Mitigation of PWR Reactor Vessel Primary Nozzle Dissimilar Metal by mikesanye

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									                                       Repair or Maintenance

     Mitigation of PWR Reactor Vessel Primary Nozzle Dissimilar Metal Butt Welds; NDE
                    Examination Techniques for the AEGIS Inlay™ Process
                            S.W. Glass, B. Thigpen, Areva, France

ABSTRACT

Alloy 600 challenges including compliance with MRP-139 [1] applicable to susceptible dissimilar-
metal (DM) but-welds of the primary nozzles have motivated AREVA’s AEGIS inlay program.
Mitigation of these DM-welds relieves utilities of expensive and frequent inspection demands imposed
by regulators based on long-term fleet experience of Primary Water Stress Corrosion Cracking
(PWSCC). The AEGIS inlay process is particularly applicable for plants that have limited access to
the outside of the welds or plants that cannot tolerate the presence of existing PWSCC or the
possibility for new PWSCC to occur. The inlay process steps include machining the pipe ID surface
near the weld location and application of new PWSCC resistant material to the pipe thus returning the
pipe geometry to the original design dimensions (figure 1). Nozzle weld inlay and nozzle replacement
have been previously performed in France and Sweden in plants with coffer dams that allow the
internals to be shielded while dry work is performed in the vessel and in the nozzles. The AGEIS
process can be performed in plants with no coffer dams.
         The process must not compromise the existing inspection qualifications. As part of this
development program, AREVA in cooperation with Westinghouse, EPRI, and the PWROG have
engaged a program to demonstrate that existing PDI inspection procedures are equivalent for detection
and sizing with or without the mitigation inlay layer. This paper outlines the AEGIS program
development focusing on the inspection equivalency demonstration program.




                     Figure 1 - Inlay weld schematic with a thin PWSCC
                     resistant layer (alloy 52) covering the susceptible weld
                     material.


INTRODUCTION – The AEGIS Primary Nozzle Mitigation Process

Many PWR plants have Reactor Vessel (RV) primary nozzles with dissimilar metal (DM) weld
configurations on both hot and cold leg primary nozzles made with materials that have been shown to
be susceptible to primary water stress corrosion cracking (PWSCC). As these plants age, the likelihood
of PWSCC occurring in the DM weld increases. In the US, MRP-139 [1] requires increased inspection
frequency of these welds unless corrective action is taken. Presently several mitigation technologies
exist including: weld inlay, full structural weld overlay, mechanical stress improvement (MSIP™),
non-structural weld overlay, and total spool-piece replacement. Each of these technologies has been
demonstrated to be effective for DM weld mitigation or replacement however the process selection is
Figure 2 - Inlay weld schematic following deep indication excavation and repair

dependant on the specific plant configuration and specific regulatory requirements. AREVA’s AEGIS
weld inlay process is particularly well suited for plants that have no room outside the pipe for external
mitigation approaches, or for plants that cannot tolerate known cracks, or even the likelihood that
PWSCC cracks will develop following the mitigation process.
        The AEGIS weld inlay of the DM weld area isolates the PWSCC-susceptible material from
the primary environment. Although there are numerous weld configurations among the nuclear fleet,
application of the protective inlay changes the condition to MRP-139 category A, and returns the
component to the current ASME required 10-year ISI frequency. The complete process is outlined
below:
    1. Pre-mitigation non destructive examination (NDE) (UT and ET) plus Laser Profile for
        indications, to locate the weld boundaries, and to determine nozzle exact geometry. If NDE is
        clear with no indications of concern, proceed to step 7.
    2. Remove surface cracks (mill up to 2 inches (50mm) (if required).
    3. Re-examin and confirm no cracks by NDE.
    4. Repair – fill in milled area(s) with PWSCC resistant alloy 52 (or 182 for deep repairs) where
        surface cracks were removed - TIG weld process. (Figure 2)
    5. Weld Crown Removal (milling)
    6. PT to confirm no crack indications
    7. Machine and Clean Surface for welding thin protective layer
    8. PT to confirm no surface cracks
    9. Weld Inlay to slightly above the original pipe geometry
    10. Machine smooth – Weld Crown Removal (for inspection and for final geometry profile)
    11. Final PT, ET, and UT to re-confirm no surface cracks or problems with the inlay. (In
        unlikely event that problem indications are found, return to step 2.)




Figure 3: European top-hat inlay system used in coffer-dam plants.

The AREVA NP French region has qualified a similar repair/mitigation process for the European
market (France and Belgium). These plants have a cofferdam in the refuel cavity that allows half of
the cavity to be flooded, while the RV side can be drained to mid-loop. The lower internals can be
removed and stored in the deep end of the flooded cavity. An inverted hat assembly, (Figure 3), is
installed on the dry cavity floor over the top of the RV. Operations are performed remotely from the
upper deck. A remote multiple process tooling system based on a mobile Staubli robot (reference 3)
was used to execute the process. Nevertheless dose is an issue since work is being performed in a dry
cavity. This process is not applicable to the U.S. market and many other plants around the world that
do not have cofferdams.
                                                        BAT Top



                                                        BAT Extension




                                                           BAT 2


                                                         Big Access
                                                         Tube (BAT) #1



                         Temporary
                            Reactor
                        Vessel Head

                            Support
                            Legs (4)

                                                            Top Hat
                                                            Extension



                               FME
                          Covers (8)                       Top Hat


                         Figure 4 - Flooded Cavity Delivery System
                         (FCDS) provides dry access to the nozzles.


The AEGIS inlay approach integrates proven technologies of dry welding, machining, NDE and
FOSAR with an ambitious delivery system that allows work in a flooded cavity and minimizes the
outage schedule impact. Unlike most plants in France, which have cofferdams in their refuel cavities,
many plants including most in the US require water shielding of the reactor vessel internals while
completing any major repairs inside the containment. The AEGIS inlay process first requires the canal
to be flooded and the internals to be removed to the far-end of the canal. The AEGIS Flooded Cavity
Delivery System (FCDS) (Figure 4) is then installed on top of the vessel and sealed to the vessel
flange. This allows the vessel to be drained below the nozzles and thereby permits robotic tooling to
be delivered to the primary pipes for dry inlay operations while the canal stays flooded. The FCDS
platform allows operations on multiple nozzles in parallel to minimize the overall outage delay. The
flexibility of the FCDS is further enhanced by easily changing between 6 and 8 nozzle configurations.
All in-pipe operations are performed by the by the common tool manipulator system (CTM) (figure 5).
Primarily based on a commercial Staubli robot specially adapted for this in-pipe service, the CTM
delivers NDE, PT, Laser metrology, machining, and welding tools. The CTM can be raised and
lowered through the FCDS for tool changes or rotated to the next nozzle for series operations as
required. Registration of the CTM from one insertion to the next is absolutely encoded and positively
registered against hard-stops for precise positioning.
                          Figure 5 - The Staubli robot common tool
                          manipulator (CTM) delivers NDE, PT,
                          machining and welding tools.

THE NDE CHALLENGE

One consideration for any mitigation process is the continued inspectibility of the DM welds. The
EPRI Performance Demonstration Initiative (PDI) qualification for this nozzle inspection is one of the
more challenging and expensive qualifications in the program. For the inlay process acceptance, it is
desirable that the PDI qualification and any other qualification of the unmitigated welds not be
affected. The NDE challenge was to demonstrate PDI equivalency testing on inlay weld geometries,
using current PDI-qualified ultrasonic examination procedures and personnel.




                          Figure 6 - UT & ET inspection head used for
                          NDE equivalency demonstrations as well as
                          ISI and post mitigation NDE.


METHOD

Under sponsorship of the Pressurized Water Owners Group (PWOG), both AREVA’s and
Westinghouse’s previously demonstrated ASME Section XI, Appendix VIII PDI procedures were
used to evaluate the influence of the AEGIS weld inlay on the inspectibility of a representative weld
inlay sample. The reference mockup contained four flaws that were implanted in a full scale
representation of a nozzle to safe-end and safe-end to pipe weld configuration. An inlay mockup was
fabricated to replicate the configuration of reference mockup including four similar flaws identified as
flaws 1, 2, 3, and 12. These flaws were implanted in three quadrants of the inlay mockup with varying
inlay thicknesses from 0.08” to 1.0”. Both mockups were scanned using the same equipment as
specified in the examination procedures. The data was then analyzed in accordance with the standard
PDI procedures for detection, length sizing, and depth sizing to identify differences between the
reference flaws and the flaws in the inlay mockup.


GENERAL OBSERVATIONS

Results from both Westinghouse and AREVA were essentially the same and are summarized in an
EPRI report prepared for the PWROG [3]. Background noise levels seemed to increase with the
thickness of inlay. This was particularly noticeable with the circumferentially directed beams.
Additional work identified the cause of the elevated noise levels to be associated with the direction of
the grain boundaries associated with the pattern of weld bead deposition. While the increased noise
levels are present, in some cases resulting in lower signal to noise ratios, the flaws were all detectable
in accordance with the procedure requirements. Furthermore, length and depth sizing results were
within the expected sizing accuracies. (Figure 7)




 Figure 7 - Representative circumferentially directed beam comparison of reference and post-inlay
 UT signal. Although S/N is worse following inlay, detection and sizing is possible within
 expected accuracies.


CONCLUSIONS

Without some mitigation process, US regulators require increased inspection of the primary pipe DM
but-welds just beyond the RPV nozzles. Several mitigation approaches have been demonstrated to be
viable and the best solution depends on the specifics of the plant and the regulatory requirements. The
AEGIS inlay mitigation process for primary nozzle DM welds has been developed specifically for
plants without coffer dams and where access to the outside of the pipe is restricted thereby eliminating
overlay or other mitigation approaches. It is also well suited to regulatory environments that cannot
tolerate known cracks or flaws.
         Although the primary goal of the mitigation process is to reduce the probability of PWSCC
and limit the frequency of inspection, the mitigation process must not compromise the ability to
inspect these welds in accordance with the normal recommended inspection interval. Furthermore,
existing rigorous qualifications for these welds are expensive to perform so it is desirable to
demonstrate the existing procedures can be applied with equivalent expectations with or without the
mitigating inlay layer in place. Although some decreased signal/noise ratio was observed following
the inlay application, both AREVA and Westinghouse demonstrated equivalency of their existing
ASME Appendix VIII, supplement 10 or 14 automated UT procedures for DM welds with and without
the AEGIS inlay process.


REFERENCES

1)      Material Reliability Program; Primary System Piping Butt Weld Inspection and Evaluation
        Guideline (MRP-139); EPRI Report 1010087 July 14, 2005
2)      Nondestructive Evaluation: Ultrasonic Equivalency Testing of Weld Inlaid Components;
        EPRI report 1016543, Technical Update, April 2008, EPRI Project Manager C. Latiolais
3)      Ninth European Nuclear Conference; Dec 11-14; Versailles France; Replacement of heavy
        components of the Main Primary System (MPS) - Recent innovations made by Framatome
        ANP : J.M. Chanussot & R. Thévenet Framatome ANP - Services Sector

								
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