ML13093A013
| ML13093A013 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 04/01/2013 |
| From: | St.Onge R Southern California Edison Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| TAC ME9727 | |
| Download: ML13093A013 (25) | |
Text
Richard l. St. Onge I EDISON SOUTHERN CALIFORNIA An EDISON INTERNATIONAL" Company Director, Nuclear Regulatory Affairs and Emergency Planning April 1, 2013 10 CFR 50.4 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555-0001
Subject:
Docket No. 50-361 Response to Request for Additional Information (RAIs 68, 69, 70 and RAI 2 Revision 1) Regarding Confirmatory Action Letter Response (TAC No. ME 9727)
San Onofre Nuclear Generating Station, Unit 2
References:
- 1. Letter from Mr. Elmo E. Collins (USNRC) to Mr. Peter T. Dietrich (SCE), dated March 27, 2012, Confirmatory Action Letter 4-12-001, San Onofre Nuclear Generating Station, Units 2 and 3, Commitments to Address Steam Generator Tube Degradation
- 2. Letter from Mr. Peter T. Dietrich (SCE) to Mr. Elmo E. Collins (USNRC), dated October 3, 2012, Confirmatory Action Letter - Actions to Address Steam Generator Tube Degradation, San Onofre Nuclear Generating Station, Unit 2
- 3. Email from Mr. James R. Hall (USNRC) to Mr. Ryan Treadway (SCE), dated March 15, 2013, Request for Additional Information (RAIs 68-72) Regarding Response to Confirmatory Action Letter, San Onofre Nuclear Generating Station, Unit 2
- 4. Letter from Mr. Richard J. St. Onge (SCE) to Document Control Desk (USNRC), dated February 25, 2013, Response to Request for Additional Information (RAIs 2, 3 and 4) Regarding Confirmatory Action Letter Response, San Onofre Nuclear Generating Station, Unit 2
Dear Sir or Madam,
On March 27, 2012, the Nuclear Regulatory Commission (NRC) issued a Confirmatory Action Letter (CAL) (Reference 1) to Southern California Edison (SCE) describing actions that the NRC and SCE agreed would be completed to address issues identified in the steam generator tubes of San Onofre Nuclear Generating Station (SONGS) Units 2 and 3. In a letter to the NRC dated October 3, 2012 (Reference 2), SCE reported completion of the Unit 2 CAL actions and included a Return to Service Report (RTSR) that provided details of their completion.
By e-mail dated March 15, 2013 (Reference 3), the NRC issued Requests for Additional Information (RAIs) regarding the CAL response. Enclosure 1 of this letter provides the response to RAIs 68, 69 and 70. SCE provided the response to RAI 2 in a letter dated 1~7~~'e P.O. Box 128 San Clemente, CA 92672 AJ IL\a~-
Document Control Desk April 1, 2013 February 25, 2013 (Reference 4). Enclosure 1 of this letter provides Revision 1 to the RAI 2 response.
There are no new regulatory commitments contained in this letter. If you have any questions or require additional information, please call me at (949) 368-6240.
Sincerely,
Enclosure:
- 1. Response to RAIs 68, 69, 70 and RAI 2 Response, Revision 1 cc: A. T. Howell III, Regional Administrator, NRC Region IV J. R. Hall, NRC Project Manager, SONGS Units 2 and 3 G. G. Warnick, NRC Senior Resident Inspector, SONGS Units 2 and 3 R. E. Lantz, Branch Chief, Division of Reactor Projects, NRC Region IV
ENCLOSURE 1 SOUTHERN CALIFORNIA EDISON RESPONSE TO REQUEST FOR ADDITIONAL INFORMATION REGARDING RESPONSE TO CONFIRMATORY ACTION LETTER DOCKET NO. 50-361 TAC NO. ME 9727 Response to RAIs 68, 69, 70 and RAI 2 Response, Revision 1
RAI 68
Reference 1, Response to RAI 2 - Provide wear depth distributions for the following, for both Unit 3 SGs:
- Anti-vibration bar (AVB) wear depth distributions for the group of AVBs, B03 through B10, for tubes without tube-to-tube wear (TTW)
- AVB wear depth distributions for the group of AVBs, B03 through B1 0, for tubes with TTW
- AVB wear depth distribution for the group of AVBs, B01, B02, B1 1, B1 2, for tubes without TTW
- AVB wear depth distribution for the group of AVBs, B01, B02, B1 1, B1 2, for tubes with TTW
RESPONSE
Note: RAI Reference 1 is SCE's "Response to Request for Additional Information (RAls 2, 3, and 4) Regarding Confirmatory Action Letter Response," dated February 25, 2013.
The distributions of tube wear depths for Unit 3 are provided in the attached figures in the form of cumulative distribution functions, and wear pattern distribution plots with actual wear location counts.
Figure 1 is a schematic of the tube support configuration for reference purposes.
Cumulative distribution functions (CDF) of the requested wear depths are provided in Figures 2 through 4. Figure 2 shows the CDF for the anti-vibration bar (AVB) wear depths at the upper supports (B03 through B10) for tubes with and without TTW. Figure 3 shows the CDF of AVB wear depths at the lower supports (B01, B02, B1 1, and B132) for tubes with and without TTW. Figure 4 shows the CDF of tube support plate (TSP) wear depths at the uppermost support plate elevation (07C and 07H) for tubes with and without TTW.
Plots that show the wear patterns and counts of the AVB and TSP support locations are also provided in Figures 5 and 6. Figure 5 shows the wear pattern (depths and numbers of affected supports) for AVB wear for the two requested tube groups; i.e., tubes with and without TTW. This figure collectively includes the AVB locations from the cold-leg side (B12 to B7) to the hot-leg side (B06 to B01).
Figure 6 shows the wear pattern (depth and number of affected supports) for TSP wear for the two requested tube groups; i.e., tubes with and without TTW. This figure collectively shows the TSP locations from the cold-leg side (01(C to 07C) to the hot-leg side (07H to 01H).
Page 2 of 23
806 807 BOB BOB 07H uli I 43.66" 04H 06C II I ,.1 43.66' 43.66" I 04H- - 04C T T 43.66"I 03H 01H -
I 03C 05C
.1 I
II 43.66" 02H S 1 - 02C 01 HI01 TE I TE 43.66" f2 I ~4282" Figure 1 - SONGS Steam Generator Tube Support Structure Schematic Page 3 of 23
Unit 3 AVB Wear Depths - Upper Supports 1.0 0.9 0.8 U-0.7 C.
.2 4-0.6 C
g.0 0.5 m
I- 0.4 E
0.3 0.2 0.1 0.0 0 5 10 15 20 25 30 35 40 NDE Depth, (%TW)
Figure 2 - Wear Depths for Tube/AVB Contacts at the Upper Supports after 0.926 Years at Power Page 4 of 23
Unit 3 AVB Wear Depths - Lower Supports 1.0 0.9 0.8 U- 0.7 0
a C 0.6 U.
U-O 0.5 E
0.4 E
0.3 0.2 0.1 0.0 0 5 10 15 20 25 30 35 40 NDE Depth, (%TW)
Figure 3 - Wear Depths for Tube/AVB Contacts at the Lower Supports after 0.926 Years at Power Page 5 of 23
Unit 3 TSP Wear Depths - Upper Support Plates 1.0 0.9 0.8 Ua. 0.7 LL C,.)
C 0.6 0
E-Z C* 0.5 0
4-
.0 0.4 0.3 0.2 0.1 0.0 0 10 20 30 40 50 60 70 80 90 100 NDE Depth, (%TW)
Figure 4 - Wear Depths for Tube/TSP Contacts at 07C and 07H Elevation after 0.926 Years at Power Page 6 of 23
SONGS-3 AVB Wear Depths in High-Wear Region - Tubes with No TTW 50 Number of Wear Locations (E-088/E-089) 45 - 2/3 9/11 123/48 154/158 2361256 3601333 402/376 307/294 214/294 125/72 16/13 3/3
__ _1 _ _ _ I [
40 0 3E-088 *3E-089 I _
35 330 w 25 w
z 15 10
_ _, 1 5
0 B12 B6l BIO B09 B08 B07 B06 B05 B04 B03 B02 B01 AVB Location SONGS-3 AVB Wear Depths in High-Wear Region - Tubes with TTW 50 Number of Wear Locations (E-088/E-089) 45 114/83 115166 118/84 122J116 120/130 80/97 93/103 127/139 113/139 118/124 133/121 105/115 40 03E-088 *3E-089 35
-> - < --- 0
- 30 2.
__0 +/-~ ~ _ _ __
uJ2
__ t i' ; I_I JI z 15 10 I _
B12 B1l B10 B09 B08 B07 B06 B05 B04 B03 B02 B01 AVB Location Figure 5 - Unit 3 AVB Tube Supports for Tubes with and without TTW Page 7 of 23
SONGS-3 TSP Wear Depths in High-Wear Region - Tubes with No TTW 100 90 80 70 60
- 0) 50 40 z~
a 30 20 10 0
OIC 02C 03C 04C OSC 06C 07C 07H 06H OSH 04H 03H 02H 01H TSP Location SONGS-3 TSP Wear Depths in High-Wear Region - Tubes with TTW 10 0 I I I I I I i I I I I I Number of Wear Locations (E-088/E-089) 90 - 0133 11136 79)65 1121126 123/118 132/143 1361147 152/155 136/152 124/144 126/94 86/79 50/78 49164 80 0*3E-088 *3E-089 J _ 1 7*
60 - ............ ...
so 0.
0) 40 z 30 __F _
20 H 01C 02C 03C 04C 05C S 06C oH07C 07H 06H 04H 03H 02H 01H TSP Location Figure 6 - Unit 3 TSP Tube Supports for Tubes with and without TTW Page 8 of 23
RAI 69
Reference 1, Response to RAI 2 - There is a statement in the middle of page 3 of 18, "The initiation-time model for TTW uses a wear index based only on tube to AVB wear in the upper supports (803 through B10)." It is further stated on the same page (in item 3), "Thus the increase in AVB wear index after TTW is mainly due to the increase in locations with AVB wear, including wear locations at the lower supports (B01, B02, B131, and B132)." The second sentence appears inconsistent with the first (i.e., new wear index definition doesn't include wear from lower supports, yet wear at lower supports is causing wear index to increase). Please clarify the apparent inconsistency.
RESPONSE
Note: RAI Reference 1 is SCE's "Response to Request for Additional Information (RAIs 2, 3, and 4) Regarding Confirmatory Action Letter Response," dated February 25, 2013.
For the purpose of estimating the initiation time, it is shown below that the total wear index reduces to a wear index calculated from tube wear depths at the AVBs in the upper supports (803 through 810). It was the intent of the first statement from RAI Reference 1 to make this point. The initiation-time analysis is a special case where only the wear in the upper supports is actually required to establish the distribution of initiation times for each tube in Unit 3 that had TTW. The technical basis for this special analysis case was discussed in the RAI 2 response.
The tube support wear patterns that support the analysis conditions are provided in the response to RAI 68.
The second statement cited (in Item 3) at the bottom of Page 3 of RAI Reference 1 was provided in the context of the total wear index. The increase in the total wear index from the contribution of AVB wear after in-plane instability is the sum of three quantities:
a) continued tube wear at AVBs that were wearing prior to TTW initiation b) additional locations with support wear at the upper AVB supports c) additional support wear in the lower AVB supports (801, B02, 811, and B12)
To eliminate the apparent inconsistency, the response to RAI 2 has been revised to remove the phrase "including wear locations at the lower supports (801, B02, 811, and B12)" from Item 3.
The revised RAI 2 response is enclosed.
The initiation-time model assumes that the AVB wear at the lower supports and the wear at the TSPs are tube degradation modes that primarily occur after in-plane tube instability. This assumption is based on comparisons of the tube support wear patterns in the high-wear region for Unit 3 tubes with and without TTW. The response to RAI 68 provides plots of the wear patterns that show this support wear behavior and forms the technical basis of the variable initiation-time model assumptions.
Page 9 of 23
To provide additional explanation on the wear index and how it was developed for this analysis, the following derivation from the total wear index definition is provided. The total wear index at the end of the operating period (EOP) is defined as WIEoP = WIPRE + WIPOST (1) where WlEoP is the total wear index at EOP, and WIPRE and WIPOST are the contributions to the total wear index during the operating period from the time up to TTW initiation and the time after TTW initiation. From the assumptions made based on the observation of support wear patterns, the WIPRE is due to wear at AVB upper supports while the WIPOST is from the wear at AVB upper supports (US), lower supports (LS), and wear at TSPs. Therefore, for WIPRE,
=AiiPre WIPRE = AVBUS The above equation is valid because of the relatively minor contributions to the wear index from thewPrAVBs lower supports and at TSPs in the non-TTW population, so that at the\lPre WIAVBLS, and WITSP can be neglected in the analysis.
e The post-initiation total wear index is the sum of the wear indices from AVB upper and lower supports and the TSP supports, t~Post AIPOSt AIPOSt WIPOST = WIV'AVB-US + WIAVB-LS + WITTSP Summing all contributions to the total wear index gives,
= WPre +AiiPost +^WiPost + IIPOSt WIEoP = WVABUS + ,AVBUS + VIAVBLS + ,TSP (2)
Since the increase in the wear index due to tube wear at the AVB lower supports and TSPs is assumed to be post-initiation occurrences based on the observations of tubes with and without TTW, Eq. 2 can be re-written in terms of the wear index at the AVB upper supports only. For the purpose of establishing the initiation time, given the pre-and post-initiation observations, the total wear index is simplified to become,
\A/rPost _us +
ePost AIEOP WIEOP - WIAVBLS - WPTOSPt ABUS + AVB-US = WIAVB-US
\^n~ot \AIPOSt The quantities WIEoP, WlAVSBLS, and WITSP are known for each tube with TTW since as discussed previously, WVABLS, and are 7,TSP negligible based on the observed wear patterns.
Therefore, for the prediction of TTW initiation time, the AVB wear index in the upper supports provides the governing empirical equation:
EOP Pre Post WIAVBUS ... AVB-US + AVB-US (3)
Figure 1, provided in RAI 70's response, is a graphical illustration of Eq. 3.
Page 10 of 23
RAI 70
Reference 1, Response to RAI 2 - It is unclear to the staff how TTW initiation times were calculated (see description on pages 4 and 5 (of 18)). Describe each individual step, in sequential order, to calculating TTW initiation time for a given tube, for a given trial. Provide (or reference) in figure form all distributions that were sampled.
RESPONSE
Note: RAI Reference 1 is SONGS "Response to Request for Additional Information (RAIs 2, 3, and 4) Regarding Confirmatory Action Letter Response," February 25, 2013.
For the purpose of describing the simulation process, Figure 1 is provided to illustrate the increase in tube-AVB wear in the upper supports before and after tube instability in terms of the wear index. The analysis variables are also illustrated to aid in the description of the probabilistic solution for TTW initiation time. The important points on the wear index lines are labeled A through E.
Governing Equations The equation that defines the initiation time is M^nOP-I 2t Y tINIT = AVB-US WIR 2 tcYc (1)
WIR 1 -WIR 2 where EOP is the end of the operating period for Unit 3, and tINIT is the initiation time (Figure 1 Point B)
WEOUS is the wear index at EOP from AVB wear in the upper supports (Figure 1 Point D)
WIR 1 is the growth rate in the WI prior to initiation (slope of Line AB in Figure 1)
WIR 2 is the growth rate in the WI after initiation (slope of Line BD in Figure 1) tcYc is the length of the operating period (0.926 years at power)
Equation 1 is the solution for the intersection of the two lines (AC and ED) and defines the time in the operating period when the change in the upper support wear index slope due to in-plane instability occurs. Point B in Figure 1 represents the calculated initiation time as determined by the model.
Page 11 of 23
The growth in the wear indices in the AVB upper supports before in-plane instability (WIR 1 ) and after in-plane instability (WIR 2 ) are calculated from the following:
,= [WR~vB-us ]i (2)
WIR2 = -*N21 [^,PoL st 1 (3)
Lvvr AVB-Si wPre adWRPost where AVBUS and AVB-US are the cumulative distribution functions (CDF) for the AVB wear rates derived from wear depth data for tubes without TTW and with TTW. The variable N1 is the number of AVB upper-support locations in the tube before in-plane instability. The parameter N2 is the number of AVB upper-support locations in the tube after in-plane instability.
Overall Simulation Process A Monte Carlo simulation was performed to solve for tINIT using Eq. 1. From this simulation process, a distribution of initiation times for each tube with TTW was developed. Each steam generator (SG) was evaluated separately. After completion of 1000 trials for each tube with TTW, a distribution of initiation times was saved in an array. The data array was used to determine the median initiation time for each tube using the calculated initiation times, ranked from shortest to longest (order-statistics).
Analysis Input For tubes with TTW in Unit 3, the following input data are known on an individual tube basis and used in the simulation deterministically:
- Total number of AVB upper support locations having detected wear after 0.926 years at power (N2).
- Wear index due to AVB wear in upper supports after 0.926 years at power, EOP WIAVBUS (Figure 1 Point D).
The variables that are treated statistically:
- Number of AVB upper support locations before in-plane instability (Figure 2)
- AVB wear rates before and after in-plane instability (Figure 3)
Procedure to Calculate TTW Initiation Time For each trial in the simulation, the following steps were followed on a tube-by-tube basis:
1.0 Determination of N, 1.1 At the start of a trial, the number of AVBs is selected by a random pick from a Poisson distribution developed from the number of affected AVBs observed in tubes without TTW. The data histogram and corresponding CDF are as shown in Figure 2. Both Unit 3 steam generators have similar distributions and these data were combined to obtain the mean parameter for the Poisson distribution.
Page 12 of 23
1.2 If the number of AVB locations resulting from the random pick for N1 is equal to zero, or if N1 exceeds the number detected at EOP (N2), the cumulative distribution in Figure 2 is re-sampled until 0 < N1 < N2.
2.0 Determination of WIR, 2.1 The wear index growth rate before instability is calculated by summing the individual AVB wear rates using Eq. 2. The AVB wear rates are determined by N, random selections from the AVB wear rate distribution in Figure 3 (pre-initiation curve).
2.2 If the slope of the Line AC exceeds the slope of Line AD, the AVB wear rates are re-sampled and a new value of WIR, is calculated from Eq. 2. The intercept point (Figure 1 Point A) is always zero. This defines the equation of Line AC.
3.0 Determination of WIR 2 3.1 The wear index growth rate after instability is calculated by summing the individual AVB wear rates using Eq. 3. The AVB wear rates are determined by N2 random selections from the AVB wear rate distribution in Figure 3 (post-initiation curve).
3.2 It is assumed that after in-plane instability is initiated, there is a re-distribution in wear patterns and corresponding wear rates at the AVBs. Wear rates were assigned for N2 locations (i.e., resetting N, AVB wear rate values as well as selecting wear rates for the new locations). This represents a physical re-distribution of support contact/interaction with the tube. The AVB wear rates are determined by N2 random selections from the AVB wear rate distribution in Figure 3 (post-initiation curve).
3.3 The intercept point (Figure 1 Point E) is calculated from the expression WOP- u sC - WIR 2
'AV B -U tCYC Y
3.4 Point E is evaluated in each trial to confirm it is < 0 on the wear index axis. This is to maintain a slope for Line ED that is always greater than or equal to the slope for Line AD. If Point E > 0, the AVB growth rates are re-sampled and WIR 2 is recalculated from Eq. 3 to satisfy this constraint. This defines the equation for Line ED, having a slope WIR 2 > WIR, and intercept < 0.
4.0 Calculation of tINIT 4.1 The point of intersection (Figure 1 Point B) is calculated from Eq. 1 and the time of initiation determined.
4.2 The above steps are repeated for 1000 trials. The 1000 initiation times (tINIT) are ranked in ascending order and the median value (500th in ranking) is recorded.
The above procedure is repeated for each tube until all tubes in the high-wear region have been evaluated and median initiation times for each tube established.
Page 13 of 23
Variable Initiation-Time Model Schematic 0
0.
0.
Co 0.
0.
CL xiL
")
CL 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Operation Time, t (Years at Power)
Figure 1 - Linear TTW Initiation-Time Model Variables Page 14 of 23
Unit 3 AVB Distribution for Tubes without TTW 1000 1.0 900 0.9 800 0.8 700 0.7 cc 600 0.6 U
500 0.5 .2 o 400 0.4 >
300 0.3 200 0.2 100 0.1 0 0.0 0 1 2 3 4 5 6 7 8 9 10 11 12 Number of AVBs Figure 2 - Distribution of Affected AVBs in Unit 3 for Determining N, Page 15 of 23
Unit 3 AVB Wear Rates 1.0 _ ------
0 .9 ------
0.8 - ----
0 04-0 5 10 5 2 S 0.5 AB WR pe rP Fiue3-LgNra Vpe uprtWa aeDsrbtosfrUi 0.0 Figur 3 -Log-NrmalAVBUpeSuor Wear
%TWrbtin Rate per Years atPoe Page 16 of 23
RAI 2
The Operational Assessment in Attachment 6, Appendix C (Reference 4), pages 3-2 and 4-12, appears to state that tube-to-tube wear (TTW) growth rates are based on the maximum TTW depths observed in Unit 3 at EOC 16 divided by the first Unit 3 operating period (0.926 years at power). Provide justification for the conservatism of this assumption. This justification should address the following:
- a. Reference 4, page 3-2 defines "wear index" for a degraded tube and states that the existence of TTIW and distribution of TTW depths are strongly correlated to the wear index. This is pictured in Figures 4-4 in terms of TTW initiation. This figure shows that TTW is not expected to have initiated until a threshold value of wear index is reached.
This threshold value varies from tube to tube according to a cumulative probability distribution shown in the figure. This figure illustrates that TTW is not expected to have initiated until sometime after BOC 16. This suggests that the observed TTW depth at EOC 16 developed over a smaller time interval than the 0.926 years assumed in the analysis.
- b. An independent analysis in Reference 3 also indicates an extremely low probability of instability onset at BOC 16 as illustrated in Figure 8-3. Reference 3, page 106 interprets this figure as indicating that the probability of instability only reaches 0.22 after 3 months and only becoming "high" after 4 months.
- c. Reference 3 also considered a variety of different wear rate models to estimate how long it took to develop the observed TTW depths at Unit 3 after instability occurred. These analyses are documented in Appendix A of Reference 3 and produced estimates in the range of 2.5 to 11 months.
RESPONSE - Revision 1 Note: RAI Reference 4 is the "Operational Assessment for SONGS Unit 2 SG for Upper Bundle Tube-to-Tube Wear Degradation at End of Cycle 16," prepared by Intertek APTECH for Areva, Report No. AES 12068150-2Q-1, Revision 0, September 2012.
RAI Reference 3 is the "SONGS U2C17 Steam Generator Operational Assessment for Tube to Tube Wear," prepared by Areva NP Inc, Document number 51-9187230-000, Revision 0, October 2012.
Basis for Zero Time Initiation Model The wear rate model in the SONGS Unit 2 operational assessment (OA) is based on the apparent tube-to-tube wear (TTW) growth rates observed in SONGS Unit 3. Industry guidelines define the apparent degradation rate as the change in observed degradation over the operating cycle length. Following standard industry practice, the wear rate model assumes that TTW began at the start of Cycle 16 for Unit 3. The wear rate distribution for SONGS Unit 2 was developed from the distribution of Unit 3 depths divided by the Cycle 16 operation period of 0.926 years at power.
Page 17 of 23
It was recognized that a variation in the time when TTW initiated would affect the wear rate distribution. The OA model for TTW includes conservative assumptions to compensate for the variation in the time when TTW may have begun. These conservative modeling conditions are:
- 1) Only the maximum TTW depths were used in the determination of wear rate. This produces a faster growth model than one developed from all of the depth data.
- 2) The TTW rates are based on growth in through-wall depth (i.e., constant-depth growth).
Wear processes are typically modeled as constant volume growth which leads to a decreasing depth growth in time.
- 3) The TTW growth rates are based on 100% power operation for Unit 3. No credit is taken for the potential reduction in TTW rate for 70% power operation.
- 4) The tube-AVB and tube-TSP growth rates are based on 100% power operation for Unit 3. No credit is taken for the potential reduction in wear rates at AVB and TSP supports for 70% power operation.
- 5) The TTW rate model is based on the wear index (WI) as an indicator of initiation and growth of TTW. The wear index is based on depth growth at the supports (AVB and TSP) which conservatively assumes constant depth growth.
By including these assumptions, the Unit 2 TTW growth rate model for 70% power operation was determined to be reasonable and conservative.
Treatment of Variable Initiation Time To address the RAI question of TTW initiation time and the impact on the allowable inspection interval for Unit 2, additional analysis was performed. A time dependent predictive model (initiation-time model) for estimating the operating period prior to TTW initiation in Unit 3 was developed. This model is illustrated in Figure 1. This figure is a schematic example for a single tube where the rate of increase in the wear index is allowed to vary.
As shown in Figure 1, the growth in the wear index occurs in two stages: pre-initiation and post initiation. The initiation-time model for TTW uses a wear index based only on tube to AVB wear in the upper supports (B03 through B10). This model's wear index (WI) starts to increase at the beginning of the cycle. The basis for this approach was developed from the Unit 3 wear data as follows:
- 1) Tubes without TTW indications have very few or no AVB wear indications at the lower support bars (B01, B02, B11, and B12).
- 2) Most tubes with TTW have wear indications in the lower support bars. These tubes typically have significant wear at all four lower support bars. The wear at the lower supports is assumed to occur after TTW has initiated.
- 3) AVB wear depths (B03 through B1 0) for tubes with TTW are similar to the wear depths for tubes without TTW. This indicates that the wear rates at individual tube/AVB contact points before TTW initiates is not significantly elevated after TTW has initiated. Thus the increase in AVB wear index after TTW is mainly due to the increase in locations with AVB wear, including ...ea. locations at the loWer SUPPOrtS (13.., B02, 1311 and B12).
Page 18 of 23
- 4) For tubes without TTW, few or no TSP wear indications were detected. Most TSP wear occurred in tubes with TTW.
Using the above Unit 3 NDE results, the initiation-time model assumes the development of wear at the lower AVB supports (801, B02, B1 1, and B1 2) and TSPs occurs after TTW initiates. The TTW initiation time was determined from the change in the wear index in the upper tube supports (B03 through B10). The initiation-time model uses AVB wear in these upper supports as a predictor of TTW since the wear at the other supports is assumed to develop after TTW initiation.
The WI growth rate prior to initiation of TTW was developed from upper AVB support wear data (803 through B10) for Unit 3 tubes without TTW. The increase in WI after TTW initiation uses AVB wear rate distributions sampled from the number of detected wear locations for tubes with TTW.
The predicted TTW initiation time is calculated by simulating two-stage growth in the WI. The initiation model is constrained to the end state for the calculated wear index from the NDE results. The slopes for the growth in the WI are also constrained to keep the two-stage process bounded by the constant WI growth case illustrated in Figure 1.
A Monte Carlo simulation was performed to compute the distribution of initiation times for each tube. After completion of 1000 trials for each tube, a distribution of initiation times was created and the median value recorded. The median value is used to provide the best-estimate time when TTW began in Cycle 16. The histogram of these median values is shown in Figure 2. By this analysis, a significant number of tubes initiated TTW early in the cycle, especially for SG 3E-088. The TTW growth rates were computed from these data using the maximum TTW depth divided by the cycle length minus the initiation time.
The zero-time initiation model in RAI Reference 4, the independently developed analysis in RAI Reference 3, and the initiation-time model developed in response to this RAI reach conclusions on either the onset of instability or the initiation of TTW in the next operating interval for Unit 2.
The approaches used in these models differ in their use of the empirical information provided by the NDE inspection data and the use of steam generator thermal hydraulic properties and support conditions from Units 2 and 3. These differences lead to some variation in predicting the onset of instability or the initiation of TTW. SCE's conclusion is all three approaches provide diverse and comprehensive evaluations supporting the proposed operating interval.
Comparison of TTW Rate Behavior The wear rate plots as a function of total wear index are shown in Figure 3 for both depth sizing techniques used in RAI Reference 4. The dotted line in each plot is the TTW rate model from Figure 4-13 in RAI Reference 4. This model assumed zero initiation time in establishing the wear rate. The solid line is the regression analysis of the results from the initiation-time model developed for this RAI. This comparison shows the TTW growth function based on the initiation-time model is more conservative than the model used in RAI Reference 4, without adjusting for the other conservative assumptions made in the development of the RAI Reference 4 model.
Page 19 of 23
Significance of Initiation Time The effect of the initiation-time model developed for this RAI on operating interval is shown in the table below. The allowable operating interval for maintaining structural integrity performance criteria margins for Unit 2 is 1.02 to 1.15 years at 70% power without changing any of the other conservative assumptions incorporated in the RAI Reference 4 model.
Unit 2 Years of Operation at 70% Power Reference 4 - Zero RAI TTW Initiation-Case Initiation Time Time Model TTW Model ETSS Sized 1.33 1.02 AREVA Resized 1.48 1.15 These results represent a margin of at least 2.4 on the planned Unit 2 inspection interval of 150 days (0.42 years at 70% power).
Page 20 of 23
SONGS-3 Initiation Time Model Illustrated Example 160 140 120 I-100 Va 80 60 460 40 20 0 ".
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Cycle 16 Operation Time, t (Years at Power)
Figure 1 - Model for Estimating TTW Initiation Time Page 21 of 23
SONGS-3 Median Initiation Times for TTW for Cycle 16 80
- 3SG88 70 03SG89 60-2 0
E z
20 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 Time of Operation Until Initiation (Years at Power)
Figure 2 - Simulation Results from Time Dependent Initiation Model Page 22 of 23
ETSS 27902.2 Sizing 110 100
- 90 0
a.
li 80 LO 70 CL 60 S50
- 40 p 30
- 20 10 0
0 50 100 150 200 250 300 350 400 450 500 550 600 650 Wear Index, WI (%TW)
AREVA Resized 110 100 90 a.0
- 80 70 60 Sso
- 40
- 30 20 10 0
0 50 100 150 200 250 300 350 400 450 500 550 600 650 Wear Index, WI (%TW)
Figure 3 - Tube-to-Tube Wear Rate Comparisons Page 23 of 23