1. Major Materials Challenges for DEMO R.J. Kurtz 1 and G.R. Odette 2 1 Pacific Northwest National Laboratory 2 University of California, Santa Barbara Harnessing Fusion Power Workshop Los Angeles, CA March 2 - 4, 2009

2. Fusion Materials Sciences Challenges 0.1 µm Challenge: Understand mechanisms controlling performance limiting materials phenomena Environment: Heat flux: 1-15 MW/m 2 ; Neutron fluence: 10-20 MW-y/m 2 ; Transmutation: ~2000 appm He/8000 appm H; High time dependent thermal and mechanical stresses. Approach: Use full suite of experimental- computational tools to model life-limiting degradation phenomena in the fusion environment. Goal: Experimentally validated, physics-based models to predict performance and improve existing materials and design better ones.

3.  Transmutants and atomic defects lead to accelerated non-equilibrium mesoscale evolutions and damage accumulation over long time > 10 8 s. Voids, bubbles, dislocations and phase instabilities (damage). Dimensional instabilities (swelling and irradiation-thermal creep). Complete loss of strain hardening capability. High-low temperature He embrittlement. Fatigue, creep-fatigue, crack growth. Corrosion, oxidation and impurity embrittlement (W, V). Bulk Phenomena He embrittlement, Thernal Creep, Corrosion Temperature Dimension al Instability Lifeti me Materials Design Window Hardenin g, Fracture N. Ghoniem & B.D. Wirth, 2002 High He may narrow or even close the window High heat, neutron fluxes and mechanical loads result in:

4. Materials Degradation in the Fusion Environment Neutron irradiation drives microstructural evolution - property changes. Fatigue, fatigue crack growth, thermal creep creep-fatigue. Effect of chemical interactions - corrosion, oxidation. >10≥0.4He Embrittlement >100.3 - 0.6Volumetric Swelling >10<0.45Irradiation Creep >10.3 - 0.6Phase Instabilities ≥0.1<0.4Hardening & Embrittlement Dose Level, dpa Temperature Range, Fraction of Melting Point Damage Phenomenon

5. fusion SiC ODS steel RAF/M steel S.J. Zinkle,OECD NEA Workshop on Structural Materials for Innovative Nuclear Energy Systems, Karlsruhe, Germany, June 2007, in press Comparison of Gen IV and Fusion Structural Materials Environments A common theme for fusion and advanced fission is the need to develop high-temperature, radiation resistant materials.

6. IFMIF Spallation neutrons Fusion reactor ITER Extrapolation to fusion regime is much larger for the fusion materials than for plasma physics program. Lack of an intense neutron source emphasizes the need for a coordinated scientific effort combining experiment, modeling and theory to a develop a fundamental understanding of radiation damage. Fusion Materials Relies Heavily on Modeling due to Inaccessibility of Fusion Operating Regime He and Displacement Damage Levels Steels

7. Theory and Modeling of Materials Performance Under Fusion Conditions Integration of modeling-theory-experiments-database development is a critical challenge in bridging the multi-physics-length-time scales. Microstructural evolution in a high-energy neutron, He-rich environment. Resulting in degradation of performance sustaining properties and stability. Effects of He are critical. Success - simulations and experiments show high-energy fusion damage events are similar to multiple, lower-energy events - provides fission-fusions dpa damage scaling. Comparison of 10 and 50 keV displacement cascades in iron 100K, iron 50 keV 10 keV 5 nm

8. Comparison of Materials Issues Fission vs. Fusion Reactor Systems Big pot and pipes dpa < 0.15, He ≈ 0 T ≈ 300°C Heat flux ≈ 0 Coolant Pure H 2 O Issue: embrittlement limits on start-up thermal shock events Intricate, large-scale, interconnected multifunctional structure with gradients, startup/shutdown and other transients, dimensional instabilities, continuous time-dependent stress redistributions. dpa ≈ 200, He ≈ 2000 appm T ≈ 400 – 600 °C Heat flux ≈ 1 – 15 MW/m 2 Coolant: He, Li, ….. Issues: possibly too many to count or even know.

9. Yield strength and strain hardening constitutive laws. Various types of ‘ductility’. Fatigue crack growth rates. Fracture toughness. Irradiation and thermal creep rate. Thermo-mechanical fatigue limits. Creep-fatigue interactions. Environmentally assisted cracking. Bulk corrosion, oxidation & compatibility. Void swelling rates. Creep rupture times and strains. Creep crack growth rates. Flaw distributions. Some Properties Needed Controlled by synergistic interactions between a many variables Fe-9Cr

10. Combinations of many environmental and material variables control in-service microstructural and property evolution in complex non- equilibrium alloys. Enormous degrees of freedom, many inherently multi-scale interacting (time - length) mechanisms - critical outcomes often depend on small differences between large competing effects (e.g., void swelling). Data must be extrapolated - therefore must be modeled - and uncertainties must be estimated. In-Service Property Changes ITER TBM Ductile Brittle

11. Low-Activation Structural Materials for Fusion None of the current reduced or low activation fusion materials existed 15 years ago. Low activation is a must!!  Materials strongly impact economic & environmental attractiveness of fusion power - basic feasibility.  Many materials are not suitable for various technical reasons.  Based on safety, waste disposal and performance considerations, the three leading candidates are:  RAF/M and NFA steels  Tungsten alloys  SiC composites

12. Impact of He-Rich Environment on Neutron Irradiated Materials  A unique aspect of the DT fusion environment is large production He and H.  He (and H?) has significant potential to create damage and cause loss of structural integrity: -High-temperature creep embrittlement. -Intermediate-temperature swelling. -Low-temperature loss of fracture toughness. Data Y. Dai Ductile Brittle Huge DBTT Shifts! Brittle intergranular fracture High He Low He

13. He Embrittlement: Unresolved Questions What is the sequence of events after He generation that controls its fate? How does He diffuse? How and where is He trapped? How does He behave and what does it do at various trapping sites? Can nanofeatures in advanced ferritic alloys stably trap He in very fine bubbles? Voids in F82H at 500°C, 9dpa, 380 appm He

14. Science-Based High-Temperature Design Criteria n Empirical high-temperature design methods are not applicable to fusion applications and damaged materials. n Thermo-mechanical challenges of first-wall/blanket & divertor structures are unprecedented even without radiation damage. RAFM Steel Poor creep-fatigue strength (cyclically soften)

15. Science-Based High-Temperature Design Criteria n Need new models of high-temperature deformation and fracture: Creep-rupture. Creep-fatigue interaction. Creep crack growth Complex time-dependent stress states and multiple failure paths. J. Aktaa & R. Schmitt, FZK, 2004 Cyclic loading far more damaging Grain boundary 316 SS @ 750°C & 100 MPa Schroeder and Batflasky, 1983

16. 52 nm Thermal creep test at 800°C and 138 MPa for 14,235 h Nanoclusters possess long-term stability at temperatures > 200°C higher than the upper temperature limit of advanced RAF/M steels. Superior Creep Strength of ODS Steels is Due to the Presence of Stable Nanoclusters SANS: r = 1.61 nm, f = 0.69%, N = 3.9x10 23 m -3 J12YWT: 12Cr-3W-0.4Ti-0.25Y 2 O 3 RAF/M Steel NFA RT tensile strength 1 - 2 GPa ORNL

17. Materials Design Strategy to Manage High He and Displacement Damage Trapping at a high-density of nanofeatures is key strategy for management of He. NFA Bubbles Dislocations Voids GB creep cavity Loops &voids GB bubbles TMS NF Climb-glide IG fracture Ductile Fracture a b G. R. Odette, M. J. Alinger and B. D. Wirth, “Recent Developments in Irradiation Resistant Steels”, Annual Reviews of Materials Research V38 (2008) 371-403 F82H MA957 NFA Ductile fracture J. Henry ICFRM13 STIP 320°C ~19 dpa 1700 appm He

18. Breaking the High Strength-Low Toughness/Ductility Paradigm  Increased strength is accompanied by reduced toughness (cracking resistance) and ductility.  Strength is increased by alloying, processing and radiation damage.  Low toughness and ductility reduce failure margins.  The benefits of simultaneously achieving high-strength and high ductility/ toughness would be enormous. Toughness or Ductility Strength Theoretical Strength ~E/50 Future Materials Current Engineering Materials

19. Fundamentals of Material-Coolant Chemical Compatibility in the Fusion Environment  The traditional approach to corrosion is empirical.  Correlations do not capture basic physics and have limited predictive capability.  Opportunities:  Controlled experiments combined with physical models utilizing advanced thermodynamics & kinetics codes.  Integrated experiments using sophisticated in situ diagnostic and sensor technologies. M. Zmitko / US-EU Material and Breeding Blanket Experts Meeting (2005) J. Konys et al./ ICFRM-12 (2005)

20. Role of Neutron Sources in Fusion Materials Science n Overcoming radiation damage degradation is the key rate-controlling step in fusion materials development. Additional factors such as joining are important, but critical radiation effects data is needed to evaluate feasibility. n Evaluation of radiation effects requires simultaneous displacement damage (~200 dpa) and He generation (~2000 appm He). Data without high fusion relevant dpa and He/dpa of limited value. n Evaluation of mechanical properties for a given material at a given temperature requires a minimum volume of ~10 cm 3 with flux gradients < 20%/cm. Innovative small-volume neutron sources would be useful but do not replace the need for a moderate-volume intense neutron source.

21. Conclusions - I  Materials and structures are a fusion power feasibility issue.  Fusion materials research has led to high-performance reduced- activation materials with radiation a resistance window to ~30 dpa/~300 appm He.  However, the structural materials scientific challenge for DEMO is managing microstructural and property evolution at ~200 dpa, ~2000 appm He.  Physical models of creep and creep-fatigue interactions are needed for development of advanced radiation damaged materials and science- based high-temperature design.  Better fundamental understanding is needed to achieve high-strength and high-ductility and toughness.  A robust theory and modeling activity is vital for understanding the complex physical phenomena associated with development of radiation- resistant fusion materials.

22. Conclusions - II  The most critical facility need is an intense neutron source. Irradiations in fission reactors combined with theory and modeling will not be able to fully address needs for DEMO (Workshop on Advanced Computational Materials Science, 2004).  N on-nuclear facilities like corrosion loops and semi-scale thermo- mechanical testing capability to:  Address component - system level issues.  Identify synergistic failure paths.  Verify computational codes for structural integrity and performance assessment.  There is growing evidence that RAF/M steels will have a limited application window for a DEMO reactor.  Tremendous opportunity to design and develop high creep strength, radiation tolerant, thermally stable nanostructured materials that may make fusion power a reality.

23. Scientific Grand Challenges: Revolutionary Technology Advances Solving the puzzle of the ductile to brittle transition: Breaking the high strength-low toughness paradigm. Understanding the transport, fate and consequences of helium and displacement damage: Radiation damage immune alloys for high-temperature, very high-dose service. Modeling the mechanisms, microstructures and mechanics of high-temperature deformation and damage: Science based performance life-cycle models for high temperature materials under complex long-term loading.

24. Greenwald Panel Research Initiatives InitiativeDescription I-1Predictive plasma modeling and validation I-2ITER – AT extensions I-3Integrated advanced physics demonstrations I-4Integrated PWI/PFC experiment I-5Disruption-free experiments I-6Engineering and materials science modeling and experimental validation I-7Materials qualification facility (intense neutron source) I-8Component development and testing program I-9Component qualification facility

25. Guiding Principles n A science-based approach is the most efficient path for developing and qualifying materials, components and structures for service in the fusion environment. n A robust theory and modeling activity (atomic to component) is vital for understanding the complex physical phenomena associated with development of radiation-resistant fusion materials and this modeling activity must be closely linked to critical experiments. n Development of low or reduced activation materials is essential to meeting the safety and environmental attractiveness goals of fusion. n Developments in related areas such as advanced fission must be leveraged to the maximum extent possible. n Research effort must begin now since structural materials development is a long-term endeavor. The historical precedent is 10-20 y with 150-200 M$ budgets.

26. Research Thrust I n Intense Neutron Source and Non-Nuclear Structural Integrity Benchmarking Facilities Identify and demonstrate approaches to improve the performance of existing and near-term materials, components and structures using the full suite of non- nuclear structural integrity benchmarking facilities (thermo-mechanical, corrosion, etc.) and the intense neutron source. Identify concept specific issues and demonstrate proof-of-principle solutions. Refine and validate predictive models of required materials, components and structural performance. Service qualification of materials, components and structures for codes, standards and regulatory requirements. Qualify large-scale, multi-physics structural and safety computational codes in preparation for intermediate nuclear facilities and DEMO. Develop foundation for surveillance of materials, components and structures including in-service inspection, maintenance and repair.

27. Research Thrust II n Development and Qualification of Advanced Structural and Functional Materials by Design With Revolutionary Properties Design of high-performance alloys and ceramics including fabrication and joining technologies. Experiments to characterize fundamental radiation damage mechanisms and thermo-mechanical degradation in next generation fusion materials. Multiscale (atomic to component level) modeling of the performance of materials, structures and components in the fusion environment. Exploration of compatibility with coolants and tritium breeders. Development of design specific functional and diagnostic materials such as coatings, insulating ceramics, functionally graded components, etc.