ML072770517
| ML072770517 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 04/12/1991 |
| From: | GPU Nuclear Corp |
| To: | NRC/SECY |
| SECY RAS | |
| References | |
| 50-219-LR, AmerGen-Applicant-23, C-1302-187-5300-011, RAS 14230 C-1302-187-5300-011 | |
| Download: ML072770517 (42) | |
Text
[IR,1 5 f/ t-3o APPLICANT'S EXH. 23
.08/28/00 11:54:39 Calc. No. C-1302-117-5300-611 Rev.
No. 0 Page 2 of 454 j
ý0 suNmbwy OP 'RESU1tT 8ý iRa Corrosion Rate M~~ean Thick~neass*
2.1
-tand-Bed Region With Cathodig Protection -All Data F-Rat i6 DOCKETED USNRC 12A
-15.6 +/-2.9 mpy 110 Top
-35.2 +/-6.8 Mpy 11C Bottom
-22.4 +/-4.3 mpy 170
.5.*O +/-.2.o Mpy 29A-.I
+/-1.5 mpy 19B
-19.0.+/-1.7 mpy 190"
-24,3 +.13 mpy 2.2 Sand Bed Reion WŽt4jCathodic 870.4 977.0 865.0 829.S 807.6
.836.9 825.1 Froec4ov*,i 5.7 milo
+/-12.5 mila 7.8 milo
+ 4.0 milo 3.0 milo
+3.2 mils 2.3 rils 5.4 4.6 4.9 29.4 39.5 21.3 66.2 October 1, 2007 (10:45am).
OFFICE OF SECRETARY RULEMAKINGS AND ADJUDICATIONS STAFF since October 1988 11C Top llC Bottom 19A Li 19B 19C Not Signif icanV Not Significant -.
Not significant*
-23.7 +4..6 mpy
-20.6 +/-3.9 mpy
-11.8+3.9 mpy
-21.5 +/-3.5 mpy 878.0 996.6 878.1 830.1 808.2 841.2 826.3
+
+..
+.
5.9 8.3 5.6 3.8 3.2 3.3 2.9 mils mils milo milo mils mile mils 2.7 2.8 0.9 3.7 2.3 Sand Bed Reaion Frame Cutout "1/19 Top
.17/19 Bottom Not Significant*
Not Significant*
986.0 4.4.A mils 100a.4
+ 3.
mils 2.4 Sand 884 Region WLthout Cathodic Protection 9D 13A 13D I5D 17A Top 17A Bottom Not Significant*
-39.1 +/--3.4 mpy Indeterminate Not Significant*
Not Significant*
.Not Signifitant*
1021.7 853.1 931.9 1056.5 1128.3 745.2
- * +/-8;9
+ 2.4
+/-22.6
+2.3
+/- 2.2
+/-.2.1 mils mile mile mile milo mile 16.3 Not Statistically significant compared to random variations in measurements
- Mean corrosion rate in mils per year +/- standard error of estimate
- Best estimate of current mean thickness in mils +/- standard error of the mean U.S. NUCLEAR REGULATORY COMMISSION
(.
Docket No
_-0 9 Official Exhibit No.
OFFERED.byensee Intervenor 001/0004.2 Other
.Witness/Pane 0A IDENTIFED WITHDRAWN OCLR00020057 lem P.(C~f~ a. cy-OQF
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. No. I Page 24Lof 454 2.0
SUMMARY
OF RESULTS Bay & Area
. corrosion Rate Impy)
Mean Thickness **
Best Estimate*
95% Conf.**
2.1 Sand Bed Recion With Cathodic Protection -All-Data
- F-r.atio N 11A 11C 11C 17D 19A 19B 19C Top Bottom
-15.6 +2.9
-35.2 +6.8
-22.4 +/-4.3
-25.0 +2.0
-21.4 +1.5
-19.0 +1.7
-24.3 +/-1.3 MPY Mpy MPY WLJY MIPY Mpy Elpy
-21.0
-48.2
-30.5
-28.7
-24.1
-22.3
-26.7 870.4 977.-0 865.0 829.5 807.6 836.9 825.1
+ 5.7 mile
+/-12.5 mile 7.8 mile 4.0 mile
+ 3.0 milo
+ 3.2 mil.s
+ 2.3 mils 5.4 4.6 4.5 29.4 39.5 21.3 66.2 9
9
.9 10 10 9
9 Yrs 3.0 3.0 3.0 3.2 3.2 3.0 3.0 1.5 1.5 1.5
- 1. 5 1.5 1.5 1.5 2.2 gand Bed Region-With Cathodic Protection - Since October 1988 11A 11C Top 11C Bottom 170 IMD 19A 19B 19C Not Significant****
Not Significant****
Not Significant****
-23.7 +/-4.6 mpy
-20.6 +3.9 mpy
-11.8 +/-3.9 mpy
-21.5 +3.5 mpy
-34.2
-29.7
-21.1
-29.5 878.0 996.6 878.1 830.1 808.. 2 841.2 826.3
+
4.
+
_+
+
+
+/-
5.9 8.3 5.6 3.8 3.2 33.3 2.9 mile mile milo mils mils mils mile 2.7 2.8 0.9 3.7 5
5 55 5
5.
(.
2.3 Sand Bed Region Frame Cutout 17/19 Top Not Significant****
17/19 Bottom Not Significant****
2.4 Sand Bed Region Without Cathodic 986.0
+/- 4.7 mile 1005.7
+ 5.6 mils Protection 5
1.3 5
1.3 Ii:
Protection 9D 13A 13D 15D 17A Top 17A Bottom Not significant****
-39.1 +/-- 3.4 mpy
-46.4 Indeterminate Not Significant****
Vot Significant****
Not Significant****
1021.7 853-1 931.9 1056.5 1i28.3 950.8
+ 8.9 mile
+ 2.4 mile
+/-22.6 mile
- 2.3 -mila
+ 2.2 mile
- .3 mile 5
16.9 6
1 S
5 S
1.3 1.4 0
1.5 1.4 1.4
.1 Mean corrosion rate in mils per year + standard error of estimate Upper bound of the one-sided.95% confidence interval Best estimate of current mean thickness in mile + standard error.0f the mean
- Not statistically significant compared to random variations in measurements N
=
Number of data sets Yrs = Years from first to-last data set 001/0004.3 OCLR00020058
08/28/00 11:54:39
[E ]Nuclear DOCUMENT NO.
C-1302-187-5300-011
- ITI1E STATISTICAL ANALYSIS OF DRYWELL. THICKNESS THRU 4-24-90 REV
SUMMARY
OF CHANGE APPROVAL DATE 1
Computed 95% upper bound of the corrosion rate in each bay where regression model is appropriate.
32-
-9 Computed maximum potential corrosion rate at 95% confidence for each bay where mean model is appropriate.
Deleted Summary of Apparent Corrosion Rates and added Summary of Maximum Potential Corrosion Rates at 95% Confidence.
Revised paragraphs 2.0, 4.5.2, and 4.10 to reflect these changes.
Ccav'<Ye~cAeta\\
-I-Lipos coy-
&Wnra,a~Se la N0036 (03,90) 0CLR00020059
08/28/00 11:54:39 Calc. No.
C-1302-187-5300-ou.
Rev. No. 0 Page 3 of 454 C--.
I ~
Bay &
ea Corrosion RatE 2.5 E. ation 5 0-12, 4.6 +/- 1. 6 5/5 Indeterminate 13/31 Indeterminate 15/23 determinate 2.6 Elevation 52
\\.
Mean Thickness ***
F-Ratio 745.2 745.1 750.8 751.2
+ 2.1 mils
- 3.2 mils
+-11.5 mile
- t 3.8 amils 1.3 7125 13/6 13/32 19/13 Indetetm 8nate Indeteimiaite Indetermina~t 715.S 724.9 698.3 712.5
+/- 2.9
+/-,2.9
+ S.1
+ 3.1 I
2.7 Elevation 87' 9
15 Not Siqnificant*
Not Significaut*
Not Significant*
619.9 636.5 636.2
+ 0.6
.. _+ 0.8
+/-. 1.1 2.5 kppargnt CorrosQin Rates These estimates of the corrosion rate aie based on a least squares fit of the data.
In those cases where the F-hatio is Less than 1.0 they.
should not be used to make future projectioua.
Por bays with cathodic protection, these apparent rates are for the-.period from October 1988 to Rpril 1990.
For the other bays, it is tor all data.
11A 11C Top 11C Bottom 19A 19B 19C 17/19 Top 17/19 Bottom Appaxent Corrosion Rate-IMM I
-16.2 +/- 8.6
-25.0 +/-10.6
-16.7 + 7.1
-23.7 1-4.6
-20.6 +/- 3.9
-11.8 +/- 3.9
-21.5 +/- 3.5
- 8.2 _-10.7
-13.1 +/-11.6 0.2 0.6 0.6 2.8 2.6 0.9 3.7 0.1 0.1 9D 13A 15D 11A Top 17A Bottom 5 EL 51' 9 EL 87' 13 EL 87' 15 EL 87' 7-ýRatio lam Apparent Corrosion Rate (my I
-21.0 +/-l1.1
-39.1 +/-. 3.4 4.6.+/- 4.8
- 6.8 3.7
-17.7 +/- 7.6 6 6 1.6 0.2 +/- 0.9 zero z Zero F-Ratio 0.1 16.9 0.1 0,3 0.01 1.-3:
z~ro
(
-I 001/0004.3 OCLROO020060
08/28/00 11:54:39
(
1 Calc. No. C-1302-187-5300-0i1 Rev. No.. 1 Page 30.of 454 Say & Area gorrosion Rate iMOV)
Mean Thickness ***
F-Ratio N Best Estimate*
95% Conf.**
2.5 Elevation 51' S/D-12 5/5 13/31 15/23 Yrs 2.5 1.1 1.2 1.1
- 4.6 + 1.6 mpy Indeterminate Indeterminate Indeterminate
-2.2 745.2 745.1 750.8 751.2
+/- 2.1 mile 1.3 3.2 mile
+11.5 mils
+/- 3.8 mile 2.6 Elevation 521 8
2 2
2 1
1 1
7/25 23/ 6 13/32 19/13 Indeterminate Indeterminate Indeterminate Indeterminate 715.5 724.9 698.3 712.5 619.9 636.5 636.2 2.7 Elevation 87'
+
.4-
+
4-
+
+
4.
2.9 2.9 5.0 3.1 mile mile fiiis mile 0
0 0
0 9
13 15 Not Significant****
Not Significant****
Not Significant****
0.6 mile 0.8 mile 1.1 mile 5
5 5
2.4 2.4 2.4 2.8 Potential Corrosion Rates at 95% Confidence For those locations where the corrosion rate is not statistically significant, the possibility does exist that the variability in the data may be masking an actual corrosion rate.
The potentially masked corrosion rate at 95% confidence is bounded by the upper bound of the 95% one-sided confidence interval about-the slope computed in the regression analysis (see Paragraph 4.10.1)..
.95% Upper Bound Corrosion Rate Bafy Elevation N
Yrs I1A (Since 10/88) 11C Top (Since 10/88) 1iC Bottom (Since 10/88) 17/19 Top 17/19 Bottom 9D 15D 17A Top 17A Bottom 9
13 is Sand Bed Sand Bed Sand Bed Frame, Cutout Frame Cutout Sand Bed Sand Bed Sand Bed Sand Bed 87' 87' 87'
-36.4
-49.9
-33.3
-33.4
-40.5
-63.4
-16.0
-15.5
-35.6
-2:.2
-2.1
-0.6 5
5 5
5 5
S
.5 5
5 1.5 1.5 1.5 1.3 1.3 1.3 1.4 1.4 1.4 2.4 2.4 2.4 f.
NOTE:
The high value for Bay 9D results from one extremely high mean value on 6/26/89.
Without this data point, the 95% upper bound is -29.2 mpy.
001/0004.4 OCLROO020061
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev.
No.
0 Page 4 of 454
. 9 2.gf Evaluation of rndividual Measurements Exceedinq 99%/9g% Tolerance Interval If One data point in Bay 5 Elev. 51' fell outside the 99%/99% tolerance interval and thus is statistically different from the mean thickness.
Based on a linear regression analysis for this point, it is concluded that the corrosion rate in this pit is essentially the same as the overall grid.
(.
- 1 001/0004.4 0CLR00020062
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. Vo. 0 Page 5 of 454
3.0 REFERENCES
3.1 GPUN Safety Evaluation SE-000243-002, Rev. 0, "Drywell Steel Shell Plate Thickness Reduction at the Base Sand Cushion Entrenchment Region" 3.2 GPUN TDR 854, Rev. 0, "Drywell Corrosion Assessment" 3.3 GPUN TDR 851, Rev. 0, "Assessment of Oyster Creek Drywell Shell" 3.4 GPu? Installation Specification IS-328227-004, Rev.
3, "Functional Requirements for Drywell Containment Vessel Thickness Examination" 3.5 Applied Regression Analysis, 2nd Edition, N.R. Draper & H. Smith, John Wiley & Sons, 1981 3.6 Statistical Concepts and Methods, G.K. Bhattacharyya & R.A.
Johnson, John Wiley & Sons, 1977 3.7 GPUN Calculation C-1302-187-5300-005, Rev.
0, "Statistical Analysis of Drywell Thickness Data Thru 12-31-88" 3.8 OPUN TDR 948, Rev.
1, "Statistical Analysis of Drywell Thickness Data!
3.9 Experimental Statistics, Mary Gibbons Natrella, John Wiley& Sons, 1966 Reprint..
(National Bureau of Standards Handbook 91) 3.10 Fundamental Concepts in the Design of Experiments, Charles C.
Hicks, Saunders College Publishing, Fort Worth, 1982 3.11 GPUN Calculation C-1302-187-5300-008, Rev. 0, "Statistical Analysis of Drywell Thickness Data thru 2-8-90" 001/0004.5 OCLROO020063
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. No. 0 Page 6 of 454 4.0 ASSUMPTIONS & BASIC DATA
4.1 Background
The design of the carbon steel drywelj includes a sand bed which is located around the outside circumference between elevations 8'-1l-1/4" and 12'-3".
Leakage was observed 'from the sand bed drains during the 1980, 1983 and 1986 refueling outages indicating that water had intruded into the annular region between the drywell shell and the concrete shield wall.
The drywell shell was inspected in 1986 during the 1OR outage to determine if corrosion was occurring.
The inspection methods, results and conclusions are documented in Ref. 3.1, 3.2, and 3.3.
As a result of these inspections it was concluded that a long term monitoring program would be established.
This program includes repetitive Ultrasonic Thickness (UT) measurements in the sand bed region at a nominal elevation of 111-3" in bays 11A, llC, 17D,
- 19A, 19B, and 19C.
The continued presence of water in-the sand bed raised concerns of potential corrosion at higher elevations.
Therefore, UT measurements were taken at the 51' and 87' elevations in November 1987"during the IIR outage.
As a result of these inspections, repetitive measurements in Pay 5 at elevation 51' and in Bays 9, 13 and 15 at the 87' elevation were added to the long term monitoring program to confirm that corrosion is not occurring at these higher elevations.
A cathodic protection system was installed in selected regions of the sand bed during the 12R outage to minimize corrosion of the drywell.
The cathodic protection system was placed in service on January 31, 1989.
The long term monitoring program was also expanded during the 12R outage to include measurements in the sand bed region of Bays 1D, 3D, 5D, 7D, 9A, 13A, 13C, 13D, 15A, 15D and 17A which are not covered by the cathodic protectionsystem.
It also includes measurements in the sand bed region between Bays.17 and 19 which is covered by the cathodic protection system, but does not have a reference electrode to monitor its effectiveness in this region.
The high corrosion rate computed for Bay 13A in the sand bed region through February 1990 (Ref. 3.11) raised concerns about the corrosion rate in the sand bed region of Bay 13D.
Therefore, the monitoring of this location using a 6"x6" grid was added to the long term monitoring program.
in addition, a 2-inch core Sample was removed in March 1990. from a location adjacent to the 6"x6" monitored grid in Bay 13A.
001/0004.6 0CLR00020064
08/28/00 11:54:39 Caic. No. C-1302-187-5300-011 Rev.
No. 0 Page 7.of 454 Measurements taken in Bay 5 Area D-12 at elevation 51' through March 1990 indicated that corrosion is occurring at his location.
Therefore, survey measurements were taken to determine the thinnest locations, at elevation 51'.
As a result, three new locations were added to the long term monitoring program (Bay 5 Area 5, Bay 13 Area 31, and Bay 15 Area 2/3).
The indication of ongoing corrosion at elevation 51' raised concerns about potential corrosion of the plates immediately above which have a smaller nominal thickness.
Therefore, survey measurements were taken in April 1990 at the 52'. elevation in all bays to determine the thinnest locations.
As a result of this survey, foir new locations were added to the long term monitoring plan at elevation 52' (Bay 7 area 25, Bay 13 Area 6, Bay 13 Area 32, and Bay 19 Area 13).
Some measurements in the long term monitoring program are to be taken at each outage of opportunity, while others are taken during each refueling outage.
'The functional requirements for these inspections are documented in Ref. 3.4.
The purpose of the UT measurements is to determine the corrosion rate and monitor it over time, and to monitor the effectiveness of the cathodic protection system.
4.2 Selection of Areas to be Monitored A program was initiated during the 2IR outage to characterize the corrosion and to determine its extent.
The details of this inspection program are documented in Ref.
3.3. The greatest corrosion was found via UT measurements in the sand bed region at the lowest accessible locations.
Where thinning was detected, additional measurements were made in a cross pattern at the thinnest section to determine the extent in the vertical and horizontal directions.
Having found the thinnest locations,
-measurements were made over a 6"x6" grid.
To determine the vertical profile, of the thinning, a trench was excavated into the floor in Bay 17 and Bay 5.
Bay 17 was selected since the extent of thinning at the floor level was greatest in that area.
It was determined that the thinning below the top of the curb was no more severe than above the curb, and became less severe at the lower portions of the sand cushion.
Bay 5 was excavated to determine if the thinning line was lower than the floor level in areas where no thinning was detected above the floor.
There were no significant indications of thinning in Bay 5.
001/0004.7 0CLR00020065
08/28/00 11:54:39 Caic. No. C-1302-187-5300-011 Rev. No.
0 Page 8 of 454 It was on the basis of these findings that the 6"x6" grids in Bays IIA, IIC, 17D, 19A, 19B and 19C were selected as representative locations for longer term monitoring.
The initial measurements at these locations were taken in December 1986 without a template or markings to identify the location of each measurement.
subsequently, the location of the 6"x6" grids were permanently marked on the drywell shell and a template is used in conjunction with these markings to locate the UT probe for successive measurements.
Analyses have shown that including the non-template data in the data base creates a significant variability in the thickness data.
Therefore, to minimize-the effects of. probe location, only those data sets taken with the template are included in the analyses.
The presence of water in the sand bed also raised concern of potential corrosion at higher elevations.
Therefore, UT measurements were taken at the 51, and 87' elevations in 1987 during the 11H outage.
The measurements were taken in a band on 6-inch centers at all accessible regions at these elevations.
Where these measurements indicated potential corrosion, the measurements spacing was reduced to I-inch on centers.
If these additLonal readings indicated potential corrosion, measurements were taken on a 6"x6" grid using the template.
It was on the basis
- "of these inspections that the 6"x6" grids in Bay 5 at elevation 51' and in bays. 9, 13 and 15 at the 87' elevation were selected as representative locations for long.term monitoring.
A cathodic protection system was installed in the sand bed region of Bays 11A, 1IC, 17D,
- 19A, 19B, 19C, and at the frame between Bays 17 and 19 during the 12R outage.
The system was placed in service on January 31, 1989.
The long term monitoring program was expanded as follows during the 12R outage:
(1)
Measurements on 6"x6" grids in the sand bed region of Bays 9D,
- 13A, 15D and 17A.
The basis for selecting these locations is that they were originally considered for cathodic protection but are not included in the system being installed.
(2)
Measurements on 1-inch centers along a 6-inch horizontal strip in the sand bed region of Bays 1D, 3D, 5D, 7D, 9A, 13C, and 15A..
These locations were selected on the basis that they-are representative of regions which have experienced nominal corrosion and are not within the scope of the cathodic protection system.
001/0004.8 0CLR00020066
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev.
No. 0 Page 9 of 454 (3). A 6"x6" grid in the curb cutout between Bays 17 and.19.
The purpose of these measurements is to monitor corrosion in this region which is covered by the cathodic protection system but.
does not have a reference electrode to monitor its performance.
The long term monitoring program was expanded in March 1990 as follows; (1) Measurements in the sand bed region of Bay 13D:
This location was added due to thb high indicated corrosion rate in the sand bed region of Bay 13A.
The measurements taken in March 1990 were taken on a l"x6" grid.
All subsequent measurements are to be taken on a 6"x6" grid.
(2)
Measurements on 6"xS" grids at the following'locations at elevation 51'-
Bay 5 Area 5, Bay 13 Area 31, and Bay 15 Area 2/3.
These locations were added due to the indication of ongoing corrosion at elevation 51', Bay 5 Area D-i.
The long term monitoring program was expanded in April 1990 by adding the following locations at elevation 52':
Bay 7 Area 25, Bay 13 Area 6, Bay 13 Area 32, and Bay 19 Area 13.
All k I measurements are taken on 6"x6" grids.
These locations were added due to the indication of ongoing corrosion at elevation 51' and the fact that the nominal plate thickuness at elevation 52' is less than at elevation.51'..
4.3 UT Measurements The UT measurements within the scope of the long term monitoring program are performed in accordance with Ref. 3.4.
This involves taking UT measurements using a template with 49 holes laid out on a 6"x6" grid with I" between centers on both axes.
The center row is used in those bays where only 7 measurements are made along a 6-inch horizontal strip.
The first set of measurements were made in December 1986 without the use'of a template.
Ref. 3.4 specifies that for all'subsequent
. readings, QA shall verify 'that locations of UT measurements performed are within. +/- 1/4" of the location of the 1986 UT measurements.
It. also specifies that.all subsequent measurements are to be within +/- 1/8" of the designated locations.
001/0004.9 OCLR00020067
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. No.
0 Page 10 of 454 4.4 Data at P lu Locations Seven core samples, each approximately two inches in diameter were removed from the drywell vessel shell.
These samples were evaluated in Ref. 3.2.
Five of these samples were removed within the 6"x6' grids for Bays I1A, 17D, 19A, 19C and Bay 5 at elevation
- 51'.
These locations were repaired by welding a plug in each hole.
Since these plugs are not r~preseatative of the drywell shell, UT measurements at these locations on the 6"x6" grid must be dropped from each data set.
The following specific grid pointsahave been deleted:
Bay Ara Points 11A 23, 24, 30, 31 17D 15, 16, 22, 23 19A 24, 25, 31, 32 19C 20, 2.6, 27, 33, 5 EL 51' 13, 20, 25, 26, 27, 28, 33, 34, 35 The core sample removed in the sand bed region of Bay 13A was not within the monitored 6"x6" grid.
4.5 Bases for Statistical Analysis of 6"x6" Grid Data 4.5.1 assumptions The statistical evaluation of the UT measurement data to determine the corrosion rate at each location Is based on the following assumptions:
" (1)
Characterization of the scattering of data over each 6"x6" grid is such that the thickness measurements are normally distributed.
(2)
Once the distribution of data for each 6"x6l grid is found to be normal, then the mean value of the thickness is the appropriate representation of the average condition, (3)
A -decrease in the mean value of the thickness with time is representative of the corrosion occurring within the 6"x6-grid.
001/0004. 10 0CLR00020068
08/28/00 11:54:39 CaLc.
No. C-1302-187-5300-011 Rev.
No.
0 Page 11 of 454 (4)
If corrosion has ceased, the mean value of the thickness will not vary with time except for random errors in the UT measurements..
(5)
If corrosion is continuing at a constant rate, the mean thickness will decrease linearly with time.
In this case, linear regression analysis can be used to fit the mean thickness values for a given zone to a straight line as a function of time.
The corrosion rate is equal to the slope of the line.
The validity of these assumptions is assured by:
(a)
Using more than 30 data points per. 6"x6" grid (b)
Testing the data for normality at each 6"x6" grid location.
(c)
Testing the regression equation as. an appropriate model to describe the corrosion rate.
These tests are discussed in the following section.
in cases where one or more of these assumptions proves to be invalid, non-parametric analytical techniques can be used to evaluate the data.
4.5.2 statistical Approach The following steps are performed to test and evaluate the UT measurement data for those locations where 6"x6" grid data has been taken at least three times:
(1) Edit each 49-point data set by setting all invalid points to zero.
Invalid points are those which are declared invalid by the UT operator or are at a plug location.
(The computer programs.used in the following steps ignore all zero thickness data points.)
(2) Perform a chi-squared goodness of fit test of each 49 point data set to ensure that the assumption of normality is valid at the 5% and 1% level of significance.
(3)
Calculate the mean thickness and variance of each 49 point data set.
.(4)
Perform an Analysis of Variance (ANOVA) F-test tb determine if there is a significant difference J
between the means of the data sets.
001/0004.11" OCLROO020069
08/2.8/00 11:54:39 P
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- 5)
Using the mean thickness values for each 6"x6" grid, perform linear regression analysis over time at each location.
(a)
Perform F-test for significance of regression at the 5% level of significance.
The result of this test indicates whether or not the regression model is more appropriate than the mean model.
In other words, it tests to see if the variation of the regression model is statistically significant over that of a mean.
model.
(b)
Calculate the ratio of the observed F value to the critical F value at 5% level of significance.
For data sets where the Residual Degrees of Freedom in ANOVA is 4 to 9, this F-Ratio should be at least 8 for the regression to be considered
-s'xas opposed to simply "significant.," "l~f Pp
.133)
(c)
Calculate the coefficient of determination (RF) to assess how well the regression model explains the percentage of total error and thus. c V.
how useful the regression lineswill be as a
/
4 M predictor.
a) 41 (d)
Determine if the residual values for the.
w3 C O12A regression equations are normally distributed..c c 4 M,*
44 OD V0 154>0 -`4 (e)
If the regression model is found to be V
0 o appropriate, calculate the y-intercept, the i 0 C
)
slope and their respective standard errors.
0 W C.
The y-intercept represents the fitted mean
-4 q 4 0 thickness at time zero, the slope represents
(.3 M*0 g
the corrosion rate, and the. standard errors represent the uncertainty or random error of
" 3 3
these two parameters.A
- 0) 4 M
.00 (6)
Use a K factor from Table A-7 of Reference 3.9 and a O E34(d.
the standard deviation to establish a one-sided 3
2 1
99%199% tolerance limit about the mean thickness 42 X4 Q.
values for each 6"x6" grid location to determine 44 2*4 whether low thickness measurements or "outliers", are
- D 0 A4 4 statistically significant..
If the data points are r-04 l
1is greater than the 99%199% lower tolerance
- limit, then Q 4 0 O deemed to be due to expected random error.
- However, if the data point is less than the.lower 99%199%
tolerance limit, this implies that the difference is.
statistically significant and is probably not due to chance.
K 4-'
(
00.1/0004.12 OCLROO020070
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0 Page 13 of 454 4.6 Analysis of Two 6"x6" Grid Data Sets Regression analysis is inappropriate when data is available at only two points-in time.
Howevet, the t-test can be used to determine if the means of the two data sets are statistically different.
4.6.1 Assumptions This analysis is based upon the following assumptions:
(1) The data in each data set is normally distributed.
(2).
The variances of the two data sets are equal.
4.6.2 Statistical Approach The evaluation takes place.in three steps:
(1)
Perform a chi-squared test of each data set at 5% and 1% levels of significance to ensure that the assumption of normality is valid.
-(2)
Perform an F-test at 5% and 1% level of significance of the two data sets being compared to ensure that the assumption of equal variances is valid.
(3)
Perform a two-tailed t-test for two independent samples at the 5% and 1% levels of significance to determine if the means of the two data sets are statistically different.
A conclusion that the means are not statistically different is interpreted to mean that significant corrosion did not occur over the time period represented by the data.
- However, if equality of the means is rejected, this implies that the difference is statistically significant and could be due to corrosion.
4.7 Analysis of Single 6"x6" Grid Data Set In those cases where a6,.x6, data set is taken at a given location for the first time during the current outage, the only other data to which they can be compared are the UT survey measurements taken at an earlier time.
For the most part, these are single-point measurements which were taken in the vicinity of the 49-point data set, but not at the exact location.
Therefore, rigorous statistical analysis of these single data sets is impossible.
However,b'by making certain assumptions, they can be compared with
-the previous data points.
if more extensive data is available at.
the location of the 49-point data set, the t-test can be used to compare the means of the two data sets as described in paragraph 4.5.
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Page 14 of 454 When additional measurements are made at these exact locations during future outages, more rigorous statistical analyses can be employed.
4.7.1 Assumptions The comparison of a single 49-point data sets with previous data from the same vicinity is based on the following assumptions:
(1) Characterization of the scattering of data over the 6"x6" grid is such that the thickness measurements are normally distributed.
(2) Once the distribution of data for the 6"x6" grid is found to be normal, then the mean value of the thickness Is the appropriate representation of the average condition.
(3)
The prior data is representative of the condition at this location at the earlier date.
4.7.2 Statistical Approach The evaluation takes place in four stepst (1) Perform a chi-squared test of each data set to ensure that the assumption. of normality is valid at the 95%
and 99% confidence levels.
(2)
Calculate the mean and the standard error of the mean of the 49-point data set.
(3)
Determine the two-tailed t value from a t distribution table at levels of Significance of 0.05 and 0.01 for n-i degrees of freedom.
(4)
Use the t value and the standard error of the mean to calculate the 95% and 99% confidence intervals about the mean of the 49-point data set.
(5)
Compare the prior data point(s) with these confidence intervals about the mean of the 49-point data sets.
If the prior data falls within the 95% confidence intervals, it provides some assurance that significant corrosion has not occurred in this region in the period of time covered by the data.
If it falls within the 99%.-
confidence limits but not within the 95% confidence limits,
- )
this implication is not as strong.
In either case, the corrosion rate will be interpreted to be "Not Significant".
.001/0004.14 OCLR00020072
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. No. 0 Page 15 of 454 If the prior data falls above the upper 99% confidence limit, it could mean either of two things: (1) significant corrosion has occurred over the time period covered.by the data, or (2) the prior data point was not representative-of the condition of the location of the 49-point data set in 1986.
There is no way to differentiate between the two.
In this case, the corrosion rate will be interpreted to be "Possible".
If the prior data falls below the lower 99% confidence limit, it means that it is not representative of the condition at this location at the earlier date.
In this case, the corrosion rate will be interpreted to be "Indeterminable".
4.8 Analysis of Single 7-Point Data Set In. those cases where a 7-point data set is taken at a given location for the first time during the current outage, the only other data to which they can be compared are the UT survey measurements taken at an earlier time to identify the thinnest regions of the drywell shell in the sand bed region.
For the most part, these are single point measurements.which were taken in the vicinity of the 7-point data sets, but not at the exact locations.
However, by making certain assumptions, they can be compared with the previous data points.
If more extensive data is available at the location of the 7-point data set, the t-test can be used to compare the means of the two data sets as described in paragraph 4.5.
When additional measurements are made at these exact locations during future outages, more rigorous statistical analyses can be employed.
4.8.1 Assumptions The comparison of a single.7-point data sets with previous data from the same vicinity is based on the following as.samptions!
(1) The corrosion in the region of each 7-point data Set is normally distributed.
(2)
The prior data is representative of the condition at this location at the earlier date.
The validity of these assumptions cannot be verified.
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Statistical Approach The evaluation takes place in four steps:
(1)
Calculate the mean and the standard error of the mean of the 7-point data set.
(2)
Determine the two-tailed t value using the t distribution tables at levels of significance of 0..05 and 0.01 for n-i degrees of freedom.
(3)
Use the t value and the standard error of the mean to calculate the 95% and 99% confidence intervals about the mean of the 7-point data set.
(4)
Compare the prior data point(s) with these confidence intervals about the mean of the 7-point data sets.
If the prior data falls within the 95% confidence intervals, it provides some assurance that significant corrosion has not occurred in this region in the period of time covered by the data.
If it falls within the 99%
confidence limits but not within the 95% confidence limits, this implication is not as strong.
In either case, the corrosion rate will be interpreted to be "Not Significant".
If the prior data falls above the upper 99% confidence interval, it could mean either of two things: (1) significant corrosion has occurred over the time period covered by the data, or (2) the prior data point was not representative of the condition of the location of the 7-point data set in 1986.
There is no way to differentiate between the two.
In this came, the corrosion rate will be interpreted to be "Possible".
If the prior data falls below the lower.99% confidence limit, it means that it~is not representative of the condition at this*location at the earlier date.
In this
- . case, the corrosion rate will be interpreted to be
- -Indeterminable,.
4.9 Evaluation of Drywell Mean Thickness This section defines the methods used to evaluate the drywell thickness at each location within the scope of the long term monitoring program.
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0 Page 17 of 454 4.9.1 Evaluation of Mean Thickness Usinq Regression Analysis The following procedure is used to evaluate the drywell mean thickness at those locations where regression analysis has been deemed to be more appropriate than the mean model.
(1) The best estimate of the mean thickness at these locations is the point on the regression line corresponding to the time when the most recent set of measurements was taken.
In the SAS Regression Analysis output (App. 6.2), this is the last value in the column labeled "PREDICT VALUE".
(2)
The best estimate of the standard error of the mean thickness is the standard error of the predicted value used above.
In the SAS Regression. Analysis output, this is the last value in the column labeled "STD ERR PREDXICT".
(3)
The two-sided 95% confidence interval about the mean thickness is equal to the mean thickness plus or minus t times the estimated standard error of the mean.
This is the interval for which* we have 95%
confidence that the true mean thickness will fall within.
The vaiue of t is obtained from a t distribution table for equal tails at n-2 degrees of freedom and 0.05 level of significance, where n is the number of sets of measurements used in the regression -analysis.
The degrees of freedom is equal to n-2 because two parameters (they-intercept and the slope) are calculated in the regression analysis with n mean thicknesses as input.
(4)
The one-sided 95% lower limit of the mean thickness is equal to the estimated mean thickness minus t times the estimated standard error of the mean.
This is the mean thickness for which we have 95%
confidence that the true mean thickness does not fall below.
In this case, the value of t is obtained from a t distribution table for one t at n-2.degrees of freedom and 0.05 level of significance.
4'.9.2 Evaluation of Mean Thickness Using Mean Model The following procedure is used to evaluate the drywell mean thickness at. those locations where the mean model is deemed to be more appropriate than the linear regression model.
This method is consistent with that used to evaluate the mean thickness using the regression model.
001/0004.17 0CLR00020075
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0 Page 18 of 454 (1) Calculate the mean of each set of UT thickness measurements.
(2)
Sum the means of the sets and divide by the number of sets to calculate the grand mean.
This is the best estimate of the mean thickness.
in the SAS Regression Analysis output, this is the value labelled "DEP MEAN"-
(3)
Using the means of thesets from (1) as input, calculate the standard error about the mean.
This is the best estimate of the standard error of the mean thickness.
(4)
The two-sided 9S% confidence interval about the mean.
thickness is equal to the mean thickness plus or minus t times the.estimated standard error of the mean.
This is the interval for which we have 95%
confidence that the true mean thickness will fall within.
The value of t is obtained from a t distribution table for eqala tails at n-i degrees of freedom and 0.05 level of significance.
(5)
The one-sided 95% lower limit of the mean thickness is equal to the estimated mean thickness minus t times the estimated standard error of the mean.
This is the mean thickness for which we have 95%
confidence that the true mean thickness does not fall below.
In this case, the value of t is obtained from a t. distribution table for one tail at.n-i degrees of freedom and 0.05 level of significance.
4.9.3 Evaluation of Mean Thickness Using Single Data Set The following procedure is used to evaluate the drywell thickness at those locations where only one set of measurements is available.
(1) Calculate the mean of the set of UT thickness measurements.
This is the best estimate of the mean thickness.
(2) Calculate the standard error of the mean for the set of UT measurements.
This is the best estimate of the standard error of the mean thickness.
Confidence intervals about the mean thickness cannot be calculated with only one data set available.
001/0004.18 0CLR00020076
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34-U) U. 0 W 0 *0 Lon.of Drywell Corrosion Rate Mean Model Calc. No. C-1302-187-5300-011 Rev. No. k/
Page 19 of 454 If the ratio of the observed F value to the crLtical F value is less than I for the F-test for the significance of regression, it indicates that the mean model is more appropriate than the regression model at the 5% level of significance..
In other words, the variation in mean.
thickness with time can be explained solely by the random variations in the measurements.
This means that the corrosion rate is not significant compared to the random variations.
Ithis
- case, an F-test is performed to compare the varq i7lity of the data set means between data sets with the. var ility of individual measurements within the data sets.
If observed F value is less than the critical F value, It con sthat theoean model proriate.
if the F-test indica B tha e"
a briablity.of the means is significant, the Lea Significant Difference (LSD) is computed.
This is the max m difference between data set mean thicknesses that can be a ributed to random variation in the measurements.
If the dif once between the means of data sets exceeds LSD, it indicat that difference is significant. The difference between me is subtracted from LSD and the result is divided by the t e between
.measurements to estimate the Significant Corr Ion Rate" in mils per year (mpy).
if the difference betwee the meats does not exceed LSD, then it is concluded thato significant corrosion occurred during that period of t' Cr*
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4.10.2 Regression Model If the ratio of the observed F value to the critical F value is 1 or greater, it indicates that the regression model is more appropriate than the mean model at the 5%
level of significance.
In other words, the variation in mean thickness with time cannot be explained solely by the random variations in the measurements.
This means that the corrosion rate is significant compared to the random variations.
Although a ratio of I or greater indicates that regression is significant, it does not mean that the slope of the regression line is an accurate prediction, of the corrosion rate.
The ratio should be at least 4 or 5 to considek the
.slope to be a useful predictor of the corrosion rate (Ref.
OCLR00020077
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No. 4 Page 20.of 454 3.5, pp. 93, 129-133).
A ratio of 4 or 5 means that the variation from the mean due to regression is approximately twice the standard deviation of the residuals of the regression.
To have a high degree of confidence in the predicted corrosion rate. the ratio should be at least 6 or 9 (Ref.
3.5, pp. 129-133).
In t instances, four sets of measurements over a periotd of abou ne year do not provide a significant re ssion
' model which
.be used to predict future thikesseB.
- However, a least ares fit of the four data points does provide a reasonable timate of the ent corrosion ae i
i rao
- o.
artic valuable for asessing the effecttivenes athodic protection and the draining of the sand bed gion.
ce a linear regression analysis lierorms a ear leas*t squar fit of the data, the best outimat f the recent corrosion e is the slope from the re ssion analysis for the period of 'terest.
The values are tabulated as the "Apparent Corrosion ell paragraph 2.5.
The upper bound of the 95% one-sided confidence interval about the computed slope is an estimate of the maximum probable corrosion rate at 95% confidence.
The 95% upper bound is equal to the computed slope plus the one-sided t-table value times the standard error of the slope.
The value of t is determined for n-2 degrees of freedom.
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GI Page 21 of 454 5.0 CALCULATIONS 5.1 6"'x6" Grids in Sand Bed Region With Cathodic Protection 5.1.1 Bay 11A 5.1.1.1 Bay IIA% 5111B7 to e2--8i-Nine 49-point data sets were available for this bay covering 4/24/90 period.
Since a plug lies within this region, four of the points were voided in each data set.
The data were analyzed as described in paragraphs 4.4, 4.5.1 and 4.6.1.
(1) The data are normally distributed.
(2) The regression model is appropriate.
(3)
The regression model explains 78.3% of the variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 870.4 + 5.7 mils.
(6)
The corrosion rate + standard error is
-15.6 +/- 2.9 mils per year.
(7)
F/F critical = 5.4.
(8)
The measurement below 800 mils was tested and determined not to be statistically different from the mean thickness.
5.1.2.2 Bay IIA:
1018/88 to 4124/90 Five 49-point data sets were available for this bay covering this period.
(1) The data are normally distributed.
.(2)
The mean model is more appropriate than the regression model.
(3)
The F-test for the significant of the difference between the means shows that the difference between the mean thickness are not significant.
j1 i........ )
. 001/0004.21 0CLR00020079
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The t-test of the last two data sets shows that the difference between the mean thickness is not significant.
(5)
The current thickness based on the mean model is 878.9 + 5.9 mile.
(6)
These analyses indicate that the corrosion rate with cathodic protection is not significant compared to random variations in the.measurements.
(7)
The best estimate of the corrosion rate during the period based on a least squares fit is -16.2 t 8.6 mile per year.
5.1.2 BaiC 5.1.2.1 Bay lIC; 5/1/87 to 4/24190 Nine 49-point data sets were available for this bay covering this period.
The initial analysis of this data indicated that the data are not normally distributed.
The lack of normality was tentatively attributed to minimal corrosion in the upper half of the 6"x6" grid with more extensive corrosion in the lower half of the grid.
To test this hypothesis, each data set was divided into two subsets, with one containing the top three rows and the other containing the bottom four rows.
Top 3 Rows (1) The data are normally distributed.
(2)
The regressiAon model is appropriate.
(3)
The regression model explains 79% of the total variation about the mean.
(4)
The residuals arenormally distributed.
(5)
The current mean thickness + standard error is 977.0 + 12.5 mils.
(6)
The corrosion rate is -35.2 + 6.8 mils per year.
(7)
F/F critical = 4.6.
001/0004.22 OCLROO020080
08/28/00 11:54:39 Calc. No. C-1302-187-5300-011 Rev. No. 0 Page 23 of 454 Bottom 4 Rows (1) Seven of the nine data sets are normally distributed.
The other two are skewed.
toward the thinner side of the mean.
The Chi-square test shows that they are close
-to being normally distributed at the 1%
level of significance.
(2)
The regression model is appropriate.
(3)
The regression model explains 80% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 865.0 + 7.8 mils.
(6)
The corrosion rate + standard. error is
-22.4 + 4.3 mile per year.
- (7)
F/F critical = 4.9 5.1.2.2 Bay llC:
10f8/88.to 4124/90 Five 49-point data sets were available for this period.
These data were divided into two subsets as described above.
Top 3 Rows (1) The data are normally distributed.
(2)
The mean model is more appropriate than
- the regression model.
(3) The F-test for the significance of the.
difference between the means shows that the differences between the mean thicknesses are not significant.
(4)
The t-test of the last two data sets shows that there is no statistical difference between their means.
(5). These analyses indicate that the current corrosion rate with cathodic protection is not significant compared'to random variations in the measurements.
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Based on the mean model, the current thickness + standard error is 996.6 +
8.3 mile.
(7)
The best estimate of corrosion rate during this period based on a least squares fit is -25.0 +/- 10.6 mile per. year.
Bottom 4 Rows (1) Four of the five data sets are normally distributed.
(See. 5.1.2.1 above).
(2)
The mean model is more appropriate than the regression model.
(3)
The F-test for the significance of the difference between the means shows that the differences between the mean thicknesses are significant.
(4)
The t-test of the last two data sets shows that there is no significant statistical difference between their means.
(5)
Based on the mean model, the current thickness +.standard error is 878.1 +
5.6 mile.
(6)
Based upon examination of the distribution of the five data set mean values, it is concluded that the current corrosion rate is not significant compared to random variations in the measurements.
The measurements alternated as follows:
- 897, 877, 891, 869, 863.
Therefore the difference must be due to variations other than corrosion.
(7)
The best estimate of the corrosion rate during this period based on a least squares fit is -16.7 + 7.1 mils per year.
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.1
(. )
5.1.3 Bay i7D 5.1.3.1 Bay 1'7D-2/17/87 to 4/24/90 Ten 49-point data sets were available for this period.
Since a plug lies within this region, four of the points were voided in each data set.
Point 24 in the 2/8/90 data was voided since it is characteristic of. the plug thickness.
(1) The data are normally distributed.
(2)
The regression model is appropriate.
(3) The regression model explains 95% of the total vairiation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 829.5 + 4.0 mils.
(6)
The corrosion rate + standard error is
-25.0 +/- 2.0 mils per year.
(7)
F/F critical
= 29.4 (8) The measurements below 800 mils were tested and determined not to be statistically different from the mean thickness.
5.1.3.2 Bay 17D-10/8/88 to 4/24190, Five 49-point data sets were available for this period.
(1) The data are normally distributed..
(2)
The regression model is more appropriate than the mean model.
(3)
- The regression model explains 90% of the variation about the mean.
- (4)
The residuals are normally distributed.
(5) The current mean thickneess + standard error is 830.1 + 3,8 mile.
001/0004.25 0CLR00020083
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Cale. No. c-1302-187-5300-011 Rev. No. 0 Page 26 of 454 (6) The corrosion rate s
standard error is
-23.7 +/- 4.6 mpy.
(7) F/F critical. = 2.7 5.1.4 Bay 19A 5.1.4.1 Bay 19A:
2/17187 to 4/24/90 Ten 49-point data sets were available for this period.
Since a plug lies within this region, four of the points were voided in each data set.
(1) The data are normally distributed at the 1% level ot significance.
(2)
The regression model is appropriate (3) The regression model explains 96% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 807.6 +/- 3.0 mils.
(6)
The corrosion rate + standard error is
-21.4 +/-.1.5 mpy.
(7)
F/F critical 39.5 (8)
The data points that were below 800 mils were tested and determined not to be statistically different from the mean thickness.
5.1.4.2 Bay 19A:
I0/8/88 to 4/24/90 Five 49-point data sets were available for this period.
(1)
The data are normally distributed.
(2)
The regression model is more appropriate than the mean model.
001/0004.26 0CLROO020084
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(4)
(5)
(6)
(7)
Calc.
No. C-1302-187-5300-011 Rev. No.
0 Page 27 of 454 The regression model explains 90% of the variation about the mean.
The residuals are normally distributed.
The current mean thickness + standard error is 808.2 + 3.2 mile.
The corrosion rate + standard error is
-20.6 + 3.9 mpy.
F/F critical
= 2.8 5.1.5 Bay 19B 5.1.5.1 Bay 19B:
5/1/87 to 4/24/90 Nine 49-point data sets were available for this period.
(1) The data are normally distributed.
(2)
The regression model is appropriate.
(3)
The regression model explains 94% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness +-standard error is 836.9 + 3.2 mile.
(6)
The corrosion rate + standard error is
-19.0 + 1.7 mpy.
(7)
F/F critical = 21.3 (8)
The measurements below 800 milswere tested and determined not to be statistically different from the mean thickness.
5.1.5.2 Bay 19B:
10/8188 to 4/24/90 Five 49-point data gets were available for this period.
(1) The data are normally distributed.
(2)
The regression model is more appropriate than the mean model.
001/0004.27
)
OCLROO020085
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The regression model explains 75% of the variation aboutý the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 841.2 + 3.3 mile.
(6)
The corrosion rate + standard error is
-11.8 + 3.9 mpy.
(7)
F/F. critical 0.9 Bay 19C:
5/1/87 to 4/24190 Nine 49-point data sets were available for this period.
Since a plug lies within this region, four of the points were voided in each data set.
(1) The data are normally distributed at the 1% level of significance, but appears to be developing two peaks.
(2) The regression model is appropriate.
(3) The regression model explains 98% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5) The current mean thickness + standard error is 825.1 + 2.3 mile.
{6)
The corrosion rate + standard error is
-24.3 + 1.3 mpy.
(7)
F/F critical = 66.2 (8)
The measurements below 800 mils were tested and determined not to be.
statisticaliy different.from the mean thickness-it 001/0004.28 0CLR00020086
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1018/88 to 4/24/90 Five 49-point data sets were available for this period.
(1) The data are normally distributed at the 1% level of significance.
(2)
The F-test for significance of regression indicates that the regression model is appropriate.
(3)
The regression model explains 93% of the total variation about the mean.
(4) The residuals are normally distributed..
(5) The current mean thickness + standard error is. 826.3 +
2.9 mils.
(6) The corrosion rate + standard error is
-21.5 + 3.5 mpy.
(6) F/F critical = 3.7.
5.1.7 Bays 17119 Frame Cutout:
12/30/88 to 4/24/90 Two sets of 6"x6" grid measurements were taken in December 1988.
The upper one is located 25" below the.top of the.
high curb and the other. below the floor.
There is no previous data.
The upper location was added to the long term monitoring program.
Five 49-point data sets were available for this period.
These data were analyzed as described in 4.4, 4.5.2 and 4.6.1.
The initial analysis of this data indicated that the first and last data sets are not normally distributed.
The lack of normality was tentatively attributed to more extensive corrosion in the upper half of the grid than the bottom half.
To test this hypothesis, each data set was divided. into two subsets, with one containing the top three rows and the other containing the bottom four rows.
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Four of the five subsets are normally distributed at the 1% level of significance but one is not.
(2)
The mean model is appropriate.
(3)
The F-test for the significance of the difference between the means shows that the differences between the mean thicknesses are not significant at 1% level of significance.
(4)
These analyses indicate that the corrosion rate is not significant compared to the random variations in the measurements.
(5)
Based on the mean model, the current thickness +
standard error is 986.0 +/-.4.7 mile.
(6)
The best estimate of the corrosion rate during this period based on a least squares fit is -8.2 + 10.7 mils per year.
Bottom 4 Rows M1) Four of the five subsets are normally distributed at the 5% level of significance, and one at the I% level of significance.
(2)
The mean model is appropriate.
(3)
The F-test for the significance of the difference between the means shows that the differences between the mean thicknesses are not significant at 1% level of significance.
(4).
These analyses indicate that the corrosion rate is not significant compared to the random variations i the measurements.
(5)
Based on the mean model, the current thickness +
standard error is 1005.7. + 5.6 mils..
(6)
The best estimate of the corrosion rate during this period based on a least squares fit is -13.1
+ 11.6 mils per year.
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0 Page 31 of 454 5.2 6"x6" Grids in Sand Bed Region Without Cathodic Protection 5.2.1 Bay 9D:
12119/88 to 4/24/90 Five 49-point data sets were available for this period.
(1) The data are normally distributed.
(2)
The mean model is more appropriate than the regression model.
(3)
The current mean thickness is 1021.7 + 8.9 mils.
(4)
The F-test for the significance of the difference between the mean thicknesses indicates that the differences between the means are significant.
The LSD analysis shows that this is due to the second measurement on 6/26/89 which is 33 to 52.3 mile higher than the other four.
(5)
The t-test of the last two data sets shows that the difference between the mean thicknesses is not significant.
(6) The overall analysis indicates that there was no significant corrosion from December 19, 1988 to April 24, 1990.
(7)
The best estimate of the corrosion rate during this period based on a least squares fit is -21.0 + 18.1 mile per year.
5.2.2 Bay 13A-12/17/88 to 4/24/90 Seven 49-point data sets were available for this period-(1)
The data are normally distributed.
(2)
The regression model is appropriate.
(3)
The regression model explains 97% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness + standard error is 853.1
+ 2.4 mils.
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The indicated corrosion rate + standard error is
-39.1
+ 3.4 mils per year.
(7)
F/F critical = 16.9 (8)
The measurements below 800 mils were tested and determined not to be statistically different from the mean thickness.
5.2.3 Bay 13D: 3/28/90 to 4/25/90 One 7-point data set and one 49-point data set are available for this bay covering this period.
(1) The 7-point data set is normally distributed at 5%
level of significance.
The 49-point data set is normally distributed at 1% level of significance.
However, there is a diagonal line of demarcation separating a zone of minimal corrosion at the top from a corroded zone at the bottom.
Thus, corrosion has occurred at this location.
(2) 'The mean of the 7-point data set is not significantly
- idifferent from the mean of the corresponding 7 points in the. 49-point data. set.
(3) The current means thickness is 931.9 + 22.6 mils.
It is concluded that corrosion has occurred at this location.
However, with minimal data over a one-month period, it is impossible to determine the current corrosion rate.
5.2.4 Bay 15D.
12/17/88 to 4/24/90 Five 49-point data sets were available for this period.
(1)
The data are normally distributed.
(2)
The mean model is more appropriate than the regression model.
(3)
The current mean thickness 4-standard error is 1056.5
+/- 2.3 mils.
(4)
The F-test for the significance of the difference between the mean thicknesses indicates that the-differences between the means are pot significant.
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-(5)
The t-test of the last two data sets shows that the difference between the mean thicknesses is not significant.
(6)
There was no significant corrosion from December 17, 1988 to April 24, 1990.
(7)
The best estimate of the corrosion rate during this period based on a least squares fit is
-4..6 mils per year.
5.2.5 Bay 17A; 12117/88 to 4/24/90 Five 49-point data sets were available for this period.
The initial analysis of this data indicated that the data are not normally distributed.
The lack of normality was tentatively attributed to minimal corrosion in the upper half of the 6"x6" grid with more extensive corrosion in the lower half of the grid.
To test this hypothesis,.each data set was divided into two subsets, with one containing the top three rows and the other containing the bottom four rows.
Top. 3 Rows (1) The data are normally distributed.
(2) The mean model is more appropriate than the regression model.
(3)
The current mean thickness + standard error is 1128.3
+/- 2.2 mile.
(4)
The F-test for the significance of the difference between the mean thicknesses indicates the differences between the means are not significant.
(5)
The t-test of the last two data sets indicates that the difference between the mean thicknesses is not significant.
(6)
There was no significant corrosion during this period.
(7)
The best estimate of the corrosion rate during this period based on a least squares fit is -6.8. + 3.1 mils per year.
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The data are -normally distributed.
(2)
The' mean model is more appropriate than the regression model.
(3)
The current mean thickness
- standard error 950.83
+ 5.3 mile.
(4)
The F-test for the significance of the difference between the mean thicknesses indicates that the differences between the means are not significant.
(5)
The t-test of the last two data sets indicates that the difference between the mean thicknesses is not significant.
(6)
There was no significant corrosion during this period.
(7)
The best.estimate of the corrosion rateduring this period-based on a least squares fit is -17.7 +'7.6
.mile per year.
5.3 61!x6" Grids at 51' Elevation 5.3.1 Bay 5 Area D-
.2
.*1' Elevation:
11/1/87 to 4124/90 Eight 49-point data sets were available for this period.
The initial analysis of this data indicated that the data are not normally distributed.
These data sets names start with E.
The following adjustments were made to the data:
(1) Point 29 in the 9/13/89 data is much greater than the preceding or succeeding measurements.
Therefore, this reading was dropped from the analysis.
(2)
Point 9 is a significant pit.
Therefore, it was dropped from the overall analysis and is evaluated separately.
(3)
Points 13 and 25 are extremely variable and are located adjacent to the plug which was removed from this grid.
They were also dropped from the analysis.
(4)
Point 43 in the 11/01/87 data is much less than any succeeding measurement.
Therefore, this reading was dropped from the analysis.
001/.0004A.5 0CLR00020092
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0 Page 35 of 454 With these adjustments, the first and last data sets are normally distributed at the 1% level of significance and the other five at 5%.
These data set names start with F.
It was noted that the D-Meter calibration at 0.750" yielded readings which ranged from.-l mil for one set of measurements to + 4 mile for another.
The data was adjusted to eliminate these biases.
These data set names start with G.
The final analyses are based on these adjusted data sets.
(1) The data are normally distributed.
(2)
The regression model is appropriate.
(3)
The regression model explains 57% of the total variation about the mean.
(4)
The residuals are normally distributed.
(5)
The current mean thickness s
Standard error is 745.2
+ 2.1 mils.
(6)
The indicated corrosion rate + standard error is
-4.6
+ 1.6 mile per year.
(7)
F/F critical = 1.3.
Thus, the regression is just barely significant.
(8)
The F-test for significance of the difference between the mean thickness indicates that the differences are significant.
(9)
The t-test of the last. two data sets shows that the difference between the mean thickness is not significant.
(10) The measurements of the pit at point 9 were 706,
- 746, 696, 694, 700, 688, 699 and 6B9 mils.
The mean value of these measurements is 702.3 + 6.5 mils.
A least squares fit shows that the best estimate of the corrosion rats during this period is -11.5 mils per year with R2 =31%.
The second measurement is much higher than the others.'
Dropping this point, the mean of the remaining measurements is 696.0 +2.4
- mile, and the best estimate of the corrosion rate is
-4.9 mile per year with R2 = 49%.
Recognizing that the variability of single measurements will be about 6 times the variability of the mean of.40 measure-melnts. it is concluded that the. corrosion rate in the pit is essentially the same as the overall grid.
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No. 0 Page 36 of 454 5.3.2 Bay 5 Area 51-5 at 511 Elevation: 3/31/90 to 4/25/90 Two 49-point data sets are available for this time period.
(1)
The data are not normally distributed.
This is due to a large corroded patch near the center of the grid, and several small patches on the periphery.
When the data less than the grand mean were segregated, it. was found that these subsets are normally distributed.
(2)
The t-tests of the two complete data sets and the two subsets indicate that the difference between the mean thicknesses are not significant.
(3)
The current mean thickness + standard error is 745.1
+ 3.2 mils..
It is concluded that corrosion has occurred at this location.
However, with minimal data over such a brief period, it is impossible to determine the current corrosion rate.
5.3.3 Bay 13 Area.31 Elevation 51': 3/31/90 to 4/25/90 Two 49-point data sets are available for this time period.
(1)
The data are to normally distributed.
This is due to a large corroded patch at the left edge of the grid.
When the data less than the grand mean were segregated, it was found that these subsets are normally distributed..
-(2)
The t-test of the two complete data sets indicate that the difference between the means is statistically significant.
However, the difference between the means of the two subsets is not statistically significant.
(3)
The current mean thickness is
+ standard error is 750.8 +'11.5 mils.
It is concluded that corrosion has occurred at this location.
However, with minimal data over such a brief period, it is impossible to determine the current corrosion rate.
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0 Page 37 of 454 5.3.4 Bay 15 Area 23 Elevation 51': 3/31/90 to 4/25190 Two 49-point data sets are available for this time period.
(1) The data.are. not normally distributed.
This is due to a large corroded patch.
When the data less than the grand mean were segregated, it was found that these two subsets are normally distributed.
(2)
The t-tests of the two complete data sets and the two subsets indicate that the differences between the mean thicknesses are not significant.
(3)
The-current mean thickness + standard error is 751.2
+ 3.8 milo.
It is concluded that corrosion has occurred at this location.
However, with minimal data over -such a brief period, it is impossible to determine the current corrosion rate.
5.4 6" x 6" Grids at 52' Elevation 5.4.1 Bay 7 Area 25 Elevation 52': 4/26/90 One 49-point data set is available.
(1) The data are not normally distributed.
-The subset of the data less than the mean thickness is not normally distributed.
When four points below 700 mlls were dropped from the data set, the remaining data was found to be normally
- distributed.
Therefore, the lack of normality of the complete data set is attributed to these thinner points.
Three of these could be considered to be pits (626, 657 and 676 mile) since they deviate from the mean by more than 3 sigma.
(2)
The current mean thickness + standard is 715.5 -
2.9
- mils.
It is concluded that corrosion has occurred at-this location.
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4/26190 One 49-point data set is available.
(1) The data are not normally distributed.
The subset of the data less than the mean thickness is normally distributed.
Thus, the lack of normality of the complete data set is attributed to a large corroded patch at the left side of the grid.
(2)
The current mean thickness + standard error is 724.9
+ 2.9 mils'.
(3)
It is concluded that corrosion has occurred at this location.
5.4.3 Bay 23 Area 32 Elevation 521: 4126190 One 49-point data set is available.
(1) The data are not normally diistributed.
The subset of the data less than the mean thickness is normally distributed.
Thus, the lack of normality of the complete data set is attributed to these corrosion patches.
(2) The current mean thickness + standard error is 698.3
+ 5.0 mils.
It is concluded that corrosion has occurred at this location.
5.4.4 Bay 19 Area 13 Elevation 52': 4/26/90 One 49-point data set is available.
(I)-
The data are normally distributed.
- However, two adjacent points differ from the mean by 3 sigma and 5 sigma.
Thus, there is a pit.
(2)
The current means thickness
- standard error is 712.5
+ 3.1 mils.
It is concluded that some corrosion has occurred at this location.
i i
001/0004A.9 OCLR00020096
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0 Page 39 of 454 5.5 6" x 6" Grids at 87' Elevation 5.5.1 Bay 9 871 Elevation: 11/6/87 to 3128/90 Five 49-point data sets were available for this period.
(I)
The data are normally distributed.
(2)
The mean model is more appropriate than the regression model.
(3)
There was no significant corrosion during this period.
(4).
The current mean thickness, standard error is 619.9
+ 0.6 milo.
(5)
The best estimate of the corrosion rate during this period based on a least squares fit is -0.2 + 0.9 mils per year.
5.5.2 Bay 13 87' Elevation: 2I/10/87 to 3/28/90 Five 49-point data sets were available for this period.
(1) The data are normally distributed.
(2)
The mean model is more appropriate than the regression model.
(3)
There was no significant corrosion during this period.
(4)
The current mean thickness + standard error.is 636.5
+ 0.8 mile.
(5)
The best estimate of the corrosion rate during this period based on a least squares fit is zero mils per year.
5.5.3 Bay 15 87' Elevation: 11/10/87 to 3/28/90 Five 49-point data sets were available for this 1period.
(1)
The data are normally distributed.
(2)
The mean model is more appropriate than the regression model.
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There was no significant corrosion during this period.
(4)
The current mean thickness + standard error is 636.2 1.1mile.
(5) The best estimate of the corrosion 'rate during this period based on a least squares fit is zero mils per year.
001/0004A.11 OCLROO020098