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.
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 defects cause changes in the microstructure of the material, and ultimately result in a number of radiation effects.
The probability of an interaction between two atoms is dependent on the thermal neutron cross section (measured in barn).
Thermal Neutron Cross Sections (Barn)[8] Microstructural evolution is driven in the material by the accumulation of defects over a period of sustained radiation.
This non-stoichiometric flux can result in significant change in local composition near grain boundaries,[10] where the movement of atoms and dislocations is impeded.
This is motivated for very similar reasons to those that cause radiation hardening; development of defect clusters, dislocations, voids, and precipitates.
[12] 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.
[13] Swelling, in combination with the embrittlement and hardening, can have disastrous effects on any nuclear material under substantial pressure.
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.
Instead, polymers deform via the movement and rearrangement of chains, which interact through Van der Waals forces and hydrogen bonding.
In the presence of high energy, such as ionizing radiation, the covalent bonds that connect the polymer chains themselves can overcome their forces of attraction to form a pair of free radicals.
Free radicals can also undergo reactions that graft new functional groups onto the backbone, or laminate two polymer sheets without an adhesive.
For example, dose rate determines how fast free radicals are formed and whether they are able to diffuse through the material to recombine, or participate in chemical reactions.
[18] The ratio of crosslinking to chain scission is also affected by temperature, environment, presence of oxygen versus inert gases, radiation source (changing the penetration depth), and whether the polymer has been dissolved in an aqueous solution.
[19] Crosslinks strengthen the polymer by preventing chain sliding, effectively leading to thermoset behavior.
[14] Polyethylene is well known to experience improved mechanical properties as a result of crosslinking, including increased tensile strength and decreased elongation at break.
[16] Thus, it has “several advantageous applications in areas as diverse as rock bolts for mining, reinforcement of concrete, manufacture of light weight high strength ropes and high performance fabrics.”[14] In contrast, chain scission reactions will weaken the material by decreasing the average molecular weight of the chains, such that tensile and flexural strength decrease and solubility increases.
In addition, “gaseous products, such as CO2, may be trapped in the polymer, and this can lead to subsequent crazing and cracking due to accumulated local stresses.
[15] The resistance of these polymers to radiation damage can be improved by grafting or copolymerizing aromatic groups, which enhance stability and decrease reactivity, and by adding antioxidants and nanomaterials, which act as free radical scavengers.
In some gaseous ionisation detectors, radiation damage to gases plays an important role in the device's ageing, especially in devices exposed for long periods to high intensity radiation, e.g. detectors for the Large Hadron Collider or the Geiger–Müller tube Ionization processes require energy above 10 eV, while splitting covalent bonds in molecules and generating free radicals requires only 3-4 eV.
The electrical discharges initiated by the ionization events by the particles result in plasma populated by large amount of free radicals.
These high molecular weight compounds then precipitate from gaseous phase, forming conductive or non-conductive deposits on the electrodes and insulating surfaces of the detector and distorting its response.
Carbon tetrafluoride can be used as a component of the gas for high-rate detectors; the fluorine radicals produced during the operation however limit the choice of materials for the chambers and electrodes (e.g. gold electrodes are required, as the fluorine radicals attack metals, forming fluorides).
A gas-free water subjected to low-LET gamma rays yields almost no radiolysis products and sustains an equilibrium with their low concentration.
They also act to help prevent diffusion, which restricts the ability of the material to undergo radiation induced segregation.
[27] Finally, by engineering grain boundaries to be as small as possible, dislocation motion can be impeded, which prevents the embrittlement and hardening that result in material failure.