ML20057C903
| ML20057C903 | |
| Person / Time | |
|---|---|
| Site: | Duane Arnold  |
| Issue date: | 09/24/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20057C900 | List: |
| References | |
| TAC-M87666, NUDOCS 9309300130 | |
| Download: ML20057C903 (7) | |
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Enclosure SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION REVIEW OF THE REQUEST FOR RELIEF FROM THE ASME CODE REPAIR l
RE0VIREMENTS FOR THE DUANE ARNOLD "D" OUTBOARD MAIN STEAM ISOLATION VALVE MATERIALS AND CHEMICAL ENGINEERING BRANCH TAC NO. M87666 IOWA ELECTRIC LIGHT AND POWER COMPANY DUANE ARNOLD ENERGY CENTER DOCKET NO:
50-331
1.0 BACKGROUND
Accordi.fg to the provisions of 10 CFR 50.55a(g)(4), activities relative to the American Society of Mechanical Engineers (ASME) Code Class 1, 2, and 3 structures, systems and components are required to comply with the in-service testing and structural integrity requirements of the ASME Code,Section XI, during the service life of the plant. However, relief from specific Code requirements may be granted under the provisions of 10 CFR 50.55a(g)(6)(1) should the Commission determine that compliance with certain Code requirements is impractical, and if the Commission determines that relief from require-ments is authorized by law and will not endanger life or property or the common defense and security, and is otherwise in the public interest giving due consideration to the burden upon the licensee that could result if the requirements were imposed on the facility.
Licensee's Relief Reouest By letter dated September 18, 1993, Iowa Electric Light and Power Company (the licensee) requested Code relief in order to perform a non-Code repair to the "D" Main Steam Isolation Valve (MSIV) at the Duane Arnold Energy Center.
Relief is requested for performing a weld repair over a group of four linear indications in the MSIV body casting.
Relief is required because such a repair fails to comply with the flaw removal requirement of the Code.
Details of the Relief Reouest The MSIV is an ASME Section XI class 1 valve. The body is fabricated from ASTM /ASME SA-216, grade WCB, cast carbon steel, procured under GE Specifi-cation 21A9230 Rev. 2.
The bore diameter for the valve disc is 15.53 inches.
i Wall thickness is 3.175 inches. The valve experiences normal BWR main steam pressures and temperatures along with design basis seismic loads.
9309300130 930924 PDR ADOCK 05000331 P
. During the ongoing refueling outage, the "D" MSIV body was rebored as part of a job to correct unacceptable seat leakage which was discovered during testing. The reboring involved removing approximately 0.015 inches on the inside diameter of the body casting. After machining, the area was examined -
for surface indications by a magnetic particle test (MT).
An unacceptable indication was found. The defect was uncovered by the machining operation. The licensee concluded that the indication was an area of subsurface casting shrinkage brought to the surface by the reboring operation. Review of the original acceptance radiographs confirmed the presence of subsurface defects, interpreted as an area of shrinkage, at the location of the newly revealed surface indication. Since the shrinkage was not originally surface connected, and was of acceptable size for a subsurface flaw, it met the valve body casting acceptance criteria.
The exposed flaw length of approximately 1-1/8 inches exceeded the acceptance criteria of the procurement specification and the Code.
Flaw removal, by grinding, was initiated. After 0.593 inches of material at the flaw location was excavated, three discontinuous flaws remained. These ranged in length frcm 1/4 inch to 1-1/8 inches.
The licensee had past experience with casting defect repairs. On a previous outage, another MSIV was weld repaired after a deep excavation.
Per the procurement specification, weld repairs performed on excavations deeper than 20% of the wall thickness, or 0.6 inch in this case, require a post weld heat treatment (PWHT). After the PWHT was performed, the licensee encountered unacceptable seat leakage due to heat induced distortion that occurred during the PWHT. Significant remachining of the valve body was necessary to restore necessary tolerances for acceptable leakage rates.
Due to this experience, the licensee desired to avoid the PWHT requirement by restricting the excavation depth to 0.6 inches or less. This avoids the potential for PWHT induced distortion but fails to accomplish the flaw removal requirement for a Code repair. The licensee maintained that the risks posed by performing a PWHT are more significant than leaving an imbedded flaw.
PWHT reight distort the valve body beyond salvage. Conversely, flaw analysis, employing embedded flaw criteria from Section XI, demonstrate that safe (Code) margins are met with the proposed weld overlay over the remaining flaws.
Section XI of the ASME Code requires that repairs to ASME Class I components be performed in accordance with the requirements of ASME Section XI, IWA/IWB 4000 or the criteria of the original design code of record. Under these circumstances, the licensee is required to perform flaw removal in order to be in accordance with the requirements of IWA/IWB 4000. The licensee seeks to repair the flaws in the valve body casting by removing up to 0.6 inch of material followed by a weld overlay to replace excavated material. The licensee believes, however, that relief under the provisions of 10 CFR 50.55a(g)(6)(1) is warranted in this case, since a Code repair would require
the performance of a PWHT to the valve body that very likely would damage the valve with the potential consequences of a lengthy plant shutdown.
Additionally, the licensee purports to show that the embedded flaws remaining after the proposed repair would be acceptable by Code criteria.
2.0 DISCUSSION OF THE RELIEF RE0 VEST Code Reauirement ASME Boiler and Pressure Vessel Code,Section XI, 1980 Edition with Summer 1981 Addenda, Articles IWA/IWB-4000.
Code Relief Reauest Relief is requested from the requirement for complete flaw removal during execution of a weld repair of the MSIV, a class I component, at Duane Arnold Energy Center.
Basis for Relief A Code repair to the MSIV would require a PWHT of the valve body casting.
Prior experience has demonstrated that unacceptable distortion of the valve body may occur. Correction of the distortion may not be feasible.
Application of imbedded flaw acceptance criteria after performing a weld overlay in the affected area should demonstrate acceptable structural integrity.
Proposed Alternative Procram The licensee proposes the following alternative in lieu of a Code repair:
1.
Excavate the flaws to a depth not to exceed 20% of the wall thickness (0.6 inch in this case). Perform an MT to document remaining flaw length (s).
2.
Using a 200 *F preheat, perform a low heat input, multipass weld to fill the excavated cavity. Verify weld integrity by performing a visual i
examination and hot MT of each layer of weld metal.
Shielded metal arc welding (SMAW), using small diameter electrodes will be employed. The fluxing action of the SMAW electrodes (E7018 in this case) will aid in removal of any casting impurities that may be present in the flawed area.
j 3.
Finish machine the weld and perform an MT of the machined surface.
4.
Radiograph the repair area to verify that no defects exist in the weld deposit and that the casting around the repaired area meets the requirements of the original construction code.
..J
. 5.
Demonstrate structural integrity for the remaining embedded flaw by performing bounding analyses using the appropriate Section XI embedded flaw evaluation methods.
Discussion of Alternative Proaram The proposed repair method meets the procurement specification and Code i
requirements for a weld repair without PWHT except for the failure to completely remove the flaw.
Flaw removal is a necessary step in a Code repair j
because it precludes subsequent failure due to existing crack propagation through the new weld. The licensee maintains (discussed later) that crack propagation is unlikely. Regardless of the conclusion for crack propagation through the proposed weld overlay, the NRC staff observes a weakness with the licensee's maintenance program.
That weakness is the lack of a qualified temper bead weld procedure.
Section XI permits weld repairs to carbon steel without PWHT if a qualified temper bead procedure is employed. Application of such a technique is fairly common in the industry.
If the licensee had such a qualified procedure, the subject repair could have proceeded as a Code repair incorporating the required flaw 1
removal and not be limited to excavation depth.
The area where the flaws are located is in the upper bore of the valve body.
This area provides the guiding surface for the piston during valve stroking j
and tha riding surface for the disc and piston in the open position. The 1
stresses in this area are primarily from system pressure, structural and seismic loading, and any thermal stresses during heatup and cooldown. The wall thickness in the proposed repair area is 3.175 inches. The minimum wall j
thickness required is 1.29 inches. The actual thickness is thus substantially i
more than that required for the design loads.
The flaw location matches the location of casting defects (shrinkage) that were documented (and dispositioned as acceptable) in the original fabrication radiographs. No previous repairs have been performed in this area, although two Code repairs are being performed in tandem with this proposed repair. The other repairs are also for correcting previously embedded casting flaws that were exposed as a result of the machining of the bore.
In those cases, flaw removal was accomplished before the maximum excavation limit was reached.
The three concurrent repairs (two Code repairs and the proposed repair) are to areas with documented embedded casting shrinkage. This strongly suggests that the observed flaws did not initiate in service. The licensee believes that the flaws did not propagate in service but were simply uncovered as a result of the machining operation. This conclusion is based upon the low stresses resulting from the extra wall thickness, the lack of previous repairs, and the fact that the flawed areas are not adjacent to the valve seat, where the material may be susceptible to thermal fatigue crack initiation or thermal fatigue induced propagation of pre-existing flaws.
. Enaineerino Evaluation of the Embedded Flaws l
The licensee performed an evaluation of tha flaws remaining subsequent to the l
grinding and weld repair using the criteria described in the Section XI of the ASME Code, Articles IWA-3000 and IWB-3000. The three remaining indications ranging in length from 1/4 inch to 1-1/8 inche were considered as a single flaw of 2 inches in length using the proximity rules of IWA-3320. As a result of the weld repair, the flaw was considered to be subsurface as per IWA-3330(b) and IWA-3320.
Using assumed flaw aspect ratios and an iterative procedure based on the crack length and valve body geometry, the licensee i
calculated a " bounding" aspect ratio and flaw depth. The conclusion was that the calculated aspect ratio (a/l - 0.086) and flaw depth (2a - 0.344) for the inservice condition constituted an allowable planar flaw by the criteria of IWB-3518(b) and Table IWB 3518-1.
The licensee also performed an analytical evaluation of the flaws as per ASME Code Section XI, IWB-3600 and Appendix A.
For this analysis, the licensee considered the following conditions:
(1)
Flaw Size - A length of 2 inches was used with aspect ratios (a/1) ranging from 1/6 to 1/2.
(2)
Fracture Toughness - As fracture toughness data for the ASTM A216 carbon steel casting were not available.
The licensee utilized a standard fracture text CVN correlation to estimate fracture toughness and then index to the K and K,, curves for alloy steels a
in Section XI of the ASME Code.
RT was used as the indexing parameter.Thefracturetoughnessv$Yuesdeterminedwereas follows:
J-R_b, Condition Fracture Touahness. Kg (ksilin) l 120 F Normal 100 (pressure test) 227 'F Faulted 200 10 F Test 40 (3)
Loading - Stresses due to thermal, pressure and valve closure loading were considered. The hydrotest case was used to bound normal conditions (P - 1000 psi, T - 180 F).
Faulted conditions were taken as the worst transient stress (AT - 265 F, A stress -
39,759 psi). The stress due to closure, 3252 psi, was evaluated at room temperature.
(4)
Fatigue Crack Growth - To account for crack growth, the licensee used the ASME Code Section XI, Figure A-4300-1, fatigue crack growth rate curve for subsurface flaws in an air environment.
. The licensee concluded that the flaws in the MSIV body were bounded by the assumed initial flaw sizes in this analysis and were within the ASME Code l
Section XI, IWB-3612 requirements for normal and faulted conditions. On the basis of the fatigue crack growth analysis, the licensee also considers the l
flaws to be acceptable for 40 years of operation per the requirements of the ASME Code Section XI, IWB-3600.
Discussion of Flaw Analyses l
The NRC staff finds that the disposition of the remaining flaws as subs' We and with the grouping of flaws being considered as a single 2 inch long flaw as per IWA-3320, is acceptable. However, as the actual depth of the flaws is l
unknown, the staff does not consider the licensee's calculated aspect ratio (a/1 - 0.086) and flaw depth (2a = 0.344) to be bounding. Due to the uncertainties in flaw depth, the staff does not consider that the IWB 3518(b) and Table 3518-1 criteria for allowable planar flaws are satisfied.
l The licensee's flaw evaluation as per ASME Code Section XI, IWB-3600 is I
acceptable. The analysis adequately demonstrates compliance with IWB-3612 l
requirements for up to a bounding flaw depth of 2 inches. Assumptions, where necessary, on initial flaw size, fracture toughness and loading were made in a conservative manner. The fatigue crack growth analysis demonstrates that growth by fatigue will not significantly impact the structural integrity of the MSIV body over the remaining service lifetime.
3.0 STAFF EVALUATION The NRC staff has determined that the licensee's relief request for a non-code repair of the flaws in the "D" outboard MSIV is acceptable for the following reasons:
1.
A Code repair to the MSIV would require a PWHT of the valve body casting. Prior experience has demonstrated that unacceptable distortion of the valve body may occur. Correction of the distortion may not be feasible.
2.
The repairs (2 Code repairs and the proposed repair) are to areas with documented embedded shrinkage flaws. This strongly suggests that the observed flaws did not initiate in service, but rather were uncovered as a result of the machining operation to remove the taper from the valve bore.
3.
An analytical flaw evaluation was performed in accordance with ASME Code Section XI, IWB-3600 requirements. This analysis demonstrates that the flaws in the MSIV body were bounded by i
postulated initial flaw geometries and were within the ASME l
Section XI, IWB-3612 requirements for normal and faulted conditions. The fatigue crack growth analysis demonstrates that growth by fatigue will not significantly impact the structural
i integrity of the MSIV body over the remaining service lifetime.
I However, radiographic inspection of the MSIV body should be accomplished after the next two scheduled refueling outages to ensure adequate structural integrity of the non-Code repair.
The NRC staff has also identified a weakness with the licensee's maintenance program. That weakness is the lack of a qualified temper bead weld procedure.
Section XI permits weld repairs to carbon steel without PWHT if a qualified i
temper bead procedure is employed. Application of such a technique is reasonably common in the industry.
If the licensee had such a qualified procedure, the subject repair could have proceeded as a Code repair j
incorporating the required flaw removal and not be limited to excavation depth.
4.0 CONCLUSION
j Accordingly, the NRC staff concludes that the Code repair rrequirements in this case are impractical because Code repair of the flaws would damage a large containment isolation valve relied upon to close under accident conditions.
Code compliance would also result in an extended shutdown to complete the necessary repairs. Therefore, pursuant to 10 CFR 50.55a(g)(6)(1), granting of the requested relief is authorized by law and will not endanger life or i
property or the common defense or security and is otherwise in the public interest, given due consideration to the burden upon the licensee and facility that could result if the Code requirements were imposed on the facility.
Therefore, pursuant to 10 CFR 50.55a(g)(6)(i), relief is granted provided a radiographic inspection of the MSIV body is accomplished after the next two scheduled refueling outages to ensure adequate structural integrity of the non-Code repair.
Principal Contributors: G. Hornseth E. Hackett j
Date: September 24, 1993 i
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