ML19239A114
| ML19239A114 | |
| Person / Time | |
|---|---|
| Site: |  |
| Issue date: | 08/19/2019 |
| From: | Shawn Smith Wolf Creek |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| ET 19-0015 | |
| Download: ML19239A114 (10) | |
Text
..
W$LFCREEK 'NUCLEAR OPERATING CORPORATION Stephen L. Smith August 19, 2019 Vice President Engineering ET 19-0015 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
References:
- 1) Letter WO 18-0031, dated July 23, 2018, from J. H. McCoy, WCNOC, to USNRC
- 2) Letter dated December 12, 2018, from R. Pascarelli, USNRC, to A. C.
Heflin, WCNOC, "Wolf Creek Generating Station, Unit 1 - Request for Relief From Requirements of the American Society of Mechanical Engineers Boiler and Pressure Vessel Code, Code Case N-666-1 (EPID L-2018-LLR-0101)"
Subject:
Docket No. 50-482: lnservice Inspection (ISi) Program Relief Request to Utilize Code Case N-666-1, "Weld Overlay of Class 1, 2, and 3 Socket
_Welded Connections,Section XI, Division 1" To Whom It May Concern:
Pursuant to 10 CFR 50.55(z)(2), Wolf Creek Nuclear Operating Corporation (WCNOC) hereby requests Nuclear Regulatory Commission (NRC) approval of relief from the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Nuclear Code Case N-666-1, "Weld Overlay of Class 1, 2, and 3 Socket Welded Connections,Section XI, Division 1."
A previous leak from a through wall flaw at Wolf Creek Generating Station (WCGS) led to the submittal of a relief request (Reference 1) to utilize Code Case N-666-1. The NRC approved safety evaluation is contained in Reference 2. Due to extent of condition from this through wall flaw, there is the potential for requiring similar weld overlays in other locations, which have the same carbon content. To perform repairs at these locations in accordance with the requirements of ASME Code and Code Case N-666-1, the E~sential Service Water (ESW) system must be isolated from these locations. Previous experience has shown that completely isolating the ESW system in these locations can be difficult. thus, draining the ESW system would likely be required to perform the repairs. This activity would render at least one train of the ESW system inoperable.
Because the ESW system is the safety related cooling medium for most safety related systems at WCGS, making one train of the ESW system inoperable would require entry into several WCGS Technical Specification (TS) action statements. It would be a significant challenge to complete the repairs and restore the ESW system to operable status within the TS completion times to P.O. Box 411 / Burlington, KS 66839 / Phone: (620) 364-8831 An Equal Opportunity Employer M/F/HCNET
ET 19-0015 Page 2 of 2 avoid a TS-required plant shutdown. WCNOC proposes that establishing the condition of ESW drained to complete a future/potential weld overlay will result in a hardship without a compensating increase in quality and safety. WCNOC requests relief from the maximum carbon content requirement to repair Class 3 components by performing water backed welding.
Additionally, WCNOC requests relief from the requirement to verify that socket weld failure is a
.result of vibration fatigue. These proposeq alternatives are being submitted in accordance with 10 CFR 50.55a(z)(2), "hardship without a compensating increase in the level of quality and safety," to allow WCNOC to perform adequate repairs as needed, while avoiding the increased risk associated with a plant shutdown. This request to utilize Code Case N-666-1 in this way on a larger set of components does not supersede or negate the need for the previous approval.
The attachment to this letter provides the reason for the request and the proposed alternative.
WCNOC requests your review and approval by June 1, 2020.
There are no commitments contained within this submittal. If you have any questions concerning this matter, please contact me at (620) 364-4093 or Ron Benham at (620) 364-4204.
Sincerely,
- Stephen L. Smith SLS/rlt
Attachment:
Relief Request from ASME Code N-666-1 cc: S. A. Morris (NRC), w/a N. O'Keefe (NRC), w/a B. K. Singal (NRC), w/a Senior Resident Inspector (NRC), w/a
Attachment to ET 19-0015 Page 1 of 8 RELIEF REQUEST FROM ASME CODE N-666-1
- 1. ASME Code Component(s) Affected:
Affected components are limited to the Component Cooling Water (CCW) Heat Exchanger (HX) (EEG01A/EEG018) raw water (Essential Service Water (ESW)) socket welded connections. The CCW HX socket welded connections are designed and constructed to the ASME Code, Section Ill, Class 3. Both the piping and the HX were to the 1974 Edition with Summer 1975 Addenda. The specific socket weld connections are as follows:
- EGV0029, CCW HXA INTERPASS WTRBX DRAIN
2. Applicable Code Edition and Addenda
ASME Code,Section XI, 2007 Edition with 2008 Addenda
- 3. Applicable Code Requirement: (Reference 1)
ASME Code Case N-666-1, Paragraph 1(b):
Use of this Case is limited to Class 1, 2, or 3, NPS 2 (ON 50) and smaller socket welded connections with base material of P-No. 1 Group 1 and P-No. 1 Group 2, or P-No. 8. For water backed welding the carbon content of P-No. 1 Group 2 materials shall be limited to 0. 30%
maximum.
Relief is requested from the maximum carbon content of the P-No. 1 Group 2 material during water backed welding. The requested change is to allow using P-No. 1 Group 2 material with a maximum carbon .content of 0.35%. This is the maximum allowed carbon content for SA-105, Specification for Carbon Steel Forgings for Piping Application.
ASME Code Case N-666-1, Paragraph 2 (a):
The owner shall verify that the socket weld failure is a result of vibration fatigue. This determination shall include review of the design, operating history, including changes in the piping system, and visual inspection of the failed socket weld.
Attachment to ET 19-0015 Page 2 of 8 Wolf Creek Nuclear Operating Corporation (WCNOC) is requesting relief from verification of socket weld failure as result of vibration fatigue. Since the history of this system has shown flaw initiation may be the result of localized internal corrosion, vibration fatigue may have resulted in the eventual failure of the socket weld. Therefore, verifying the failure was a result of vibration fatigue may be inconclusive.
4. Reason for Request
A previous leak from a through wall flaw at Wolf Creek Generating Station (WCGS) led to the submittal of a relief request (Reference 2) to utilize Code Case N-666-1 on an NPS 2 drain line upstream of valve EGV0029. Due to extent of condition from this through wall flaw, there is the potential for requiring similar weld overlays in other locations. Therefore, this request
- is asking for relief from the sanie conditions of N-666-1 as the previous request. This relief request does not supersede or negate the need for the approval of the.previous request.
WCNOC has attempted to isolate the "A" Train ESW from the "A" Train CCW HX utilizing the
- isolation valves near the HX. Attempts to isolate were unsuccessful. The "B" Train ESW to "B" Train CCW HX has the same isolation valves and has been previously identified to have similar leak-by conditions while trying to isolate. Reducing the ESW inventory so that the HX tubes are drained to perform the socket weld repair/replacement activities would render the affected ESW train non-functional. With either train completely non-functional, WCGS would
.be placed into an elevated, undesirable risk condition.
ESW is the safety related cooling medium for most safety related systems at WCGS. The following equipment would be affected if the "A" train or "B" train ESW system was rendered non-functional because some form of cooling is provided by ESW:
, a) Emergency Core Cooling System Room Coolers b) Containment Spray and Cooling (room cooler and Containment Coolers) c) Motor Driven Auxiliary Feedwater Pump Room Cooler and emergency makeup to Auxiliary Feedwater Pumps d) Component Cooling Water (HX and Emergency Makeup) e) Control Room Air Conditioning system (Condenser cooling)
.f) Emergency Diesel Generator (lntercooler, Jacket Water and Lube Oil cooling) g) Class IE.Air Conditioning (Condenser cooling) h) Electrical Penetration Room Cooler i) Spent Fuel Pool Cooling HX Cooled by CCW which is cooled by ESW j) Residual Heat Removal (Cooled by CCW which is cooled by ESW)
One alternative is to place the plant in Cold Shutdown (Mode 5) prior to draining the ESW system. Inherent risk is assumed when maneuvering the power plant from 100% power to
- Mode 5 and then back to 100%. Additionally, only one train of Residual Heat Removal would be available for shutdown cooling if the "A" Train ESW or "B" Train ESW system were drained.
WCNOC proposes that establishing the condition of ESW drained to complete a future/potential weld overlay will result in a hardship without a compensating increase in quality and safety.
- Attachment to ET 19-0015 Page 3 of 8
- 5. Proposed Alternative and Basis for Use:
Background:
In July 2018, a through wall leak was detected through an NPS 2 (Schedule 80) socket weld drain line attached to the CCW system HX (EEG01A). Attempts to isolate the HX and drain line to perform the replacement were not successful. Accordingly, the use of Code Case N-666-1 was implemented to perform a Code and NRC approved repair.
The purpose of this request is to install a repair, if necessary, in accordance with Code Case N-666-1, applicable conditions as identified in Regulatory Guide 1.147, and as identified herein due to the extent of condition from the July leak and repair. This request may be used only on the items listed in Section 1 and may be installed preemptively.
- For the locations listed in Section 1 above, the connecting drain lines (SA-106 Gr 8) and coupling materials (SA-105)_ are P-No. 1 Group 1 and P-No. 1 Group 2, respectively (Reference 3). The system design temperature and pressure are 200 °F and 200 psig, respectively and normal operating temperature and pressure are 110 *°F and 60 psig, respectively.
However, in review of the Code Case there are limitations on the carbon content P-No.1 Group 2 materials of 0.30% for water backed welding. From review of the material Specification, the maximum carbon content is 0.35%. Thus, the elevated carbon content placed the coupling outside the general requirements of Case N-666-1. The chemical composition of the coupling is found in Table 1, as well as the Carbon Equivalent (CE). The CE is being used because it helps compare and contrast hardenability of different materials and their chemistry via a single value.
Table 1 Chemistry and Carbon Equivalents for the selected materials based on SA-105 material maximums Material C% Mn% Mo% Ni% Cr% Cu% Carbon E uivalent*
SA-105 0.35 1.05 0.525
- CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 This evaluation is intended to demonstrate that the elevated carbon content is not thought to have a detrimental effect compared to similar materials.'
Degradation Mechanism:
Due to the history of the ESW system (raw lake water system), WCNOC has identified and repaired numerous flaws including through wall flaws, such as the leak from July 2018. The pronounced degradation mechanism in this system is localized under deposit corrosion (pit
.corrosion).
Potential Concerns:
There are believed to be two potential technical concerns related to elevated carbon content which are listed below with each evaluated separately.
Attachment to ET 19-0015 Page 4 of 8
- 1. Brittle Microstructure
Brittle Microstructure P-No. 1 Group 2 materials are considered to be low hardenable materials. This does not mean that a high hardness cannot be achieved with these materials, but that the ability to form martensite across a span of cooling rates is less. Thus, it is observed that the hardness decreases faster than more hardenable materials with similar cooling rates. This can be seen via Jominy End Quench data for various P-No. materials of varying carbon equivalents. The use of Jominy End Quench data for various P-No. materials is provided in Figures 1 and 2 reproduced from EPRI research. Of particular importance is P-No. 1 Group 2 data that had higher CE values than the coupling in question. Figures 1 and 2 show that the P-No. 1 materials have the lowest hardenability of the groups tested and do not sustain their hardness compared to other P-Nos. Note that CE values are provided in the Figures for each material and the coupling CE is lower than all the materials tested.
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0 5 10 15 20 25 30 Distance from Quenched End (sixteenths)
Figure 1 - Jominy End Quench Hardness Curves (1/16 to 1-3/4 inches) (Reference 3)
Attachment to ET 19-0015 Page 5 of 8 50
- SA516 C:< 70 P1 G1/ 020C/ 0.46 CE
- SA515 G-70 As f<<llled 45 P1 G2/0.26C/0.48CE
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- SA:335-11 P4 G1 / 0.11C/0.65 CE 30
.....,._ SA387-22 P5A G1 / 0.12C/0.88 CE 25 - - - SA335-P22 P5A G1 /0.11C/0.89 CE 1 2 3 4 5 6 7 8 Distance from Quenched End (sixteenths)
Figure 2 - Jominy Hardness Curves Close to Quenched End (1/16 to 1/2 inches) (Reference 3)
The expectation is that the coupling will follow a similar behavior trend to the other P-No. 1 Group 2 materials with high hardness dropping rapidly.
The same research also shows the effect of multiple weld passes and layers to the hardness of the Heat Affected Zone (HAZ) microstructure. Once more, of particular importance is the work on 1.5 inch SA-516 Grade 70 base material (P-No. 1 Group 2 material). As shown in Figure 3, three hardness plots were developed for 1 layer, 2 layer, and 3 layer weld beads on plate. The hardness traverses were then plotted as presented in Figure 4. The main observation from this research is for multiple weld layers a tempering effect is achieved. This same effect would be expected from a socket weld overlay per N-666-1 because the requirements specify a minimum of 2 layers be deposited.
1 Bead 1 Layer 3 Beads 2 layers 4 Beads 3 layers (1B1L) (3B2L) (4B3L)
Figure 3 - Bead-on-Plate/Bead-on-Pipe Configuration with Typical Bead Placement (Reference 3)
Attachment to ET 19-0015 Page 6 of 8 Bead--00-Plate Hardness -11/2" SA 516 Cs 70 500~~~~~~~~~~~~~~~
g 450 + - - - - - - - - -- - - - - - - - i a
~ 400 +---t"-:-- - -- - -- -- - -- ----;
> -+--1B1l
- e. 350 - + - - - ~ - - - - - -- - - -- - -3B2L
- 300---~~~~.,...._~~~~~~~~--,
-A-4B3L
~ 250 +---"'-,=--- ......"'=..--- - - -- ----;
!e 200 +---'::=,~~~=:::llt==:E~~~::§s:d i 150 + - - -- - - - - - -- - - - -
100 +-~-~~-~--~-~~ -~ --i 0 10 20 30 40 50 60 70 80 90 100 Distance from fusion line (thousandths)
Figure 4 - SA-516-70 Bead-on-Plate Hardness Curves for 1-1/2" Plate (Reference 3)
While the CE value is thought to bound the behavior of the coupling as previously described, the main effect of higher carbon content is the potential for higher peak hardness. This was observed via past research where it was found that a shift of carbon content of 0.30% to 0.35%
shifts the peak hardness from approximately 500 to approximately 560 Vickers (Reference 3, Figure 5). While the 0.35% carbon content coupling could achieve a higher peak hardness, this is a relatively minor shift when comparing the two levels . Consideration should also be taken that welding on the coupling and drain line will not be an ideal quench as the water inside is not ambient (due to leak by of the isolating valves to the heat exchanger); therefore, this level of hardness is not expected.
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- 0 Ma1erkowskl (36) 10 IIII 0
X 100 O o., 0.2 0 .3 0 .4 0 .5 0 .6 0 .7 0 .8 0 .9 1 .0 1 .1 1. 2 1 .3 Weight perc*nt carbon Figure 5 - Summary of Hardness of Martensite as a Function of Carbon Content in Fe-C alloys and steels (Reference 3)
Attachment to ET 19-0015 Page 7 of 8 Hydrogen Induced Cracking (HIC) Susceptibility:
The welding plan for repairing a socket weld via N-666-1 includes sealing the leak via Shielded Metal Arc Welding (SMAW) and then using low hydrogen electrode, E7018 SMAW, or ER70S-2 or ER70S-6 Gas Tungsten Arc Welding (GTAW), for the weld overlay. The plan includes instructions for stringer weld beads (no weaving) thus promoting beneficial tempering effect of the coupling. Thus, the weld will be dry once the actual layers of the overlay are applied for structural credit. The use of low hydrogen electrodes and the minimum of 2 layers of weld metal should provide two means of eliminating HIC. The use of low hydrogen electrodes will reduce the amount of hydrogen available via the welding process and multiple weld layers will soften the HAZ. Thus, formation of HIC is not a concern.
Examination and Testing:
All examinations and testing are in accordance with N-666-1. Walk downs will be used to identify leakage post installation.
Conclusion:
This evaluation finds that a coupling with the maximum carbon content of 0.35% is expected to behave in a similar manner to other P-No. 1 group 2 materials with regards to hardness. In addition, the two potential concerns with a brittle microstructure and HIC do not appear to be justified when the welding plan and Code Case N-666-1 are followed. Specifically, even with very rapid cooling rates the P-No. 1 group 2 materials can have high peak hardness but quickly diminish compared to high hardenability materials. Additionally, hardness was seen to drop with multiple weld layers which will be done per N-666-1. HIC is also not believed to be an issue because low hydrogen consumables and practices will be used, as well as multiple weld layers that will have a tempering effect on the HAZ. There does not appear to be the concurrent factors necessary for HIC to occur. Thus, the concerns associated with welding on the coupling with 0.35% carbon do not seem to be warranted.
No current defects or degradation have been identified on any of the components listed in Section 1. This request for relief is intended for future installation or repair activities only.
6.. Duration of Proposed Alternative:
When a weld overlay is installed, it will be removed from service prior to the end of the next refueling outage following the installation. The socket weld connection will be reworked/repaired to meet the requirements of the ASME Code.
- 7. Precedent:
WCNOC's request was approved by NRG Safety Evaluation (SE), ADAMS Accession No. ML18215A176 (Reference 2). This request utilizes a similar approach.
- 8.
References:
Attachment to ET 19-0015
. Page 8 of 8
- 1. ASME Code Case N-666-1, 'Weld Overlay of Class 1, 2, and 3 Socket Welded ConnectionsSection XI, Division 1," March 13, 2012.
- 2. WCNOC Letter WO 18-0031, "lnservice Inspection (ISi) Program Relief Request from the Requirements of American Society of Mechanical Engineers Code Case N-666-1, "Weld Overlay of Class 1, 2, and 3 Socket Welded Connections,Section XI, Division 1,"" July 23, 2018. ADAMS Accession No. ML18215A176.
- 3. Steels: Processing, Structure, and Performance, by George Krauss, ASM International, 2005.