Proactive Residual Stress Management Craig Stover Technical Leader EPRI - Advanced Nuclear Technology
ILWR MRP Conference August 4, 2016 Chicago, IL © 2016 Electric Power Research Institute, Inc. All rights reserved.
What if we could prevent cracks up front... Could we save ourselves from costly inspections and repairs...
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Objectives Review EPRI Report: – 3002005402 - Guidance for ALWR Primary System Component Residual Stress Management
Review ongoing EPRI research on residual stress modeling
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Residual Stress Guidelines Purpose: Optimize performance and life-cycle management of Residual Stress (RS) related degradation and failures in advanced nuclear plants Currently, nuclear industry uses Chemistry, Inspection, & Repair to manage material degradation Opportunity to create another “tool” to strategically & proactively manage material degradation – Residual Stress – Non-prescriptive, asset management tool that quantifies RS risk and provides guidance on mitigation techniques
– Allows the utility to identify susceptible locations & determine which tool is the best solution
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Core Shroud 304L/316L SCC
Methodology – ALWRs Developed Residual Stress Guidelines for ALWRs FMEA methodology for identifying & quantifying RS risk – Screen out low risk locations, focus on higher risk welds
– Insight on value of mitigation
Guidance on management strategies – Mitigation options: technical basis & critical factors – How to choose appropriate technique
Significant review and input from industry – WRTC advisors, industry experts – Hosted three TAGs (project meetings) 5 © 2016 Electric Power Research Institute, Inc. All rights reserved.
Methodology – SMRs Updated RS Guidelines for SMRs Partnered with NuScale to review current methodology
Made minor changes, documented light water SMR applicability, included a worked example (nozzle to safe end DMW) – Minimal changes due to very similar operating environments and materials, biggest difference is “inspectability” i.e. access concerns
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Process for Determining if RSM Technique Should be Applied - Simplified 1. Select components / locations for evaluation 2. Screen out low risk items – Functionality, replaceability, “as-manufactured”, degradation mechanism
3. Collect data – Material, environment, fabrication & welding process, geometry, repair?
4. Assess RS risk (FMEA) – – – –
Influence of RS on initiation and crack growth Surface RS & Thru-wall RS (configuration and fabrication) Weighting factors on occurrence and severity IASCC (if applicable)
– Factor of Improvement (how much impact from RS mitigation?)
5. Integrity Risk – ISI + Flaw tolerance 7 © 2016 Electric Power Research Institute, Inc. All rights reserved.
Example Application – Advanced PWR: DVI Piping Weld 1. Location – weld joining DVI nozzle safe end to SS piping 2. Does NOT screen out –
Passive, long lived, shop & field welds, SCC & thermal fatigue applicable
3. Data assumed, not specific to design or vendor 4. Assess Risk – Assumed GTAW, standard single V groove, etc… – RPN: 161 x FOI 1.6 = 190 – Integrity Category: 4 – RSM Category: C Assume deep OD weld repair: – RPN x FOI = 1856 – RSM Category: B
5. Evaluate RSM Technologies –
Heat sink welding, dry ID compression welding, induction heat stress improvement, Mechanical Stress Improvement (MSI), shot peening, laser jet peening, etc….
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Interpreting Results
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RSM Guidelines Summary Advanced Light Water Reactors
Small Modular Reactors
Methodology for identifying and quantifying risk of residual stress (FMEA)
Partnered with SMR vendor to review current methodology Expanded scope, documented light water SMR applicability, documented a worked example (nozzle to safe end DMW)
– Screen out low risk locations, focus energy on higher risk welds
– Insight on value of mitigation
Guidance on management strategies – Mitigation options: technical basis and critical factors – Technique selection (e.g. different welding techniques, mechanical stress improvement process, weld overlay, peening, and polishing
– Minimal changes due to very similar operating environments and materials, biggest difference is “inspectability” i.e. access concerns
See EPRI report 3002005402, ANT: Guidance for ALWR Primary System Component Residual Stress Management
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Residual Stress Modeling Measuring & Modeling How can we incorporate Residual Stress into the design phase? Need improved modeling & measurement, lack of confidence and raw data to benchmark (Gap identified in Report) Partnering with WRTC to fabricate mock-ups for modeling and measuring RS – WRTC fabricating several ALWR mock-ups
– ANT fabricating an SMR mock-up (partnership with SMR Vendor)
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Modeling Project Methodology Fabricating 4 RPV Nozzle mockups – Mockups will simulate both long and short nozzles – Nozzles will be welded to safe-ends utilizing both narrow and wide groove welds – WRS finite element modeling conducted to predict stresses – Residual Stresses will be measured and benchmarked against the stresses predicted by the model
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FEA Preliminary Results - PWHT As-Welded at 70 F
First step shows significant aswelded residual stress buildup
End of Hold Time at 1100 F
Second step shows the majority of the residual stresses are relieved at the end of PWHT Thirst step shows residual stress re-appear – Since the stress relieving occurs at a high temperature, “new” residual stresses are created when the materials are cooled – The stainless steel cladding wanted to shrink more than the vessel during cooling resulting in tensile RS in the cladding
End of PWHT at 70 F
Cooling 100°F/hr Hold time 1.5 hrs
Air cool from 800 F
Heating 100°F/hr
ksi
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FEA Preliminary Results – Dissimilar Metal Welds (DMW) HOOP STRESS
DMWs result in extensive compressive hoop residual stresses in the weld roots
Long nozzle Wide Groove Long nozzle
Short nozzle Wide Groove Short nozzle
Narrow Groove
Narrow Groove Max
Max
DMWs result in insignificant tensile axial residual stress in the weld roots This is a reverse trend from a typical butt AXIAL STRESS weld. – The reason for the reverse trend is because the nozzle has a very thick wall. Therefore, the R/t ratio becomes smaller leading to compressive residual stresses on the ID
Max
HOOP STRESS
Max
ksi
Long nozzle Wide Groove
Short nozzle Wide Groove Max
Max
The most susceptible crack initiation location is from the nozzle OD surface near the DMW and buttering interface
AXIAL STRESS ksi
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Mockup Status Mockups are going through final machining – Safe-ends will be welded by the end of August.
Residual Stress measurements and benchmarking against the model will be completed by the end of 2016 A final report will be published by the end of first quarter of 2017
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Together…Shaping the Future of Electricity
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