☢️Nuclear Fusion Technology Unit 7 – Fusion Fuels and Fuel Cycles

Key Concepts and Terminology

• Fusion fuels consist of light atomic nuclei that combine to form heavier nuclei, releasing energy in the process
• Fusion reactions require high temperatures and pressures to overcome the repulsive Coulomb force between positively charged nuclei
• Plasma is a highly ionized gas in which fusion reactions occur, as it can sustain the necessary high temperatures
• Confinement systems, such as magnetic or inertial confinement, are used to contain the hot plasma and maintain fusion conditions
• Tritium, a radioactive isotope of hydrogen, is a key component of many fusion fuel mixtures due to its high reactivity
• Breeding blankets surrounding the reactor vessel are designed to generate tritium from lithium to ensure a sustainable fuel supply
• Fuel cycle encompasses all stages of fuel production, handling, injection, extraction, and recycling
• Neutron activation occurs when materials are exposed to high-energy neutrons from fusion reactions, potentially creating radioactive waste

Fusion Fuel Basics

• Fusion reactions release energy when light nuclei combine to form heavier nuclei, with the most energy released when forming helium-4
• Hydrogen isotopes, particularly deuterium (D) and tritium (T), are the most commonly used fusion fuels due to their relatively low Coulomb barrier
• The D-T reaction is currently the most promising for fusion power due to its high reactivity and energy yield at lower temperatures compared to other fuel combinations
• Fusion fuels must be heated to extremely high temperatures (around 100 million degrees Celsius) to overcome the repulsive Coulomb force and achieve ignition
• High-density plasma is necessary to increase the likelihood of fusion reactions occurring, as it increases the collision frequency between nuclei
• Fusion reactions produce high-energy neutrons, which can be used for energy generation, material irradiation, and tritium breeding

Types of Fusion Fuels

• Deuterium (D), an isotope of hydrogen with one proton and one neutron, is a stable and abundant fusion fuel that can be extracted from seawater
• Tritium (T), a radioactive isotope of hydrogen with one proton and two neutrons, is a key component of the D-T fusion reaction but must be bred from lithium
• Helium-3 (³He) is a rare, non-radioactive isotope of helium that can be used in fusion reactions with deuterium, producing fewer high-energy neutrons compared to D-T
  • ³He is scarce on Earth but is thought to be abundant on the lunar surface, potentially making it a valuable future fusion fuel
• Advanced fuel cycles, such as D-D, D-³He, and p-¹¹B (proton-boron), are being researched as alternatives to D-T, as they produce fewer neutrons and less radioactive waste
• Lithium, both in its natural isotopic form and enriched in lithium-6, is used in breeding blankets to generate tritium fuel via neutron capture reactions

Fuel Cycle Overview

• The fusion fuel cycle involves the production, handling, injection, extraction, and recycling of fusion fuels
• Fuel production focuses on the extraction and purification of deuterium from water and the breeding of tritium from lithium in reactor breeding blankets
• Fuel handling systems ensure the safe storage, transportation, and processing of fusion fuels, particularly the radioactive tritium
• Injection systems introduce the fuel into the reactor vessel, typically as frozen pellets or high-speed gas streams, to maintain the desired plasma density
• Exhaust systems remove the fusion reaction products (helium ash) and unburned fuel from the reactor vessel to maintain optimal plasma conditions
• Tritium extraction systems recover unburned tritium from the exhaust gas for reprocessing and reuse in the fuel cycle
• Breeding blankets, containing lithium compounds, surround the reactor vessel to generate new tritium fuel via neutron capture reactions
• Fuel recycling aims to minimize waste and optimize resource utilization by reprocessing unburned fuel and breeding tritium for future use

Fuel Production and Handling

• Deuterium production involves the extraction of deuterium from water through processes such as distillation, electrolysis, and chemical exchange
  • Distillation exploits the slight difference in boiling points between heavy water (D2O) and regular water (H2O) to separate the isotopes
  • Electrolysis of heavy water produces deuterium gas (D2) at the cathode, which can be collected and purified
• Tritium production primarily occurs through the irradiation of lithium-containing breeding blankets in fusion reactors
  • Lithium-6 undergoes a neutron capture reaction to produce tritium and helium-4: $^6Li + n → ^3H + ^4He$
• Tritium handling requires special precautions due to its radioactivity and ability to permeate through materials
  • Storage systems use multiple containment barriers and inert atmospheres to prevent tritium release
  • Tritium-compatible materials, such as low-permeability alloys and ceramics, are used in fuel handling systems
• Fuel pellet production involves the solidification of deuterium and tritium mixtures into small, uniform pellets for injection into the reactor
  • Pellets are typically formed by cryogenic extrusion or drop-freezing techniques
• Inertial confinement fusion (ICF) fuel capsules are manufactured with precise layers of deuterium-tritium ice surrounding a central gas cavity
• Stringent safety protocols and monitoring systems are in place to minimize the risk of tritium release during fuel handling and processing

Reactor Fuel Injection Systems

• Fuel injection systems introduce the fusion fuel into the reactor vessel while maintaining the desired plasma density and confinement conditions
• Gas puffing involves the rapid injection of fuel gas at the plasma edge, typically using fast valves or piezoelectric actuators
  • Gas puffing is useful for maintaining plasma density but can lead to impurity accumulation and reduced confinement
• Pellet injection uses high-speed pellets of frozen deuterium-tritium fuel to penetrate deep into the plasma core
  • Pellet size, velocity, and injection frequency are optimized to maintain plasma density and fuel replenishment
  • Pneumatic, centrifugal, or electromagnetic accelerators are used to propel the pellets at speeds of several kilometers per second
• Neutral beam injection (NBI) involves the acceleration and neutralization of fuel ions, which can then cross the magnetic field lines and enter the plasma
  • NBI is used for both fueling and plasma heating, as the high-energy neutral particles transfer their energy to the plasma through collisions
• Compact toroid injection (CTI) uses self-contained plasma rings, called compact toroids, to deliver fuel to the reactor core
  • CTI can efficiently deposit fuel in the plasma core without disrupting the magnetic confinement field
• Advanced injection techniques, such as pellet-triggered ELM pacing and shattered pellet injection, are being developed to improve fueling efficiency and mitigate edge instabilities

Fuel Behavior in Plasma

• Fusion fuel undergoes ionization, dissociation, and recombination processes in the high-temperature plasma environment
• Plasma confinement systems, such as magnetic or inertial confinement, are designed to contain the hot fuel ions and maintain fusion conditions
• Fuel ions undergo Coulomb collisions with each other and with electrons, leading to thermalization and energy transfer within the plasma
• Bremsstrahlung radiation occurs when electrons are accelerated in the electric fields of ions, leading to energy loss from the plasma
• Fusion reactions between fuel ions produce high-energy products, such as alpha particles (helium nuclei) and neutrons
  • Alpha particles transfer their energy to the plasma through collisions, contributing to plasma heating and potentially enabling self-sustaining fusion reactions (ignition)
  • Neutrons escape the plasma and are absorbed in the surrounding structures, depositing their energy for power generation and tritium breeding
• Plasma instabilities, such as magnetohydrodynamic (MHD) modes and edge localized modes (ELMs), can affect fuel confinement and lead to rapid energy and particle losses
• Plasma-wall interactions, including sputtering and erosion, can introduce impurities into the plasma and degrade the performance of plasma-facing components
• Advanced plasma diagnostics, such as Thomson scattering and charge exchange recombination spectroscopy, are used to measure fuel ion temperatures, densities, and velocities in the plasma

Waste Management and Recycling

• Fusion reactors generate radioactive waste primarily through neutron activation of reactor components and tritium contamination
• Low-level waste, such as contaminated equipment and protective clothing, is compacted and stored in designated repositories
• Intermediate-level waste, including activated reactor components and tritium-containing materials, requires shielding and long-term storage
  • Recycling and reuse of valuable materials, such as beryllium and tungsten, can reduce the amount of waste generated
• High-level waste, mainly consisting of activated structural materials near the reactor core, requires deep geological disposal
  • Advanced materials, such as low-activation steels and ceramics, are being developed to minimize the production of long-lived radioactive isotopes
• Tritium waste management involves the safe handling, storage, and disposal of tritium-contaminated materials
  • Tritium recovery systems are used to extract tritium from waste streams for recycling and reuse in the fuel cycle
  • Immobilization techniques, such as cementation and vitrification, are used to stabilize tritium-containing waste for long-term storage
• Decommissioning and dismantling of fusion reactors at the end of their operational lifetime requires careful planning and execution to minimize waste generation and environmental impact
• International collaborations and guidelines, such as the ITER project and the IAEA fusion waste management principles, aim to establish best practices and standards for fusion waste management

Future Fuel Developments

• Advanced fuel cycles, such as D-D, D-³He, and p-¹¹B, are being investigated to reduce neutron production and radioactive waste generation
  • These fuel cycles require higher plasma temperatures and confinement times compared to D-T, presenting additional technical challenges
• Aneutronic fusion reactions, which produce little to no neutrons, are of interest for their potential to minimize neutron activation and radioactive waste
  • The p-¹¹B reaction is a promising aneutronic candidate, but it requires extremely high plasma temperatures (>1 billion degrees Celsius) to achieve significant fusion rates
• Tritium self-sufficiency is a key goal for future fusion power plants, aiming to produce enough tritium in breeding blankets to sustain the fuel cycle
  • Advanced breeding materials, such as lithium ceramics and liquid lithium-lead alloys, are being developed to optimize tritium production and extraction
• Direct energy conversion techniques, such as magnetohydrodynamic (MHD) generators and electrostatic convertors, are being explored to efficiently harness the kinetic energy of fusion products
  • These techniques could potentially increase the overall efficiency of fusion power plants and reduce the size and complexity of the balance of plant
• Laser-driven fusion, using high-power lasers to compress and heat fuel capsules, is an alternative approach to magnetic confinement fusion
  • Inertial confinement fusion (ICF) experiments, such as the National Ignition Facility (NIF) in the USA, aim to demonstrate the feasibility of laser-driven fusion for power production
• Magnetized target fusion (MTF) is a hybrid approach that combines elements of magnetic and inertial confinement to achieve fusion conditions
  • MTF concepts, such as the plasma liner experiment (PLX), use magnetically confined plasma to compress and heat a fusion fuel target
• Fusion-fission hybrid reactors are being considered as a way to enhance the performance of both fusion and fission systems
  • Fusion-generated neutrons could be used to drive subcritical fission blankets, increasing energy output and reducing radioactive waste production compared to conventional fission reactors