Radiation damage is the effect of ionizing radiation on physical objects including non-living structural materials. It can be either detrimental or beneficial for materials.
Radiobiology is the study of the action of ionizing radiation on living things, including the health effects of radiation in humans. High doses of ionizing radiation can cause damage to living tissue such as radiation burning and harmful mutations such as causing cells to become cancerous, and can lead to health problems such as radiation poisoning.
Causes This radiation may take several forms: Cosmic rays and subsequent energetic particles caused by their collision with the atmosphere and other materials. Radioactive daughter products (radioisotopes) caused by the collision of cosmic rays with the atmosphere and other materials, including living tissues. Energetic particle beams from a particle accelerator. Energetic particles or electro-magnetic radiation (X-rays) released from collisions of such particles with a target, as in an X ray machine or incidentally in the use of a particle accelerator. Particles or various types of rays released by radioactive decay of elements, which may be naturally occurring, created by accelerator collisions, or created in a nuclear reactor. They may be manufactured for therapeutic or industrial use or be released accidentally by nuclear accident, or released intentionally by a dirty bomb, or released into the atmosphere, ground, or ocean incidental to the explosion of a nuclear weapon for warfare or nuclear testing.
Effects on materials and devices Radiation may affect materials and devices in deleterious and beneficial 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. On the other hand, radiolysis can also be used to induce crosslinking of polymers, which can harden them or make them more resistant to watering. By formation of reactive compounds, affecting other materials (e.g. ozone cracking by ozone formed by ionization of air). By ionization, causing electrical breakdown, particularly in semiconductors employed in electronic equipment, with subsequent currents introducing operation errors or even permanently damaging the devices. Devices intended for high radiation environments such as the nuclear industry and extra atmospheric (space) applications may be made radiation hard to resist such effects through design, material selection, and fabrication methods. By introducing dopants or defects by ion implantation to modify their electrical functionality in desired ways To treat cancer by electron, gamma or ion irradiation or via boron neutron capture therapy.
Many of the radiation effects on materials are produced by collision cascades and covered by radiation chemistry.
Effects on metals and concrete
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 as well as 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 transgranular. This increases the strength of the material, mitigating the embrittling effect of radiation. 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.
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 in order 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. These trends were also shown to exist in reinforced concrete, a composite of both concrete and steel.
The knowledge gained from current analyses of materials in fission reactors in regards 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.
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. 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.
Radiation damage event Interaction of an energetic incident particle with a lattice atom Transfer of kinetic energy to the lattice atom, giving birth to a primary displacement atom Displacement of the atom from its lattice site Movement of the atom through the lattice, creating additional displaced atoms Production of displacement cascade (collection of point defects created by primary displacement atom) Termination of displacement atom as an interstitial
Radiation cross section The probability of an interaction between two atoms is dependent on the thermal neutron cross section (measured in barn). Given a macroscopic cross section of (where is the microscopic cross section, and is the density of atoms in the target), and a reaction rate of (where is the beam flux), the probability of interaction becomes . Listed below are the cross sections of common atoms or alloys.
Thermal Neutron Cross Sections (Barn) Magnesium 0.059 Lead 0.17 Zirconium 0.18 Zircaloy-4 0.22 Aluminum 0.23 Iron 2.56 Austenitic Stainless Steel 3.1 Nickel 4.5 Titanium 6.1 Hafnium 104 Boron 750 Cadmium 2520 Gadolinium 48,890
Microstructural evolution under irradiation Microstructural evolution is driven in the material by the accumulation of defects over a period of 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 a 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. 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, 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.
Thermo-mechanical effects of irradiation
Hardening Radiation hardening is the strengthening of the material in question by the introduction of defect clusters, impurity-defect cluster complexes, dislocation loops, dislocation lines, voids, bubbles and precipitates. For pressure vessels, the loss in ductility that occurs as a result of the increase in hardness is a particular concern.
Embrittlement Radiation embrittlement results in a reduction of the energy to fracture, due to a reduction in strain hardening (as hardening is already occurring during irradiation). This is motivated for very similar reasons to those that cause radiation hardening; development of defect clusters, dislocations, voids, and precipitates. Variations in these parameters make the exact amount of embrittlement difficult to predict, but the generalized values for the measurement show predictable consistency.
Creep Thermal creep in irradiated materials is negligible, by comparison to the irradiation creep, which can exceed 10−6sec−1. The mechanism is not enhanced diffusivities, as would be intuitive from the elevated temperature, but rather interaction between the stress and the developing microstructure. Stress induces the nucleation of loops, and causes preferential absorption of interstitials at dislocations, which results in swelling. Swelling, in combination with the embrittlement and hardening, can have disastrous effects on any nuclear material under substantial pressure.
Growth Growth in irradiated materials is caused by Diffusion Anisotropy Difference (DAD). This phenomenon frequently occurs in zirconium, graphite, and magnesium because of natural properties.
Conductivity Thermal and electrical conductivity rely on the transport of energy through the electrons and the lattice of a material. Defects in the lattice and substitution of atoms via transmutation disturb these pathways, leading to a reduction in both types of conduction by radiation damage. The magnitude of reduction depends on the dominant type of conductivity (electronic or Wiedemann–Franz law, phononic) in the material and the details of the radiation damage and is therefore still hard to predict.
Effects on polymers