Materials Reliability Program Thermal FatigueThe MRP 2015-019 (and subsequently, MRP-146 Rev. 2...

© 2019 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m

Michael Hoehn IIMRP ChairAmeren

NRC-EPRI Technical Exchange MeetingMay 21, 2019

Materials Reliability ProgramThermal Fatigue

© 2019 Electric Power Research Institute, Inc. All rights reserved.w w w . e p r i . c o m2 © 2018 Electric Power Research Institute, Inc. All rights reserved.

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

Incidences 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 is implemented by:

– MRP-146 Cyclic stratification in non-isolable RCS branch lines– MRP-192 Thermal mixing tees in RHR systems


Fatigue Management of Thermal Stratification and Mixing

Normally Stagnant Branch Lines, MRP-146Addresses interaction of hot swirl penetration into non-isolable horizontal pipe

Thermal Mixing Tees, MRP-192Addresses interaction from mixing of thermally different fluids

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


2013 – 2016: 13 Operating Experience

– 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 (1 US Plant)

– 13 events in US between 2013 and 2016 (3 years) 2 through-wall leaks in branch line cracks 8 part-through branch line cracks 1 through-wall leak in RWCU mixing tee 2 part-through RHR mixing tee

Date Component Method of Detection

Nov 2013 Cold Leg Drain Elbow (DH)

Found during planned MRP-146 examination

Nov 2013Cold Leg HPI Nozzle weld (H)(primary vibration)

Through-wall leakage & forcedoutage

Apr 2014 Cold Leg HPI Nozzle weld (H)

Found during planned MRP-146 examination using upgraded NDE

Sep 2014

Loop-B Cold Leg HPI Nozzle weld (UH) Found during Extent of Condition

examsLoop-C Cold Leg HPI Nozzle weld (UH)

Sep 2014 RHR Mixing Tee Found during planned, calendar based examination

Oct 2014 Reactor Water Clean Up Tee

Through-wall leakage & forcedsystem outage

Nov 2014 Cold Leg Drain Elbow (DH) Found during planned examination

Dec 2014 Cold Leg Drain Elbow (DH)

Through-wall leakage & forcedoutage

May 2015 RHR Mixing Tee Found during planned examination

May 2016Cold Leg Drain Elbow (DH)

Found during planned MRP-146 examination

March 2016 Charging lineFound during planned MRP-146 examination

May 2016 RHR Suction Line Found during planned examination


More Recent Thermal Fatigue OE

Date Component Flaw Depth Method of Detection MRP Action

March 2016 Drain Line 58% Part-wall Extent of Condition* IG -2019

March 2016 Charging Line 44% Part-wall MRP-146 Inspection IG -2019

May 2016 RHR Suction Line 24% Part-wall Section XI Program IG -2019

February 2017 Safety Injection Line Through-wall Obstruction prevented area of leak from inspection – through-wall leak caused forced plant shutdown IG -2019

May 2017 Safety Injection Line 21% Part-wall MRP-146 Inspection IG -2019

April 2017 RHR Suction Line 14% Part-wall Extent of Condition* IG -2019

September 2018 Drain Line &HHSI Pipe-to-Reducer

19% Part-wall39% Part-wall

MRP-146 InspectionMRP-146 Inspection IG -2019

- *Inspection methods consistent with MRP-146 inspection guidance.

- IG = interim guidance

• Review and Actions for recent observations (2016-2018)– 1 through-wall – 6 part-wall (cracking discovered during planned MRP-146 examinations or extent of

condition examinations)– 1 part-wall (detected via planned Section XI examinations)


Thermal Fatigue OE The events revealed a potential need for modified guidance:

– Drain line was screened out based on MRP-146 generic guidance

– One cracked location was not considered likely due to line size and geometry, discovered during a one-time inspection

– RHR suction lines were screened out because these lines run fully hot

– Safety injection (SI) OE identified a possible source of in-leakage caused by SI system cross-flow

– Some observed flaws were detected outside of the MRP-146 inspection zone


Thermal Fatigue OEMRP Response to OE (2/3)

The MRP Thermal Fatigue Focus Group (TFFG) reviewed each event to understand relevant factors and to make a determination if guidance changes were necessary– Interim Guidance was determined to be necessary


Thermal Fatigue OEResolved Inspection Challenges from Recent OE Cracking morphologies

– February 2017 RCS high pressure safety injection nozzle cracking (3-inch outside diameter) Axial cracks on the nozzle side without craze cracking Examination from the nozzle side is difficult due to geometry

– NDE Challenge: single-side examination for axial crack detection– March 2016 RCS drain cracking (2-inch outside diameter) Weld cracks initiated in the heat-affected zone then propagated into the weld

– NDE challenge: difficult to detect the crack tip inside the weld– NDE challenge: weld complexity may trigger false calls

Elbow skewed cracks – NDE challenge: cracks with complex skewed paths are difficult to detect


Thermal Fatigue OEMRP Thermal Fatigue Activities (2017-2019) 2017

– Review and discuss potential for cross-flow (out-flow) as cause of the February 2017 thermal fatigue incident

– The MRP issued a letter to its member utilities in December 2017 to ensure awareness of cross-flow as a potential cause of thermal fatigue

2018– Review of recent OE to: Prepared and endorsed interim guidance

– Copy of MRP-146 provided to NRC for information Based on request from NRC at May 2018 Materials R&D Technical Exchange Meeting

2019– Issued interim guidance


MRP Actions

2017: Issued MRP information letter on cross-flow as potential cause of thermal fatigue2018: Reviewed valve maintenance, performance testing requirements and best

practices 2018: Technical Committee reviewed the current generic analysis and developed

guidance2018: Technical Committee developed augmented inspection guidance

2018: Prepared interim guidance to address recent operational experience

2019: Issued thermal fatigue interim guidance

Thermal Fatigue OEMRP Actions and Guidance Revision Considerations


MRP 2019-006 Letter: NEI 03-08 Needed Interim Guidance for Management of Thermal Fatigue

Note: The requirements of this guidance shall be implemented within two refueling outages after June 30, 2019. Results of the exams shall be reported to the EPRI MRP within six months after completion.

IG# Interim Guidance Basis for Interim Guidance1 DH lines previously exempted by the Generic Analysis

option described in MRP-146, paragraph 2.1.5.4 shall be inspected every other RFO if all the following conditions are true:

a) cracking has been identified

a) the cause of the cracking could not be identified and mitigated / eliminated.

The MRP 2015-019 (and subsequently, MRP-146 Rev. 2 Section 2.1.5.4) requires that sites with DH lines evaluated using the MRP-146, Appendix A generic evaluation complete a one-time examination and report the results to MRP. If cracking is then identified on these lines and the cause evaluation is unable to identify and mitigate the cause, then it should be conservatively assumed that a degradation mechanism is present. Therefore, a routine exam is justified consistent with other MRP-146 screened-in DH lines.

2 For those large bore (> 4”) DH lines that previously screened-out as “HOT” (per MRP-146) a one-time examination of the piping at the first 45-degree (or 90-degree) elbow is required (as represented on Figure 1A). As a minimum, the inspection volume shall include the base metal and welds.

Note: credit may be taken for previous exam if: Previous exam volume requirements bound those of

this IG

During a 2016 risk-informed in-service inspection of a 14” RHR Suction line (Class 1 piping), a weld flaw extending 24% of through-wall was identified by ultrasonic testing. This flaw was located in the Heat Affected Zone of the pipe to 45-degree elbow weld on the upstream end of the elbow. This pipe line previously was exempt from examination since it was screened out as “HOT” per MRP-146 guidelines and MRP-170 software. Using an array of thermocouples at the flaw location and at additional downstream locations, high-frequency temperature oscillations were recorded at 100% power operations. An extent-of-condition examination on the sister unit in Spring 2017 revealed a similar (though less extensive) crack depth in a similar location.


Note: The requirements of this guidance shall be implemented within two refueling outages after June 30, 2019. Results of the exams shall be reported to the EPRI MRP within six months after completion.

3 Sites shall review MRP-146 “screened” out UH/H lines to determine susceptibility to in-leakage from cross-flow. To perform this determination, interconnected lines, or lines sharing a common header, with only check-valve isolation between RCS loops, shall be “screened-in” as potentially susceptible to in-leakage / cross-flow. For those new “screened-in” lines a one-time inspection shall be performed using the volumetric requirements of MRP-146 Rev. 2, and as amended by IG #4 below.

Note: credit may be taken for previous exam if: Previous exam volume requirements bound those

of this IG

From MRP-146 Rev. 2, Section 2.1.1, it is possible that a UH/H branch line previously screened out of MRP-146 because no high pressure source (> RCS pressure) is present; however, such lines could still be susceptible to in-leakage from cross-flow if the following two conditions exist:a) The Loop branch lines are interconnected via a common header,

ANDb) A differential pressure exists between the interconnected RCS

loops.

4 Future fatigue examinations of the bottom inner third thickness of base metal as indicated in Figure 2-20 of MRP-146, Rev. 2 shall be 1” wide.

For the Boron Injection leakage event at a 4-loop Westinghouse-designed plant in 2017, the Metallurgical Examination showed cracking extending beyond the 1/2” width bottom dead center, as indicated in Figure 2 below. Therefore, TFFG recommends an increase to 1” width.

MRP 2019-006 Letter: NEI 03-08 Needed Interim Guidance for Management of Thermal Fatigue


2018 Thermal Fatigue Deliverables

MRP-146S, Revision 1, “Management of Thermal Fatigue in Normally Stagnant Non-Isolable Reactor Coolant System Branch Lines – Supplemental Guidance,” Product ID 300203264

MRP-192, Revision 3, “Assessment of Residual Heat Removal Mixing Tee Thermal Fatigue in PWR Plants,” Product ID 300203266

MRP-85, Revision 2,“Operating Experience Regarding Thermal Fatigue of Piping Connected to PWR Reactor Coolant,” Product ID 300203263

MRP-433, “PWR Residual Heat Removal Mixing Tee Thermal Fatigue Guidance Update: Current Experience, Selection of Modeling Tools, Input Data Identification, and Planned Approach,” Product ID 300201277


2019 Thermal Fatigue Deliverables MRP-365 Revision 1, Temperature Monitoring Data Evaluation for RCS Branch Lines

Subject to Thermal Fatigue MRP-32, Revision 1, Thermal Fatigue Monitoring Guidelines MRP-385 Revision 1, Finite Element Analysis of Drain Weld Cracking


Thermal Fatigue R&D Projects

Category/Title

Fatigue Research

Evaluation Criteria for Craze CrackingExperiments and Computational Fluid Dynamics Thermal Fatigue Model DevelopmentReducing High Vibration FatigueNDE for Thermal Fatigue Damage in Small Bore PipingThermal Fatigue Virtual Mockups for Computer Based TrainingInvestigate and Define Mixing Tee Conditions for Upstream Thermal Fatigue CrackingDevelopment of Mitigation ConceptsSupport for Thermal Fatigue OE Investigations (as needed)

Support for Subsequent License Renewal (SLR)-Focused Aging Management (as needed)

MRP and NEI-03-08 Standards and GuidanceThermal Fatigue Guidance Maintenance and Revisions

Current thermal fatigue research programs:


Summary Thermal stratification fatigue cracking OE has occurred recently Effective fatigue management programs are in place and are frequently reviewed

and revised – MRP-146 Rev. 2 issued October 2016– MRP 2019-008 issued March 2019

Comparative studies performed to understand fatigue mechanisms and benchmark US experience/guidance with other countries

MRP Research programs for better understanding, prediction tools and to improve thermal fatigue management are ongoing


Thermal Fatigue Management Summary

Overall, the MRP Thermal Fatigue Program has proven successful in detecting flaws before leakage occurs or when cracking presents a challenge to existing structural margins

Based on continued MRP R&D program improvements in existing thermal fatigue programmatic guidance will be evaluated and implemented as necessary


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Materials Reliability Program Thermal FatigueThe MRP 2015-019 (and subsequently, MRP-146 Rev. 2...

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