analysis and impact of recent thermal fatigue operating ... · oct 2014 reactor water clean up tee...

© 2016 Electric Power Research Institute, Inc. All rights reserved.

Bob McGillStructural Integrity Associates

Terry ChildressDuke Energy

Michael HoehnAmeren Missouri

Mike McDevittEPRI

Analysis and Impact of Recent

Thermal Fatigue Operating

Experience in the USA

2016 International LWR Material Reliability Conference and ExhibitionAugust 2016

2© 2016 Electric Power Research Institute, Inc. All rights reserved.

Presentation Content

• MRP Fatigue Management of Thermal Stratification and Mixing

• Operating Experience 2013 – 2015

• EPRI Response and Analysis Summary

• Additional Fatigue Events Spring 2016

• Program Impact and Next Steps


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 03-08 is implemented by:

– MRP-146 Cyclic stratification in non-isolable RCS branch lines

– MRP-192 Thermal mixing tees in RHR systems


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

Thermal Mixing Tees, MRP-192

Addresses interaction from mixing of thermally different fluids

“UH” Configurations “H” Configurations “DH” Configurations


2013 – 2015: 10 Operating Events – An Alarming Trend

– 5 Events leading into NRC Bulletin 88-08 (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


Overview of 2013 – 2015 Operating Experience

Date Component Method of Detection

Nov 2013

TMI-1Cold Leg Drain Elbow (DH)

Found during planned MRP-146

examination

Nov 2013

Oconee-1

Cold Leg HPI Nozzle weld (H)

(primary vibration)Through-wall leakage & forced outage

Apr 2014

McGuire-2Cold Leg HPI Nozzle weld (H)

Found during planned MRP-146

examination using upgraded NDE

Sep 2014

McGuire-1

Loop-B Cold Leg HPI Nozzle

weld (UH)Found during Extent of Condition exams

Loop-C Cold Leg HPI Nozzle

weld (UH)

Sep 2014

McGuire-1RHR Mixing Tee

Found during planned, calendar based

examination

Oct 2014

Perry (BWR)Reactor Water Clean Up Tee

Through-wall leakage & forced system

outage

Nov 2014

Oconee-1Cold Leg Drain Elbow (DH) Found during planned examination

Dec 2014

North Anna-1Cold Leg Drain Elbow (DH) Through-wall leakage & forced outage

May 2015

Catawba-2RHR Mixing Tee Found during planned examination


Fatigue Crack Locations October 2013 – May 2015Stagnant Branch Lines, MRP-146

Thermal Mixing Tees, MRP-192

Oct 2014, 100% t-w, 2.5” l, Circ

May 2015, shallow, Circ

Sep 2014, 14% t-w, 0.5” l, 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 2013, 100% t-w, Circ

Nov 2014, 20% t-w,

3/8” l, Axial

Dec 2014, 100% t-w, 2” l, Axial

Nov 2013,

35% t-w, ¾” l, Circ


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 2015-019)


Analysis Results

Management Program Failures Identified

Date Component Management Program Breakdown

Nov 2013 1B 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 2014Loop B HPI Nozzle Weld (UH)

Were exempted by MRP-146 screening

Cracks located outside of exam zoneLoop C HPI Nozzle Weld (UH)

Sep 2014 RHR Mixing TeeLarger than expected for low usage

Likely missed in prior NDE

Oct 2014 Reactor Water Clean Up TeeRepeat of 2008 through-wall crack

Growth exceeded plant expectations

Nov 2014 Loop B C.L. Drain ElbowRepeat of 2000 through-wall crack

Mitigating insulation was compromised

Dec 2014 Loop B C.L. Drain ElbowThrough-wall leakage

Frequent water chemistry sampling

May 2015 RHR Mixing Tee Active degradation or better NDE?


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


Analysis Recommendations

Fatigue Management Program Changes

Examine vertical section of UH branch piping subject to cold in-leakage

Socket Welded

Construction

Butt Welded

Construction


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


Analysis RecommendationsResearch & 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


Analysis RecommendationsResearch & 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)


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


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)


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


D.C. Cook Temperature Observations Revealed Possible

In-leakage Condition

590°F300°F

250°F

120°F

T∞ = 120°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

Possible Leakage Flow


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


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


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


Together…Shaping the Future of Electricity

analysis and impact of recent thermal fatigue operating ... · oct 2014 reactor water clean up tee...

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