ML051950570
| ML051950570 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 07/14/2005 |
| From: | Hartz L Virginia Electric & Power Co (VEPCO) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| Download: ML051950570 (15) | |
Text
VIRGINIA ELECTRIC AND POWER COMPANY
- RICHMOND, VIRGINIA 23261 July 14, 2005 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555 Serial No.05-384 NLOS/GDM RO Docket No.
50-281 License No.
DPR-37 VIRGINIA ELECTRIC AND POWER COMPANY SURRY POWER STATION UNIT 2 ASME SECTION XI INSERVICE INSPECTION PROGRAM EMBEDDED FLAW EVALUATION During the ten-year reactor vessel inservice inspections performed during the recently completed Surry Unit 2 Spring 2005 refueling outage, an embedded circumferential indication was detected in the Unit 2 reactor vessel inlet nozzle to shell weld region.
The dimensions of the embedded flaw exceeded the allowable flaw size specified in ASME Section XI paragraph IWB-3512, and consequently required a flaw evaluation to be performed to determine whether the detected flaw is acceptable for continued plant operation without being repaired. The flaw evaluation was performed by Westinghouse Electric Company in accordance with the guidelines of paragraph IWB-3600 of the code and concluded that the detected indication in the Surry Unit 2 inlet nozzle to shell weld is acceptable for continued operation without repair. The flaw evaluation was previously provided to the Surry NRC Senior Resident Inspector and is included in the attach men t for your i nformation.
If you have any questions or require additional information, please contact Mr. Gary D.
Miller at (804) 273-2771.
Very truly yours, Leslie N. Hartz u Vice President - Nuclear Engineering Attachment Commitments made by this letter: None
Serial No.05-384 Docket 50-281 Page 2 cc:
U. S. Nuclear Regulatory Commission Region I I Sam Nunn Atlanta Federal Center 61 Forsyth St., SW, Suite 23T85 Atlanta, Georgia 30303 Mr. S. R. Monarque U. S. Nuclear Regulatory Commission One White Flint North 11 555 Rockville Pike Mail Stop 8H12 Rockville, MD 20852 Mr. R. E. Martin NRC Lead Project Manager - North Anna and Surry U. S. Nuclear Regulatory Commission One White Flint North 1 1555 Rockville Pike Rockville, Maryland 20852 Mr. N. P. Garrett NRC Senior Resident Inspector Surry Power Station Mr. R. A. Smith Authorized Nuclear Inspector Surry Power Station
Serial No.05-384 Docket 50-281 Attachment Embedded Flaw Evaluation for the Surry Unit 2 Reactor Vessel Inlet Nozzle to Shell Weld Region Virginia Electric and Power Company (Do mini on)
Surry Power Station Unit 2
Embedded Flaw Evaluation For The Surry Unit 2 Reactor Vessel Inlet Nozzle To Shell Weld Region Revision 1 June 2005 Author: -Q&-
C. K. Ng, /
Piping Analysis & Fracture Mechanics 3
Verifier:
p->+
(s,J+ikraisorn, Piping Analysis & Fracture Mechanics Approved:
S. Swamy, Manager, Piping Analysis & Fracture Mechanics
1.O Introduction Revision 0
During the Spring 2005 Outage, an embedded circumferential indication was detected in Surry Unit 2 reactor vessel inlet nozzle to shell weld region. The indication is classified as an embedded flaw since it meets the ASME Section XI IWA-3300 guidelines [l]. The embedded indication is located near the outside surface and can be detected from outside the reactor vessel shell and the inlet nozzle bore region. The dimensions of the embedded flaw detected exceeded the allowable flaw size given in Section XI Table IWB-3512 [l]. The purpose of this flaw evaluation is to demonstrate using the Section XI IWB 3600 flaw evaluation guidelines that the detected embedded flaw is acceptable for continual plant operation without repair.
Description Date Original Issue May 2005 Record of Revision 1
Revised the ASME code edition to 1989 Edition June 2005 Page 2 of 12
2.0 ASME Code Acceptance Criteria There are two alternative sets of flaw acceptance criteria for continued service without repair in paragraph IWB-3600 of ASME Code Section XI. Either of the criteria below may be used, at the convenience of the analyst.
- 1.
- 2.
Acceptance Criteria Based on Flaw Size (IWB-3611)
Acceptance Criteria Based on Stress Intensity Factor (IWB-3612)
The most beneficial criteria have been used in generating the flaw evaluation chart for the as-found indication in the inlet nozzle to shell weld region of the Surry Unit 2 reactor vessel.
2.1 Criteria Based on Flaw Size The code acceptance criteria stated in IWB-3611 of Section XI are:
af <.1 a, and af <. 5 a where af
=
a,
=
For Normal Conditions (Upset & Test Conditions Inclusive)
For Faulted Conditions (Emergency Condition Inclusive)
The maximum size to which the detected flaw is calculated to grow at the end of a specified period, or until the next inspection time.
The minimum critical flaw size under normal operating conditions (Upset And Test Conditions Inclusive)
The minimum critical flaw size for initiation of nonarresting growth under postulated faulted conditions.
(Emergency Conditions Inclusive)
To determine whether a surface flaw is acceptable for continued service without repair, both normal and faulted condition criteria must be met.
2.2 Criteria Based on Stress Intensity Factor The term stress intensity factor (K,) is defined as the driving force on a crack. It is a function of the size of the crack and the applied stresses, as well as the overall geometry of the structure. In contrast, the fracture toughness (Kla, K,,)
is a measure of the resistance of the material to crack propagation. It is a material property and a function of temperature.
Page 3 of 12
The criteria are:
K1'i For normal conditions (upset & test conditions inclusive)
K, <x K
K, < 2 For faulted conditions (emergency conditions inclusive) 47 where KI =
The maximum applied stress intensity factor for the flaw size af to which a detected flaw will grow, during the conditions under consideration, for a specified period, or to the next inspection.
KI, =
Fracture toughness based on crack arrest for the corresponding crack tip temperature.
KI, =
Fracture toughness based on fracture initiation for the corresponding crack tip temperature.
To determine whether a flaw is acceptable for continued service without repair, both normal and faulted condition criteria must be met.
Page 4 of 12
3.0 Allowable Flaw Size Determination 3.1 Critical Flaw Size One of the key parameters used in the evaluation of the indication discovered are two critical flaw depths. The first of these critical flaw depths is calculated using the thermal transient stresses from the governing normal, upset, and test conditions. The second is calculated based on stresses for the governing emergency and faulted conditions. The thermal transient stresses used were obtained from the Westinghouse generic 3-loop plant thermal transient stress database [2]. Thermal transient stresses were selected from the generic stress database to provide a conservative flaw evaluation for Surry Unit
- 2. Critical flaw depths are calculated based on the two sets of conditions using the ASME Code criteria as discussed in Section 2.0.
3.2 Fracture Toughness The other key element in the evaluation of the indication discovered is the fracture toughness of the material. The fracture toughness for ferritic steel has been taken directly from the reference curves of Appendix A in Section XI of the ASME Code. In the transition temperature region, these curves can be represented by the following equations:
KI, = 33.2 + 2.806 exp. [0.02 (T-RTNDT + 1 OO"F)]
K,, = 26.8 + 1.233 exp. [0.0145 (T-RTNDT + 1 60°F)]
where Ki, and KI, are in ksi &.
The upper shelf temperature regime requires utilization of a shelf toughness which is not specified in the ASME Code and a value of 200 ksi&is used here This value is consistent with general practice in such evaluations, as shown for example in Reference
[3], which provides the background and technical basis of Appendix A in ASME Section XI Code.
Once the critical flaw sizes, such as a, under normal operating conditions, or ai under faulted conditions for the IWB-3611 criteria and the stress intensity factors, KI, for the IWB-3612 criteria have been determined, the allowable flaw size can then be obtained based on the most beneficial results from both the flaw size and stress intensity factor criteria.
Page 5 of 12
4.0 Fatiaue Crack Growth In applying the ASME Code acceptance criteria as introduced in Section 2.0, the final flaw size (at) is defined as the maximum flaw size to which the detected flaw is calculated to grow at the end of a specified period, or until the next inspection time. The allowable initial flaw size at the time of detection is obtained by taking into account the fatigue crack growth of this initial flaw for a given time duration. Fatigue crack growth analysis has been performed for the vessel inlet nozzle to shell region to determine crack growth as a function of service life. The design thermal transients and cycles for Surry Unit 2 have been considered in the fatigue crack growth analysis.
4.1 Crack Growth Rate Reference Curves The crack growth rate curves used in the analyses were taken directly from Appendix A of ASME Section XI Code [l]. The air environment curve was used for the embedded indication detected.
The crack growth rate reference curve for air environment is a single curve, with growth rate being a function of applied AKI.
and with da/dN = C,(AK1)3.07 c, = 1.99 x 10"O s S =25.73(2.88 - R)".07
- where,
~ da
= Crack growth rate, micro-inchedcycle dN AK, = Stress intensity factor range, ksi&
R
= Kim,,/
Kim,,
4.2 Stress Intensity Factor The stress intensity factor for an embedded flaw was calculated using the procedures given in Section XI Article A-3000 [l] which is applicable to an embedded flaw in a finite medium subjected to an arbitrary stress profile. It is a function of the stresses at the cross-section where the indication is located, along with the material properties. This stress intensity factor can be expressed in terms of the effective membrane and bending stress components as follows:
Page 6 of 12
where Om, Ob
Membrane and bending stresses Mm, Mb
Correction factors for the membrane and bending stresses a
=
Half crack depth Q
= Flaw shape parameter 4.3 Analysis Methodology The methods used in the crack growth analysis reported here are the same as those recommended by Section XI of the ASME Code. The analysis procedure involves postulating an initial flaw at specific regions and predicting the growth of that flaw due to an imposed series of loading transients. The input required for a fatigue crack growth analysis is basically the information necessary to calculate the parameter AKI which depends on the crack and structure geometry as well as the range of applied stresses in the area where the crack exists. Once AKI is calculated, the growth due to that particular stress cycle can be calculated by the reference crack growth curves in Section XI Appendix A. This increment of growth is then added to the original crack size, and the analysis proceeds to the next transient. The procedure is continued in this manner until all the transients predicted to occur in the period of evaluation have been analyzed.
Page 7 of 12
5.0 Flaw Evaluation Flaw Parameters Flaw Depth (2a) 5.1 Embedded Flaw Evaluation Chart Nozzle Bore Vessel Shell 0.65 in 0.59 in An embedded flaw evaluation chart is generated by first determining the allowable flaw depths as a function of proximity to the surface. These flaw depths were determined directly using the flaw evaluation guidelines in ASME Section XI Appendix C as discussed in Section 2.0. The allowable flaw depths were determined based on a flaw shape aspect ratio of 20.1, which has been found to be a conservative value based on the UT inspection result at the vessel inlet nozzle to shell weld region. Note that for indications that are very close to the surface, the allowable flaw depth is small because it is limited by the surface proximity rules in Section XI IWA-3300. The initial allowable flaw depth is then determined by subtracting the fatigue crack growth for a given time duration from the final allowable flaw size determined using the methodology discussed in Section 2.0. The embedded flaw evaluation chart is then generated by plotting the initial allowable flaw depth as a function of proximity to the surface as shown in Figure 1.
The maximum allowable initial flaw depth plotted on the embedded flaw evaluation chart cannot exceed the surfacelembedded flaw demarcation line, because above that line, the flaw would have to be considered as a surface flaw.
Flaw Length (4) a I t Wall Thickness (t) 5.2 Evaluation Results 5.5 in 5.5 in 0.06 0.05 9.13 in 9.13 in Two sets of flaw parameters were obtained for the indication detected at the inlet nozzle to shell weld region. One from the outside vessel shell and the other from the inlet nozzle bore region:
I Distance from Surface (S) I 4.25 in 3.71 in I
I Page 8 of 12
Using the bounding values from both sets of flaw parameters, a / t =
0.3319.13 =0.036 a10 =
0.05 S
=
3.71 in 6
=
S + a
=
4.04in 61t =
0.442 The bounding values for a/t and 6/t for the detected indication are then plotted in Figure
- 1. It can be seen that the indication detected is acceptable since it lies below the acceptable initial flaw size limit lines.
Page 9 of 12
6.0 Conclusion The embedded circumferential indication detected at the Surry Unit 2 inlet nozzle to shell weld have been evaluated in accordance with the ASME Section XI IWB 3600 flaw evaluation guidelines.
As demonstrated in Figure 1, the detected indication is acceptable for continual operation without repair.
Page 10 of 12
7.0 References
- 1.
ASME Code Section XI 1989 Edition, Rules for In-service Inspection of Nuclear Power Plant Components, The American Society of Mechanical Engineers, New York, New York, USA.
- 2.
CB&I Stress Report Contract No. 71-2632 Rev. 2 dated December 1981 including Addendum dated April 12, 1985.
- 3.
Marston, T. U. et. al. Flaw Evaluation Procedures: ASME Section XI Electric Power Research Institute Report EPRI-NP-719-SR, August 1978.
Page I I of 12
0.1C 0.09 0.08 h s v
Q 0.07 rc!
?x m
m I%? 0.0s c
Y 0
.c.'
.- e 3
5 n
3 cc
- u. 0.03 Y-E I
= 0.05 m
0.04 I). 02 0.(11 0."
Figure 1 Surry Unit 2 Sub-surface Flaw Evaluation far Reactor Vessel Inlet Nozzle to Shell Weld Flaws above Demarcation line must be and below the Acceptable per criteria of IWB 3600 Initial Flaw Size are acceptable -
.:p tl>
.n 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.49 0.45 0.59 Distance from Surface @;tj