ML22301A138
| ML22301A138 | |
| Person / Time | |
|---|---|
| Issue date: | 09/22/2022 |
| From: | Neil Day, Kenneth Kolaczyk NRC/RGN-III, NRC/NRR/DRO/IRAB |
| To: | |
| References | |
| Download: ML22301A138 (36) | |
Text
Mechanical Stress Improvement Process Neil Day September 22, 2022
Disclaimer/Ground Rules This training session is being recorded for future use in the NRCs knowledge management program. The recorded contents of the session, including any questions Video is being Recorded for future use posted by audience members, will be preserved in accordance with I promise to be professional and ask the the NRCs record management same program and are subject to FOIA I allow Questions/Comments/Insights at disclosure. Please refrain from any time including any sensitive information (i.e., SUNSI) in any questions that Use the knowledge of the room, Im more you may ask. of a facilitator Please leave your chat box open to view all questions and answers posed during the session.
Acknowledgements Edison Fernandez, Reactor Inspector in R3 Onsite doing ISI inspection, saved me Jim Melfi, Project Engineer in R4 DORS Onsite helping with the Outage, saved me Jim Drake, Senior Reactor Inspector in R4 DORS Followed up reviewing Westinghouse Root Cause Eval, always saves me
Scope of Todays Presentation Key Takeaways (Starting with the End First)
Mechanical Stress Improvement Process (MSIP) - What is it?
Where is it useful?
What are the advantages/disadvantages vs other options?
What happened at Comanche Peak Unit 2?
Key Takeaways MSIP can be a useful tool in reducing the required nondestructive examinations needed on welds Payback is time/money/dose/predictability It is a high energy process Takes planning and can be in tight locations Interferences removed/reinstalled Training helpful in real size mockups Key time in outage (defueled)
Vendor performed with licensee oversight and responsibility Where the breakdown happened at Comanche Peak Unit 2 Fall 2021 Outage
Short Commercial Video from NUVISION Engineering https://www.youtube.com/watch?v=EVoMswp6vJ0 Keep in mind this is a marketing video
Mechanical stress improvement process (MSIP)
Mitigates Stress Corrosion Cracking in Nickel Alloy Butt Welds in Piping Systems Approved for Leak-Before-Break MSIP uses a hydraulically operated clamp to slightly compress the pipe on one side of the weldment.
Over 3000 welds, including those with pre-existing crack indications, have been treated in BWR and PWR nuclear power plants worldwide.
To date, no cracks have initiated in any welds treated.
Cracks in weldments with pre-existing flaws have been arrested.
Westinghouse Advertisement Link to NS-IMS-0051
Alternatives to MSIP 3 Different Methods to dealing with stress corrosion cracking MSIP (Of course)
Increase volumetric NDE inspection frequency Weld Overlays MSIP is a proven Technology that if implemented, saves long term cost/outage duration and time Each have advantages/disadvantages Westinghouse/NuVision offers services Interferences was big drawback - hot work required
Typical Locations of Alloy 82/182 Butt Welds
RPV Hot Leg Nozzle, Nozzle Weld, and Safe End Clamping device Comanche Peak Unit 2 MSIP Hot Leg 2 MSIP Tooling - Intact Box Press on Left, Failed Box Press on Right
What it does/How it works MSIP modifies the existing inner-diameter (ID) residual tensile stresses in the weld metal and heat-affected zone (HAZ) of butt welds in piping.
A load is applied to the outside surface of the pipe with a large two-piece mechanical clamshell hydraulic clamping device connected by two pairs of tangentially positioned studs.
The studs are tightened with hydraulic tensioners.
Displacement is controlled to pre-assigned limits by using shims, providing safety from over-straining the pipe.
Loading plastically strains the pipe in compression, which creates compressive residual stress around the weld and counterbore region.
Stainless steel, waffled contact plates are placed between the tool and the pipe outer surface to aid in uniformly distributing the load.
History of MSIP Check Out TEMPORARY INSTRUCTION 2515/172 Alloy 82/182/600 materials used in reactor coolant system (RCS) pressure boundaries of pressurized water reactors (PWRs) are susceptible to primary water stress corrosion cracking (PWSCC).
Cracking can initiate at the inside surface of these materials, in part, because of tensile residual stresses introduced by welding In December 2005, the NRC sent a letter to ASME requesting that the Section XI standards body take actions necessary to develop improvements to the existing Code requirements to address the potential for PWSCC in Alloy 82/182 weld materials Class 1 PWR piping butt welds.
Development of an ASME Code Case N-770 on appropriate inspection requirements to address PWSCC in Alloy 82/182Class 1 butt welds.
Code Case revised in 2009 to address NRC concerns and Code Case N-770-1 was issued later that year (ASME 2009a).
The NRC incorporated ASME Code Case N-770-1 by reference into 10 CFR 50.55a (76 FR 36232) in June 2011.
Requirements from Code Case N-770-1 The mitigation technique has to minimize the likelihood of crack initiation.
The effect on the susceptible weld material produced by the mitigation process has to be permanent.
The capability to perform UT of the required inspection volume of the component cannot be adversely affected by the mitigation.
The mitigation process cannot have degraded the component or adversely affected other components in the system.
The mitigated weld has to be inspectable by a qualified process.
Existing flaws, if any, have to be addressed as part of the mitigation.
The effect of the mitigation on any existing flaws has to be analyzed.
History of MSIP continued MSIP was first used in 1986 for PWR units.
MSIP was approved by the NRC for BWR applications in 1988.
The current holder on the patent for MSIP, NuVision Engineering, has indicated that MSIP has over 25 years of successful operating experience in the United States with more than 1000 welds treated (NuVision 2011).
Where used MSIP has been implemented on many PWR reactor vessel nozzle Dissimilar Metal Welds.
V.C. Summer, Salem Unit 1 Seabrook D.C. Cook, Unit 1 Watts Bar, Unit 2.
RPV Hot Leg Nozzle Weld Axial Scan CRV Coverage Assessment RPV Cold Leg Nozzle RPV Cold Leg Nozzle Weld Axial Scan CRV Coverage Experience at Comanche Peak First attempt - Unable to complete due to access limitations, rescheduled to 2RF19 Failures of NuVision Engineerings Mechanical Stress Improvement Process (MSIP) hardware during the Fall of 2021 refueling outage.
20 October 2021, Unit 2 Hot Leg 2 had Press frame failure of both legs.
23 October 2021, Unit 2 Cold Leg 1 had Press Frame failure of one leg.
Licensee and vendor did not understand cause of first failure before continuing process.
Acknowledged proceeding At Risk
Hot Leg 2 MSIP Tooling layout Including Approximate Failure Location
Description of Events at Comanche Peak On October 20, 2021, a hydraulic box press failed completely during implementation of the Mechanical Stress Improvement Process (MSIP) project; one of two press clamps had separated into two pieces (CR-2021-006952). The press was installed on Unit 2 RCS Hot Leg #2 for compression of the Hot Leg #2 to reactor vessel weld. As the tooling was pressurized to nearly full pressure, a loud noise was heard and felt throughout Unit 2 containment. Upon initial inspection of Hot Leg #2, indications on the outside diameter of the Hot Leg #2 pipe segment and associated sandbox liner were identified. In addition, some non-plant equipment was damaged. In a similar manner on October 23, 2021, a second hydraulic box press partially failed during the MSIP compression of the Unit 2 RCS Cold Leg #1 to reactor vessel weld (CR-2021-007061).
Upon initial inspection of Cold Leg #1, no plant damage due to tool failure was identified; Subsequently, an indication was identified on the outside diameter of the pipe segment. This indication had been pre-existing as evidenced by pre-event photographs
Description of Events at Comanche Peak (Cont.)
Failures of the hydraulic box presses resulted in shocks directly transmitted to Reactor Coolant System Cold Leg #1 and Hot Leg #2 with the shocks potentially affecting these loops and other reactor coolant loops and associated piping, welds, components, supports and the containment internal structure.
THESE WERE HIGH ENERGY EVENTS
Sand Box Damage Large indication of damage on the floor below the hot leg 2 MSIP failure. This damage was likely from the sudden ejection downward of the smaller part of the box press that failed, while the upper portion ejected into the ceiling. The large indication in the floor shows a significant amount of energy was absorbed by the impact.
Loop 2 Hot Leg Damage Failed Box Press Lower Portion Failed Box Press Upper Portion Inspector Followup Perform Containment Walkdown to look for structural damage CP Engineering Performed a Post Seismic Event Walkdown Independently assess nozzle damage Operability Evaluation Inspection - Thanks again Jim!!!!
Note during MSIP Squeezes, core was offloaded (Lower Risk)
Why are they called sandboxes anyways?!?!
Included internal vessel/nozzle submarine inspections Licensee cause evaluations After 1st Failure - SRI discussed with licensee NRC concerns of proceeding without condition/cause being understood and corrected.
The reactor coolant loop piping and supports system was assessed regarding the MSIP tool failures observed at the Comanche Peak Unit 2, Loop 2 hot leg and Loop 1 cold leg locations. It was concluded that the plant is safe for continued operation with no significant damage to the reactor coolant system piping that would impact operability.
Loop 2 Hot Leg Thickness Measurements Post Buffing Repair
Longer term actions Inside containment closed ventilation (Neutron Monitoring Well)
High DP alarm after about a month of operation post outage (unexpected)
Previous cleaning frequencies were 18 months Resident Team entered containment at power to assess Sand like debris internal to ventilation, correlated to concrete aggregate from MSIP failures
Direct and Root Causes Inadequate processes and procedures Design Changes No engineering calculations for entire tooling assembly Entire assembly not modeled Offset contour rings generated unanticipated and unanalyzed loading Changes in stresses not understood or analyzed Proceeded in the face of uncertainty Assumed since offset not as large on Cold leg clamp, could proceed on the cold leg MSIP.
Still working it Still monitoring containment ventilation Pursing several performance deficiencies SHARING THE KNOWLEDGE
Questions?