ML20210A822
| ML20210A822 | |
| Person / Time | |
|---|---|
| Site: | Brunswick  |
| Issue date: | 06/19/1986 |
| From: | Marisa Herrera, Mehta H, Ranganath S, White M GENERAL ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML19292F889 | List: |
| References | |
| DRF-137-0010, DRF-137-10, MDE-80-0686, MDE-80-0686-R01, MDE-80-686, MDE-80-686-R1, SASR-86-39, NUDOCS 8609170352 | |
| Download: ML20210A822 (24) | |
Text
MDE #80-0686 SASRf86-39 DRF-#137-0010 Re v. l EVALUATION OF THE BRUNSWICK 2 RECIRCULATION OUTLET N0ZZLES Al AND B1 AXIAL INDICATIONS JUNE 19, 1986 PERFORMED BY: dMe Margarft A. White Engineer Structural Analysis Services
/
% rcos L. Herrera Senior Engineer Structural Analysis Services VERIFIED BY:
- b Hardayal S. Mehta Principal Engineer l
Stuctural Analysis Services APPROVED BY:
~ ~ ^[ ^
Sampath Ranfanath, Manager Structural Analysis Services h
P a
TABLE OF CONTENTS PAGE 1.
INTRODUCTION 1
2.
FLAW LOCATION 2
3.
APPLIED STRESS 3
3.1 Pressure Stress 3
3.2 Weld Residual Stress 4
3.3 Applied Stress Intensity 5
4.
CRACK GROWTH ANALYSIS 6
4.1 Crack Growth Rate Behavior 6
4.2 Crack Growth Prediction 8
5.
FLAW ACCEPTANCE EVALUATION 9
5.1 Assessment of Flaw in Ni-Cr-Fe 9
Alloy 182 Mbterial 5.2 Assessment of Flaw in Low Alloy 9
Material 5.3 Assessment of Flaw in Stainless Steel 11 Material 6.
SUMMARY
OF CONSERVATISMS 12 7.
CONCLUSIONS 13 8.
REFERENCES -
14 TABLES FIGURES
- 1. INTRODUCTION
~
During ultrasonic testing (UT) inspection of the Brunswick 2. reactor recirculation outlet nozzles, indications were discovered in nozzles Al and Bl. The indications were discovered in the recirculation outlet safe end to nozzle Ni-Cr-Fe Alloy 182 weld butter material.
Both indications were axially oriented with a depth of.25 inch.
The lengths of the indications were approximately.30 inch.
An evaluation of the indications was performed to determine if continued operation without repair can be justified for an additional fuel cycle (18 months, 12000 hours). Crack growth by IGSCC mechanism was calculated and comparison with end-of-cycle allowable flaw sizes was performed.
The results of the fracture mechanics analysis show that cwt?.nued operation for at least 12000 hours without repair is justified. The ASME Code Section X1 safety margins would be maintained throughout. this period. In addition, substantial leak-before-break margin against pipe as well as nozzle failure is assured.
l l
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2.
FLAW LOCATION The detected indications were found in recirculation outlet nozzles Al and Bl.
The two indications were located in the Ni-Cr-Fe Alloy 182 weld butter material.
The indications were axially oriented.
Figure 1 shows a schematic of the veld area and approximate location of the indications.
The indications were located on the low alloy side of the weld.
The indications were determined to be. 25" deep with lengths of of approximately. 3 inch.
The approximate wall thickness at the weld location is 2-1/16 inch.
Therefore, the indications are approximately 12% of wall thickness in depth. -, _ -
3.
APPLIED STRESS Only the sustained stress contributes to the crack growth by Intergranular Stress Corrosion Cracking (IGSCC)
The primary contributors to the sustained stress are pressure, veld residual and thermal expansion.
However, since the detected indications were axially oriented, only the pressure and weld residual stress will produce significant stress in the hoop direction.. All other hoop stresses including the secondary stresses are negligible.
3.1 Pressure Stress To determine the stress due to pressure, the design pressure was conservatively used. The design pressure is 1250 psi.
The corrasponding hoop stress is:
O "E
h 2t where P = internal pressure = 1250 psi D = Diameter = 29.875" t = pipe thickness = 2.0625" (1250)(29.875)
=
h 9.05 ksi
=
2(2.0625)
As mentioned earlier the stress due to pressure is the only primary stress and all secondary hoop stress is negligible.
The hoop stress due to pressure must be combined with the weld residual stress to determine the total driving force for any subsequent crack extension by ICSCC...
1 3.2 Weld Residual Stress Weld residual stress is the primary contributor to the sustained stress.
Therefore, the determination of the weld residual stress is important.
The veld residual stress can be determined by.either experimental or analytical methods.
Analytical determination of weld residual stress has been shown to reliably predict the general trends and magnitudes of the stress distributions.
An analytical evaluation of weld residual stress at a recirculation outlet safe end to nozzle weld similar to Brunswick 2 was performed previously (Reference 1).
Therefore,-
the veld residual stress distribution used in the crack growth evaluation was obtained from this analysis. A brief description of this weld residual stress analysis is provid,ed next.
The elastic-plastic finite element analysis to determine weld residual stress consisted of three main parts:
1)
Finite element modeling 2)
Thermal analysis 3)
Residual stress analysis The analysis was performed using the ANSYS computer program.
The axisymmetric finite element model used in the thermal stress analysis is shown in Figure 2.
Also shown in the figure are the stainless steel piping material, low alloy nozzle material, and Ni-Cr-Fe Alloy 82 and 182 weld material.
Figure 3 shows the residual hoop and axial stress on the inside surface of the pipe.
Also shown on the same figure are the interfaces between the three materials present.
The results show that the hoop stress is approximately 20 ksi greater than the axial stress in the area of cracking.
The axial cracking in the Brunswick 2 nozzle confirms that residual heap stress was a major contributing factor. _
Figure 4 shows the distribution of through-wall hoop and axial residual stresses at the cracked cross-section. The residual hoop stress remains highly tensile throughout the thickness.
The residual axial
-stress decreases rapidly and becomes compressive at approximately 30% of pipe wall depth.
It should be noted that the residual stress evaluation was based on assumed yield strength values which were on the high side so that the predicted residual stresses would be overestimates.
If one used Code minimum yield strength values '(which are about half the values used in the analysis) the predicted residual stresses would be significantly lower. Clearly the residual hoop membrane stress of 70 ksi used in this analysis represents a conservative upper bound.
The total sustained stress is the combination of the stress due to pressure and weld residual.
T p
Residual
= 9.0 + 70.0 280 ksi 3.3 Applied Stress Intensity The applied stress intensity factor for an applied stress of 80 ksi can be calculated using the methodology given in Section XI, Appendix A of the ASME Code (reference 2).
I? sing the ASME Code methodology an applied stress intensity factor of 52 ksi /in is obtained for the.25 inch deep,
.30 inch long indication.
I '
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4.
CRACK GROWTH ANALYSIS 4,.1 Crack Growth Rate Behavior To determine the expected crack growth during the next operating cycle, the crack growth behavior of the Ni-Cr-Fe Alloy 182 material must ne known.
Figure 5 shows the accumulated crack growth data developed by General Electric (Company Proprietary) for Ni-Cr-Fe Alloy 182.
For purposes of comparison, Figure 5 also shows crack growth rates for sensitized stainless steel material.
The data shown in Figure 5 were obtained from two sets of ' specimens (i) constant load specimens -(ii) bolt loaded WOL specimens.
The constant load tests were run at starting stress intensity levels of 23, 32 and 40 ksi /in. With crack growth the actual K values are somewhat higher.
Tests were run with IT-WOL specimens in the As Welded (AW) + Post Weld Heat Treated (PWHT) + Low Temperature Sensitization (LTS) condition in 0.2 ppm oxygenated water at 550*F.
The loading consisted of constant load + slow cyclic loading to assure IGSCC growth.
Crack growth was monitored by compliance techniques.
Results of the constant load tests show relatively flat crack growth rates for the range of K levels tested.
The bolt load WOL tests were tested at an initial K value of 16 ksi /in in 0.2 ppm oxygenated water at 550*F.
Tests were run both in the AW and AW + PWHT conditions.
The observed crack growth was somewhat lower than that in the const nt load tests.
The results shown in Figure 5 show a bounding
-5 value of 4.5X10 in/ hour for Alloy 182 in 0.2 ppm oxygenated water at_
550*F.
In addition to the General Electric information, data have been obtained by other investigators.
Table 1 shows a summary of the available data for Ni-Cr-Fe Alloy 182 from various references.
The data includes a wide range of test environments.
Specifically, for 0.2 ppm oxygenated water, the maximum observed crack growth rate in all the tests
-5 is 4.5X10 in/ hour at K value of 42 ksi /in.
This was generated using constant load CT specimens which tend to over-estimate the crack growth -.
rate because of plasticity effects.
Studies on stainless steel specimens have shown that the equivalent K value considering elastic-plastic behavior is much higher than the values calculated using linear elastic f'racture mechanics.
Thus, if the applied K value is calculated on a J-integral basis the equivalent K value would be in the range of 50-60 kai /in.
Since the stresses intensity factor in the safe end is predominantly due to displacement controlled. veld residual stresses, it is conservative to use test data from load controlled CT tests.
Another way ~ to assess the maximum crack growth rate in Figure 5 is to compare the results for 0.2 ppm 0 veter with more severe environment 2
conditions (i.e., higher oxygen content).
.It is seen that even with 8 ppm oxygenated water the maximum crack growth rate at K = 44.5 kai /in is
-5 7X10 in/ hour.
Since crack growth' rate for 8 ppm oxygenated water is expected to be 2-5 times higher than that for 0.2 ppm 0 (based on 2
available data for stainless steel and ECP correlations)' the SWRI data
-5
[
would suggest a value not higher than 3.5X10 in/ hour for 0.2 ppm 02
-5 water.
Thus the bounding value of 4.5X10 in/ hour from the GE data appears to be conservative.
Slow strain rate test results (item 2 in Table 1) are really not relevant for purposes of crack growth prediction since the slow strain tests (or CERT) are done under increasing strain (like in a strain controlled tension test) and are used only to determine IGSCC susceptibility.
The high crack growth rate is expec'ted because of the increasing applied strain.
This information is provided only in the interest of completeness since Table 1 shows all available data on Alloy 182.
Based on the significant data in Figure 5 and Table 1, a bounding
~
~5 value of 4.5X10 in/ hour is reasonable for K values up to 44.5 ksi in.
Furthermore the data.in Figure 5 show that the crack growth rate is not a strong function of the applied K for K values up to 44.5 ksi /in.
Thus it is reasonable to extrapolate the curve to a K value of 52 ksi /in.
(corresponding to the indication in the safe end weld). h
For the purposes of the analysis described here a bounding crack
-5 growth rate of SX10 in/ hour (or approximately 0.4 in/ year)' is conservatively used here.
-4.2 Crack Growth Prediction The total crack growth and end-of-cycle crack size can be determined by multiplying the plateau crack growth rate and the number of opera' ting
-5 hours.
For purposes of this evaluation, a crack growth rate of 5 X 10 in/hr will be used.
For 18 months (12000 hours), the crack growth is
-5 (5X10 in/hr)(12000 hours) =.6 inch. Therefore, the final end-of-cycle crack depth is.6 inch +.25 inch =.85 inch.
m
l 1
- 5. FLAW ACCEPTANCE EVALUATION l
In this section, the end of cycle predicted crack depth is evaluated to assure adequate margin against failure. Two evaluations 're performed;
- 1) Assuming crack growth into the Ni-Cr-Fe Alloy 182 material, and
- 2) Crack growth into the Low Alloy nozzle material.
5.1 Assessment of Flaw in Ni-Cr-Fe Alloy 182 Material Section. XI, paragraph IWB-3640 of the ASME code (reference 2),
provides acceptance criteria for austenitic piping. The same criteria can be used for Ni-Cr-Fe Alloy 182. For axial flaws during normal operation, subparagraph IWB-3641.3 (table IWB-3641-3) gives, allowable flaw sizes for given stress ratios and I /(Rt) ratio (1 = end of life crack length, g
g R= pipe radius, t= pipe thickness). Conservatively using a stress ratio of.6.-and i /(Rt)I
=1, the allowable flaw depth is 1.55 inches.
Even f
if one applied a conservative factor of 2/3 for weldments (Reference 3),
the allowable end of cycle crack depth is 1.03 ie 't.
Comparison with the
.85 inch end of cycle crack depth predicted in section 4.3 shows that the flaws are acceptable for an additional cycle of operation without repair.
5.2 Assessment of Flaw in Low Alloy Material The crack growth evaluation of section 4.3 considered radial crack through the thickness in the~ Ni-Cr-Fe Alloy 182 material only.
Nevertheless, an alternate evaluation is performed in this section by assuming a through-wall crack with the crack tip in the low alloy steel nozzle.
It should be noted that the crack is short and is contained in the weld.
However, for purposes of this analysis, the crack is assumed to extend into the low alloy material.
_9
Although the analysis described here considers the case of a crack in the low alloy steel section, it should be emphasized that the extent
,o.f any crack growth in the nozzle is minimal.
If there is any crack propagation in the Low Alloy steel nozzle material, it is likely to be due to fatigue cycling.
However, for the recirculation outlet nozzle, cyclic stresses are small (mainly due to change of pressure due to startup/ shutdown). The residual stresses do not contribute to the stress intensity range.
For a hoop stress of 9 kai, the stress intensity range
~
for a through-wall flaw of 0.3 in length is approximately 6 ksi /in.
For
-5 this range, the expected crack growth rate is less than 10 in/ cycle.
Therefore, even 'if one considers 20 pressurization events, the crack advance into the low alloy steel is less than 0.2 mil during one fuel cycle.
Therefore, operation 'as is' without repair will not lead to significant crack extension into the low alloy steel.
Thus degradation of the reactor vessel, or problems in subsequent repair would not arise.
The following analysis further demonstrates that large through cracks can be tolerated and that potential cracking in the nozzle poses no structural margin concerns.
To reliably predict limiting crack lengths in a material, a knowledge of the fracture mode is required.
For materials such as low alloy steel the fracture occurs in a ductile mode at the reactor operating temperature.
Linear Elastic Fracture Mechanics (LEFM) can be used when the fracture occurs in the brittle mode.
If failure is characterized by significant yielding, limit load methodology is appropriate.. Evaluation. using LEFM is conservative and is used in this l
analysis.
To determine the. allowable axial through-wall crack length in the nozzle, the stress intensity factor for various crack lengths were calculated.
Figure 6 shows the K through-wall' distribution.
Figure 7 7
shows the allowable crack arrest fracture toughness obtained from Figure A-4200-1 of Section XI of the ASME Boiler and Pressure Vessel Code (Reference 2). Although the value for (T-RTET) exceeds the value for the
given curve, a conservative.value of 200 ksi /in for the arrest fracture toughness can be assumed.
Figure 6 shows that a through-wall axial crack length of 30 inches is needed to exceed the arrest fracture toughness.
This evaluation also demonstrates that a sizeable leak-before-break margin exists against unstable crack propagation.
5.3 Assessment of Flav in Stainless Steel Material The propagation of the crack into the stainless steel safe end is controlled by two effects: 1) the weld residual hoop stress, and 2) the heat affected zone.
The residual hoop stress drops significantly with distance from the veld interface.
The reduction of the residual hoop stress removes the driving force for crack extension. Furthermore, since the safe end is made of low carbon stainless steel (0.02% carbon) IGSCC crack extension through the heat affect zone is expected to be small.
Any crack extension in the material beyond the heat affected zone would be negligible. Thus the crack is not likely to extend into the safe end material.
Nevertheless, the critical crack length in the safe end is large so that a substantial leak-before-break margin is maintained.
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- _ _ ~,
6.
SUMMARY
OF CONSERVATISMS It is 'useful to identify the conservatisms implicit in the evaluation of the veld indications.
The inherent conservatisms and the additional conservatisms assumed in the analysis are identified here.
The observed cracks are short and' shallow. 0.25 in depth in a section thickness of approximately 2.1 in.
This is approximately 12% of the wall, almost equal to the acceptance standards for piping velds for which no evaluation is required.
The length of the crack is inherently limited because of the surrounding low carbon stainless steel safe end and the low alloy nozzle material where crack extension is not expected.
Because of the axial orientation, even if the cracks were through wall, substantial margins are still assured.
Thus there are no crack sizing concerns.
The evaluation assumed extremely high residual stresses (because of limiting yield stress estimates) and thus over-estimated the crack growth.
l The assumed crack growth rate was the upper bounding value based on extensive GE and EPRI data.
i The allowable flaw sizes included the NRC factor of 2/3 over and above the ASME Code allowable values. -
I 7.
CONCLUSIONS A f racture mechanics evaluation of the axial indications found in the Brunswick 2 Recirculation outlet nozzle Al to safe end Ni-Cr-Fe 182 weld butter material was performed. The evaluation considered crack growth in the Ni-Cr-Fe Alloy 182 material as well as the Low Alloy material.
The results of the fracture mechanics analysis show that continued operation for at least 12000 hours without repair is justified.
The ASME Code Section II safety margins would be maintained throughout this ' period. In addition, substantial leak-before-break margin at the cracked location is assured.
From a practical viewpoint it is important to recognize that the observed indications are relatively benign since they are short, shallow creviced and oriented axially.
The axial flaw is. expected to arrest
_ ce it extends beyond the veld into annealed material.
Even if the crack were sign'ificantly orienteil circumferentially, still safety margins are assured. Furthermore even if the observed crack were assumed to be a through wall crack, (axial or circumferential) substantial structural margins are still maintained.
Thus operation with the observed indications is fully justified.
8.
REFERENCES 6
1.
General ' Electric Report Number NEDC-30730-P,
DRF B31-00106, Class III, September 1984, Proprietary Information 2.
American Society of Mechanical Engineers Boiler and Pressure Vessel Code, 1983 Edition 3.
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