ML18059A433
| ML18059A433 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 10/04/1993 |
| From: | Rogers D CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9310180081 | |
| Download: ML18059A433 (14) | |
Text
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consumers Power POW ERi Nii
/llllCHlliAN"S PROliRESS Palisades Nuclear Plant:
27780 Blue Star Memorial Highway, Covert, Ml 49043 October 4, 1993 Nuclear Regulatory Commission Document Control Desk Washington, DC 20555 David W. Rogers Plant Safety and Licensing Director DOCKET 50-255 - LICENSE DPR PALISADES PLANT - PRESSURIZER SAFE END CRACK REPAIR ACTION PLAN On September 16, 1993 a primary system through-wall crack developed in the pressurizer power operated relief valve (PORV) nozzle safe end.
The crack was in the Inconel safe end connected to the stainless steel pipe.
In the two weeks since the crack was identified numerous inspections and engineering evaluations have been undertaken to determine the root cause so that appropriate corrective actions can be taken to allow the Palisades Plant to be safely returned to service. Metallurgical examination of the crack indicates that it was caused by primary water stress corrosion cracking (PWSCC) in the Inconel side heat affected zone of the safe end to pipe weld and that the crack initiated from the inside wall of the safe end.
An evaluation of system conditions and materials was performed to identify other weld joints that may be susceptible to the same failure cause. These joints are being examined using radiography and dye penetrant tests.
In addition, ultrasonic examinations, using fPRI qMalified IGSCC procedures and examiners, are being performed on those weld joints where access and geometry permit.-
Understanding the significance of the crack that occurred, Consumers Power Company has elected to go beyond the minimum code examination requirements to assure plant safety and reliability. A code repair is in progress to the pressurizer nozzle safe end to pipe weld to restore structural integrity.
Improvements were made to the weld joint to_ address certain deficiencies noted in the original weld.
An engineering evaluation of the root cause of the crack is also being performed. This evaluation will address the three factors necessary to cause PWSCC:
material susceptibility, environment and stress.
Corrective actions associated with these three factors are being evaluated.
A CMS' ENERGY COMPANY
2 The following is a summary of Consumers Power Company's approach to resolution of this issue.
We are close to completing our inspections and engineering investigations, and as stated in our letter on October I, 1993, we plan to meet with the NRC staff when our work is complete and prior to the plant leaving cold shutdown.
The general methodology which is being implemented to address the Palisades pressurizer nozzle safe-end crack and resultant steam leak and to ensure that similar failures in similar safe-ends are not probable is as follows:
A.
Conduct an engineering evaluation to identify the root cause{s} of the leak in the safe end using results of metallurgical examinations of the failed safe end, as well as industry experience with Inconel 600.
B.
Identify appropriate corrective actions for the safe end that failed based on the results of the engineering evaluation.
C.
Identify appropriate corrective actions for nozzle safe ends which may be susceptible to the same failure cause.
D.
Complete those corrective actions which are necessary, in light of the pressurizer safe end crack, to assure safe operation of Palisades during the next operating cycle prior to returning the plant to service.
E.
Identify long term corrective actions which are necessary to assure long term safe operation of Palisades' systems which have welded safe ends similar to the failed safe end.
The specific actions that have been or are being taken to address the pressurizer safe end crack are summarized in the following action plan. All actions are not complete at this time as our engineering evaluation of the root cause of the failure is not complete.
We will keep the NRC staff appraised of our progress and additional actions we identify as necessary to ensure continued safe operation of Palisades.
Action Plan I. Conduct an engineering evaluation of the failure.
An engineering analysis and root cause evaluation is underway.
Consumers Power Company engineering and metallurgy personnel are being assisted in this effort by ABB Combustion Engineering, Inc. personnel with expertise in lnconel 600.
The specific elements of this engineering analysis include the following:
- a.
Metallurgical analysis of the failed safe end to identify the cause of the crack. This evaluation is nearing completion and has identified primary water stress corrosion cracking {PWSCC} as the proximate cause of the failure.
- b.
Analysis of factors contributing to PWSCC in the pressurizer nozzle safe end.
This includes evaluation of material properties and stresses that may have contributed to the failure. Piping stresses and weld residual stresses are being evaluated and will be reviewed by an independent third party.
3
- c.
Evaluation of other nozzle safe ends in the primary coolant system, based on the engineering evaluation of the failure, to identify other locations which may be susceptible to the same failure cause. This evaluation includes both nozzles with lnconel 600 safe ends and other safe end materials. Contributing factors to PWSCC will also be evaluated for the nozzle safe ends that are identified as being susceptible to the same failure cause. This will include evaluation of material properties and stresses.
- d.
Evaluation of appropriate non-destructive examination techniques to identify similar flaws in other susceptible safe ends. A summary of our non-destructive examination efforts is provided in the attachment to this letter.
- 2.
Identify corrective actions for the spec if i c safe end that failed.
A code acceptable repair to the failed pressurizer safe end is being made at this time.
This repair is only considered to be a short term solution.
An engineering evaluation of this repair, based on the root cause analysis of the failure, has shown that the lifetime of the repaired safe end well exceeds the length of the next operating cycle.
Long term actions are addressed in action plan item 5 below.
- 3.
Identify corrective actions for nozzle safe ends that may be susceptible to the same failure.
Nozzle safe ends that may be susceptible to the same failure will be inspected for flaws using appropriate non-destructive examination techniques.
- 4.
Complete necessary corrective actions to ensure safe operation of Palisades during the next operating cycle prior to returning the plant to service.
- a.
Repair of the failed pressurizer safe end (action item 2).
- b.
Non-destructive examinations of other safe ends potentially susceptible to the same failure cause (action item 3).
- 5.
Identify corrective actions necessary to ensure long term safe operation of Palisades.
4
- a.
Engineering evaluation of additional repairs which may be necessary for long term operation of the pressurizer safe ends will be performed and additional repairs and corrective actions, if required, will be completed by the end of the next refueling shutdown.
- b.
Further evaluation of non-destructive examination techniques in light of the pressurizer safe end crack will be conducted.
Enhanced ultrasonic techniques will be employed in an augmented inspection program for safe ends beginning in the next refueling shutdown.
To provide assurance that we are performing the most effective NOE possible the following actions will be completed:
- i.
Evaluation of the June 1993 NOE results and implementation of lessons learned.
ii.
Review the ISi program, in an effort to ensure the effectiveness of radiography interpretation, for the second ISi interval and determine if radiography has been performed of other Class 1 welds.
Perform independent Level III examiner review of the identified sample.
iii.
Develop an appropriate mockup and subsequent qualification of specific ultrasonic examination techniques.
In summary, Consumers Power Company recognizes the significance of the through-wall crack that developed in the pressurizer safe end nozzle.
We are systematically evaluating the root cause of the crack and taking the corrective actions that are necessary to allow the Palisades Plant to be safely returned to service.
We intend to fully understand the problem and be confident that our corrective actions will be effective before returning the plant to service.
We also intend to submit a report to the NRC and meet with the NRC staff on the results of our actions prior to returning the.plant to service.
In addition, given the significance of this issue and the dynamic nature of the ongoing evaluation, we will provide periodic telephone updates to the staff on our progress and schedule.
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David W Rogers Plant Safety and Licensing Director CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades Attachment
ATTACHMENT Consumers Power Company Palisades Plant Docket 50-255 NON-DESTRUCTIVE EXAMINATION October 4, 1993 8 Pages
NOE EFFECTIVENESS AND ADDITIONAL NOE METHOD EVALUATION To increase our confidence in the NOE methods applied and in their effectiveness, the following items were evaluated or performed.
I I. The inside diameter {ID) dye penetrant {PT) examinations performed on the pressurizer relief valve nozzle welds have been repeated.
Prior to re-examination, the examination surface was prepped to remove surface oxide to assure the best possible examination. Additionally, better lighting and fiber optic equipment was used to augment the examination.
The combined result was a PT exam which was considered to be fully effective.
The same process will be applied to the remaining pressurizer relief valve nozzle weld.
The original ID PT examination performed on the PORV nozzle to safe end weld was considered to be fully effective and was not repeated.
- 2.
To address the sensitivity of the PT examinations, the following mockups were utilized:
A PT examination was performed on Inconel 600 tubes that contained various known levels of intergranular attack {IGA).
These samples were originally developed for qualification of eddy current techniques for the original steam generators.
The PT examination demonstrated that it was capable of detecting IGA flaws in Inconel 600 material.
A second PT examination was performed on the crack sample from the failed PORV safe end weld.
This sample contained multiple facets of the crack on both the ID and OD surfaces.
The PT examination was performed to help demonstrate the extent of the crack and to determine what affect, if any, the oxidation on the ID pipe surface would have on the examination.
The examination demonstrated numerous indications of various sizes from the very pronounced through wall portion of the crack to the very fine crack tips. It was also demonstrated that the oxidation on the pipe ID surface had no affect on the.examination.
- 3.
To further evaluate the effectiveness of our standard radiographic {RT}
examinations and address the usefulness of off axis RTs, RTs were performed on the same crack sample utilized for PT examination.
The RTs were conducted by mocking up the same variables that were used during the in situ RT examinations on the pressurizer welds.
The crack sample was radiographed simulating the double wall exposure single wall viewing technique. Three separate exposures were taken as follows:
A)
Source centered on the weld centerline.
B)
Source offset a half inch to the Inconel side of the weld.
C)
Source offset a half inch to the stainless side of the weld.
2 Results of the examination revealed that the standard techniques utilized (source centered on centerline of the weld) was as sensitive as either of the offset exposures and in one case more sensitive. Based on this mockup examinatian it is our opinion that even with flaws preferentially positioned at either side of a weld bevel, the standard RT examination method is as sensitive as offset techniques.
In addition to the mockup RTs several industry experts were canvassed for their opinions of offset RTs.
The consensus was that offset RT would not provide any greater confidence than the standard source centered technique and would not be beneficial especially considering the additional time and radiation exposure to complete the examination.
- 4.
The effectiveness and confidence in the ultrasonic testing performed on the pressurizer relief valve nozzle welds is greatly enhanced for the following reasons.
A.
These are 6-inch pipe welds.
Examiner confidence and experience with 6-inch welds versus smaller welds is much greater.
B.
These welds are machined on the 1.0. and therefore, there is no mismatch or other root geometry to complicate ultrasonic examination.
C.
In addition to the 1.0. machining, the 0.0. surface has also been machined providing a good examination surface for ultrasonic examination.
Summary Given the examinations performed to date and currently scheduled to be completed and the results of our review of the methods and their effectiveness, we conclude that we have performed the most effective NOE possible.
We have a high degree of confidence in our examinations and their ability to detect PWSCC.
No further examinations or special methods are planned this outage other than those previously indicated.
PALISADES POSITION ON THE NOE METHODS FOR THE PORV NOZZLE DISSIMILAR WELDS, SPRAY NOZZLE DISSIMILAR WELDS AND THE SAFETY NOZZLE ASSEMBLY DISSIMILAR WELDS.
PRESSURIZER POWER OPERATED RELIEF VALVE (PORV} NOZZLE ASSEMBLY WELDS Weld number 1, Nozzle To Safe End Weld Examinations performed on this weld joint include: Dye Penetrant (PT) examination of the prepped inside diameter (ID) root area of the weld when the joint was exposed to create the new safe end to pipe weld; Radiography (RT) of the weld; and PT examination of the outside diameter (OD) surface.
Results of the PTs and the RT did not reveal any cracking.
With the ability to do a complete PT from the ID surface coupled with the RT of the weld volume we have a high degree of confidence that there is no cracking generating from the ID surface or any other cracking within the weld volume.
3 There is no need to perform additional radiography of this weld.
The geometry of this weld precludes the use of ultrasonic techniques for the examination of this weld joint.
Weld number 2, Safe End To Pipe Weld This is the weld joint that failed and has been replaced.
During the replacement of this weld the prep areas of this joint were PT examined before welding.
The root weld was examined by radiography and accepted.
The ID surface of the weld joint was ground smooth to remove any surface roughness which may contribute to the growth of potential flaws.
The acceptance of this new joint is based on standard RT and OD-PT methods.
These examinations have been performed and are acceptable. Since this a new weld, there is not concern for stress corrosion cracking.
No further NOE examinati9ns are scheduled for this weld joint.
PRESSURIZER SPRAY NOZZLE ASSEMBLY WELDS Weld number 21, Safe End To Elbow Weld This weld joint has been examined by RT and a dye penetrant examination on the OD surface. These examinations did not reveal any cracking and the examination results were acceptable.
The ID of this joint is not accessible for ID-PT examinations.
The geometry of this weld joint precludes the use of ultrasonic techniques for the examination.
No further NOE examinations are scheduled for this weld joint.
Weld 20, Safe End To Nozzle This weld joint has been examined by RT and a dye penetrant examination on the OD surface. These examinations did not reveal any cracking and the examination results were acceptable.
The ID of this joint is also not accessible for ID-PT examination.
The geometry of this weld joint precludes the use of ultrasonic techniques for the examination.
No further NOE examinations are scheduled for this weld joint.
PRESSURIZER SAFETY RELIEF VALVE NOZZLE ASSEMBLY WELDS There are a total of three nozzle to Inconel flange welds.
The pressurizer code safety relief valves mount to the flanges.
The ID surface of these weld joints have been machined smooth.
For these three weld joints the following examinations have been performed.
Standard RT of all three welds, PT of the ID and OD surfaces after the surfaces were cleaned by a mechanical process. These exams performed to date did not reveal any cracking and the examinations results were acceptable.
4 An ultrasonic examination of the weld joints from the OD surface using IGSCC techniques is scheduled to be performed.
This examination will be performed from the Inconel side of the weld only. This UT examination will be performed by three separate individuals; CPCO's UT Level III examiner; ABB Combustion Engineering {CE) UT Level II and Virginia Corporation UT Level III examiners.
The Combustion Engineering and Virginia Corporation examiners are certified through EPRI for their ability to detect and size IGSCC types of indications.
It is noted that these are 6-inch pipe to flange welds which greatly enhances the ability to perform UT.
Additionally, the examiners have greater experience with 6-inch pipe as compared to 4-inch pipe.
No further NOE methods are proposed for these welds.
WALL THICKNESS WELD #
DIA PIEL SEIF PORV #1 6"..
1.5" SE TO NOZZLE WELD #2 4-6"
.438" 1.5" SE TO PIPE SPRAY #20 4"
.625"
.625" ELBOW TO SE
- 21 NOZZLE 4-8"
.625" TO SE RELIEF VALVES RV-1039 #1 6"
1.3" NOZZLE TO IN FLANGE RV-1040 #1 6"
1.3" NOZZLE TO IN FLANGE RV-1041 #1 6"
1.3" NOZZLE.TO IN FLANGE Do
= Scheduled But Not Complete Done = Completed No Decision Made Not To Examine N
1.8!'
2.5"
. 1.6" 1.6" 1.6" SURFACE PROFILE ID OD Smooth Moderate**
Bore Transition Taper Heavy Bore Transition As No Welded Transition Smooth Heavy Bore Transition Smooth Moderate Bore Transition Smooth Moderate Bore Transition Smooth Moderate Bore Transition P/EL = Pipe or Elbow SE
= Safe End N
= Nozzle F
= Flange I
RT OD ID IGSCC CODE C/L PT PT UT UT COMMENTS Done Done Done No*
No*
- Configura-.
tion pre-eludes UT
- Done Done Done No*
No*
- New Weld Qone Done No No*
No*
- Configura-ti on precludes UT Done Done No No*
No*
- Configura-ti on precludes UT Done Done Done Do No Done Done Done Do**
No
- Circumfer-ential Scan
- to do Done Done Do Do No Effective as of 10/04/93
WALL THICKNESS WELD #
DIA PIEL SEIF PORV PIPING WELDS WELD #4 4"
.438 ELBOW TO PIPE WELD #5 4"
.438 PIPE TO ELBOW WELD #5 4"
.438 ELBOW TO PIPE Do
= Scheduled But Not Complete Done = Completed No
= Decision Made Not To Examine N
SURFACE PROFILE ID OD As-No Welded Transition As-No Welded Transition As-No Welded Transition P/EL = Pipe or Elbow SE
= Safe End N.
= Nozzle F
= Flange I
RT OD ID IGSCC CODE CIL PT PT UT UT COMMENTS Done Done No No No Done Done No No No Done Done No No No Effective as of 10/04/93
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