ML18059A940
| ML18059A940 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 04/06/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML18059A939 | List: |
| References | |
| NUDOCS 9404180194 | |
| Download: ML18059A940 (5) | |
Text
UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION ON THE CHECK VALVE LEAK AND REPAIR CONSUMERS POWER COMPANY PALISADES NUCLEAR PLANT DOCKET NO. 50-255 I.O INTRODUCTION I.I Purpose The purpose of this evaluation is to determine whether the licensee may *use an alternative to the ASME Boiler and Pressure Vessel Code,Section XI requirements for overlay repairs of a crack in the containment sump check valve (CK-ES3166) and of subsurface indications in a second check valve (CK-ES3181).
I.2 Background In its submittal of March 6, 1994, the licensee, Consumers Power Company (CPCo),
reported:
a leak in a 24-inch austenitic stainless steel check valve, CK-ES3166... between the containment sump and the high pressure safety injection, low pressure safety injection, and containment spray pumps suction.
The leak was identified by a small accumulation of boron on the valve body.
The valve was declared inoperable and the plant entered the appropriate technical specification action statement and the reactor was shut down.
Non-destructive examination [NOE] of the check valve CK-ES3166 revealed that, in addition'*.t~ the ~hrough wall leak, various other indications exist at and near
- the l~ak;Jocation*.*:- Additional inspection of the same area on the second containm~nt sump check valve, CK-ES3181, has revealed subsurface indications that we;'.have conser*vatively decided to repair.
In ac~ordance ~Uh... IO CFR 50.55a, we request approval for the use of Code Case N-504-I for the repair of the... valves.... we believe that Code Case N-504-I provides an acceptable alternative repair to the repair techniques presently approved for use by the NRC.
To support its request the licensee submitted a root cause analysis for the leak identified on check valve CK-ES3166, a flaw stability calculation, the repair plan, design calculations and drawings for the repairs.
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, 2.0 DISCUSSION ASME Code Case N-504, "Alternative Rules for Repair of Class 1, 2,and 3 Austenitic Stainless Steel Piping," provides for using weld overlays on the outside of austenitic stainless steel pipe found to have defects.
Its provisions correspond to those the staff has long approved for the disposition of intergranular stress corrosion cracks (IGSCC) per Generic Letter 88-01, "NRC Position on IGSCC in BWR Austenitic Stainless Steel Piping." The staff reviewed the submittal to determine whether the proposed fix is appropriate for the failure mechanism which the licensee found was not IGSCC.
2.1 Root Cause Analysis The licensee considered the following possible root causes: multiple heat input and repairs at leak site causing highly sensitized casting material, intergranular attack (IGA), a casting flaw/defect, a welding flaw, chemically induced pitting/corrosion, and IGSCC.
The licensee states:
The affected valves are Chapman A351-CF8M 316 cast stainless 24-inch tilting disk check valves.
The through wall flaw is in a heat affected zone created by casting weld repairs and adjacent butt weld.
No evidence of stress corrosion cracking was observed... Ferrite measurements taken on the outlet body of CK-ES3166 averaged 8.9%.
The butt weld measured 12.7% and the casting repair weld area closest to the leak point measured 16.6%.
Per BWR Industry Experience these ferrite levels provide the casting with high resistance to IGSCC.
The licensee performed a literature search of industry experience to determine if any other plants experienced leakage from identified IGA or IGSCC in stainless steel components.
Results showed IGSCC occurred under certain conditions at other plants not applicable to these valves.
The licensee determined that the cause was IGA, stating:
Casting weld repairs by the vendor and two butt weld operations to install the valve {original and following valve removal for seat leakage repairs in 1969) created a highly sensitized heat affected zone on CK-ES3166.
The continuously sensitized grain boundaries were observed in several surface replicas associated with CK-ES3166.... The non-continuous corrosion attack indicated that the damage mechanism is IGA.
the licensee considered the other possible causes of failure and determined that an overlay would still be appropriate. It states:
Code Case N-504-1 is an acceptable corrective action for the CK-ES3166 and CK-ES3181 check valve repair for all the possible root causes previously listed.
The reasons are as follows:
1 A.
The overlay weld metal has improved IGC (IGA) and IGSCC corrosion resistance over the casting.
B.
The overlay will conservatively meet the ASME Section XI Code structural reinforcement requirements.
C.
The overlay will be periodically inservice inspected (ultrasonically) to verify that the flaw is not propagating.
D.
The code case does not limit the flaw to any specific characteristic or nature, i.e., whether it's a crack or localized corrosion.
E.
The overlay application to these valves is not subjected to any cyclic loading which may cause fatigue crack growth.
2.2 Weld Overlay Design and Repair The licensee stated:
The weld overlay repairs to be applied to check valves CK-ES3166 and CK-ES3181 shall meet the requirements of American Society of Mechanical Engineers (ASME)
Boiler and Pressure Vessel Code (BPVC) Case N-504-1 and Paragraph IWB-3640 of ASME BPVC Section XI, 1983 Edition with Winter 1985 Addenda or later Editions and Addenda.
The designs of the weld overlay repairs for both valves are based on an assumed 100%
through wall by 360 degrees long flaw.
The major points about the weld procedure are: the repair will be made using the gas tungsten arc welding process; the filler metal is 308L; the overlay will be welded while the system is full of water, thus providing a favorable residual stress distribution; the first two design layers will have a ferrite number of 7.5 or greater.
2.4 Corrective Actions The licensee determined that the identified root cause has generic implications for welds on stainless steel cast valves in boric acid systems and components that have received multiple welds, particularly welding performed before current controlled heat procedures.
The licensee proposed the following short-term actions:
Complete visual examination (VT-2) leak inspections on stainless steel cast valves on the suction piping for ESS pumps to ensure no evidence of other through wall leaks Review Engineering Safeguard System (ESS) pumps suction piping vendor correspondence file to determine existence of similar weld repaired cast valves Complete flaw stability calculation for CK-ES3166 to ensure past capability to perform its safety function
t e Apply approved N-504-1 overlay to both valves followed by associated NDE and hydro tests Complete VT-2 leak inspections during the hydro test on all cast valves and associated welds within the hydro test boundary For long-term actions, the licensee proposes to:
Develop a prioritized list of similarly susceptible valve welds for inspections and evaluation to determine generic existence of other possible IGC. flaws Perform destructive testing of similarly susceptible valves that are planned for replacement to determine extent of IGC Reinforce to plant walkdown groups the importance associated with prompt identification of potential through wall leaks Based upon results of planned inspections implement programmatic controls if necessary to prevent excessive sensitization due to multiple welding operations 3.0 EVALUATION The staff finds applying a weld overlay appropriate for repair of the check valves.
The licensee's plans conform to the Code Case N-504-1.
This case contains the same measures we accept for austenitic piping containing IGSCC per Generic Letter 88-01.
Numerous overlay repairs have been applied in many plants over the years to piping containing IGSCC.
Service experience has shown that ~verlays are a safe and effective solution for IGSCC.
The licensee determined that the failure was caused by IGA.
We are not convinced that IGA is the failure mode.
To our knowledge, IGA has never been found to be a cause of failure in cast stainless steel valves operating in borated water at ambient temperatures.
The licensee apparently did not find any citations from its literature search to support its determination.
However, the licensee showed that even if the cause is not IGA, but rather welding defects, casting defects, chemically induced pits or IGSCC, the overlays will not have any adverse effects.
At the March 8, 1994, meeting with the NRC staff~ the licensee verbally committed to inspect the overlaid areas every refueling outage.
The licensee will use ultrasonic techniques demonstrated to be effective.
We find the ~se of the overlays for these valves acceptable based on the following:
the favorable service experiences with overlays for defects in austenitic stainless steel piping, fatigue not being regarded as a possible cause, and the licensee's commitment to inspect every refueling outage using ultrasonic techniques demonstrated to*be effective, provided analyses show shrinkage stresses are within tolerances.
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CONCLUSION The staff conclud.es that the licensee's proposed alternative for repair is acceptable in that it will provide an acceptable level of quality and safety and authorizes its use pursuant to 10 CFR 50.55a(a)(3)(i).
After the repair the licensee should submit an analysis of the shrinkage stresses done according to the criteria of Code Case N-504-1, the as-built dimensions and measured delta ferrite content.
The licensee should also send its written commitment to inspect these valves every refueling outage using ultrasonic techniques demonstrated to be effective. This information should be submitted within 30 days of completing the outage.
Principal contributor:
M. Banic Date:
April 6, 1994