ML13220A063

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Dsd ED2013 3267 Presentation, Primary Water Stress Corrosion Cracking Tests and Metallurgical Analyses of Davis-Besse Control Rod Drive Mechanism Nozzle #4.
ML13220A063
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Site: Davis Besse Cleveland Electric icon.png
Issue date: 08/30/2013
From: Alexandreanu B, David Alley, Bruemmer S, Jay Collins, Darrell Dunn, Toloczko M
Argonne National Lab (ANL), Office of Nuclear Regulatory Research, Pacific Northwest National Laboratory
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Dunn D
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Primary Water Stress Corrosion Cracking Tests and Metallurgical Analyses of Davis-Besse Control Rod Drive Mechanism Nozzle #4 D.S. Dunn1, J. Collins1, D. Alley1, B. Alexandreanu2, S.M. Bruemmer3, M.B. Toloczko3 1 United States Nuclear Regulatory Commission, Washington DC 2 Argonne National Laboratory, Argonne, IL 3 Pacific Northwest National Laboratory, Richland, WA Disclaimer: The work reported in this paper was supported by the Office of Nuclear Regulatory Research, U.S. Nuclear Regulatory Commission. The views expressed in this paper are not necessary those of the U.S. Nuclear Regulatory Commission

Outline

  • Background
  • Test materials
  • Crack growth rate testing
  • Metallurgical analyses
  • Summary 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 2

Background

- Through wall PWSCC in Nozzle #3

- Significant corrosion of the low alloy steel reactor pressure vessel head (RPVH)

- Replaced with RPVH from the cancelled Midland, MI PWR

- Operation resumed in 2004

- Observed after 5.5 effective full power years of operation

- Bare metal visual: 13 potential leaking nozzles, Nozzle #4 confirmed leaker

- Volumetric: 11 axial indications, 1 small circumferential indications, and 2 leak paths Nozzles #4 & #67

- Surface: 12 indications; 6 were potential leakers 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 3

2010 Inspection Results 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 4

Continued Operation

  • Licensee performed 1/2 nozzle repairs on 24 nozzles
  • Sample of Nozzle #4 provided to the NRC for testing and analyses
  • On June 18, 2010, the licensee informed NRC that Davis-Besse would shut down on October 1, 2011 to replace the head 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 5

Test Materials Alloy 600 heat Ni Cr Fe Mn C P Cu Co Si S B*

(appm)

SB-167 72.0 14.0- 6.0- 1.0 0.15 0.5 0.5 0.015 N/A N/A N/A Specification min 17.0 10.0 max max max max max M3935 77.89 15.58 6.25 0.27 0.028 0.004 0.01 0.01 0.37 0.0022 69 M7929 75.28 16.12 7.24 0.26 0.03 N/A 0.01 0.05 0.45 0.003 77

  • Boron concentration measured by PNNL Alloy 600 Yield Strength, Tensile Strength, Elongation, Hardness, Vickers*

heat MPa MPa percent Min Max Ave +/- SDEV SB-167 205 min 550 min 35 min N/A N/A N/A Specification M3935 334 590 60 146.6 190.7 160.2 +/- 6.5 M7929 296 668 53 166.6 209.5 186.5 +/- 9.6

  • Hardness in the crack growth plane for M3935-RPVH #1-CRDM #1 and M7929-RPVH #2-CRDM #4 measured by PNNL 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 6

Nozzle #4 Section

  • 90o section of Nozzle #4 cut from below the J-groove weld
  • Penetrant test revealed no surface cracking indications
  • 5 compact tension (CT) test specimens machined from this section 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 7

Test Specimen Machining

  • Layout for compact tension specimens
  • Sample for metallurgical analyses (next to Specimen DB-3)
  • All specimens were free releasable after machining 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 8

Test Specimens

  • 2 - 1/2 thickness (T)-

compact tension (CT) specimens

- Tested at Argonne National Laboratory (ANL)

  • 3 - 1/4 T-CT specimens

- 1 tested at ANL

- 2 tested at Pacific Northwest National Laboratory (PNNL) 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 9

Testing and Analyses

  • Machined compact tension test specimens supplied to Argonne National Laboratory (ANL) and Pacific Northwest National Laboratory (PNNL)
  • Primary objective was to obtain crack growth rates for the replacement RPVH alloy 600 nozzle material (heat M7929).
  • Secondary objective was to characterize Alloy 600 material microstructure and correlate microstructure to crack growth rates 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 10

Crack Growth Rate Testing Crack growth rate test systems located Testing in autoclaves at ANL used to obtain accurate crack under simulated PWR growth rates conditions 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 11

Crack Growth Rate Testing

  • Corrections applied to measured CGR to account for the formation of ligaments 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 12

Crack Growth Rate Testing

  • Formation and breaking of ligaments that affected CGR measurements was also observed at constant K 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 13

Crack Growth Rate Testing

  • Formation of ligaments that affected CGR measurements was confirmed by examination of the fracture surfaces 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 14

Crack Growth Rates 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 15

Temperature Sensitivity Temperature (°C) 352 333 315 298 283 10-9 Alloys 600 Heat M7929 PWR Water 10-45 cc/kg H 2 10-10 CGR (m/s)

Q = 145 kJ/mol (35 kcal/mol) 10-11 Kmax = 27 MPa m 1/2 10-12 M7929 1/2T CT DB-5 (DB600-CL-2)

M7929 1/2T CT DB-4 (DB600-CL-1) 1.55 1.60 1.65 1.70 1.75 1.80 1000/T (°K)

  • Activation energy typical of alloy 600 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 16

Fracture Surfaces

  • Predominately intergranular cracking on fracture surface of the CT specimens 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 17

Fracture Surfaces

  • Transgranular extension during air precracking at room temperature.
  • Rapid intergranular engagement
  • High degree of crack branching 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 18

Metallurgical Analyses 20 µm 20 µm Replacement Davis-Besse RPVH CRDM Nozzle #4 - Alloy 600 Heat M7929

  • Red arrows: grain boundaries
  • Blue Arrows: carbides

Conclusion:

carbides located on prior grain boundaries; not an ideal microstructure for PWSCC resistance 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 19

Metallurgical Analyses Original Davis-Besse RPVH CRDM Nozzle #3 - Alloy 600 Heat M3935

  • Carbides present at grain boundaries
  • Material was susceptible to PWSCC (NUREG/CR-6921, November 2005) 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 20

Atomic Probe Tomography M3935 M7929 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 21

Davis-Besse Alloy 600 Original RPVH Replacement RPVH

  • Alloy 600 heat M7929
  • Alloy 600 heat M3935 - Transgranular carbides on prior grain boundaries

- Grain boundary carbides with 500 - 700 nm spacing 30 micron grain size

- 200 - 400 micron grain size - 186.5 +/- 9.6 Hv

- 160.2 +/- 6.5 Hv - 2.5 atomic percent boron at

- 6 atomic percent boron at grain grain boundaries boundaries with significant chromium depletion 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 22

Summary

  • Laboratory crack growth rates in the replacement Davis-Besse RPVH were typically between the 25% and 95% of the MRP-55 disposition curves
  • Fracture surface examinations show a high degree of intergranular engagement consistent with materials susceptible to PWSCC
  • Alloy 600 heat M3935 from the original RPVH were found to have significant enrichment of boron on grain boundaries that were depleted in chromium
  • Microstructure of the alloy 600 M7929 heat from the replacement RPVH likely contributed to the increased PWSCC susceptibility 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 23

Acknowledgements The authors gratefully acknowledge the work by Mr. Jim Hyres at BWXT to decontaminate some of the samples and helpful suggestions provided by Drs. Mirela Gavrilas and Rob Tregoning 8/13/2013 Environmental Degradation of Materials in Nuclear Power Systems - 2013 24

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