Analysis and Impact of Recent Thermal Fatigue Operating Experience in the USA

Transcription

1 Analysis and Impact of Recent Thermal Fatigue Operating Experience in the USA Bob McGill Structural Integrity Associates Terry Childress Duke Energy Michael Hoehn Ameren Missouri Mike McDevitt EPRI 2016 International LWR Material Reliability Conference and Exhibition August 2016

2 Presentation Content MRP Fatigue Management of Thermal Stratification and Mixing Operating Experience EPRI Response and Analysis Summary Additional Fatigue Events Spring 2016 Program Impact and Next Steps 2

3 Fatigue Management of Thermal Stratification and Mixing Program Evolution Original plant designs and inspection programs did not conceive of all potential thermal fatigue vulnerabilities Thermal stratification Thermal mixing OE during the mid 1980s alerted Industry to the need for management of thermal fatigue Industry responded - collaborative research led to a better understanding of system behavior MRP strategy focuses on component identification, inspection and repair MRP thermal fatigue management under NEI is implemented by: MRP-146 Cyclic stratification in non-isolable RCS branch lines MRP-192 Thermal mixing tees in RHR systems 3

4 MRP Fatigue Management of Thermal Stratification and Mixing Normally Stagnant Branch Lines, MRP-146 Addresses interaction of hot swirl penetration into non-isolable horizontal pipe UH Configurations H Configurations DH Configurations Thermal Mixing Tees, MRP-192 Addresses interaction from mixing of thermally different fluids 4

5 : 10 Operating Events An Alarming Trend 5 Events leading into NRC Bulletin (11 years) 5 through-wall leakage events 6 Events subsequent to Bulletin thru 2000 (11 years) 5 through-wall branch lines (1 US plant) 1 through-wall RHR Mixing Tee (International) 3 Events after industry guidance thru 2012 (12 years) 2 part-through branch line cracks (1 US Plant) 1 part-through RHR Mixing Tee (US Plant) 10 events in US between 2013 and 2015 (2 years) 2 through-wall leaks in branch line cracks 5 part-through branch line cracks 1 through-wall leak in RWCU mixing tee 2 part-through RHR mixing tee 5

6 Overview of Operating Experience Date Component Method of Detection Nov 2013 TMI-1 Nov 2013 Oconee-1 Apr 2014 McGuire-2 Sep 2014 McGuire-1 Sep 2014 McGuire-1 Oct 2014 Perry (BWR) Nov 2014 Oconee-1 Dec 2014 North Anna-1 May 2015 Catawba-2 Cold Leg Drain Elbow (DH) Cold Leg HPI Nozzle weld (H) (primary vibration) Cold Leg HPI Nozzle weld (H) Loop-B Cold Leg HPI Nozzle weld (UH) Loop-C Cold Leg HPI Nozzle weld (UH) RHR Mixing Tee Reactor Water Clean Up Tee Cold Leg Drain Elbow (DH) Cold Leg Drain Elbow (DH) RHR Mixing Tee Found during planned MRP-146 examination Through-wall leakage & forced outage Found during planned MRP-146 examination using upgraded NDE Found during Extent of Condition exams Found during planned, calendar based examination Through-wall leakage & forced system outage Found during planned examination Through-wall leakage & forced outage Found during planned examination 6

7 Fatigue Crack Locations October 2013 May 2015 Stagnant Branch Lines, MRP-146 Nov 2013, 100% t-w, Circ Sep 2014, 50% t-w, ¾ l, Circ Sep 2014, 50% t-w, 1½ l, Axial Apr 2014, 85% t-w, 1.1 l, Axial Nov 2014, 20% t-w, 3/8 l, Axial Thermal Mixing Tees, MRP-192 Nov 2013, 35% t-w, ¾ l, Circ Dec 2014, 100% t-w, 2 l, Axial Oct 2014, 100% t-w, 2.5 l, Circ Sep 2014, 14% t-w, 0.5 l, Circ May 2015, shallow, Circ 7

8 Thermal Fatigue Operating Experience - EPRI MRP Response EPRI Thermal Fatigue Focus Group (TFFG) established to assess impact of OE and recommend corrective actions TFFG objectives: Publish Interim Guidance to prevent thermal fatigue degradation from exceeding Regulatory limits Identify actions for program improvement TFFG Process: Review each event Identify specific program failures Identify potential causes and determine mitigation alternatives Develop Interim Guidance to detect cracks at an early stage Identify knowledge gaps and propose specific research to close gaps & improve program Interim Guidance issued in May 2015 (MRP ) 8

9 Analysis Results Management Program Failures Identified Date Component Management Program Breakdown Nov B Cold Leg Drain Elbow Repeat location of 1995 TF leak Nov 2013 Loop B HPI Nozzle Weld (H) Through-wall leakage Crack was missed in prior NDE Apr 2014 Loop D HPI Nozzle Weld (H) Likely missed in prior NDE Sep 2014 Sep 2014 Oct 2014 Nov 2014 Dec 2014 Loop B HPI Nozzle Weld (UH) Loop C HPI Nozzle Weld (UH) RHR Mixing Tee Reactor Water Clean Up Tee Loop B C.L. Drain Elbow Loop B C.L. Drain Elbow Were exempted by MRP-146 screening Cracks located outside of exam zone Larger than expected for low usage Likely missed in prior NDE Repeat of 2008 through-wall crack Growth exceeded plant expectations Repeat of 2000 through-wall crack Mitigating insulation was compromised Through-wall leakage Frequent water chemistry sampling May 2015 RHR Mixing Tee Active degradation or better NDE? 9

10 Analysis Conclusions Fatigue Management Program Deficiencies Thermal fatigue in small diameter UH lines was not adequately addressed within existing programs Upstream welds in thermal mixing tees were not conservatively managed Cyclic outflow of thermally stratified fluids was not addressed Cause determinations and mitigation actions were not validated and monitored to ensure effectiveness NDE systems were not reliably detecting cracks prior to exceeding Code allowable limits Observed trend may indicate that a population of components are reaching their fatigue life 10

11 Analysis Recommendations Fatigue Management Program Changes Examine vertical section of UH branch piping subject to cold in-leakage Socket Welded Construction Butt Welded Construction 11

12 Analysis Recommendations Fatigue Management Program Changes (2) Examine or analyze lines with cyclic outflow (to address possible stratified laminar flow) Validate fatigue analysis assumptions & corrective actions (ensure at-risk assumptions remain valid) Strengthen NDE process - require essentially 100% coverage of target volumes Expand mixing tee exam volumes to include upstream welds 12

13 Analysis Recommendations Research & Development Initiatives Improve engineering technologies Construct mockups similar in design to a recent operating event; investigate OE and generate computational fluid dynamic (CFD) validation data Develop computational simulation models to investigate sensitivities and mitigations Update thermal fatigue guidance Benchmark fatigue management in France & Japan Consolidate thermal fatigue guidelines and improve usability Develop understanding & guidance for evaluating craze cracking 13

14 Analysis Recommendations Research & Development Initiatives (2) Strengthen implementation tools - improve examination systems & mockups Maintain plant staff expertise Updated EPRI Fatigue Management Handbook (MRP-235) Provide updated fatigue management training (conducted Monday and will be available electronically on EPRI MRP website) Support international initiatives (e.g., this conference) 14

15 Spring 2016 Fatigue Operating Experience North Anna Power Station, Unit 2 McGuire Nuclear Station, Unit 1 D.C. Cook Inquiry Diablo Canyon Power Plant, Unit 2 15

16 North Anna Power Station, Unit-2 Cold Leg Drain, 2-inch Outage examination results Circumferential indication in elbow-to-horizontal pipe weld Approximately 1 inch long and 58% deep (0.194 depth) Prior history Examined in 2009 with no rejectable indications identified Infrequently used for chemistry sampling Very similar configuration to Unit 1 Loop B through-wall leak Dec 2014 Industry impact Fatigue driving conditions not well understood Role of chemistry sampling is less probable than thought after Unit 1 event Thermal fatigue analysis (swirl penetration) does not predict cracking Involvement of other fatigue modes is unknown (vibration) 16

17 McGuire, Unit 1 Charging Line, 3-inch UT examination during March 2016 outage revealed unexpected growth 0.15 through-wall (44% wall) and greater than 5 long at ID Indication was identified in prior outage (September 2014) Code acceptable (0.038 through-wall and 2.75 in length) Similar to location of two cracks in 1½ NPS HPI nozzle-to-pipe welds Cause of flaw growth during past cycle is uncertain Spring check valve leakage (through isolation valve bypass line) could create a thermal fatigue condition Significant thermal shock transients are possible Possible HCF mixing at pipe to nozzle Industry impacts Thermal fatigue (swirl penetration) models do not predict this crack location Relevance to one-time examination of nozzle-to-pipe welds in 2-4 NPS UH configured lines needs assessment 17

18 D.C. Cook Temperature Observations Revealed Possible In-leakage Condition 120 F 300 F 590 F Possible Leakage Flow T = 120 F 250 F Plant engineers observed elevated temperatures in Safety Injection piping Temperature distribution suggested cross-loop flow of ~ 0.1 gpm circulation flow Circulation flow return temperature estimated ~150 F 18

19 D.C. Cook Inquiry Inquiry panel conclusions Thermal stratification cycling and/or mixing is possible from cross communication between RCS piping that results in cold in-leakage Thermal fatigue guidelines MAY be used to assess & manage the thermal fatigue risk This fatigue mode should be evaluated for potential inclusion in MRP-146 TFFG will develop recommendation for assessing response Actual susceptibility and consequence to plant designs is not known Identify population of mode-susceptible designs Identify relationship between bypass flow and in-leakage temperature 19

20 Diablo Canyon Unit 2 RHR Hot Leg Suction Line, 14-inch Identified in May 2016 Circumferentially oriented in the upper portion of 45 elbow to horizontal pipe Approximately 8 in length, 0.34 deep, ID surface connected Not observed in 2004 examinations Accepted by flaw tolerance for return to service Apparent cause under evaluation Thermal fatigue was considered very unlikely due to analytical model predictions Thermocouples have revealed unexpected thermal stratification 20

21 Program Impact due to Spring 2016 Events Relevance to industry guidance All four 2016 events reveal a potential need for modified guidance NAPS-2 OE was not expected based on low chemistry sample usage and would not be expected based on the Generic Evaluation of MRP-146S MNS-1 crack location was not considered likely in UH lines greater than 2 NPS COOK Inquiry Panel recommended evaluation of MRP-146 screening scope DCPP-2 RHR suction line was predicted to run fully hot (no stratification) Fatigue conditions & risk is not well understood These configurations will screened out after one-time examinations expire Actual conditions leading to cracking need to be understood Potential support Install instrumentation to monitor strain, vibration, temperature investigations to understand the actual loading conditions Perform data analysis and report 21

22 Together Shaping the Future of Electricity 22