Assessment of correlations between NDE parameters and tube structural integrity for PWSCC at U-bends

Transcription

1 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 35 th EPRI Steam Generator Nondestructive Evaluation and Tube Integrity Workshop Clearwater Beach, FL, USA July 18 20, 2016 Program Manager: Matthew Rossi

2 Background 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 Detachable flexible rotating probe with a midrange +Point TM probe Tube ovalization Wall thickness 2

3 Objective & Approach 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 Destructive examination of flawed tube 3

4 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 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 4

5 Assessment of correlations between NDE parameters and tube structural integrity (Contd.) Eddy current examination of specimens Data collected with flexible U-bend +Point TM 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) 5

6 Depth Depth Flow (GPM) Leak Rate (GPM) Pressure (PSI) Examination of pre- and post-pressurized specimens On-going Activity Pressure 2 Pressure Time (s) Axial ID (est. pressure) ============== Depth: % Length: in Center: in PCR: ksi PSC: ksi ============== Axial ID (est. pressure) ============== Depth: % Length: in Center: in PCR: ksi PSC: ksi ============== Time (s) Pressure (PSI) Axial (in) Axial (in) 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). 6

7 Leak Rate (GPM) Leak Rate (GPM) Examination of pre- and post-pressurized specimens (Contd.) Pressure (PSI) , tangent rupture Pressure (PSI) , apex rupture Shown here are eddy current data displayed in Eddynet TM (left), pressure and leak rate data (middle) and photos of the failed tubes with PWSCC at tangent and apex regions (right). 7

8 Assessment of correlations between NDE parameters and tube structural integrity Correlation curves were generated for failure pressure vs. different sizing parameters obtained from analysis of +Point TM 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 Voltage parameter. Correlation of failure pressure with Depth*Length parameter. 8

9 Correlation of NDE parameter with failure pressure: %Depth U-bend radius: 2.25-in Correlation type: Pressure vs. %Depth Dataset: Alternate flaws selected by criteria: 7 (28%) Fewer misses than most other criteria Standard deviation from fit: kpsi More than double the standard deviation of the other techniques 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. 9

10 Correlation of NDE parameter with failure pressure: Length U-bend radius: 2.25-in Correlation type: Pressure vs. Length Dataset: Alternate flaws selected by criteria: 13 (52%) Worst selection criteria Standard deviation from fit: 0.78 kpsi One of the better standard deviations in the final dataset Shown on the right: (Top) Plot of failure pressure as a function of estimated 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. 10

11 Correlation of NDE parameter with failure pressure: %Depth*Length U-bend radius : 2.25-in Correlation type: Pressure vs. %Depth*Length (D*L) Dataset: Alternate flaws selected by criteria: 10 (40%) Standard deviation from fit: kpsi 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 ; Std. dev kpsi) Shown on the right: (Top) Plot of failure pressure as a function of estimated Depth*Length of PWSCC at U-bend. Also marked are the data points obtained towards the end of pressure tests. (Bottom) Same plot as above with the added points for the predicted flaw to fail based on the NDE parameter. 11

12 Correlation of NDE parameter with failure pressure: %Depth*Length (Depth as selection criteria) U-bend radius : 2.25-in Correlation type: Pressure vs. %Depth*Length (Depth selected) Dataset: Alternate flaws selected by criteria: 7 (28%) 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 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. 12

13 Correlation of NDE parameter with failure pressure: %Depth*Length (Amplitude as selection criteria) U-bend radius: 2.25-in Correlation type: Pressure vs. %Depth*Length (Amplitude selected) Dataset: Alternate flaws selected by criteria: 6 (24%) Standard deviation from fit: kpsi As data accumulated, the fit came more in line with other criteria 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. *The SCC that failed at ~3.5 kpsi was discussed on Slide 6. 13

14 Correlation of NDE parameter with failure pressure: Amplitude U-bend radius: 2.25-in Correlation type: Pressure vs. Amplitude Dataset: Alternate flaws selected by criteria: 6 (24%) Early analysis (dataset ) showed this result in the fewest misses (26.3%) Standard deviation from fit: kpsi Early analysis (dataset ) 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 Shown on the right: (Top) Plot of failure pressure as a function of estimated amplitude 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. 14

15 Concluding Remarks Preliminary results indicate Among the NDE parameters examined so far, amplitude (V pp ) and Depth*Length of PWSCC at U-bend obtained from analysis of +Point TM 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 15