7.2 Advanced Fusion Fuel Concepts

Advanced fusion fuels like deuterium-deuterium and deuterium-helium-3 offer exciting possibilities for nuclear fusion. These fuels promise higher energy yields and reduced neutron production compared to traditional deuterium-tritium fusion, potentially leading to more efficient and cleaner energy generation.

However, harnessing these advanced fuels comes with significant challenges. Higher plasma temperatures, improved confinement, and material compatibility issues must be overcome. Despite these hurdles, the potential benefits of aneutronic fusion and direct energy conversion make advanced fusion fuels a promising area of research.

Advanced Fusion Fuel Properties and Reactions

Properties of advanced fusion fuels

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• Deuterium-deuterium (D-D) fusion
  • Fuses two deuterium nuclei, isotopes of hydrogen with one proton and one neutron
  • Two main reactions: $D + D \rightarrow {}^3He + n + 3.27 MeV$ and $D + D \rightarrow T + p + 4.03 MeV$
  • Generates both neutrons (n) and charged particles (helium-3, tritium, and protons)
  • Lower energy output per reaction compared to D-He3 fusion (3.27 MeV and 4.03 MeV vs 18.3 MeV)
• Deuterium-helium-3 (D-He3) fusion
  • Fuses a deuterium nucleus with a helium-3 nucleus, an isotope of helium with two protons and one neutron
  • Reaction: $D + {}^3He \rightarrow {}^4He + p + 18.3 MeV$
  • Produces only charged particles (alpha particles and protons), no neutrons
  • Higher energy yield per reaction (18.3 MeV) compared to D-D fusion (3.27 MeV and 4.03 MeV)

Benefits vs drawbacks of fusion fuels

• Benefits of advanced fusion fuels
  • Increased energy release per fusion event (18.3 MeV for D-He3 vs 17.6 MeV for D-T)
  • Lower neutron production, especially in D-He3 fusion, reducing neutron-induced damage to reactor components and minimizing radioactive waste
  • Enables direct energy conversion, converting charged particle kinetic energy directly into electricity, improving power plant efficiency
  • Reduced radiation shielding requirements due to fewer neutrons, leading to more compact reactor designs
• Drawbacks of advanced fusion fuels
  • Requires higher plasma temperatures for ignition (around 1 billion ℃ for D-He3 vs 100 million ℃ for D-T)
  • Lower fusion reaction cross-sections, meaning lower probability of fusion occurring, necessitating better plasma confinement and higher plasma densities
  • Scarcity of helium-3 on Earth, primarily found in lunar regolith, requiring mining operations on the Moon
  • More challenging to achieve and maintain the necessary plasma conditions for advanced fuel fusion

Concept of aneutronic fusion

• Aneutronic fusion involves reactions that produce no neutrons or very few neutrons
• Examples of aneutronic or nearly aneutronic reactions:
  1. D-He3: $D + {}^3He \rightarrow {}^4He + p + 18.3 MeV$
  2. Proton-Boron-11 (p-B11): $p + {}^{11}B \rightarrow 3{}^4He + 8.7 MeV$
  3. He3-He3: ${}^3He + {}^3He \rightarrow {}^4He + 2p + 12.86 MeV$
• Advantages of aneutronic fusion include reduced radioactivity, minimal neutron damage to reactor components, and the potential for direct energy conversion
• Applications in compact, high-efficiency power plants with reduced radiation shielding and radioactive waste
• Potential use in space propulsion systems with high specific impulse (Isp) and lower propellant mass compared to chemical rockets

Challenges in harnessing fusion fuels

• Plasma confinement challenges
  1. Higher magnetic field strengths needed to confine the hotter plasmas required for advanced fuel fusion (e.g., superconducting magnets)
  2. Improved plasma stability and control techniques to maintain optimal fusion conditions
  3. Development of advanced magnetic confinement concepts (stellarators, reversed-field pinches) to better suit advanced fuel fusion
• Material compatibility challenges
  1. Reactor components must withstand extreme temperatures and high heat fluxes generated by the fusion process
  2. Materials need to resist damage from high-energy particle bombardment (charged particles and any remaining neutrons)
  3. Advanced materials like tungsten alloys and carbon composites must be developed and tested for use in fusion reactors