Nuclear Reactor irradiation damage of materials
In the field of nuclear energy, fuel is typically composed of a mixture of U235 and U238, placed in the core of the reactor. This core is then kept in operation for a certain period of time and produces neutrons. The neutrons produced by the fission reaction will interact with the reactor materials, further irradiating them. As a result of this irradiation, the atomic structure of the materials will be altered and the chemical and physical properties will degrade. These chemical and physical changes, known as irradiation damage, may affect the performance and stability of the reactor in the long term.
Irradiation withstand capability is one of the key factors influencing the performance of advanced nuclear power systems. Research on the irradiation- resistance of materials is therefore of paramount importance. This article provides an overview of the basics of irradiation-induced damage and methods of analysis to characterize it.
Differences exist in the radiation resistance of different materials, but all materials exposed to radiation may encounter a common suite of damage processes. Low- dose irradiation leads to the formation of Point Defects (PDs), which are the most significant contributor to irradiation-induced changes in materials. PDs form when atoms in the crystal structure are displaced from their lattice sites. Various types of PD may be formed depending on the extent of the atomic movement, ranging from simple vacancy-interstitial pairs, to more complex defect structures like dislocation loops, interstitial clusters, and Frenkel pairs.
PDs can lead to molecular changes, namely molecular displacement. Molecular displacement occurs when atoms adjacent to a defect site are displaced, creating a new molecular arrangement. Additionally, PDs may alter the chemical reactivity of materials since they often lead to new chemical species formed due to introduction of mobile interstitials. This can further promote chemical change in the structure.
In addition to PDs, Atomic Swaps resulting from energetic incident radiation may also occur. These are characterized by an exchange of atoms between lattice sites, creating Frenkel defect pairs. Unlike PDs, which occur in the same crystal lattice, these defects occur between two distinct crystals and can result in a variety of secondary damage effects, such as the formation of interstitial atoms and stacking faults.
Another significant source of irradiation- induced damage is Neutron Activation. This occurs when atoms in the material absorb neutrons produced by the fission reaction, causing them to become unstable and form radioactive isotopes. Radioactive isotopes produced in this way can lead to a variety of secondary damage effects, including mechanical stress and localized changes to the material’s properties.
Finally, gamma-ray irradiation can also result in irradiation-induced damage. Gamma-ray irradiation may lead to the formation of helium bubbles and voids in the material. Helium bubbles are formed when the helium produced by the fission reaction is trapped in the crystal lattice, creating a spherical void, while neutron irradiation may also lead to the formation of voids in the material.
Analytical methods to evaluate the impact of irradiation-induced damage in materials are generally divided into three categories, namely:
• Damage Density Determination – Damage density determination seeks to measure the number of PDs formed in a material. This is done using various forms of microscopic or spectroscopic analysis.
• Helium bubble/Void Analysis – Helium bubble or void analysis seeks to measure the formation and growth of voids in the material due to gamma-ray irradiation. This can be done by employing X-ray diffraction or scanning electron microscopy.
• Elemental Analysis – Elemental analysis seeks to measure the elemental profiles of the material with the aim of determining the amount and nature of the radioactive isotopes present in the material. This is typically done through the use of flame spectroscopy.
In conclusion, irradiation-induced damage is a primary factor impacting materials used in nuclear power systems. Research is ongoing to better understand and reduce the impact of this damage and to develop preventative approaches. Analytical methods exist to measure the damage caused by irradiation and help guide materials selection decisions.
