9.2 High-Temperature and Plasma-Facing Materials

Fusion reactors demand materials that can withstand extreme conditions. High temperatures, intense radiation, and corrosive environments pose unique challenges. This section explores the properties and challenges of materials used in fusion applications.

Plasma-facing components are at the forefront of material challenges in fusion reactors. Various materials, from tungsten to liquid metals, are being considered. Each has its strengths and weaknesses, driving the development of advanced materials to meet fusion's demanding requirements.

High-Temperature Materials for Fusion Applications

Properties of fusion materials

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• High-temperature strength and creep resistance enable materials to withstand high thermal and mechanical stresses and maintain structural integrity at elevated temperatures (>500°C)
• Thermal conductivity allows efficient heat transfer to coolant systems, minimizing thermal gradients and thermal stresses
• Low activation minimizes radioactivity induced by neutron irradiation, reducing radioactive waste and facilitating reactor maintenance
• Compatibility with coolants such as liquid metals and molten salts ensures resistance to corrosion and erosion and maintains material properties and integrity
• Radiation damage resistance enables materials to withstand high neutron fluxes without significant degradation, maintaining mechanical properties and dimensional stability under irradiation

Challenges for plasma-facing materials

• High heat flux exposes materials to intense thermal loads from the plasma (>10 MW/m²), requiring high thermal conductivity and resistance to thermal shock
• Erosion and sputtering cause physical and chemical erosion due to particle bombardment from the plasma, leading to material loss and contamination
• Tritium retention involves absorption and retention of tritium fuel, affecting fuel efficiency and raising safety concerns due to radioactivity
• Neutron irradiation exposes materials to high-energy neutrons from the fusion reaction, causing radiation damage, embrittlement, and activation
• Thermal fatigue and cracking result from cyclic thermal stresses due to repeated plasma exposure and cooldown, potentially leading to the formation and propagation of cracks

Plasma-Facing Materials and Their Suitability

Suitability of plasma-facing components

• Tungsten (W) offers high melting point and thermal conductivity, low sputtering yield and erosion rate, but exhibits brittleness and susceptibility to neutron embrittlement
• Carbon-based materials like graphite and carbon fiber composites provide high thermal conductivity and resistance to thermal shock, low atomic number reducing plasma contamination, but have high erosion rates and tritium retention
• Beryllium (Be) has low atomic number, good thermal conductivity, and oxygen gettering properties improving plasma purity, but presents toxicity and limited operational temperature range
• High-Z refractory alloys (Mo, Ta, Nb) feature high melting points and thermal conductivity, moderate resistance to erosion and sputtering, but undergo activation and transmutation under neutron irradiation
• Liquid metals (Li, Sn, Ga) offer self-healing and renewable surfaces with high heat removal capacity, but face magnetohydrodynamic effects and material compatibility challenges

Development of advanced fusion materials

• Nanostructured materials like oxide dispersion strengthened (ODS) steels and nanocrystalline tungsten alloys exhibit enhanced mechanical properties and radiation damage resistance
• Composite materials such as silicon carbide fiber-reinforced silicon carbide (SiC/SiC) and copper-based composites with reinforcing fibers or particles demonstrate improved thermal and mechanical properties compared to monolithic materials
• Functionally graded materials (FGMs) feature gradual transitions in composition and properties, optimized for specific requirements (heat flux, neutron irradiation), reducing thermal stresses and improving overall performance
• Advanced manufacturing techniques including additive manufacturing (3D printing), plasma spraying, and vapor deposition enable fabrication of novel material architectures and compositions
• Irradiation testing and characterization involve in-pile and out-of-pile testing in fission reactors and ion beam facilities, evaluating material properties and performance under fusion-relevant conditions, complemented by modeling and simulation to predict long-term behavior and guide material development