Radiation material science

Radiation materials science is a subfield of materials science which studies the interaction of radiation with matter: a broad subject covering many forms of irradiation and of matter.

Some of the most profound effects of irradiation on materials occur in the core of nuclear power reactors where atoms comprising the structural components are displaced numerous times over the course of their engineering lifetimes.

The consequences of radiation to core components includes changes in shape and volume by tens of percent, increases in hardness by factors of five or more, severe reduction in ductility and increased embrittlement, and susceptibility to environmentally induced cracking.

All of these forms of radiation have the capability to displace atoms from their lattice sites, which is the fundamental process that drives the changes in structural metals.

The effect of irradiation on materials is rooted in the initial event in which an energetic projectile strikes a target.

While the event is made up of several steps or processes, the primary result is the displacement of an atom from its lattice site.

This event is composed of several distinct processes: The result of a radiation damage event is, if the energy given to a lattice atom is above the threshold displacement energy, the creation of a collection of point defects (vacancies and interstitials) and clusters of these defects in the crystal lattice.

is the total number of displacements that the primary knock-on atom goes on to make in the solid; Taken together, they describe the total number of displacements caused by an incoming particle of energy

The result is the total number of displacements in the target from a flux of particles with a known energy distribution.

To generate materials that fit the increasing demands of nuclear reactors to operate with higher efficiency or for longer lifetimes, materials must be designed with radiation resistance in mind.

This leads to increased vulnerability to normal mechanical failure in terms of creep resistance as well as radiation damaging events such as neutron-induced swelling and radiation-induced segregation of phases.

By accounting for radiation damage, reactor materials would be able to withstand longer operating lifetimes.

This is of particular interest in developing commercial viability of advanced and theoretical nuclear reactors, and this goal can be accomplished through engineering resistance to these displacement events.

Face-centered cubic metals such as austenitic steels and Ni-based alloys can benefit greatly from grain boundary engineering.

By increasing populations of low energy boundaries without increasing grain size, fracture mechanics of these face centered cubic metals can be changed to improve mechanical properties given a similar displacements per atom value versus non grain boundary engineered alloys.

This method of treatment in particular yields better resistance to stress corrosion cracking and oxidation.

This can have a huge impact on total damage especially when comparing the materials of modern advanced reactors of zirconium to stainless steel reactor cores, which can differ in absorption cross section by an order of magnitude from more-optimal materials.

[4] Example values for thermal neutron cross section are shown in the table below.

[5] For nickel-chromium and iron-chromium alloys, short range order can be designed on the nano-scale (<5 nm) that absorbs the interstitial and vacancy's generated by primary knock-on atom events.

This occurs through generating a metastable phase that is in constant, dynamic equilibrium with surrounding material.

This metastable phase is characterized by having an enthalpy of mixing that is effectively zero with respect to the main lattice.

This allows phase transformation to absorb and disperse the point defects that typically accumulate in more rigid lattices.

This extends the life of the alloy through making vacancy and interstitial creation less successful as constant neutron excitement in the form of displacement cascades transform the SRO phase, while the SRO reforms in the bulk solid solution.

Image sequence of the time development of a collision cascade in the heat spike regime produced by a 30 keV Xe ion impacting on Au under channeling conditions. The image is produced by a classical molecular dynamics simulation of a collision cascade. The image shows a cross section of two atomic layers in the middle of a threedimensional simulation cell. Each sphere illustrates the position of an atom, and the colors show the kinetic energy of each atom as indicated by the scale on the right. At the end, both point defects and dislocation loops remain.