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EVALUATION OF CRAZE CRACKING DEGRADATION
Steve Fyfitch (AREVA)
Mike McDevitt and Paul Crooker (EPRI)
EPRI International LWR Materials Reliability Conference and Exhibition
Chicago, Illinois
August 1-4, 2016
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.2
Background
MRP Thermal Fatigue Guidelines (MRP-146 and MRP-192)
establish requirements in thermal fatigue examinations
Provide guidance to look for and report instances of craze
cracking
Currently no specific guidance for how to evaluate as-found craze
cracking condition
Overall objective of project to develop guidance for utility
engineers use in recognizing, understanding, and evaluating
as-found craze cracking conditions
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.3
Project Workscope (1/2)
Phase 1 Task 1
Literature search to identify state of knowledge and technical areas
to be addressed to develop consensus guidance for dispositioning
craze cracking indications
Phase 1 Task 2
Recommendations for resources (potential expert panel members
and research opportunities) to assess knowledge gaps
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.4
Project Workscope (2/2)
Phase 2 Task 1
Identify and assemble appropriate Expert Panel members
Discuss and develop guidance for disposition of craze cracking
Phase 2 Task 2
Conduct Expert Panel meeting
Discuss and develop utility engineer disposition guidance of craze
cracking identified during NDE examinations
Phase 2 Task 3
Prepare summary and guidance MRP report
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.5
Literature Search Results (1/10)
[Stress Corrosion Cracking Instances]
Craze cracking observed in reactor
vessel head control rod drive
mechanism (CRDM) nozzles
Identified at numerous units in
U.S. and Europe
Attributed to PWSCC
Flaws remained shallow
[< 2 mm (0.079 inch)]
Not associated with any known
through-wall flaws that caused
leakage
Fluorescent
Dye Penetrant
Results of
Ringhals Unit 2
CRDM Nozzle
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.6
Literature Search Results (2/10)
[Thermally Driven – French Studies]
S. Taheri and others evaluated thermal fatigue craze cracking
observed in residual heat removal (RHR) systems of French 900
and 1300 MWe units
Provide explanation of differences between high cycle thermo-
mechanical and mechanical fatigue being related to difference
between spatial stress gradients
Stress gradients are higher near inner surface and negligible farther
away for high frequency thermal loading
Craze crack arrest explained as result of decreasing stress intensity
factor reaching threshold level
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.7
Literature Search Results (3/10)
[Thermally Driven – French Studies]
Craze cracking example
observed in French RHR
system piping
Operating temperature
between 20 and 300˚C (68
and 572˚F)
Temperature variation
around 140˚C (284˚F)
(a) Craze Cracking on a
French Residual Heat
Removal Pipe Inner
Surface
(b) Crack Depth Observed in
the Crazing at (a)
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.8
Literature Search Results (4/10)
[Thermally Driven – French Studies]
Necessary Conditions
for Crack Arrest
Parametric Study for Crack
Arrest at 2.5 mm Crack Depth
Effect of Tube Thickness
on Crack ArrestQualitative determination
of fatigue life using
assumptions that lead to
crack arrest at 2.5 mm
(0.098 inch) for RHR piping
in French 1300 MWe units
Crack arrest depth is
deeper for a thin tube
than a thick tube
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.9
Literature Search Results (5/10)
[Thermally Driven – Japanese Studies]
Evaluations performed by Japanese in response to thermal
fatigue operating experience observed
If relatively high frequency fluid temperature fluctuation
sinusoidal waveform exists, possibility of crack arrest occurs at
less than 10 Hz
On contrary though, for cases where cracking propagates
through-wall, waveform deduced to be trapezoidal shape and
crack arrest frequency is relatively low
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.10
Literature Search Results (6/10)
[Thermally Driven – Japanese Studies]
Results show that crack
could initiate at fluid
temperature range of more
than 120 K (i.e., 120C˚ or
216F˚) regardless of
magnitude of membrane
constraint
Growth of initiated crack
would be difficult to stopCalculated Fluid Temperature and Frequency
Conditions for Failure, Crack Arrest, and No Cracking
(Trapezoidal Temperature Fluctuation)
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.11
Literature Search Results (7/10)
[Thermally Driven – ASME Code Fatigue Study]
Thermal fatigue testing and analyses
of stainless steel pipes performed by
Jones et alia
Purpose to provide data for
improvements in analytical methods
and ASME Code fatigue curves
Water chemistry controlled to very
low dissolved oxygen content,
consistent with PWR specifications
At periodic intervals, testing
interrupted to ultrasonically inspect
for cracking
Type 304 Stainless Steel Test Sample
(Dimensions in Inches)
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.12
Literature Search Results (8/10)
[Thermally Driven – ASME Code Fatigue Study]
Thermal cycling test performed as follows:
Pipes pressurized to 17.2 MPa (2500 psi) and temperature cycled
between 38˚C (100˚F) and 343˚C (650˚F) in three seconds
Followed by holding at 343˚C for 237 seconds and then quenching
to 38˚C in three seconds
Followed by another 237 second hold time
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.13
Literature Search Results (9/10)
[Thermally Driven – ASME Code Fatigue Study]
Liquid Penetrant Inspections for the Type 304
Stainless Steel Pipe Test Specimens
Craze cracking present at
all circumferential
locations in two thickest
sections with no preferred
orientation
No cracking seen in two
thinner sections
This cracking supports
assumption initial
cracking due to local peak
thermal stress on pipe
inner wall
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.14
Literature Search Results (10/10)
[Thermally Driven – MRP Efforts]
MRP-23, Revision 1 provides
guidance for thermal fatigue
inspections to support
implementation of NEI 03-08
requirements
“Needed”
(MRP-146, Revision 1)
“Good Practice”
(MRP-192, Revision 1)
Dye Penetrant Photograph
of Typical Pipe Crazing
and Crack Patch
Dye Penetrant Photograph
of Typical Elbow Crazing
and Crack Patch
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.15
Key Drivers For Craze Cracking (1/2)
Material surface characteristics
Surface finish (roughness)
Cold-work (e.g., resulting from grinding, damage from a loose part,
or shot peening)
Metallurgical features (e.g., grain size tends to influence cyclic
plasticity and fatigue crack initiation)
Material cracking susceptibility (e.g., Alloy 600 is susceptible to
PWSCC)
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.16
Key Drivers For Craze Cracking (2/2)
Mixing of hot and cold fluids
Stratification of fluids with sufficiently high temperature changes (ΔT)
capable of exceeding the yield strength of the material
Thermal cycles exceeding the fatigue endurance limit of the material in
the environment of concern (i.e., PWR water conditions)
High residual stresses near welds
High levels of mean stress
Geometrical discontinuities (e.g., weld root and counterbore) near
mixing areas
Evaluation of Craze Cracking Degradation – Fyfitch, McDevitt, Crooker – August 1-4, 2016 - p.17
Next Steps
MRP Technical Support Advisory Committee (TS-
TAC) to organize and convene Expert Panel to
investigate thermally-driven craze cracking
Critically evaluate state of knowledge regarding craze
cracking PWRs
Reach consensus for developing suggested guidance for
craze cracking disposition
Prepare and issue MRP report containing overall
summary and utility guidance