7.1 Deuterium-Tritium Fuel Cycle

The deuterium-tritium fuel cycle is a key component of nuclear fusion technology. It offers the highest fusion reaction cross-section at lower temperatures, enabling more efficient energy production. However, challenges like tritium's radioactivity and the need for breeding complicate reactor design and operation.

Nuclear reactions in the D-T cycle involve the primary fusion reaction and tritium breeding in the lithium blanket. Neutrons play a crucial role in energy transfer and tritium production. Fuel burnup and helium ash accumulation require careful management to maintain plasma performance and stability.

Deuterium-Tritium Fuel Cycle

Advantages vs challenges of deuterium-tritium fuel

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• Advantages of using deuterium-tritium (D-T) fuel
  • Highest fusion reaction cross-section at lower temperatures compared to other fusion fuels (D-D, D-He3) enables more efficient fusion reactions
  • Requires lower confinement time and plasma temperature to achieve ignition allows for smaller, less expensive reactor designs
  • Deuterium is abundant in seawater (0.015%), ensuring a virtually inexhaustible fuel supply for long-term energy production
• Challenges of using D-T fuel
  • Tritium is radioactive with a half-life of 12.3 years, requiring careful handling and storage to minimize radiation exposure and environmental risks
  • Tritium is rare in nature (trace amounts in cosmic rays) and must be bred from lithium in the reactor blanket adds complexity to reactor design and operation
  • High-energy neutrons (14.1 MeV) produced in D-T reactions cause radiation damage to reactor components (first wall, blanket) limiting their lifetime and increasing maintenance requirements
  • Neutron activation of reactor materials leads to radioactive waste disposal challenges necessitates specialized waste management and long-term storage solutions

Nuclear reactions in deuterium-tritium cycle

• Primary D-T fusion reaction: $D + T \rightarrow \alpha (3.5 MeV) + n (14.1 MeV)$
  • Deuterium and tritium fuse to form an alpha particle (helium-4 nucleus) and a high-energy neutron releases energy from the mass defect between reactants and products
  • Total energy release of 17.6 MeV per reaction, with the neutron carrying 80% of the energy allows for efficient energy extraction and tritium breeding
• Tritium breeding reactions in the lithium-containing blanket
  1. $^6Li + n \rightarrow \alpha + T + 4.8 MeV$
     • Lithium-6 absorbs a neutron to produce an alpha particle and tritium exothermic reaction that generates additional heat
  2. $^7Li + n \rightarrow \alpha + T + n - 2.5 MeV$
     • Lithium-7 absorbs a neutron to produce an alpha particle, tritium, and a lower-energy neutron maintains neutron economy in the reactor
     • Endothermic reaction that requires a high-energy neutron (> 2.5 MeV) to overcome the energy threshold

Neutrons in deuterium-tritium fusion

• Neutron energy spectrum in a D-T fusion reactor
  • 14.1 MeV neutrons produced from the primary D-T fusion reaction carry the majority of the fusion energy
  • Lower-energy neutrons resulting from scattering and moderation in the blanket and structural materials contribute to tritium breeding and heat generation
• Neutron interactions in the reactor
  • Tritium breeding in the lithium-containing blanket to maintain the fuel supply ensures a closed fuel cycle and reduces external tritium requirements
  • Neutron multiplication through (n, 2n) reactions in beryllium or lead to enhance tritium breeding increases the overall tritium production rate
  • Neutron moderation using materials like graphite or water to optimize tritium breeding and reduce radiation damage improves reactor performance and component lifetimes
• Impact on reactor design
  • Blanket design optimization for efficient tritium breeding and heat extraction requires careful selection of materials (Li, Be) and geometry (pebble bed, liquid metal)
  • Selection of materials with low activation (vanadium alloys) and high resistance to radiation damage (SiC composites) minimizes radioactive waste and extends component lifetimes
  • Shielding requirements to protect superconducting magnets and other sensitive components from neutron flux necessitates the use of thick, high-density materials (tungsten, boron carbide)

Fuel burnup and ash accumulation

• Fuel burnup
  • Fraction of D-T fuel consumed in the fusion reactions during plasma confinement typically ranges from 1-10% depending on plasma conditions
  • Depends on factors such as plasma temperature, confinement time, and fuel density higher values lead to more efficient fuel utilization
  • Higher burnup leads to more efficient fuel utilization but also increases ash accumulation requires optimization to balance fuel efficiency and plasma performance
• Ash accumulation
  • Helium-4 (alpha particles) produced as a byproduct of the D-T fusion reaction accumulates in the plasma over time
  • Accumulation of helium ash in the plasma reduces the fusion reactivity and dilutes the fuel by occupying space and lowering the effective fusion cross-section
  • Helium ash removal is crucial for maintaining plasma performance and stability to prevent fuel dilution and instabilities (kink modes, disruptions)
• Strategies for managing fuel burnup and ash accumulation
  1. Continuous or periodic exhaust of the plasma to remove helium ash using divertors or gas puffing
  2. Magnetic divertors to guide the plasma exhaust and separate the ash from the main plasma by leveraging the difference in gyroradii between helium and D-T ions
  3. Fuel replenishment through pellet injection or gas puffing to maintain optimal D-T ratio compensates for fuel depletion and helps control plasma density