0 1.8-2.5 Min imum Liis ratio 10 10 10 N/A Notes:
1.
Limits are for bulk RCS - no water in OTSG's at this time.
2.
Sulfate and oxygen were monitored but no limit was applied.
TDR 638 R::v ' O Page 35 of 48 l
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