ML20210C941
| ML20210C941 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 07/31/1999 |
| From: | GENERAL ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML20210C932 | List: |
| References | |
| SG-99-07-002, SG-99-07-002-R01, NUDOCS 9907270087 | |
| Download: ML20210C941 (57) | |
Text
p as a su SG-99-07-002 Revision 1 SOUTH TEXAS UNIT - 1 CYCLE 9 VOLTAGE-BASED REPAIR CRITERIA
~ '
90-DAY REPORT July 1999 6
Westinghouse Electric Company LLC Nuclear Services P.O. Box 158 Madison, Pennsylvania 15663-0158 9907270007 990723 PDR ADOCK 05000498 P
SG-99-07-002 Revision 1
.9 SOUTH TEXAS UNIT - 1 CYCLE 9 VOLTAGE-BASED REPAIR CRITERIA 90-DAY REPORT July 1999
i Table of Contents Page No.
I 1.0 Introduction 1-1 2.0 Summary and Conclusions 2-1 1
3.0 EOC-8 Inspection Results and Voltage Growth Rates 3-1 3.1 EOC-8 Inspection Results 3-1 3.2 Voltage Growth Rates 3-2 3.3 NDE Uncertainties 3-4 3.4 Probability of Prior Cycle Detection (POPCD) 3-4 3.5 Assessment of RPC Confirmation Rates 3-6 3.6 Probe Wear criteria 3-7 4.0 Database Applied for Leak and Burst Correlations 4-1 5.0 SLB Analysis Methods 5-1 6.0 Bobbin Voltage Distributions 6-1 6.1 Calculation of Voltage Distributions 6-1 6.2 Probability Of Detection (POD) 6-2 6.3 Limiting Growth Rate Distribution 6-2 6.4 Cycle Operating Period 6-2 6.5 Projected EOC-8 Voltage Distributions 6-3 6.6 Comparison of Actual and Projected EOC-8 Voltage Distributions 6-3 j
7.0 SLB Leak Rate and Tube Burst Probability Analyses 7-1 7.1 Leak Rate and Tube Burst Probability for EOC-8 (Condition 7-1 Monitoring Assessment) 7.2 Leak Rate and Tube Burst Probability for EOC-8 (Operational 7-2 Assessment) 8.0 References 8-1 Q:\\epe\\tgx99\\EOC990d. doc i
y South Texas Unit - 1 Cycle 9 Voltage-Based Repair Criteria 90-Day Report 1.0 Introduction This report provides a summary of the South Texas Unit-1 steam generator bobbin and rotating pancake coil (RPC) probe data for tube support plate (TSP) intersections from the end of Cycle 8 (EOC-8) inspection, together with postulated
)
Steam Line Break (SLB) leak rate and tube burst probability analysis results. The results support continued implementation of the 1.0 volt voltage-based repair criteria for Cycle 9 as outlined in the NRC Generic Letter 95-05 (Reference 8-1).
Information required by the Generic Letter is provided in this report including SLB leak rates and tube burst probabilities calculated using the EOC conditions for the recently completed cycle (Cycle 8) and projection of bobbin voltage 41stributions, leak rates and burst probabilities for the ongoing cycle (Cycle 9).
This will be the last report to support 1-volt repair criteria for Unit-1 as the steam generators will be replaced at the next refueling outage.
Analyses for Cycle 8 were carried out using the actual bobbin voltage distributions measured during the EOC-8 outage and the results compared with corresponding values from projections performed based on the last inspection (EOC-7) bobbin voltage data. The methodology used in these evaluations is in accordance with that utilized for the analyses performed during the last outage (Reference 8-2),
which is consistent with the Westinghouse generic methodology presented in Reference 8-3.
Analyses were also performed to project leak rates and tube burst probabilities for postulated SLB conditions at the end of the ongoing cycle (Cycle 9) based on the 1.0 volt repair criteria. These analyses utilized bobbin voltage distributions measured during the recent (EOC-8) inspection and a limiting growth rate distribution from the last two inspections (EOC-7 and EOC-8 mspections).
Two other supplemental evaluations are also presented in this report. One of them examines probability of detection for the Cycle 7 inspection (probability of prior cycle detection - POPCD) and the other assesses the fraction of the indications that showed no degradation during RPC inspection in the 1997 EOC-7 inspection, were left in service at beginning of Cycle 8 (BOC-8), and were RPC confirmed in 1999 at EOC-8.
Q:\\ ape \\tgx99\\EoC890d. doc 1-1
2.0 Summary and Conclusions A total of 1552 indications were found in the EOC-8 inspection of which 61 indications were over 1 volt. Seventy-four indications were inspected with a RPC probe (including all 61 indications above 1 volt), and 43 were confirmed as flaws.
SG-C had the largest number ofindications among the four SGs with 614 bobbin indications, of which 25 were above 1.0 volt. All 25 indications above 1 volt in SG-C plus 6 other indications below 1 volt in SG-C were inspected by RPC and 23 were confirmed as flaws.
No ID indications at dented TSP intersections or circumferential indications at the TSP intersections or indications extending outside the TSP were found in this inspection. Mixed residual indications (MRIs) with a bobbin voltage above 1.5 volts were RPC inspected. An indication with a bobbin voltage less than 1 volt was detected in a MRI of 1.57 volts. The MRI RPC inspection plan was expanded to 20% of the MRIs between 1.3 to 1.5 volts, and no indication was detected in the expansion program.
SLB leak rate and tube burst probability analyses were performed for all 4 steam generators (SGs) based on their actual measured EOC-8 bobbin voltage distributions as well as the projected EOC-9 bobbin voltage distributions. With the exception of 2 indications, one each in SGs A and C, the actual measured EOC-8 bobbin voltages are all equal to or lower than the corresponding projections performed using the EOC-7 outage bobbin voltage data, for both a constant probability of detection (POD) of 0.6 and voltage-dependent POPCD. As noted in the last 90-day report (Reference 8-2), because the peak projected EOC-8 voltages were relatively small, occurrence of a few indications in the voltage range of 3 to 4 volts, resulting in the actuals exceeding their projections, is to be expected. Two such indications did occur during Cycle 8 (one each in SGs A and C). The actual number of indications detected in all SGs during the EOC-8 inspection are well beks the corresponding projections. The two indications in SGs A and C that exceed their respective maximum projected EOC-8 voltage account for about 90%
of the SLB leak rate and tube burst probability calculated for those SGs based on the actual measured voltages. Without the contribution of those two indications, SLB leak rate and tube burst probability values for SGs A and C based on the actual conditions are 50% to 80% below the corresponding projections based on a constant POD =0.6 as well as POPCD. The corresponding margins for SGs B and D, with all indications considered, are between 10% to 50%.
The absolute magnitude of the SLB leak rates and tube burst probabilities based on the actual EOC-8 voltages, including the contribution from the 2 relatively large indications, are small, and they are 1 to 2 orders of magnitude below their respective allowable limits.
The leak rate and tube burst probability projections at the EOC conditions for the current cycle (Cycle 9) are also well within their acceptable limits. The limiting SLB leak rate projected for the EOC-9 conditions using the NRC SER endorsed probability of detection of 0.6 is 0.036 gpm. This value is projected for SG-C that Q:\\ ape \\tgx99\\EOC890d. doc 2-1
had the largest number ofindications as well as the largest indication found in the EOC-8 inspection. The total SLB leak rate should include a leakage allowance for F* tubes. SG-C has only one F* tube '. eft in service for Cycle 9 (SGs B and D have the remaining two F* tubes left in service) and the leakage contribution from it is negligibly small (about 2.3x10+ gpm).
Thus, the limiting EOC-9 leak rate projected is over 2 orders of magnitude below the allowable leakage limit of 15.4 gpm (room temperature). The highest tube burst probability, 4.2x104, is also predicted for SG-C, and it is more than a factor 20 below the NRC reporting guideline of10-2, Probability of detection (POPCD) for EOC-7 inspection was assessed using EOC-7
)
and EOC-8 inspection data. The POPCD values for EOC-7 inspection are below the generic POPCD in the voltage range 0.6 to 1.5 volts. There were only a total of 8 indications in the voltage range 1.0 to 1.5 volts which is insufficient to provide a
, reliable POPCD distribution in that voltage range. With sparse data, a single missed indication can exaggerate the POPCD value. However, the POD values for the 0.6 to 1 volt range are low, but comparable to the 0.6 value specified in the GL 95-05 for tube integrity evaluations. Six of the 9 EOC-7 RPC NDF indications left in service during Cycle 8 were RPC tested during the EOC-8 inspection and 3 were confirmed. Including similar data from the previous inspections for both units,9 out 15 RPC NDF indications have been confirmed in the subsequent inspection thus yielding a confirmation rate of 60%. This database is still too small to recommend a confirmation rate for use in projection analyses. All RPC NDF indications are included in the EOC-9 projections presented in this report.
Q:\\ ape \\tgx99\\ EOC890d. doc 2-2 i
r 3.0 EOC-8 Inspection Results and Voltage Growth Rates 3.1 EOC-8 Inspection Results According to the guidance provided by the NRC Generic Letter 95-05, the EOC-8 inspection of the South Texas Unit-1 SGs consisted of a complete,100% eddy current (EC) bobbin probe examination of all tube support plate intersections in all four SGs. A 0.610 inch diameter probe was used for all hot and cold leg TSPs whe~ the voltage-based repair criteria were applied.
Subsequently, RPC l
examination was performed for all indications with an amplitude above 1 volt (61 indications, all on the hot leg side) plus 13 indications under 1 volt. Forty-three indications were confirmed as flaws. All cold leg indications had a bobbin voltage' below 1 volt. No ODSCC indications were found at the flow distribution baflie elevation. Only four indications were found in tubes near the wedge supports for which the voltage-based repair criteria do not apply, and they all had a bobbin voltage less than 0.5 volt. They were inspected with a RPC probe, and no degradation was detected.
There were no RPC circumferential indications at the TSPs and no RPC indications with potential ID phase angles. Also, no signal interference from copper deposits was found during this inspection. Nine single volumetric indications (SVI) were reported at the edges of TSP intersections. These TSP intersections were RPC inspected because they had a non-quantifiable bobbin signal (NQI, dent or MRI). The SVI signals were evaluated as part of the condition monitoring assessment. All are low voltage (s 0.8 bobbin volts, s 0.69
+ Point volts) and could not be clearly associated with tube degradation.
If assumed to be tube degradation, the signal responses would be more typical of shallow patches of axial ODSCC (with no structural or leakage implications) than wall thinning, such as MBMs, which have higher voltages. The indications could also be attributed to sludge / scale irregularities or small dings / dents. In either case, these indications can be ignored for the evaluations of this report.
Tubes containing these 9 SVI indications were repaired, i
A total of 59 dents with a bobbin voltage over 5 volts were found at TSPs in all 4 SGs combined. (Dents called within 0.5"in the topmost TSP and within 0.4" in the remaining TSPs are considered to be within TSP.)
All dented TSP intersections above 5 volts were inspected with a RPC probe in this inspection, and no degradation was found at those locations. Mixed residual indications (MRIs) with a bobbin voltage above 1.5 volts were RPC inspected. An indication with a bobbin voltage less than 1 volt was detected in a MRI of 1.57 volts. The MRI RPC inspcetion plan was expanded to 20% of the MRIs between 1.3 to 1.5 volts, and no indication was detected in the expansion program.
{
A summary of EC indications for all four SGs is shown on Table 3-1, which Q:\\npc\\tgx99\\EOc890idoc 3-1
tabulates the number of field bobbin indications, the number of those indications that were RPC inspected, the number of RPC confirmed inaications, and the l
l number ofindications removed from service due to tube repairs. The indications that remain active for Cycle 9 operation is the difference between the observed and the ones removed from service.
A total of 72 indications were removed from service of which 39 indications were repaired because of failing to meet the 1 volt repair critera. The remaining 33 indications were present in tubes repaired for non-ODSCC causes. No cold leg TSP ODSCC indications required repair.
Consistent with the 1 volt repair critera, indications with bobbin amplitude less than or equal 1.0 volt were not considered for removal from service, regardless of RPC data. A review of Table 3-1 indicates that more indications (a quantity of 566 indications, with 4 above 1.0 volt) were returned to service in SG-C than tlm other SGs, thereby it potentially will be the limiting SG at EOC-9 from the standpoint of SLB leak rate and tube
-burst probability.
Figure 3-1 shows the actual bobbin voltage distribution determined from the EOC-8 EC inspection; Figure 3-2 shows the population distribution of those EOC-8 indications removed from service due to tube repairs; Figure 3-3 chows the distribution for indications returned to service for Cycle 9. The distribution of EOC-8 indications as a function of support plate location is summarized in Table 3-2 and plotted in Figure 3-4. The data show a strong predisposition of ODSCC to occur in the first few hot leg TSPs (1379 out of 1552 indications occurred at hot leg intersections in the first three TSPs above the flow distribution baffle plate),
although the mechanism extended to higher TSPs. Only 15 indications were detected on the cold leg side.
This dist 'bution indicates the predominant temperature dependence of ODSCC at Souch Texas Unit-1, similar to that observed at other plants.
3.2 Voltage Growth Rates For projection of leak rates and tube burst probabilities at the end of Cycle 9 operation, voltage growth rates were developed from EOC-8 inspection data and a reevaluation of the EOC-7 inspection EC signals for the same indications.
Table 3-3 shows the cumulative probability distribution (CPDF) for growth rate in each South Texas Unit-1 steam generator during Cycle 8 (September '97 to April '99) on an EFPY basis, along with the corresponding Cycle 7 growth rate distributions. Cycle 8 growth data are also plotted in Figure 3-5.
The curve labelled ' cumulative' in Figure 3-5 represents averaged composite growth data from all four SCs.
Average growth rates for each SG during Cycle 8 are summarized in Table 3-4.
It is evident that the absolute magnitude of average voltage growth in all SGs is relatively small. Among the four steam generators, SG-B had a slightly larger Q:\\ ape \\tgx99\\EOC890d doc 3-2
I magnitude for average voltage growth during Cycle 8, and SG-C had the indication with the largest voltage growth (see Table 3-3). Steam generator B also had the largest average growth during Cycle 6, but SG-A had the largest average voltage growth rate during the last (EOC-7) inspection. The average growth rates over the entire voltage range for Cycle 8 vary between 10% and 20%
(of the BOC voltage) per EFPY, between SGs, with an overall average of 14.8%
per EFPY. The small average BOC voltages (< 0.4 volt) leads to the relatively large percentage growth even though the average growth (< 0.06 volt per EFPY) is very small. The average growth for indications greater than or equal to 0.75 volt is comparable to that for indications less than 0.75 volt (14.4% vs.14.8% per EFPY).
In the past, other plants with %" tube SGs have experienced growth rates that are dependent on the BOC voltage. To determine if South Texas Unit-1 exhibited a similar trend during Cycle 8, growth rate data for Cycle 8 was plotted against l
BOC voltage, and the resulting plot is shown in Figure 3-6. The growth in the indications experiencing the largest growth in SGs A and C appear to show a i
dependency on the BOC voltage. The large growth for those two indications may represent a random occurrence or could represent the beginning of a trend. The impact of growth dependency on BOC voltage relative to the EOC-9 leak rate and l
tube burst probability projection is addressed in Section 7.0.
Averaged composite voltage growth data from all four steam generators for the last three operating periods are summarized in Table 3-5. It is evident that the average growth rate during Cycle 8 is slightly smaller than that during Cycle 7.
j In Figure 3-7, the cumulative probability distribution (CPDF) for composite i
growth rate data from all SGs during Cycle 8 is compared with that for Cycle 7.
l Although the average growth rate for Cycle 8 is less than that for Cycle 7, growth l
values corresponding to cumulative probability over about 0.9 are slightly higher for Cycle 7. Also, the top 4 growth values for individual tubes in Cycle 8 are higher than the largest growth observed in Cycle 7. Therefore, Cycle 8 has a more limiting growth distribution since the larger tail of the growth distribution is more important than the average growth for tube integrity analyses.
The guidelines in Generic Letter 95-05 require the use of the more conservative growth rate distributions from the past two inspections for projecting EOC distributions for the next operating cycle. As noted above, Cycle 8 has the limiting growth distribution for the last two cycles, and it was used to develop EOC-9 predictions. Cycle 8 growth rates for SG C are slightly higher than the I
composite growth distribution, and it includes the largest growth for the cycle.
Per the Monte Carlo methodology described in Reference 8-3, SG-specific growth l
rates are to be used for SG C while the all SG composite growth rates should be applied for the remaining 3 SGs.
Table 3-6 lists the top 30 indications on the basis of Cycle 8 growth rates in l
l Q:\\ ape \\tgx99\\EOC890d. doc 3-3
descending order. Twenty-two of those indications were RPC confirmed and the remaining 8 were not inspected (all well below 1 volt). Nine of the 30 indications shown, including the two indications with the top 2 growth values, are new indications and EOC-7 voltages used to estimate growth rates for them were 1
obtained by revaluating the last inspection data.
3.3 NDE Uncertainties The NDE uncertainties applied for the Cycle 8 voltage distributions in the Monte Carlo analyses for leak rate and burst probabilities are the same as those previously reported in the South Texas Unit-1 voltage-based repair criteria report of Reference 8-2 and NRC Generic Letter 95-05 (Reference 8-1).
They are presented in Table 3-7 as well as graphically illustrated in Figure 3-8. The probe wear uncertainty has a standard deviation of 7.0 % about a mean of zero and has a cutoff at 15 % based on implementation of the probe wear standard. The analyst variability uncertainty has a standard deviation of 10.3% about a mean of zero with no cutoff. These NDE uncertainty distributions are included in the Monte Carlo analyses for SLB leak rates and tube burst probabilities based on the EOC-8 actual voltage distributions as well as for the EOC-9 projections.
3.4 Probability of Prior Cycle Detection (POPCD)
The inspection results at EOC-8 permit an evaluation of the probability of detection (POD) at the prior EOC-7 inspection. For voltage-based repair criteria applications, the important indications are those that could significantly contribute to EOC leakage or burst probability. These significant indications can be expected to be detected by bobbin and confirmed by RPC inspection. Thus, the population of interest for voltage-based repair criteria POD assessments is the EOC RPC confirmed indications that were detected or not detected at the prior inspection.
The probability of prior cycle detection (POPCD) for the EOC-7 inspection can then be defined as follows.
EOC-7 cycle reported
+ Indications confirmed indications confirmed by and repaired in EOC-7 RPC in EOC-8 inspection inspection POPCD =
l (EOC-7)
{ Numerator}
+
New indications RPC confirmed in EOC-8 inspection 1
POPCD is evaluated at the 1997 EOC-7 voltage values (from 1999 EOC-8 reevaluation for growth rate) since it is an EOC-7 POPCD assessment. The indications detected at EOC-7 that were RPC confirmed and plugged are included Q:\\apc\\tgx99\\EOC890d. doc 3-4 i
l as it can be expected that these indications would also have been detected and confirmed at EOC-8. It is also appropriate to include the plugged tubes for voltage-l based repair criteria applications since POD adjustments to define the BOC l
distribution are applied prior to reduction of the EOC indication distribution for plugged tubes.
It should be noted that the above POPCD definition includes all new EOC-8 indications not reported in the EOC-7 inspection. The new indications include EOC-7 indications present at detectable levels but not reported, indications present at EOC-7 below detectable levels and indications that initiated during Cycle 8.
Thus, this definition, by including newly initiated indications, differs from the traditional POD definition. Since the newly initiated indications are appropriate for voltage-based repair criteria applications, POPCD is an acceptable definition and eliminates the need to adjust the traditional POD for new indications.
The above definition for POPCD would be entirely appropriate if all EOC-8 indications were RPC inspected. Since only a fraction of bobbin indications are generally RPC inspected, POPCD could be distorted by using only the RPC inspected indications. Thus, a more appropriate POPCD estimate can be made by i
assuming that all bobbin indications not RPC inspected would have been RPC confirmed. This definition is applied only for the EOC-8 indications not RPC inspected since inclusion for the EOC-7 inspection could increase POPCD by
{
including indications on a tube plugged for non-ODSCC causes which could be RPC NDF indications. In addition, the objective of using RPC confirmation for POPCD is to distinguish detection ofindication at EOCo.1 that could contribute to burst at EOCo so that the emphasis is on EOCo RPC confirmation. This POPCD can be obtained by replacing the EOC-8 RPC confirmed by RPC confirmed plus not RPC
{
inspected in the above definition of POPCD.
For this report, both POPCD 4
definitions are evaluated for South Texas Unit-1.
The POPCD evaluation for the 1997 EOC-7 inspection data is summarized in Table 3-8 and illustrated on Figure 3-9. As seen from Table 3-8, only a limited amount of data is available for RPC confirmed indications from EOC-7 inspection and it is insufficient to define a POPCD distribution based on the RPC confirmed indications only. Therefore, only results based on RPC confirmed plus not RPC inspected indications are shown in Figure 3-9.
A generic POPCD distribution developed by analyses of 18 inspections in 10 plants and presented in Table 7-4 of Reference 8-4 is also shown in the figure. It is seen from Figure 3-9 that the POPCD values for South Texas Unit-1 are below the generic POPCD in the voltage range 0.6 to 1.5 volts. There were only a total of 8 indications in the voltage range 1.0 to 1.5 volts in the EOC-7 inspection which is insuflicient to provide a POPCD distribution in that voltage range. With sparse data, a single missed indication can exaggerate the POPCD value. However, the POD values for the 0.6 to 1 volt range are low, but comparable to the 0.6 value specified in the GL 95-05 for tube Q.\\apc \\tgx99\\ EOC890d. doc 3-5
r integrity evaluations.
l 3.5 Assessment of RPC Confirmation Rates This section tracks the 1997 EOC-7 indications were left in service at BOC-8 relative to RPC inspection results in 1999 at EOC-8. The composite results for all SGs are given in Table 3-9.
For EOC-7 bobbin indications left in service, the s
. indications are tracked relative to EOC-7 RPC confirmed, EOC-7 RPC NDD, EOC-1 l
- 7 bobbin indications not RPC inspected, and EOC-7 bobbin indications with no indication found in EOC-8. Also included are new EOC-8 indications. The table shows, for each category of indications, the number of indications RPC inspected
)
I and RPC confirmed in EOC-8, as well as the percentage of RPC confirmed I
I indications. A total of 74 EOC-7 bobbin indications were called NDD in the EOC-8 inspection.
..s.
Of the 9 EOC-7 RPC NDF indications left in service at BOC-8,6 were RPC tested i
during the EOC-8 inspection and 3 were confirmed. Therefore RPC confirmation rate for prior RPC NDF indications is 50%. Including the similar data from the pmvious inspections for both units,9 out 15 NDF indications have been confirmed in the subsequent inspection, thus yielding a confirmation rate of 60%. It has been recommended by industry that the largest RPC NDF confirmation rates over the prior two cycles be used for projections. However, the database for STP SGs is still too small to recommend a confirmation rate.
All RPC NDF indications are l
included in the EOC-9 projections presented in this report.
S.6 Probe Wear Criteria 1
l An alternate probe wear criterion approved by the NRC (Reference 8-5) was l
applied during the EOC-8 inspection. When a probe does not pass the 15% wear i
limit, this alternate criteria requires that only tubes with indications above 75% of l
- the repair limit since the last successful probe wear check be reinspected with a good probe. As the repair limit is 1 volt, all tubes containing indications for which worn probe voltage was above 0.75 volts were inspected with a new probe. An L
evaluation of worn probe and new probe data is presented in the following
)
paragraphs.
In accordance with the guidance provided in Reference 8-5, voltages measured with a worn probe and a new probe at the same location were analyzed to ensure that i
the voltages measured with worn probes are within 15% of the new probe voltages.
No new large indications were detected with new probes; thus, worn probes did not miss significant indications. Figure 3-10 shows plots of the worn probe voltages plotted against the new probe voltages for all four SGs. SGs A, B and D had only a
)
i limited number of indications in the tubes retested for probe wear (5, 4 and 7, respectively), and therefore, their data are combined in the plot at the top of Figure i
l Q:\\ ape \\tgx99\\EOC890d. doc 3-6
{
l l
3-10. SG-C had 24 indications in the tubes retested and data for those indications are plotted separately. The data in Figure 3-10 show a consistent relationship between the two voltages. Composite data from all 4 SGs are plotted in Figure 3-
- 11. Also shown in Figure 3-11 as a solid line is a linear regression for the data and I
dashed lines representing tolerance limits that bound 90% of the population at 95%
)
confidence. The mean regression line has about 430 slope indicating that, on the average both the worn probe and new probe yielded essentially the same voltage.
l The dotted horizontal line at 0.75 worn probe volts demarcates indications l
requiring retest from those that do not. The shaded area at the bottom shows the l
region where a tube requiring repair may be left in service because of probe wear.
In the South Texas Unit-1 EOC-8 inspection, there are no occurrences for which a worn probe was less than 0.75 volts and the new probe voltage exceeded the plugging limit, i.e., no pluggable tubes were missed due to probe wear l
considerations.
l
-Among the indications requiring retesting (worn probe volts > 0.75 volts), only 3 indications fall outside the 90%/95% tolerance limit bands.
For all those 3 l
indications, the worn probe voltages are higher than the corresponding new probe voltages, i.e., the worn probe voltages are conservative. No indications lie below the lower 90%/95% tolerance band. Therefore, data for the 3 indications outside the 90%/95% tolerance band are acceptable.
Overall, it is concluded that the criteria to retest tubes with worn probe voltages above 75% of the repair limit are adequate. The alternate probe wear criteria used in the EOC-8 inspection is consistent with the NRC guidance provided in Reference 8-5.
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T' Table 3-6 South Texas Unit 1 April 1999 Summary of Largest Voltage Growth Rates for BOC-8 to EOC-8 Steam Generator Bobbin Voltage RPC New SG Row Col Elevation EOC BOC Growth Confirmed ?
Indication ?
C 38 35 0311 4.5 1.36 3.14 _
Y Y
A 19 99 02H 3.75 0.76 2.99 Y
Y C
27 27 02H 1.98 0.55 1.43 Y
N C
15 29 02H 2.11 0.77 _
l.34 Y
N
, A 21 32 02H 1.46 0.27 1.19 Y
Y C
16 9
03H 1.3 0.29 1.01 Y
N D
24 38 02H 1.42 0.45 0.97 Y
N C
24 95 0211 1.46 0.6 0.86 Y
N D
15 26 02H 1.21 0.36 0.85 Y
N D
24 40 02H 1.29 0.45_,
0.84 Y
N B
22 32 0211 1.18 0.39 0.79 Y
N C
16 9
02H 1.38 0.68 0.7 Y
Y C
29 77 0311 1.1 0.45 0.65 Y
Y B
35 55 05H 1.43 0.79 0.64 Y
Y C
42 50 02H 0.83 0.19 0.64 N
N B
23 84 02H 1.11 0.5 0.61 Y
N C_
42 71 0411 1.51 0.93 0.58 Y
N C_
41 40 02H 1.03 0.46 0.57 Y
Y B
41 57 0411 0.87 0.32 0.55 N
Y C
23 102 0211 0.94 0.39 0.55 N
N B
21 26 02H 1.22 0.69 0.53 Y
N C
14 92 0211 1.36 0.83 0.53 Y
Y C
44 57 0411 1.02 0.49 0.53 Y
N C
43 63 04H 0.88 0.36
__0.52 N
N C
31 87 02H 1.17 0.67 0.5 Y
N B
21 104 _.
0211 _ _
0.94 0.46
_ _0 48. _
N N
C 25 22 0211 0.82 0.34 0.48 N
N C
37 _
81 02H 1.02 0.55 0.47 Y
N C
18 12 02H 0.99 0.53 0.46 N
N C
29 101 02H 0.62 0.16 0.46 N
N Growth Tabic 5 6/15/99 4 52 N 3-15
E Table 3-7 Probe Wear and Analyst Variability - Tabulated Values I
Analyst Variability Probe Wear Variability Std. Dev = 10.3% Mean = 0.0%
Std. Dev = 7.0%
Mean = 0.0%
No Cutoff Cutoff at +/- 15%
Value Cumul. Prob.
Value Cumul. Prob.
-40.0%
0.00005
< -15.0%
0.00000
-38.0%
0.00011
-15.0% __
0.01606
-36.0%
0.00024
-14.0%
0.02275 ~
-34.0o5 0.00048
-13.0%
0.03165
-32.0%
0.00095
-12.0%
0.04324
-30.0%
0.00179
-11.0%
0.05804
-28.0% _
0.00328
-10.0%
0.07656
-26.0%
0.00580
~~ -9.0%
0.09927
-24.0%
0.00990
-8.0%
0.12655
-22.0%
0.01634
-7.0%
0.15866
-201)5 ~
0.02608
-6.0%
0.19568
-18.0%
0.04027
-5.0%
0.23753
- 16.0%
0.06016
-4.0%
0.28385
-14.0%._
__ 08704
-3.0%
0.33412
~
0.
- 12.0%
0.12200
-2.0%
0.38755
-10.0%
0.16581
-1.0%
0.44320
-8.0%
0.21867 0.0%
0.50000
-6.0%
0.28011 1.0%
0.55680
-4.0%
0.34888 2.0%
0.61245
-2.0%
0.42302 3.0%
0.66588 0.0%
0.50000 4.0%
0.71615 2.0%
0.57698 5.0%
0.76247 4.0%
~~ 0.65112
~~
' 6.0 %
0.80432 6.0%
0.71989 7.0%
0.84134 8.0%
0.79133 8.0%
0.87345 10.0 %
0.83419 9.0%
0.90073 12.0 %
0.87800 10.0 %
0.92344 14.0 %
0.91296 11.0%
0.94196 16.0 %
_ _ 93984 12.0 %
0.95676 0.
18.0 %
20.0 % _
0.95973 13.0 %
0.96835 22.0 %
~
0.98366
~
0.97725
~
0.97392 14.0 %
15.0 %
0.98394 24.0 %
0.99010
> 15.0%
1.00000 26.0 %
0.99420 28.0 %
0.99672 30.0 %
_.. _ _0.99821 32.0 %
0.99905 34.0 %
0.99952 36.0 %
0.99976 38.0 %
0.99989 40.0%___ _0.99995 NDEuncert Table 3-7 6/15/99 5 09 PM 3-16
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4.0 Database Applied for Leak and Burst Correlations The database used for the leak and burst correlations that are applied in the analyses of this report is the same as the voltage-based repair criteria database for %" tubes approved recently by the NRC (Reference 8-6), and it is documented in Reference 8-4. Plant S pulled tube indication R28C41 is included in the leak rate correlation at a revised SLB leak rate of 1250 lph consistent with Reference 8-6. Leak rate data for Model Boiler specimens 598-3 and 604-2 are excluded from the repair criteria database based on application of EPRI data exclusion Criterion 3a which permits exclusion of leak rate data that lie below the one-sided, 99-percent statistical confidence intervals of the mean regression line relating leak rate to both throughwall crack length and bobbin coil voltage.
South Texas pulled tube data from 1993 and 1995 inspections are included in the database applied. Data from 3 TSP indications in tubes pulled from the South Texas Unit-2 SGs during the EOC-6 outage are not yet included in the EPRI database. The database update to include this Unit-2 data has been completed, but the updated database has not been reviewed and approved by the NRC.
However, an evaluation of the effects of adding the new South Texas Unit-2 data to the reference database in Reference 8-4 indicates that the burst pressure, leak rate and the probability of leak correlations to the common logarithm of the bobbin amplitude would not be significantly changed.
The database for %" tubes meets the NRC requirement that the p value obtained from the regression analysis ofleak rate be less than or equal to 5%. Therefore, a SLB leak rate versus voltage correlation is applied for the leak rate analyses of this report. The following are the correlations for burst pressure, probability of leakage and leak rate used in this report (Reference 8-4).
Burst Pressure (ksi) 7.4282 - 2.9121 x log (Volts)
=
Probability of Leak
=
-( 5.2246 -8.8034 x log (volts))
s 10 Leak Rate (I/hr)
=
The upper voltage repair limit applied at the EOC-8 inspection, documented in Reference 8-7, was developed using the NRC-approved database presented in Reference 8-4. The structural limit (Ve) for the TSP indications established using 1.43 times the SLB AP of 2407 psid is 5.8 volts. Per Gencric Letter 95-05, allowance for voltage growth was taken to be the larger of 30%/EFPY or the largest single SG growth from the two previous inspections. Growth rates for all Q.\\ ape \\tgx99\\Eoc890d doc 4-1
r SGs during Cycles 6 and 7 were less than 30%/EFPY; therefore, the upper repair I,*mit for the EOC-8 inspection was determined using 30%/EFPY growth rate.
This is consistent with the individual SG growth data for Cycle 8 which are well below 30%/EFPY. For an expected Cycle 9 duration of 0.82 EFPY (301 EFPD),
the growth allowance becomes 25%. The allowance for NDE uncertainty is 20%
per Generic Letter 95-05. The upper voltage repair limit for TSP indications then becomes 4.0 volts. This value was applied at the EOC-8 inspection to assure that indications exceeding these limits were repaired independent of RPC confirmation.
l I
i l
l l
l Q \\apc\\tgx99\\EOC890d doc 4-2
i 5.0 SLB Analysis Methods Monte Carlo analyses are used to calculate SLB leak rates and tube burst probabilities for both actual EOC-8 and projected EOC-9 voltage distributions. The Monte Carlo analyses account for parameter uncertainty.
The analysis methodology is described in the Westinghouse generic methods report of Reference l
8-3, and it is consistent with the methodology applied to leak and burst analyses l
performed during the EOC-7 outage. The licensing submittal for the Unit-1 voltage-based repair criteria referenced Revision 0 of Reference 8-3 for methodology. Revision 1 incorporates NRC comments on Revision 0 and thus is the current NRC approved methodology.
In general, the methodology involves application of correlations for burst pressure, probability ofleak and leak rate to a measured or calculated EOC distribution to estimate the likelihood of tube burst and primary-to-secondary leakage during a postulated SLB event. NDE uncertainties and uncertainties associated with burst pressure, leak rate probability and leak rate correlations are explicitly included by considering many thousands of voltage distributions through a Monte Carlo sampling process. The voltage distributions used in the projection analyses for the next operating cycle are obtained by applying growth data to the BOC distribution.
The BOC voltage distributions include an adjustment for detection uncertainty and occurrence of new indications, in addition to the adjustments for NDE uncertainties. Comparisons of projected EOC voltage distributions with actual distributions after a cycle of operation have shown that the Monte Carlo analysis technique yields conservative estimates for EOC voltage distributions and as well as leak and burst results based on those distributions. Equation 3.5 in Reference 8-3 was used to determine the true BOC voltage.
Q:\\ ape \\tgx99\\ EoC890d. doc 5-1
I I
i 6.0 Bobbin Voltage Distributions This section describes the salient input data used to calculate EOC bobbin voltage distributions and presents results of calculations to project EOC-9 voltage distributions. Also, EOC-8 voltage projections performed during the last outage based on the EOC-7 inspection bobbin voltage data are compared with the actual bobbin distributions from the current inspection.
6.1 Calculation of Voltage Distributions The analysis for EOC voltage distribution starts with a cycle initial voltage distribution which is projected to the end of cycle conditions based on the growth rate and the anticipated cycle operating period.
The number of indications assumed in the analysis to project EOC voltage dintributions, and to perform tube leak rate and burst probability analyses, is obtained by adjusting the number of reported indications to account for detection uncertainty and birth of new indications over the projection period. This is accomplished by using a POD factor, which is defined as the ratio of the actual number ofindications detected to total number ofindications present. A conservative value is assigned to POD based on I
historic data, and the value used herein is discussed in the next section (Section 6-2). The calculation of projected bobbin voltage frequency distribution is based on a net total number ofindications returned to service, defined as follows.
NTet aTs = Ni/ POD - Nrepain.d + Ndeplugged
- where, Nrot ars =
Number of bobbin indications being returned to service for the next cycle, Ni Number of bobbin indications (in tubes in service)
=
identified after the previous cycle, POD Probability of detection,
=
Nrepairea Number of Ni which are repaired (plugged) after the last
=
- cycle, Ndepiugged Number ofindications in tubes deplugged after the last
=
cycle and returned to service in accordance with voltage-based repair criteria.
There are no deplugged tubes returned to service at BOC-8; therefore, Ndeplugged = 0.
The methodology used in the projection of bobbin voltage frequency predictions is described in Reference 8-3, and it is same as that used in performing EOC-8 predictions during the last (EOC-7) outage (Reference 8-2). Salient input data Q:\\ ape \\tgx99\\ EOC890d doc 6-1
j r
i used for projecting EOC-9 bobbin voltage frequency are further discussed below.
6.2 Probability of Detection (POD) l l
The Generic Letter 95-05 (Reference 8-1) requires the application of a constant POD value of 0.6 to define the BOC distribution for EOC voltage projections, unless an alternate POD is approved by the NRC. A POD value of 1.0 represents the ideal situation where all indications are detected. A altage-dependent POD may provide a more accurate prediction of voltage distributions consistent with voltage-based repair criteria experience.
In this report both NRC mandated constant POD of 0.6 as well as a voltage-dependent POD developed for EPRI (POPCD) are used. The EPRI POPCD is developed by analyses of 18 inspections in 9 plants and is presented in Table 7-4 of Reference 8-4. The POPCD values applied represent lower 95% confidence bound, and their distribution is graphically illustrated in Figure 6-1.
6.3 Limiting Growth Rate Distribution The NRC guidelines in Generic Letter 95-05 stipulate that the more conservative growth rate distributions from the past two inspections should be utilized for projecting EOC distributions for the next cycle. As discussed in Section 3.2, Cycle 8 has a more limiting growth distribution for the last two cycles, and it was used to develop EOC-9 predictions. Cycle 8 growth rates for SG C are slightly higher than the composite growth distribution and, per the methodology described in Reference 8-3, SG-specific growth rates are to be used for SG C while the all SG composite growth rates should be applied for SGs A, B and D.
Since the largest growth during Cycle 8 occurred in SG-C, on an EFPY basis, a hybrid distribution composed of SG-C data plus the largest growth in SG-A was applied to SG-C as the largest growth for the cycle can occur randomly in any SG.
6.4 Cycle Operating Period The operating periods used in the growth rate /EFPY calculations and voltage projections are as follows.
Cycle 8 BOC-8 to EOC-8
- 525 EFPD or 1.44 EFPY(actual)
Cycle 9
- BOC-9 to EOC-9 333 EFPD or 0.91 EFPY(estimated)
The projected Cycle 9 duration shown above (333 EFPD) is slightly higher than l
that assumed in Reference 8-7 (301 EFPD) to calculate the upper repair limit.
This Cycle 9 duration is the maximum cycle length considered in the fuel design, and therefore it was conservatively used to project SLB leak rate and tube burst probability for the EOC-9 condition.
Q:\\apc \\tgx99\\Eoc890d. doc 6-2 i
t 6.5 Projected EOC-9 Voltage Distribution Calculations for EOC-9 bobbin voltage projections were performed for all four SGs based on the EOC-8 distributions shown in Table 6-1. The BOC distributions were adjusted to account for probability of detection as described above, and the adjusted number ofindications at BOC-9 are also shown in Table 6-1. Calculations l
were performed using a constant POD of 0.6 as well as the EPRI POPCD distribution (shown in Figure 6-1). The larger growth rates for the last two cycles of operation, which are the EOC-8 growth rates shown in Table 3-3, were applied.
l The EOC-9 voltage distributions thus projected for all four SGs are summarized on Table 6-2.
These results are also shown graphically on Figures 6-2 to 6-5.
In 3
general, results based on a constant POD of 0.6 are more conservative than those using the voltage-dependent EPRI POPCD.
-6.6 Comparison of Actual and Projected EOC-8 Voltage Distributions i
Table 6-3, and Figures 6-6 and 6-7 provide a comparison of the EOC-8 actual measured bobbin voltage distributions with the corresponding projections performed using the last (EOC-7) inspection bobbin voltage data.
EOC-8 projections based on a constant POD of 0.6 as well as the EPRI POPCD distribution are shown. As reported in Reference 8-2, SG-C was projected to have the highest number ofindications and it was confirmed to have the highest number ofindications as well as the largest indication measured in the EOC-8 inspection.
Total number ofindications as well as the number ofindications over 1 volt found in the EOC-8 inspection for all SGs are well below their projections based on both POD =0.6 and POPCD. With the exception of the largest indications in SGs A and C, the actual peak voltages measured for all SGs are equal to or less than their projected value. Because the peak projected EOC-8 voltages were relatively small, cecurrence of a few indications in the modest voltage range of 3 to 4 volts, resulting in the actuals exceeding their projections, is to be expected. This was noted in the last 90-day report (Reference 9-2).
A comparison of the actual and projected voltage distributions in Figures 6-6 and 6-7 show that in general the indication population is overpredicted with both POD =0.6 and POPCD. Above 1 volt, predictions based on the voltage-dependent POPCD are more realistic, although still conservative, in comparison to those obtained with a constant POD of 0.6. Below 0.5 volt, the results for POD =0.6 are i
non-conservative, and POPCD results provide a better representation of the actual I
voltage distributions since POPCD values are less than 0.6 in that voltage interval.
Q \\apc\\tgx99\\EOc890d. doc 6-3
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Table 6-2 South Texas Unit 1 April 1999 Voltage Distribution Projection for EOC - 9 l
Steam Generator A Steam Generator B Steam Generator C Steam Generator D Voltage Projected NumberofIndications at EOC-9 in P
POPCD OND O6 06 06 0.1 1.05 2.12 0.97 1.97 3.20 6.06 1.84 3.44 0.2 14.84 24.24 1221 19.93 52.28 85.54 32.85 53.74 0.3 56.42 78.92 43.28 60.77 144.36 209.88 95.58 138.72 0.4 96.19 118.93 70.80 87.66 185.30 235.93 132.04 165.54 0.5 97.57 108.08 74.30 81.56 160.08 181.51 122.80 139.37 0.6 75.30 75.55 61.52 61.15 122.82 124.33 98.96 97.82 0.7 50.02 46.05 41.14 37.83 93.05 85.19 71.50 66.17 0.8x 3226 27.58 26.46 22.61 67.46 56.97 47.21 40.27 0.9 20.40 16.51 17.44 14.04 4823 38.22 27.74 23.55 1.0 12.07 9.38 11.39 8.75 34.47 25.83 15.51 12.07 1.1 7.01 525 7.37 5.40 23.76 17.12 9.55 7.12 1.2 4.31 3.08 4.65 3.19 15.10 10.38 6.86 4.78 1.3 2.88 1.92 2.91 1,81 8.74 5.58 524 3.34 1.4 2.08 1.27 1.88 1.06 4.84 2.75 3.88 229 1.5 1.51 0.83 125 0.67 2.76 1.35 2.69 1.49 1.6 1.06 0.52 0.82 0.43 1.63 0.68 1.77 0.93 1.7 0.72 0.33 0.51 0.12 0.98 0.36 1.14 0.59 1.8 0.51 0.24 0.09 0.00 0.62 0.20 0.76 0.41 1.9 0.41 021 0.00 0.70 0.42 0.12 0.53 0.30 2.0 0.36 0.20 0.70 0.00 0.33 0.09 0.38 022 2.1 0.33 0.22 0.00 0.00 0.47 0.39 0.30 024 2.2 0.34 0.11 0.00 0.00 0.79 0.85 0.19 0.12 2.3 0.33 0.00 0.00 0.00 0.86 0.86 0.00 0.00 2.4 026 0.70 0.00 0.30 0.65 0.57 0.70 0.70 2.5 0.10 0.00 0.30 0
0.46 0.02 0.00 0.00 2.6 0.00 0.30 0
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0 0
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TOTAL 479.33 522.54 379.99 409.95 975.35 1091.78 680.32 763.52
>1V 23.21 15.18 20.48 13.68 64.10 42.32 34.29 22.83
>2V 2.36 1.33 0.30 0.30 4.92 3.69 1.49 1.36 i
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Figure 6 2 South Texas Unit 1 SG-A Predicted Bobbin Voltage Distribution for Cycle 9 Combined Data for Ilot and Cold Leg Indications POD = 0.6 120 100 O B O C-9 e 80 E
5 Pred EOC-9 3e y go.
o Z 4o.
20
~-
0
^
~ ~ - " -
Bobbin Voltage EPRI POPCD 160 140 O B O C-9 120 e
jotoo 3 Pred E O C-9 lii M
y go.
o 60 3Z go 20
"^
^
^-
0 Bobbin Voltage G-8
_ w... ~
Figure G-3 South Texas Unit 1 SG-B Predicted Bobbin Voltage Distribution for Cycle 9 Combined Data for IIot and Cold Leg Indications POD = 0.6
~
so
~
O BOC-9 eo
- 50 5 Prod EOC-9 i
4o ;
$o 3
20 10 N^
0 01 02 03 04 0.5 06 07 08 09 10 1.1 1.2 13 14 15 16 17 18 20 2.5 l
Bobbin Voltage EPHIPOPCD 120 C BOC-9 5 Pred EOC-9 3
5
- s 20 l b---- -- ------ ---
m O
0.1 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 19 2.4 Bobbin Voltage evues as pu i
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Figure 6 4 South Texas Unit 1 SG-C Predicted Bobbin Voltage Distribution for Cycle 9 Combined Data for Hot and Cold Leg Indications POD = 0.6 250 200 l
O BOC-9 150 5 Pred EOC-9 o
100-1.
So 0
Bobbin Voltage EPRI POPCD 350 300 -
O BOC-9 250 E
8
~
~
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o 150-gon.
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Bobbin Voltage a-G 10
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f Figure 6 5 South Texas Unit 1 SG D Predicted Bobbin Voltage Distribution for Cycle 9 Combined Data for liot and Cold Leg Indications POD = 0.6 160 140 0 BOC-9 120 o 100 5 Pred EOC-9 li!
~
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60 40 -
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7.0 SLB Leak Rate and Tube Burst Probability Analyses This section presents results of analyses carried out to predict leak rates and tube burst probabilities for postulated SLB conditions using the actual voltage distributions from EOC-8 inspection as well as for the projected EOC-9 voltage distributions. The methodology used in these analyses is described in Section 5.0.
SG-C with the largest total number ofindications as well as indications over 1 volt is expected to yield the limiting SLB leek rate and burst probability for Cycle 9.
7.1 Leak Rate and Tube Burst Probability for EOC-8 (Condition Monitoring Assessment)
Condition monitoring analysis to estimate EOC-8 SLB leak rates and tube burst probabilities were performed using the actual bobbin voltage distributions presented in Table 6-1. Results of Monte Carlo calculations are summarized in Table 7-1. A comparison of the EOC-8 actuals in Table 7-1 with the corresponding predictions performed after the EOC-7 inspection, presented in Reference 8-2, indicates the following.
a)
Projection analysis performed at BOC-8 for the EOC-8 conditions using a POD of 0.6 predicted SG-C to have the largest SLB leak rate and SG-A the largest tube burst probability, although the difference between tube burst probability predicted for SGs A and C was very small (about 1.1x104).
Analysis based on actual EC bobbin measurements at EOC-8 show that SG-C has both the highest SLB leak rate and tube burst probability.
b)
For SGs A and C, the leak rate and tube b;rst probability calculated using the EOC-8 actual measured bobbin measurements exceed the corresponding projections based on the EOC-7 inspection data, whereas for SGs B and D the projected results exceed those based on the actual voltage data.
Underprediction of EOC-8 leak rate and burst probability for SGs A and C is due to occurrence of a single indication in each SG that exceed the maximum projected voltage. The largest indications in SGs A and C, which exceed the projected peak voltage, account for about 90% of the SLB leak rate and tube burst probability based on the actual measured voltages.
Without the contribution of those two indications, the SLB leak rate and tube burst probability values based on the actual conditions for all SGs are 10% to 80% below the corresponding projections based on a constant POD =0.6 as well as POPCD. Occurrence of 2 indications over the predicted maximum voltage is not unexpected and this possibility was discussed in the last 90-day report (Reference 8-2).
c)
Leak rate and tube burst probability predictions for all four SGs based upon Q \\ ape \\tgx99\\EOC890d. doc 7-1
EOC-8 actual bobbin measurements are well within the allowable limits.
In summary, with the exception of the largest indications in SGs A and C, the actual measured EOC-8 bobbin voltage distributions for all SGs are below the corresponding projections obtained using the NRC mandated probability of
' detection of 0.6. Limiting values for SLB leak rate (3.8 x 10 2 gpm) and tube burst probability (4.3 x 104) obtained using the actual measured voltages are 1 to 2 orders of magnitude below the allowable Cycle 8 SLB leakage limit of 15.4 gpm (room temperature) and the NRC reporting guideline of 102 for the tube burst probability.
7.2 Leak Rate and Tube Burst Probability for EOC-9 (Operational Assessment)
Calculations to project SLB leak rate and tube burst probability for all 4 SGs at the
-EOC-9 condition were carried out using two values for POD: 1) NRC required constant value of 0.6, 2) voltage dependent EPRI POPCD distribution.
The methodology used for these predictions is the same as previously described for analysis based in the EOC-8 actual voltages. Projected results for the EOC-9 conditions are summarized on Table 7-2. With a constant POD of 0.6, the limitia EOC-9 SLB leak rate projected is 3.4x10 2 gpm (room temperature) and the.
corresponding peak tube burst probability is 4.2x104; these values are predicted for SG-C which has the largest number ofindications returned to service for Cycle 9 operation. The total SLB leak rate should include a leakage allowance for F*
tubes. SG-C has only one F* tube left in service for Cycle 9 (SGs B and D have the remaining two F* tubes left in service) and the leakage contribution from it is negligibly small (about 2.3x104 gpm). Thus, the limiting EOC-9 SLB leak rate value (3.4x10 2 gpm) is over 2 orders of magnitude below the allowable Cycle 9 SLB leakage limit of 15.4 gpm (room temperature); the limiting tube burst probability is a factor of 20 below the NRC reporting guideline of 10 2, As noted in Section 3.2, Cycle 8 growth appears to show a dependency on the BOC voltage. Therefore, EOC-9 SLB leak rate and tube burst probability projection for the limiting SG (SG-C) was repeated by treating growth to be voltne dependent.
A methodology described in Reference 8-4 was used to input voltage-dependent growth data into the Monte Carlo analysis for leak rate and tube burst probability analysis. Since SG-A had the 2nd largest growth during Cycle 8, that value was added to the SG-C growth data to ensure the top 3 growth values were included in the growth data applied. The results for voltage-dependent growth and a constant POD of 0.6 are also shown in Table 7-2. Inclusion of growth dependency on the BOC voltage has only a modest effect on the EOC-9 prediction for SG-C (Limiting SG-C). The peak voltage predicted increased by 0.1 volt and SLB leak rate increased by 6%, which is negligible in comparison to the margin available to the allowable leak rate limit. A small reduction in the tube burst probability is Q \\ ape \\tgx99\\EOC890d. doc 7-2 i
l attributed to the differences in the random numbers used the Monte Carlo simulations.
With EPRI POPCD total number ofindications predicted are higher than those for POD =0.6. The reason for this is that below about 0.5 volt, the detection probability calculated from the EC inspection data could be significantly below 0.6 as shown by the EPRI POPCD distribution in Figure 6-1.
Nearly 80% percent of the indications returned to service for Cycle 9 operations are below 0.5 volt. However, SLB leak rate and burst probability based on EPRI POPCD for all SGs are less than those with POD =0.6.
In summary, SLB leak rates and tube burst probabilities predicted for EOC-9 are 1 to 3 orders of magnitude smaller than their respective limits.
l Q:\\apc\\tgx99\\EOC890d doc 7-3 L
Table 7-1 South Texas Unit-1 April 1999 EOC-8 Outage Summary of Calculations of Tube Leak Rate and Burst Probability Based on Actual Bobbin Voltage - 250k Simulations 4
stb Steam Number Max.
Burst Probability Leak j
Generator POD ofIndi.
Volts (2)
Rate cationsm 1 Tube 1 o*> More (gpm)(3)
Tubes EOC - 8 Projections Reported in Reference 8-2 A
0.6 335 2.3 9.4x10-5 9.4x10-5 3.2x10.a B
0.6 255 1.7 3.1x10-5 3.1x10-5 8.1x104 C
0.6 800 2.3 8.3x104 8.3x10-5 5.3x10-3 D
0.6 512 1.8 4.7x10-5 4.7x10-5 2.0x10-3 A
POPCD 408 2.2 7.5x10 5 7.5x10-5 2.4x10 3 B
POPCD 325 1.7 3.3x10-5 3.3x10-5 1.1x10 3 i
C POPCD 1022 2.2 7.6x10 5 7.6x10-5 4.2x10 3 D
POPCD 664 1.8 4.7x10-5 4.7x10-5 2.1x10-3 EOC-8 Actuals A
1 293 3.75 2.1 x 104 2.1 x 104 1.8x10 2 B
1 231 1,43 1.2 x 10-5 1.2 x 104 3.5x104 C
1 614 4.5 4.3 x 104 4.3 x 104 3.8x10 2 D
1 414 1.78 4.2 x 10-5 4.2 x 10-5 9.6x104 i
H9.tto (1) Number ofindications adjusted for POD.
(2) Voltages include NDE uncertainties from Monte Carlo analyses and exceed measured voltages.
(3) Equivalent volumetric rate at room temperature.
l Q:\\ ape \\tgx99\\EOC890d. doc 7-4 L
Table 7-2 South Texas Unit-1 April 1999 Outage Summary of Projected Tube Leak Rate and Burst Probability for EOC 9 - 250k Simulations SLB Steam No. of Max.
Burst Probability Generator POD Indic.
Volts (2)
Rate ationsm 1 Tube 1 or More (gpm)(3)
Tubes EOC-9 PROJECTIONS A
0.6 479 4.1 1.9x104 1.9x104 1.5x10-2 B
0.6 380 2.5 2.5x10-5 2.5x10 5 1.4x10-3 C
0.6 975 4.8 4.2x104 4.2x104 3.4x10-2 D
0.6 678 2.6 6.3x10-5 6.3x10 5 3.2x104 A
POPCD 523 2.6 3.1x10-5 3.1x10-5 2.0x104 B
POPCD 410 2.4 1.9 <10-5 1.9x104 1.1x10-3 C
POPCD 1092 2.8 7.Sx104 7.3x10-5 5.3x10-3 D
POPCD 760 2.6 5.3x10 5 5.8x104 2.4x10 3 EOC - 9 Projections Using Voltage-Dopendent Growth (o C
0.6 975 4.9 3.9x104 3.9x104 3.6x10-2 Hoks (1) Number ofindications adjusted for POD.
(2) Voltages include NDE uncertainties from Monte Carlo analyses and exceed measured voltages.
(3) Equivalent volumetric rate at room temperature.
(4) SG-C groveth data plus the largest growth in SG-A (to ensure the top 3 growth values from all SGs are included) are used per methodology described in Section 8.2 of Reference 8-4.
Q:\\apc\\tgx99\\EOC890d. doc 7-5
l 8.0 References 8-1 NRC Generic Letter 95-05, " Voltage-Based Repair Criteria for the Repair of Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking," USNRC Office of Nuclear Reactor Regulation, August 3,1995.
8-2 SG-97-12-006, " South Texas Unit-1 Cycle 8 Voltage-Based Repair Criteria 90-Day Report," Westinghouse Electric Corporation, December 1997.
8-3 WCAP-14277, Revision 1, "SLB Leak Rate and Tube Burst Probability Analysis Methods for ODSCC at TSP Intersections", Westinghouse Nuclear Services Division, December 1996.
8-4 EPRI Report NP 7480-L, Addendum 2, " Steam Generator Tubing Outside Diameter Stress Corrosion Cracking at Tube Support Plates Database for Alternate repair Limits," Electric Power Research Institute, April,1998.
8-5 Letter from B. W. Sheron, Nuclear Regulatory Commission, to A. Marion, Nuclear Energy Institute, dated February 9,1996.
86
" Evaluation of Proposed Update to SGDSM Database and Modifications to the Methodology to Assess Steam Generator Tubing Outside Diameter Stress Corrosion Cracking," G. C. Lainas (USNRC) to D. J. Modeen (NEI),
November 20,1998.
8-7 SG-99-02-002, " South Texas Unit 1, SG Degradation Assessment 1RE08 Refueling Outage," Westinghouse Electric Company, February 1999.
Q:\\apc\\tgx99\\EOC890d. doc 8-1