Radiation materials science is a special area of materials science that studies how radiation affects materials. It covers many types of radiation and different kinds of materials.

Main aim of radiation material science

Irradiation has important effects on materials used in the core of nuclear power reactors. Over time, the atoms that make up the structural parts of these reactors are moved many times throughout their lifetimes. Radiation can cause these materials to change shape and size by up to 10%, become much harder, lose flexibility, become more brittle, and be more likely to crack when exposed to certain environmental conditions. To ensure these structures work properly, it is important to understand how radiation affects materials. This knowledge helps engineers design better structures, adjust reactor operating conditions to reduce harm, and develop new materials that can better handle radiation.

Radiation

The types of radiation that can change the structure of materials include neutron radiation, ion beams, electrons (also called beta particles), and gamma rays. These forms of radiation can move atoms out of their normal positions in a material's structure. This movement is the main reason structural metals change over time. When ions are part of the radiation, it connects to other areas of study, such as using accelerators to change nuclear waste or creating new materials through methods like ion implantation, ion beam mixing, plasma-assisted ion implantation, and ion beam-assisted deposition.

When radiation hits a material, the first event is an energetic particle striking the material. This process involves several steps, but the main result is an atom being moved from its normal position. This movement leaves an empty space (called a vacancy) where the atom was, and the displaced atom eventually stops in a spot between other atoms, becoming an interstitial atom. The combination of a vacancy and an interstitial atom is important in radiation effects and is called a Frenkel pair. The presence of Frenkel pairs and other radiation-related changes influence how materials behave. When stress is applied, these changes can lead to effects like swelling, growth, phase changes, or segregation.

In addition to moving atoms, high-energy charged particles can transfer energy to electrons in the material through a process called electronic stopping power. This energy transfer can cause damage in non-metallic materials, such as creating ion tracks or fission tracks in minerals.

Radiation damage

A radiation damage event happens when energy from an incoming particle moves into a solid, causing atoms in the material to shift from their normal positions. This event includes several steps that lead to changes in the structure of the solid.

If the energy given to an atom in the crystal structure is greater than a specific energy level called the threshold displacement energy, the atom moves out of its place. This movement creates small defects in the material, such as vacancies (empty spots where atoms are missing) and interstitials (atoms that are squeezed into spaces between other atoms). These defects can group together to form clusters.

To measure radiation damage in a solid, scientists calculate the number of displacements (when atoms move from their normal positions) per unit volume per unit time, represented as R. This calculation uses several factors:

• N: The number of atoms in a given volume.
• E_max and E_min: The highest and lowest energy levels of the incoming particles.
• ϕ(Ei): The number of particles with a specific energy level.
• T_max and T_min: The highest and lowest energy levels transferred during a collision between a particle and an atom in the solid.
• σ(Ei, T): The probability that a particle with a specific energy will transfer a certain amount of energy to an atom in the solid.
• υ(T): The total number of displacements caused by a single atom that has been moved by the incoming particle.

The two most important parts of this calculation are σ(Ei, T) and υ(T). The term σ(Ei, T) describes how likely it is for an incoming particle to transfer energy to the first atom it hits (called the primary knock-on atom). The term υ(T) shows how many times the primary knock-on atom moves other atoms in the solid. Together, these values explain how many total displacements occur from a single incoming particle. The equation accounts for the different energy levels of the incoming particles and calculates the total number of displacements in the material.

In radiation material science, displacement damage is often measured as displacements per atom (dpa), which tells how many times each atom in the material has been moved by radiation. This measurement is more useful for understanding how radiation affects a material’s properties than another measure called fluence, which tracks the total energy of radiation particles (measured in MeV).

See also: Wigner effect.

Radiation-resistant materials

To create materials that meet the growing needs of nuclear reactors, which must operate more efficiently or last longer, materials must be designed to resist radiation damage. Generation IV nuclear reactors function at higher temperatures and pressures than modern pressurized water reactors, which are used in many reactors in Western countries. These conditions increase the risk of mechanical failure, such as reduced resistance to gradual deformation (creep) and damage from radiation, including neutron-induced swelling and the separation of material phases. By considering radiation damage, reactor materials can last longer, allowing reactors to operate for extended periods before needing to be shut down. This improves the cost-effectiveness of reactors without reducing safety. This is especially important for developing new types of nuclear reactors, which can be achieved by engineering materials to resist radiation-related damage.

Face-centered cubic metals, such as austenitic steels and nickel-based alloys, can benefit from grain boundary engineering. This process aims to increase the number of special grain boundaries, which occur when grains have favorable orientations. By increasing the number of low-energy boundaries without changing the grain size, the mechanical properties of these metals can be improved, even when exposed to similar levels of radiation damage as non-engineered alloys. This method enhances resistance to stress corrosion cracking and oxidation.

Using advanced material selection methods, materials can be evaluated based on their neutron-absorption cross-sectional area. Choosing materials with low neutron-absorption reduces the number of times atoms are displaced over a reactor’s lifetime. This slows the process of radiation embrittlement by limiting how often atoms move, as materials that absorb fewer neutrons interact less with nuclear radiation. This can significantly reduce overall damage, especially when comparing materials like zirconium used in modern reactors to stainless steel, which can differ in neutron-absorption by a factor of ten compared to more optimal materials.

Example values for thermal neutron cross sections are shown in the table below.

For nickel-chromium and iron-chromium alloys, nano-scale structures (<5 nm) can be designed to absorb interstitials and vacancies caused by radiation events. This helps reduce swelling in materials exposed to high levels of radiation damage, keeping the overall volume change below 10%. This occurs by forming a metastable phase that remains in balance with the surrounding material. This phase has an enthalpy of mixing that is nearly zero compared to the main lattice, allowing it to absorb and spread point defects that would otherwise accumulate in rigid lattices. This extends the alloy’s lifespan by making it harder for vacancies and interstitials to form, as neutron impacts continuously alter the nano-scale structures while they reform in the bulk material.