Radiation is well known to cause an increase of corrosion in the materials it comes into contact with. The increase in the corrosion rate leads to machines and equipment becoming inefficient more quickly, and components needing to be replaced more often. Radiation breaks the chemical bonds in the material, thereby weakening the material. Additionally, radiation can cause the material to swell, leading to cracking and overall changing the properties of the material in a negative manner.
However, recent research into the process of how radiation speeds up corrosion surprised researchers. The results were contrary to the expected outcomes and indicate that an increase in corrosion due to radiation is not constant across all materials. According to the research team of K. Woller, P. Stahle, G. Q. Zheng, Y. Yang, and A. M. Minor, the corrosion effects of radiation differ greatly between molten salt and water-based systems.
The unexpected results were tested numerous times, incorporating a wide variety of conditions. However, the result remained the same; radiation can slow down the rate of corrosion of some materials.
Carrying out the experiments was difficult for the researchers to complete due to the fact that it is impossible to directly measure the extreme temperatures involved. Therefore, the research team needed to measure the temperature indirectly by placing sensors all around the material involved.
While the researchers were actually trying to quantify the rate of corrosion caused by radiation in certain alloys of chromium and nickel, which can be employed as cladding for nuclear fuel facilities, they found this surprising result; radiation slows the rate of corrosion in certain alloys that could be used in fusion or fission reactors. As companies work on creating new, leading-edge reactor designs, including new fusion reactors and molten-salt-cooled fission reactors, this startling development provides impetus to continue the push forward.
The team’s experimental simulations occurred where lithium, molten sodium, and potassium salt work as a coolant for the nuclear fuel rods in fission reactors as well as the vacuum vessel, which surrounds the plasma in possible future fusion reactors. Corrosion occurs rapidly when hot molten salt comes into contact with metal. However, when the metal was a nickel-chromium alloy, the corrosion slowed down, taking up to twice as long as the corrosion of other materials.
To determine the mechanism behind the phenomenon, the researchers used transmission electron microscopy. Close examination revealed that radiation typically creates more dents or defects in the structure of the alloys of nickel and chromium, which provide the atoms with pathways along which they can flow more easily to fill in the defects. The forward loop of radiation corrosion creates a backward loop of self-healing.
Those designing reactors for the future are looking at fusion reactors and molten-salt-cooled fission reactors. The fission process involves splitting a large, unstable nucleus into two smaller nuclei. Molten-salt-cooled fission reactors are a step forward from the light-water reactors commonly used today. These new reactors operate at atmospheric pressure, while light-water reactors work at 75 to 100 times the atmospheric pressure. Benefits of the new technology include the fact that the new structures can be smaller (a significant cost saving), a potential explosion risk is eliminated, and the potential of improved efficiency in generating electricity.
Fusion reactors are not yet in use. Fusion power would be a form of power generation that generates electricity by utilizing the heat from nuclear fusion reactions. The fusion process is the opposite of the fission process and involves combining two lighter atomic nuclei to create a heavier nucleus, releasing energy at the same time.
Scientists are looking at fusion reactors to produce electricity in the future because of the many possible advantages over fission reactors. Fusion reactors create less radioactive material than fission reactors do. In addition, fusion reactors have an almost unlimited supply of fuel. The advantages all lead to improved safety for everyone involved. However, the combination of the time, pressure, and temperature required to make a fusion reactor viable remains a drawback to producing electricity in an economical manner. Scientists continue their efforts to overcome the problems and design a fusion reactor for the future that will overcome the difficulties and take advantage of all the benefits.
In the future, the ability to determine the rate of corrosion caused by radiation in different components will increase the lifespan of reactors as problems can be rectified before they occur and cause damage.
Further information: Proton irradiation-decelerated intergranular corrosion of Ni-Cr alloys in molten salt, Weiyue Zhou, Yang Yang, Guiqiu Zheng, Kevin B. Woller, Peter W. Stahle, Andrew M. Minor & Michael P. Short, https://doi.org/10.1038/s41467-020-17244-y.