☢️Nuclear Fusion Technology Unit 8 – Neutronics and Blanket Design

Key Concepts and Principles

• Neutronics involves the study of neutron interactions, transport, and effects in fusion reactor systems
• Blanket design focuses on the components surrounding the fusion plasma that absorb neutrons, breed tritium fuel, and extract heat for power generation
• Neutron multiplication enhances the number of neutrons available for tritium breeding through reactions with materials like beryllium or lead
• Tritium self-sufficiency is a critical goal in fusion reactor design, requiring a breeding ratio greater than 1 to sustain the fuel cycle
• Radiation damage to materials is a significant challenge due to the high-energy neutrons produced in fusion reactions (14.1 MeV)
• Shielding is essential to protect reactor components and personnel from harmful radiation
• Activation of materials by neutron irradiation creates radioactive isotopes that must be considered in safety and waste management
• Computational modeling and simulation play a crucial role in designing and optimizing neutronics and blanket systems

Neutron Physics Fundamentals

• Neutrons are electrically neutral subatomic particles with a mass of approximately 1.675 × 10^-27 kg
• Neutron interactions with matter include elastic scattering, inelastic scattering, capture, and fission
• Cross sections quantify the probability of neutron interactions and depend on the incident neutron energy and target material
• Neutron energy spectrum in fusion reactors spans from thermal energies (0.025 eV) to high energies (14.1 MeV)
• Neutron transport involves the movement of neutrons through materials, considering scattering, absorption, and leakage
• The neutron mean free path is the average distance a neutron travels between interactions and depends on the material density and cross sections
• Neutron moderation reduces the energy of fast neutrons through collisions with light nuclei like hydrogen or carbon
• Neutron absorption removes neutrons from the system through capture reactions, which can be beneficial (tritium breeding) or detrimental (parasitic absorption)

Fusion Reactor Components

• Plasma chamber: Contains the high-temperature fusion plasma where the fusion reactions occur
• First wall: The innermost layer of the blanket directly facing the plasma, subjected to high heat and particle fluxes
• Blanket: Surrounds the plasma chamber and serves multiple functions, including neutron absorption, tritium breeding, and heat extraction
• Divertor: Handles the exhaust of fusion reaction products and impurities from the plasma
• Vacuum vessel: Provides a high-vacuum environment for the plasma chamber and acts as a first barrier for confinement
• Magnets: Generate strong magnetic fields for plasma confinement and stability (in magnetic confinement fusion reactors)
• Cryostat: Encloses the vacuum vessel and superconducting magnets, maintaining a cryogenic environment
• Tritium processing systems: Extract, purify, and recycle tritium from the blanket and fuel cycle

Blanket Design Considerations

• Tritium breeding: The blanket must efficiently breed tritium through neutron interactions with lithium to maintain a self-sufficient fuel cycle
  • Lithium-6 undergoes an exothermic reaction: $^6Li + n → ^4He + ^3H + 4.8 MeV$
  • Lithium-7 undergoes an endothermic reaction: $^7Li + n → ^4He + ^3H + n - 2.5 MeV$
• Neutron multiplication: Incorporating neutron multipliers like beryllium or lead in the blanket enhances the neutron population for improved tritium breeding
• Heat extraction: The blanket must efficiently remove the heat generated by neutron interactions and transfer it to a power conversion system
• Structural integrity: Blanket materials must withstand high temperatures, thermal stresses, and radiation damage over the reactor lifetime
• Compatibility: Blanket materials must be compatible with the coolant (e.g., water, helium, or molten salts) and not undergo excessive corrosion or degradation
• Activation: The choice of blanket materials should minimize the production of long-lived radioactive isotopes to facilitate decommissioning and waste management
• Maintenance and replacement: Blanket design should allow for efficient maintenance, repair, and replacement of components, considering the high radiation environment

Materials for Neutronics and Blankets

• Lithium: Essential for tritium breeding, used in the form of lithium ceramics (Li2O, Li4SiO4, Li2TiO3) or molten salts (LiF-BeF2, LiPb)
• Beryllium: Excellent neutron multiplier due to its low atomic mass and high (n,2n) cross section, also used as a plasma-facing material
• Lead: Used as a neutron multiplier and coolant in the form of molten lead or lead-lithium eutectic (LiPb)
• Graphite: Used as a neutron moderator and reflector to improve neutron economy and tritium breeding
• Steels: Structural materials for blanket components, such as reduced activation ferritic/martensitic (RAFM) steels or oxide dispersion strengthened (ODS) steels
• Vanadium alloys: Promising structural materials with low activation, high-temperature strength, and good compatibility with liquid metal coolants
• Tungsten: Used as a plasma-facing material in divertors and first wall components due to its high melting point and thermal conductivity
• Copper alloys: Used for heat sink applications and as a structural material in high heat flux components

Neutron Multiplication and Breeding

• Neutron multiplication increases the number of neutrons available for tritium breeding and energy multiplication
• Beryllium is an effective neutron multiplier through the (n,2n) reaction: $^9Be + n → 2^4He + 2n$
  • The cross section for this reaction is significant at high neutron energies (> 1 MeV)
• Lead also undergoes (n,2n) reactions, but with a higher threshold energy compared to beryllium
• Tritium breeding ratio (TBR) is the number of tritium atoms produced per fusion neutron
  • A TBR > 1 is necessary for self-sufficient tritium fuel cycle, typically targeting TBR ≈ 1.1 to account for losses
• Neutron multipliers are strategically placed in the blanket to optimize the spatial distribution of neutrons and enhance the TBR
• Breeding blanket design must balance neutron multiplication, tritium extraction efficiency, and heat removal

Radiation Shielding and Safety

• Radiation shielding protects reactor components, personnel, and the environment from the intense neutron and gamma radiation generated in fusion reactions
• Shielding materials attenuate radiation through absorption, scattering, and moderation processes
• Concrete, water, and heavy metals (e.g., lead, tungsten) are commonly used shielding materials
  • Concrete provides effective shielding against both neutrons and gamma rays due to its high hydrogen content and density
  • Water is an excellent neutron moderator and shield, but requires additional shielding for gamma rays
  • Heavy metals are effective for gamma ray attenuation due to their high atomic number and density
• Layered shielding designs optimize the attenuation of different radiation types while minimizing the overall shielding thickness and mass
• Activation of shielding materials is a concern, as it can lead to the production of radioactive waste
  • Low-activation materials, such as boron carbide or tungsten carbide, are preferred for their reduced long-term radioactivity
• Remote handling and maintenance systems are necessary for operating in the high-radiation environment of a fusion reactor
• Comprehensive safety analysis, including accident scenarios and release pathways, is crucial for licensing and public acceptance of fusion power plants

Simulation and Modeling Techniques

• Neutron transport codes, such as MCNP (Monte Carlo N-Particle) or Serpent, are used to simulate neutron interactions and distribution in complex 3D geometries
  • These codes use stochastic methods to track individual neutron histories and estimate quantities of interest (e.g., flux, reaction rates, heating)
• Deterministic transport methods, like discrete ordinates (SN) or spherical harmonics (PN), solve the neutron transport equation numerically
  • These methods discretize the phase space (space, energy, and angle) and provide detailed spatial and energy-dependent neutron flux solutions
• Activation calculations predict the production and decay of radioactive isotopes in materials exposed to neutron irradiation
  • FISPACT is a widely used activation code that couples with neutron transport codes to estimate activation, transmutation, and decay
• Multiphysics modeling couples neutronics with other physics phenomena, such as thermal-hydraulics, structural mechanics, or plasma physics
  • Codes like COMSOL Multiphysics or ANSYS provide platforms for integrating different physics models in a single simulation environment
• Sensitivity and uncertainty analysis quantifies the impact of input parameters (e.g., cross sections, material compositions) on the simulation results
  • Perturbation theory and adjoint methods are used to efficiently compute sensitivity coefficients and propagate uncertainties
• Experimental validation is essential to benchmark and improve the accuracy of simulation models
  • Integral experiments, like mock-up blanket assemblies or neutron source facilities, provide valuable data for code validation and calibration