Study of interaction of radiations with materials has always been a popular scientific research domain. Broadly, radiations can be classified as ionizing and non-ionizing depending on their ability to ionize materials. The ionization potential of isolated atoms ranges from a few eV for alkali elements to 24.6 eV for noble gases 1. Ionizing radiation can interact with material directly or indirectly and carry enough energy to ionize the medium it passes through. While, non-ionizing radiation cannot ionize material due to its low ionization potential. Figure 1 presents the classification of radiations.
Figure 1: Classification of radiations 2.
The types of radiation that can alter structural materials consist of neutrons, ions, electrons, and gamma rays. 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 inclusion of ions among the irradiating particles provides a tie-in to other fields and disciplines such as the use of accelerators for the transmutation of nuclear waste, or in the creation of new materials by ion implantation, ion beam mixing, plasma assisted ion implantation and ion beam assisted deposition.
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. Irradiation displaces an atom from its site, leaving a vacant site behind (a vacancy) and the displaced atom eventually comes to rest in a location that is between lattice sites, becoming an interstitial atom.
The vacancy-interstitial pair is central to radiation effects in crystalline solids and is known as a Frenkel pair (FP). The presence of the Frenkel pair and other consequences of irradiation damage determine the physical effects, and with the application of stress, the mechanical effects of irradiation by the occurring of interstitial, phenomena, such as swelling, growth, phase transition, segregation, etc., will be affected. In addition to the atomic displacement, an energetic charged particle moving in a lattice also gives energy to electrons in the system, via the electronic stopping power.
This energy transfer can also for high-energy particles produce damage in non-metallic materials, as so-called ion tracks. 34.
Radiation may affect materials in deleterious ways;
? By causing the materials to become radioactive (mainly by neutron activation, or in presence of high-energy gamma radiation by photodisintegration).
? By nuclear transmutation of the elements within the material including, for example, the production of Hydrogen and Helium which can in turn alter the mechanical properties of the materials and cause swelling and embrittlement.
? By radiolysis (breaking chemical bonds) within the material, which can weaken it, cause it to swell, polymerize, promote corrosion, cause belittlements, promote cracking or, Otherwise, change its desirable mechanical, optical, or electronic properties 5.
2. Effects of Radiation on Solid Material
Radiation can have harmful effects on solid materials as it can degrade their properties so that they are no longer mechanically sound. This is of special concern as it can greatly affect their ability to perform in nuclear reactors and is the emphasis of radiation material science, which seeks to mitigate this danger.
As a result of their usage and exposure to radiation, the effects on metals and concrete are particular areas of study. For metals, exposure to radiation can result in radiation hardening which strengthens the material while, Subsequently embrittling it (lowers toughness, allowing brittle fracture to occur). This occurs as a result of knocking atoms out of their lattice sites through both the initial interaction and a resulting cascade of damage, leading to the creation of defects, dislocations (similar to work hardening and precipitation hardening. Grain boundary engineering through thermomechanical processing has been shown to mitigate these effects by changing the fracture mode from intergranular (occurring along grain boundaries) to trans granular. This increases the strength of the material, mitigating the embrittling effect of radiation 6.
Radiation can also lead to segregation and diffusion of atoms within materials, leading to phase segregation and voids as well as enhancing the effects of stress corrosion cracking through changes in both the water chemistry and alloy microstructure 78.
As concrete is used extensively in the construction of nuclear power plants, where it provides structure as well as containing radiation, the effect of radiation on it is also of major interest. During its lifetime, concrete will change properties naturally due to its normal aging process, However nuclear exposure will lead to a loss of mechanical properties due to swelling of the concrete aggregates, and, Thus, damaging the bulk material. For instance, the biological shield of the reactor is frequently composed of Portland cement, where dense aggregates are added to decrease the radiation flux through the shield.
These aggregates can swell and make the shield mechanically unsound. Numerous studies have shown decreases in both compressive and tensile strength as well as elastic modulus of concrete at around a dosage of around 1019 neutrons per square centimeter. 9 These trends were also shown to exist in reinforced concrete, a composite of both concrete and steel. 10.
The knowledge gained from current analyses of materials in fission reactors in regard to the effects of temperature, irradiation dosage, materials compositions, and surface treatments will be helpful in the design of future fission reactors as well as the development of fusion reactors. 11. Solids subject to radiation are constantly being bombarded with high energy particles. The interaction between particles, and atoms in the lattice of the reactor materials causes displacement in the atoms. 12 Over the course of sustained bombardment, some of the atoms do not come to rest at lattice sites, which results in the creation of defects.
These defects cause changes in the microstructure of the material, and ultimately result in a number of radiation effects.
3. Atomic Structural Evolution Under Irradiation
Atomic structural evolution is driven in the material by the accumulation of defects over a sustained radiation. This accumulation is limited by defect recombination, by clustering of defects, and by the annihilation of defects at sinks. Defects must thermally migrate to sinks, and in doing so often recombine, or arrive at sinks to recombine.
In most cases, Drad = DvCv + DiCi >> Dtherm, that is to say, the motion of interstitials and vacancies throughout the lattice structure of a material as a result of radiation often outweighs the thermal diffusion of the same material.
One consequence of flux of vacancies towards sinks is a corresponding flux of atoms away from the sink. If vacancies are not annihilated or recombined before collecting at sinks, they will form voids. At sufficiently high temperature, dependent on the material, these voids can fill with gases from the decomposition of the alloy, leading to swelling in the material 13. This is a tremendous issue for pressure sensitive or constrained materials that are under constant radiation bombardment, like pressurized water reactors. In many cases, the radiation flux is non-stoichiometric, which causes segregation within the alloy. This non-stoichiometric flux can result in significant change in local composition near grain boundaries 14.
where the movement of atoms and dislocations is impeded. When this flux continues, solute enrichment at sinks can result in the precipitation of new phases.
4. Applications and Data Analysis
the promise of advanced reactor designs, and the acceptance of used nuclear fuel disposition options are dependent on the performance of materials in extreme nuclear environments. The effects of irradiation on materials properties and performance are critical to safe and reliable reactor operations, efficient use of uranium resources, reduced production of nuclear waste, and acceptable used nuclear fuel disposition 15.
Irradiation damage in materials for nuclear applications primarily results from the production of energetic particles in fission, nuclear reaction and radioactive decay events. The interaction of these energetic particles (fission products, fast neutrons, protons, alphas, and recoil nuclei) with materials results in the production of atomic-scale defects from ballistic collisions and introduction of new chemical elements. About high-energy fission products, the intense ionization along the fission product path can also introduce defects or damage 16.
Research reactors play a crucial role in such developments for testing and characterization of new fuels and structural materials. In-core facilities are needed to provide realistic conditions for testing new fuels and materials. Neutron beams play a unique role both in the characterization and development of materials.
Neutrons are a bulk probe, penetrating deep into a sample without damaging the structure of the material under examination. Some of the specific applications of neutron beams are 17;
? Measurement of residual strain formed during manufacturing processes and in service.
? Measurement of particle size and distribution, void distribution, helium bubbles, phase, and texture analysis.
? Transmission techniques such as neutron radiography or tomography.
? Study of surfaces, thin films and buried interfaces.
The atomic and electronic structure of polymer films undergoes deep modifications during high energy (KkeVMeV) ion irradiation, from molecular solid to amorphous material. Low energy density (1022-1024 eV/cm3) typical effects include chain scissions, ccrosslinks. Molecularemission and double bonds formation. In hydrocarbon polymer (polystyrene, polyethylene) the main effect of irradiation is the formation of new bonds as detected by molecular weight distribution, solubility and optical measurements.
Horeover, the concentration of trigonal carbon sp2) in the polymer changes with an ion fluence 1011-l014 ions/cm2 and stabilizes to a value of 20% independently on the initial chemical structure of the irradiated sample.
Photoemission spectroscopy shows evolution of valence band states from localized to extended sstates. Athigh energy density (1024-1026 eV/cm3) the irradiated polymer continues to evolve showing spectroscopic characteristics close to those of hydrogenated amorphous carbon.
Trigonal carbon concentration changes with an ion fluence (1014-1016 Ions/cm2) reaching the steady state value of 60% , and the hydrogen concentration decreases to 20%. Horeover, the values of the optical gap (7.5-0.5 eV) suggest the presence of medium range order in the obtained hydrogenated amorphous carbon. These values are consistent with the formation of graphitic clusters, whose size goes from 5 to 20 A0 by changing the ion fluence (or energy density) 18.
5. Scope of Research
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. For these structures to fulfill their purpose, a firm understanding of the effect of radiation on materials is required to account for irradiation effects in design, to mitigate its effect by changing operating conditions, or to serve as a guide for creating new, more radiation-tolerant materials that can better serve their purpose.
6. Future Prospects
One problem is that the components cannot be tested under the same conditions as the ones they will be exposed to when the facility is in operation. The correlations between data obtained under different conditions need to be understood.
For these projects, it is essential to know how long the heavily irradiated components will survive. In addition, improvement of the lifetime of components needs knowledge about the underlying mechanism of radiation damage and its relation to the changes in material properties.
The macroscopic effects on structural materials caused by radiation damage are the following;
? hardening, which leads to a loss of ductility;
? embrittlement, which leads to fast crack propagation;
? growth and swelling, which lead to dimensional changes of components, and can also induce additional mechanical stress;
? increased corrosion rates, in particular in contact with fluids;
? irradiation creep, which leads to deformation of components;
? phase transformations in the material or segregation of alloying elements, which leads to changes in several mechanical and physical properties.
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