Radiation effects in materials Free

This Special Topic “Radiation Effects in Materials” is a broad collection of scientific studies addressing the interaction of radiation with condensed matter, a long-standing issue that is relevant for a range of key energy technologies.

One of the earliest observations of the interaction of radiation with matter concerned the dark pleochroic halos surrounding some accessory minerals inclusions in micas as revealed in petrographic thin sections of igneous rocks. This phenomenon was explained by J. Joly in 1907, who attributed the halos to the radioactive decay of uranium and thorium that are incorporated in enclosed accessory minerals such as zircon, monazite, and titanite.¹ This was in the same year as Rutherford demonstrated that an alpha particle is a helium nucleus. The size of the halos corresponds to the length of an alpha particle track in the surrounding mica mineral, along which it causes damage to the crystal lattice and a concomitant change in the transmission of the light. Subsequently, Henderson and Bateson showed that the halos are composed of several rings corresponding to the energy (range) of the different alpha particles in the decay chains of uranium and thorium.² Scintillation, the luminescence resulting from the interaction of particles with matter, was also discovered in the early 20th century, when W. Crookes exposed ZnS to a radium source,³ but attributed this to electron radiation not alpha radiation.

With the discovery of nuclear fission and its use for energy production, the study and understanding of the effects of radiation on materials got a strong engineering dimension. In nuclear reactors, the radiation sources are multiple (particles such as neutrons and alpha nuclei, gamma rays, and heavy ions) and intense, which means that all reactor components are heavily exposed: the fuel, the cladding, the control rods, the moderator, the core internals, and the reactor vessel. Like for the halos in minerals, atomic displacements are a key result of the interaction of particles with engineered materials, leading to changes in the structural ordering and the concomitant degradation of the material properties. A dramatic consequence of the accumulation of atomic displacements was evidenced by the fire in the Windscale Reactor in the UK in 1957, which was caused by the sudden release of the potential energy stored in the neutron-induced defects in the graphite moderator of the reactor.⁴ The effect of intense electromagnetic radiation, which does not result in atomic displacements, also cannot be neglected when evaluating materials degradation. This is best exemplified by the radiolytic decomposition (breaking of the chemical bonds) of the used fluoride fuel of the Molten Salt Reactor Experiment (MSRE) at Oak Ridge National Laboratory during the spent fuel storage after experiment termination.⁵ 

The effect of radiation on the material behavior has proved to be critical for the performance assessment of reactor materials and its understanding assists the engineering of materials with improved properties. For nuclear fission systems, the material performance in radiation fields is determined by the combination of physical, mechanical, and chemical processes that take place in materials and at the interfaces of materials, depending on temperature, pressure, and chemical potentials. Some striking examples for the current generation of light water reactors (LWR) are as follows:

• accumulation of radiation induced defects and fission products in the colder rim of the LWR fuel pellets, causing a substantial change of the microstructure, from the original fine grain microstructure to a nanocrystalline structure with large pores filled with the fission gases;⁶ 

• the exposure of the zirconium-based cladding of LWR fuel to neutron radiation, leading to changes in the microstructure by the formation of voids, dislocation loops and precipitates in the material,⁷ whereas radiolysis of the cooling water causes oxidative conditions that cause the formation of a zirconium oxide layer at the outside of the cladding tube and precipitation of zirconium hydride;

• the neutron irradiation of the reactor vessel causing embrittlement as a result of atomic displacements in the steel, diffusion of defects, and the creation of extended defects and precipitates that cause a change in the fracture toughness.⁸ 

During the approximately 75 years of commercial power reactor operation, the knowledge of the relevant material degradation processes has continually improved, originally through advances in experiments and post-irradiation examinations, but in recent years also through the rapid progress in modeling and simulation. As a result, the scientific scale of atomistic processes and the macroscopic engineering scale have come closer together. Experimental techniques such as transmission electron microscopy (TEM) and atom probe tomography (APT) can now reveal changes in the material structure at micro-meter and lower scales, whereas computational techniques such as molecular dynamic (MD) and kinetic Monte Carlo (KMC) are able to simulate systems with millions of atoms in time scales of picoseconds to seconds.⁹ These length and time scales are typical for the interaction of radiation with condensed matter, and therefore the understanding of the critical mechanisms affecting material properties and behavior has reached not only a high descriptive level but also a predictive capacity. This understanding is needed to improve safety and economics of the plant operation, for example, by using nuclear fuel to higher burn-up, the long-term operation of nuclear reactors, development of so-called accident-tolerant fuel, or even the design of small modular reactors (SMR).

It is obvious that radiation effects also play a role in the waste from nuclear fuel cycles. The fission products and actinides, the latter formed by activation of the fuel, can have half-lives ranging from minutes to centuries and thus will continue to decay and lead to radiation exposure of the materials and the environment in which they are contained. This is especially true for the actinides such as plutonium, americium, and curium, and other long-lived radioactive elements found in used fuel or vitrified high-level waste that will be stored in intermediate or final nuclear waste repositories for centuries. There, the waste forms, packaging and engineered barriers will have to maintain their integrity over long times, during which materials will experience degradation due to a combination of radiation and chemical corrosion.¹⁰ Also, the performance assessment of these waste management concepts need a strong scientific basis, from atomistic to macroscopic engineering scale.

The future generations of nuclear fission systems (generally called Generation IV) pose even higher demands on the materials as they operate in a faster (more energetic) and more intense neutron radiation field, at higher temperatures, and in more corrosive environments.¹¹ Moreover, for these concepts newly engineered materials are foreseen (e.g., oxide-dispersed steels or multi-principal component alloys), whose radiation resistance is not sufficiently known. Typical examples are fast reactors with liquid metals (sodium, lead) as coolant, molten salt reactors with liquid fluoride or chloride salts as fuel, and/or coolant or rector cooled by supercritical water. These types of reactors are currently the focus of research and development worldwide. This has resulted in a multitude of start-up companies that aim to push the concepts to technology readiness in shorter periods than are historical. The rapid development of these Generation IV concepts is reliant on materials performance simulations to condense the extent of in-reactor experimental material testing to a limited number of critical experiments.

Similarly, fundamental understanding of fusion environments impact on materials is important to enhance our understanding of solar wind through the development of fusion energy systems. The conditions of plasma, temperature, and radiation field in fusion reactor concepts are extreme. Finding structural and functional materials that can withstand these environments currently limits the performance fusion reactor designs. For example, the plasma facing components of the International Thermonuclear Experimental Reactor (ITER) must be able to withstand temperatures in the several thousand K range, a high flux of low-energy alpha particles, and high-energy neutron (14.1 MeV). The coupled and complex degradation processes make experimental research efforts to deconvolute the governing factors and underlying materials degradation mechanisms significantly more challenging and as such informed computational approaches are essential in the fusion materials science.¹² 

Although traditional fission and fusion reactor core materials make up a substantial fraction of the papers in this collection, it is interesting to notice the attention for radiation effects in other classes of materials and applications, for which also other radiation sources such as charged particles are relevant:

• electronic devices (semiconductors, junctions, quantum dots, quantum phonon circuits), particularly for applications in hostile environments, such as particle accelerators, satellites and spacecrafts;

• photovoltaic cells that are exposed to radiation and cold temperatures in space, where exposure to electrons (ionization) and protons (atomic displacements) plays a much stronger role than for terrestrial applications, and repair mechanisms are slow;

• polymers that are employed in nuclear environments, and whose radiation resistance (degradation) are an important aspect of the lifetime extension of nuclear power plants, which may reach 60 or 80 years;

• radiation detectors to measure radiation exposure. As noted above, the principle of scintillators is based on the interaction between radiation and matter, and the increasing uses of radiation in our society stimulate the search for better performing materials.

Finally, several papers report fundamental aspects of the effects of radiation on materials, such as stability and properties of nanoparticles or the changes in magnetic properties, and help improve the understanding of the damage mechanisms.

Overall, this Special Topic shows that the research field continues to have a high significance in nuclear science. The mix of experimental and theoretical studies and the nature of the reported work reflects the current trend of a multi-scale, multi-physics, multi-disciplinary approach to the engineering of materials with the goal improving their properties and functionalities to enable technologies that answer the societal challenges with respect to energy and climate change.

The authors have no conflicts to disclose.