ML20064E530

Evaluation of Effectiveness of Code Case N-504-1 Repair for Proposed Root Causes for Containment Sump Suction Check Valves

ML20064E530
Person / Time
Site:	Palisades
Issue date:	03/06/1994
From:	Deboo G, Flaherty J, Louden J SARGENT & LUNDY, INC.
To:
Shared Package
ML18059A868	List: ML20064E445 ML20064E462 ML20064E483 ML20064E491 ML20064E501 ML20064E530
References
CMED-058617, CMED-058617-R00, CMED-58617, CMED-58617-R, NUDOCS 9403140336
Download: ML20064E530 (9)

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ATTACHMENT 7  ;

Consumers Power Company Palisades Plant Docket 50-255 l

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EVALVATION OF EFFECTIVENESS OF CODE CASE N-504-1 REPAIR FOR PROPOSED ROOT CAUSES FOR l CONTAINMENT SUMP SUCTION CHECK VALVES .

March 6, 1994 l

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9403140336 940306 l PDR ADOCK 05000255 '

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l Project No. 9400-040 l

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Nuclear Safety-Related i

Evaluation of the Effectiveness of Code l Case N-504-1 Repair for Proposed Root Causes l for Containment Sump Suction Check Valves l

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Prepared for Palisades Nuclear Station l

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l CMED-058617 i Revision 0-March 5,1994  !

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CMED-058617 Sargent & Lundy N ' '#^"

Prepared by: /Il ' A Date 3 'b I lj J. T. Loud'en J

Reviewed by: O_D '\d _1 Date 3-5-%

'J. F1aberty 'N Approved by: u l) I4h h Date 3/6[94

' G. H dDeBoo Page 2

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CMED-058617 Introduction This evaluation will consider the possible root causes for-the leak in CK-ES3166 and the indications identified in CK-ES3181, and the resultant effect of a Code Case N-504-1 overlay.

The possible root causes as postulated by CPCo are:

1. Multiple Heat Input and Repairs at Leak Site Causing Highly Sensitized Casting Material

2. Intergranular Corrosion (IGC)

3. Intergranular Stress Corrosion Cracking (IGSCC)

4. Casting Flaw

5. Welding Flaw

6. Chemically Induced Pitting Corrosion A review of these root causes concluded that possible root cause 1 was a contributing factor of root causes 2 and 3; therefore it is not addressed' individually,.but as part of root causes 2 and 3. Also,. root causes 4 and 5 were addressed together since the resultant effect on a Code Case N-504-1 overlay is considered the same.

Consequently, the possible root causes evaluated are:

1. Intergranular Corrosion (IGC)

2. Intergranular Stress Corrosion Cracking (IGSCC)'

3. Casting Flaw or Weld Flaw

4. Chemically Induced Pitting Corrosion The attributes of a Code Case N-504-1 weld overlay as applied to thesh valves J

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CMED-058617 are presented and their ability to mitigate the propagation of the identified indications is discussed for each possible root cause. It is important to-emphasize that cyclic loading and the associated stress driven flaw propagation is not a factor to be considered for this overlay application as stated in CMED-058599.

This evaluation demonstrates that the Code Case N 504-1 overlay is a technically valid corrective action for all the possible root causes stated above.

Code Case N-504-1 Corrective Action 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:

A. The overlay weld metal has improved IGC 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.

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D. The code case does not limit the flaw to any specific characteristic or l nature, i.e., whether its a crack or localized corrosion.

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CMED-058617 E. The overlay application to these valves is not subjected to any cyclic loading which may cause fatigue crack growth.

The use of Code Case N-504-1 for the check valve repair will be an acceptable

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repair for a probable root cause of intergranular corrosion or for any of the other possible root causes as discussed below: ,

1. Intergranular Corrosion (IGC) - The leak in the 24" cast stainless steel check valve, CK-ES3166, has been attributed to intergranular corrosion 1 (IGC).

IGC is a selective form of corrosion that attacks the grain boundaries with the majority of the grain unaffected. it can affect certain alloys (e.g., several of the austenitic stainless steels) that are highly resistant to general and localized attack. These alloys are made susceptible to IGC by sensitization. Sensitization of stainless steel is the precipitation of chromium carbides _at the grain boundaries which depletes the chrome content ir. the adjacent grain boundary. For regular grade stainless steels this occurs during the welding operation.

The overlay will be a low carbon, high ferrite stainless steel, and will not be sensitized by the welding / overlaying process. Without sensitization IGC cannot occur in the overlay. However, the casting surface below the overlay (and at the ends) will be sensitized. Thus, .

IGC could eventually progress from the leak area horizontally to the end of the overlay and then go upward through the casting surface as 'a leak.

However, this path would be substantially longer than the original through wall flaw. Based on the corrosion rate for the past 25 years, the overlay will last for the life of the plant.

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CMED-058617

2. Intergranular Stress Corrosion Cracking (IGSCC) - As with IGC, this mechanism requires a sensitized material. Howevei, to propagate, it requires significant stress perpendicular to the crack. For reasons discussed above, the overlay is immune to IGSCC because it is not sensitized. It also has a high resistance to IGSCC because of the required minimum ferrite content. The weld deposition of the overlay will be performed with the pipe filled with water. This produces a residual compressive stress in the casting material below the overlay.

The compressive stress will mitigate IGSCC crack propagation. Since the weld metal is a ductile stainless steel any possible crack propagation through the overlay would be minimized. Additionally, propagation through the sensitized casting under the overlay is unlikely since there -

is very little through-thickness stress to cause propagation. The overlay will last for the life of the plant.

3. Casting or Weld Metal Flaw - The high quality overlay (deposited using the machine gas tungsten arc welding process) will present a new defect-free material as verified by the code case required preservice ultrasonic examination. The design of the overlay is of ample thickness to keep the stresses low and thus prevent stress driven propagation. As discussed in Item 2, the residual compressive stress below the overlay will mitigate flaw propagation. The overlay will last

- for the life of the plant.

4. Chemically Induced Pitting Corrosion - The low carbon non-sensitized stainless steel overlay is as resistant to chemical attack or pitting as the casting. The overlay will add another layer of material that the pitting corrosion would have to penetrate. While the overlay only adds additional time for pitting to penetrate, it should be emphasized that Page 6

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l CMED-058617 i

this is the least likely root cause. Chemical analysis of the water and metallurgical examination of the surface replica (as detailed in CMED-058599) effectively makes pitting corrosion improbable as a root cause.

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Effect of the Overlav on the Valve Since there are recordable indications on the casting a seal layer will be deposited as stipulated by the code case. Depositing this seal layer may expose new indications. The Code Case N-504-1 requires liquid penetrant examination to be performed after the seal layer is deposited. Indications greater than 1/16" will be repaired as necessary.

Additionally, the weld overlay will induce residual stress fields and system shrinkage. The IWB-3640 flaw evaluation will be performed using as built conditions and combine these effects with the applied loads to demonstrate the acceptance of the overlay for the life of the plant.

Conclusion The findings presented in Report CMED-058599 show that pitting corrosion is a highly unlikely root cause, however the additional material would extend the time for pitting corrosion to penetrate the overlay. Since these valves are not subject to cyclic loading or brittle fracture, propagation of the flaw through the overlay would have to be driven by a corrosion mechanism. The low carbon stainless steel overlay weld metal has better IGC and IGSCC resistance than the casting. Periodic inservice inspections will monitor any flaw growth or propagation. Therefore, with the exception of pitting corrosion, the Code Page 7

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i Case N-504-1 overlay provides an effective corrective action for each of the stated possible root causes for the life of the plant, b 'coden504 tmp Final Page i

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