Thermonuclear reactors, where energy is generated by combining hydrogen isotopes, are one of the potential means to overcome the global energy crisis.
Jana Paju. Image Credit: Tallinn University.
Although the temperature on the walls of plasma chambers in thermonuclear reactors ought to be less than 1000 °C under normal conditions, in certain cases, it can reach values higher than 3400 °C — tungsten’s melting temperature.
Jana Paju, a doctoral student from Tallinn University, explored how plasma irradiation damages materials and how the damage propagates. The use of electrical conductivity as an analytical method will help predict the long-term accumulation of damage.
Both destructive and non-destructive techniques were used as part of the research. The aim was to identify the overlaps and shortcomings of these analytical techniques. At the same time, the focus was also on whether and how rare but extreme events can impact the behavior of materials than regular conditions.
The research proved that the synergistic effects of irradiation emerge even in case of mild secondary irradiation. It was the first time the damage in plasma irradiated materials was analysed using electrical conductivity and it enabled to describe the levels of damage that cannot be described using other methods.
Besides melting, erosion of the melted material and vaporization, the transition from cold to hot temperatures and vice versa between irradiation cycles tends to damage the sample surface due to thermal expansion cracking of the surface.
However, the surface damage extension does not always offer sufficient insights about the extension of actual material damage. This is because internal damage caused by mechanical plasma shockwaves can stay hidden. The study demonstrated that though various irradiation cycles cause identical damage on the surface, the physical properties within the material differed greatly.
The most crucial part of the study was the comparison between the analytical techniques employed. The conclusion was that the electrical conductivity analysis, which has never been used on plasma irradiated samples previously in this way, allows the comparison of irradiated samples.
Another conclusion was that there was a decrease in the electrical conductivity within the objects, at a depth where the damage cannot be viewed from the images taken by a scanning electron microscope, and the material’s microhardness also decreased similarly. The insights into this correlation enable the materials to be investigated without any damage to them.