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Corrosion and Deuterium Pickup in Zr-2.5Nb:
Twenty Years of In-Reactor Testing at the OECD
Halden Boiling Water ReactorH.M. Nordin, R. Szőke
19th International Symposium on Zirconium in the Nuclear Industry
May 19 - 23, 2019, Manchester, UK.
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Outline
1. Introduction
• CANDU Reactor
• Pressure Tube
• Pressure Tube Fabrication
• Corrosion and Deuterium Ingress
2. Experiment
• Materials
• Test Loop
3. Results
4. Summary
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CANDU Reactor
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CANDU Reactor Fuel Channel
Pressuretube
Calandriatube
Feederpipe
Temperature
Flow
Fast neutron flux
[D]
•Temperature: 250°C – 310°C
•Pressure: 10 MPa
•Flux: 0 – 3.5x1017 n/m2s
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Pressure Tube Fabrication1990’s
Zirconium Feedstock
Two vacuum arc melts (Double Melt)
ingot
1960 – 1980’s
Press forge at 1015°CRotary forge at 815°C
Heat to 1015°C and β-Quench
Extrude at 815°C
Cold draw 27%
Steam Autoclave at 400C for 24 hours
1980’s
1970’s
Four vacuum arc melts (Quad Melt)
ingot
Press forge at 1015°CRotary forge at 815°C
Heat to 1015°C and β-Quench
Extrude at 815°C
Cold draw 27%
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Pressure Tubes• Historically, extensive research programs have been under
taken to fabricate improved pressure tubes:
• ~ 1960’s started investigating Zr-2.5Nb – improved mechanical properties and corrosion
• ~ 1980’s introduced β-quenching – improve strength
• ~ 1990’s introduced quad melting and lowered initial hydrogen content – reduce Cl and F for improved fracture toughness
• ~ 2000’s increased Fe concentration to 1080 ppm and reduced C concentration to 80 ppm – improve corrosion, deformation and fracture toughness
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Pressure Tubes• The extrusion process is known to result in microstructure and
texture variations along the pressure tube which in turn affects deformation, mechanical properties, oxidation and deuterium pickup.
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Corrosion and D Ingress• Corrosion and the associated deuterium ingress may be life
limiting degradation mechanisms in Zr-2.5Nb pressure tubes.
• Corrosion and deuterium uptake in pressure tubes can be influenced by:
- environmental factors (flux, temperature or pH).
- material properties (microstructure, microchemistry, texture and state of the β-phase) which are influenced by fabrication processes.
• To support pressure tube improvement programs, in-reactor corrosion and deuterium ingress studies were conducted at the OECD Halden Boiling Water Reactor over a 20 year period.
• Effect of environment and fabrication variables investigated:
- ingot melting (double melted or quadruple melted)- non--quenched versus -quenched- cold work (12% or 27%)- pH- In-flux versus out-flux
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Materials• Corrosion test coupons were machined from twenty-one different
Zr-2.5Nb pressure tubes.
- variations in fabrication history (e.g. β-quenched) and minor element concentration, illustrating the development history of pressure tube fabrication processes.
• The machined coupons tested had different surface finishes:
Machined coupons ~ 10 mm wide, 30 mm long and 1 mm thick
Machined
PickledHF, HNO3 and H2SO4 solution
Pickled + Pre-filmed400°C steam for 24 hours
Machined + Pre-filmed400°C steam for 24 hours
Note: Many variables!
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Schematic Diagram of In-Flux Test Loop 9
• Operates under CANDU HTS conditions (pHa 10.2-10.8)
• Two independently heated in-flux test channels, three out-flux autoclaves
• Temperature range: 250°C – 335°C
• Neutron flux: 3-5x1013 n/cm2/s
• Ratio of thermal flux\fast flux: 2:1
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Oxygen and Deuterium Pickup
• Oxidation kinetics were linear after 150 days.
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Effect of Flux on Oxide Growth Rate
• The in-flux oxide growth rate tends to be greater than the out-flux oxide growth rate (P-value = 0.0004).
• Enhancement of in-flux oxide growth rate dependent on initial microstructure and surface finish.
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• No effect of flux on deuterium pickup (P-value = 0.96).
Effect of Flux on D Pickup Rate
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• Oxide growth and deuterium pickup rate similar between Kroll and electrolytic.
• Larger extruded tube front-back differences in Kroll ingots.
Ingot Production
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Effect of Ingot Melting
• Quad melting result in lower deuterium ingress rates.
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Effect of β-Quenching
• β-quenching results in lower deuterium ingress rate.
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Effect of β-Quenching
Non-β-Quenched β-Quenched
• More uniform grain structure in β-quenched material.• Curly nature of non-β-quenched grains may cause β-Zr filaments to be
oriented perpendicular to free surface leading to faster oxidation along these filaments.
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Comparison of Fabrication Methods
• Fabrication modifications reduced oxide growth rate.
1960 - 1980 1980 - 1990 1990 - 2000 2000 - 2005
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Comparison of Fabrication Methods
1960 - 1980 1980 - 1990 1990 - 2000 2000 - 2005
• Deuterium ingress reduced by a factor of 5.
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Discussion• A large number of confounding variables in tests making
comparison of data and interpretation of the results difficult.
• The synergistic interactions between fabrication processes, microstructure, texture, the state of the β-phase and the resulting pressure tube properties are difficult to assess.
• With the tube improvements, the net total concentration of hydrogen isotopes has been significantly reduced making Zr-2.5Nb pressure tubes much less susceptible to the deleterious effects of hydrides on fracture properties.
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Conclusions• Research programs undertaken to improve pressure tube
properties were successful in reducing corrosion and deuterium ingress.
• Deuterium ingress reduced by a factor of 5.
• This reduction should make Zr-2.5Nb pressure tubes much less susceptible to the deleterious effects of hydrides on fracture properties.
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Acknowledgements• CNL’s (formerly AECL’s) in-reactor corrosion test program lasted for
approximately 20 years. • The contributions of many individuals need to be acknowledged:
• From AECL/CNL (including former employees): V. F. Urbanic, R.A. Ploc, G. McDougall, I. J. Muir, G.A. McRae, A. A. Bahurmuz, A.J. Elliott, A. Shaddick, S. Bergin, V. Hilton, C. Davis, M. Seguin, D. Wilkins, R. MacLeod, A. Britton, R. Stuthers, P. Sullivan, D. Irvine, R. Beier, J. Hamel, Y. Andrews and M. Godin.
• From the Halden Reactor Project (including former employees): M. A. McGrath, K-L Moum, I. Thoresen, H. Devold, H. Valseth, H. Thoresen, C. Helsengreen, M. Lundgren, K-W. Eriksen, C. Vitanza.
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