Assessment of correlations between NDE parameters and … · Assessment of correlations between NDE...

Assessment of correlations between NDE

parameters and tube structural integrity

for PWSCC at U-bends

S. Bakhtiari, T. W. Elmer, Z. Zeng and S. Majumdar Nuclear Engineering Division Argonne National Laboratory

35th EPRI Steam Generator Nondestructive Evaluation and Tube Integrity Workshop Clearwater Beach, FL, USA July 18–20, 2016

Program Manager: Matthew Rossi

Work Sponsored by the US Nuclear Regulatory Commission


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Under the SG tube integrity and inspection program at ANL, research is being conducted to assess the ability of eddy current (EC) inspection techniques to detect and characterize flaws in SG tubes

Cracking has been observed in the U-bend region of SG tubes

Detection and characterization of SCC in small radius U-bends could be challenging due to such factors as – Large background noise associated with tube geometry

variations (ovalization and change in wall thickness)

– Non-uniform probe motion and coupling

Studies are being conducted at ANL on inspection techniques and integrity models for U-bends

Background

Detachable flexible rotating probe with a mid-range +PointTM probe

Tube ovalization

Wall thickness


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Objective of NDE related research activities

– Evaluate the ability of EC inspection techniques to detect and size flaws

• Focus of recent studies on PWSCC in U-bend region of SG tubes

• Utilize data and results from past studies by industry on ODSCC at U-bends

– Provide experimental data for tube structural integrity studies

• NDE results to help guide laboratory sample production and testing

• Evaluation of predictive models for tube structural integrity based on NDE data

Approach — Evaluation of correlations between NDE parameters and tube structural integrity

– Perform EC inspection and analyze data at different stages of the flaw manufacturing process

– Obtain data from structural integrity tests on U-bend tube specimens

– Evaluate potential correlations between independent variables associated with EC estimates of crack size and tube failure pressure

Objective & Approach

Destructive examination of flawed tube


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Assessment of correlations between NDE parameters and tube

structural integrity

Assembled a library of lab-produced specimens with PWSCC at U-bend region

– Collected EC inspection data (bobbin, rotating and array probe) on all tube specimens

– Performed data analysis in accordance with common industry practices

– Evaluated alternative signal processing and data analysis methods

Available database of U-bend specimens with laboratory produced PWSCC

– 2.25-in. radius Alloy 600 U-bend tubes

• 0.75-in. OD and 0.043-in. nominal wall thickness

• All failed tubes have primarily axially oriented PWSCC

– 6.0-in. radius U-bends are not included in this study

• Fewer number of available samples

• PWSCCs are primarily circumferential

• Large majority of the tubes did not fail up to the limit of the pressure test facility

• Failed tubes all had excessively deep flaws (i.e., unreliable failure pressure data)

+Point data from a section of a U-bend tube sample examined before (top) and

after (bottom) pressurization test


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structural integrity (Contd.)

Eddy current examination of specimens

– Data collected with flexible U-bend +PointTM rotating probe

– PWSCC sizing results based on manual analysis of data

• Results based on analysis of EC inspection data collected following termination of flaw manufacturing process

• Analysis performed on unprocessed data

• Signals measurements were made with a priori knowledge of intended flaw locations and by using historical comparisons

• Measurements could be subjective due to influence of large background noise, particularly for smaller radius bends and low amplitude signals (low S/N)


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20 40 60 80 100 120-500

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Time (s)

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ssu

re (

PS

I)

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Time (s)

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w (

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M)

-500 0 500 1000 1500 2000 2500 3000 3500 4000-2

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Pressure (PSI)

Le

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Pressure 2

Pressure 1

0 0.2 0.4 0.6 0.8 1 1.2 1.4-100

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Depth

Axial (in)

Axial ID (est. pressure)==============Depth: 70.6875%Length: 0.39024 inCenter: 0.67073 inPCR: 5.2826 ksiPSC: 5.4671 ksi==============

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6-100

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Depth

Axial (in)

Axial ID (est. pressure)==============Depth: 76.2137%Length: 0.78049 inCenter: 0.96341 inPCR: 3.1705 ksiPSC: 3.6176 ksi==============

Data for a U-bend specimen with laboratory grown PWSCC at apex (left) and at tangent (middle)

region, both at extrados.

Based on the ERC model using EC depth profile, the flaw at the apex (left) was predicted to fail first.

Ligament rupture, however, occurred for the shorter PWSCC located at the tangent (middle & right).

Examination of pre- and post-pressurized specimens On-going Activity


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Examination of pre- and post-pressurized specimens (Contd.)

Shown here are eddy current data displayed in EddynetTM (left), pressure and leak rate data (middle)

and photos of the failed tubes with PWSCC at tangent and apex regions (right).

150930-02, tangent rupture -500 0 500 1000 1500 2000 2500 3000-1

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150930-03, apex rupture -500 0 500 1000 1500 2000-2

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Pressure (PSI)

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structural integrity

Correlation curves were generated for failure pressure vs. different sizing parameters obtained from analysis of +PointTM data

– Independent variables included estimates of maximum amplitude (voltage), maximum depth, length, and depth*length

– Different criteria were used in selecting the candidate flaw for failure

– All PWSCCs within the U-bend region which failed were included in the analyses

Correlation of failure pressure with Depth*Length parameter. Correlation of failure pressure with Voltage parameter.


U-bend radius: 2.25-in

Correlation type: Pressure vs. %Depth

Dataset: 2015-11-12

Alternate flaws selected by criteria: 7 (28%)

– Fewer misses than most other criteria

Standard deviation from fit: 1.664 kPSI

– More than double the standard deviation of the other techniques

Correlation of NDE parameter with failure pressure:

%Depth

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Shown on the right:

(Top) Plot of failure pressure as a function of estimated

depth of PWSCC at U-bend.

(Bottom) Same plot as above with the added points for

the predicted flaw to fail based on the NDE parameter.



Correlation type: Pressure vs. Length

Dataset: 2015-11-12


– Worst selection criteria


– One of the better standard deviations in the final dataset

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Length

Shown on the right:


length of PWSCC at U-bend.




U-bend radius : 2.25-in

Correlation type: Pressure vs. %Depth*Length (D*L)

Dataset: 2015-11-12



– As more points were added, the fit came closer to that of the other techniques

– Early analysis seemed to indicate this would be the best fit (Dataset 2015-10-22; Std. dev. 0.554 kPSI)


%Depth*Length

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Shown on the right:


Depth*Length of PWSCC at U-bend. Also marked are the

data points obtained towards the end of pressure tests.




U-bend radius : 2.25-in

Correlation type: Pressure vs. %Depth*Length (Depth selected)

Dataset: 2015-11-12


– Fewer overall misses than D*L as selector

For a subset of flaws, 4 (16%), alternate selection criteria is less conservative than the main parameter

– These are cases in which using D*L would have been more conservative than depth as the selection criteria

– Tighter fit than D*L as selector


%Depth*Length (Depth as selection criteria)

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Shown on the right:

(Top) Plot of failure pressure as a function of estimated Depth*Length of PWSCC at U-bend.

(Bottom) Same plot as above with the added points for the predicted flaw to fail based on the NDE parameter.



Correlation type: Pressure vs. %Depth*Length (Amplitude selected)

Dataset: 2015-11-12



– As data accumulated, the fit came more in line with other criteria


%Depth*Length (Amplitude as selection criteria)

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Shown on the right:


Depth*Length of PWSCC at U-bend.



*The SCC that failed at ~3.5 kPSI was discussed on Slide 6.



Correlation type: Pressure vs. Amplitude

Dataset: 2015-11-12


– Early analysis (dataset 2015-10-22) showed this result in the fewest misses (26.3%)


– Early analysis (dataset 2015-10-22) showed this to have lower standard deviation than most (0.56 kPSI), but slightly worse than D*L (0.55 kPSI)

– In the final dataset, this resulted in the best fit with the fewest number of misses


Amplitude

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Shown on the right:


amplitude of PWSCC at U-bend.




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Concluding Remarks

Preliminary results indicate

– Among the NDE parameters examined so far, amplitude (Vpp) and Depth*Length of PWSCC at U-bend obtained from analysis of +PointTM data exhibited the highest degrees of correlation with tube failure pressure (rupture or burst)

– Voltage, both as the selection parameter and the independent variable, produced the highest degree of correlation with tube structural integrity

– In some cases, a smaller flaw (based on signal amplitude) at tangent failed before a larger flaw at apex

• No visible difference between flaw morphologies

• This phenomenon is currently being investigated

Follow-on and future studies

– Further evaluate the results once destructive examination of the specimens is completed

– Examine EC data collected with other probes

– Evaluate the reliability of correlations using a more statistically significant dataset

– Examine correlation of alternative NDE parameters with tube structural integrity

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