When accelerated neutrons or ions bombard a material, the surface layer undergoes dramatic physical and chemical transformations. The National Center for Nuclear Research in Swierk, Poland, has managed to learn in detail about the processes that occur in polymers in such situations. The knowledge gathered was used by physicists to develop a method to produce super seals. They also proposed a simple and fast way to detect dangerous cables whose polymer insulation is beginning to lose its insulating properties.
The safe and reliable operation of nuclear reactors, and in the future also of fusion reactors, depends strictly on the quality of their cabling. However, research conducted by the National Center for Nuclear Research (NCBJ) in Świerk, Poland, shows that polymer cable insulations, which have been exposed to high doses of radiation for decades, gradually lose their insulating properties. A group of physicists from NCBJ, led by Prof. Jacek Jagielski, not only learned the details of this process, but also proposed an easy-to-use technique to detect dangerous cables.
Prof. Jagielski’s team has been working on problems related to the modification of the surface layer of materials with ion beams for a long time. A few years ago, the attention of researchers was drawn to polymer seals. One of the Polish companies was in trouble at that time because of the seals in the manufactured military equipment. The machines were exported to a country with a tropical and rainy climate. The seals in engines, which worked without major problems in Poland, began to overheat and leak under the new conditions. Meanwhile, the contract forced the manufacturer to replace the entire failed drive unit because of one gasket.
Overheating of gaskets used in moving mechanisms is a result of the high coefficient of friction of the polymers from which they are generally made. Prof. Jagielski’s group decided to investigate whether ion irradiation affects the coefficient of friction. It was found that a thin surface layer of polymer, about a micrometer thick, hardens significantly as a result of bombardment. The coefficient of friction even decreased ten times – and this despite the fact that it was quickly covered with a network of cracks.
In practice, a significant reduction in the value of the coefficient of friction of polymer gaskets means a reduction in the wear of mechanical elements. Mechanisms equipped with modified gaskets will therefore not only run longer, but also more efficiently, especially since the surface cracks can be used as reservoirs of lubricant. In some cases, such as pneumatic drives, the machine can run faster, leading to an increase in productivity.
“During the research on gaskets, we noticed that the electrical properties of polymers started to change due to radiation defects,” says Prof. Jagielski. “So it seemed logical to ask another question: what happens to the insulation of cables that are exposed to radiation, if their insulation is also made of polymers?”
It may seem like a niche issue, but it takes on a new meaning at a time when the efficiency and safety of nuclear energy is beginning to be valued again. Modern nuclear power plants are designed for a minimum of 60 years of operation, increasingly with the possibility of extending this to 100. At the same time, each reactor must be equipped with several thousand kilometers of smaller and larger cables. Some of them will be exposed for decades to neutron bombardments released in nuclear reactions. In this situation, the question of the fate of the polymers that guarantee their insulation becomes a question of the energy security of millions of people.
In nuclear reactors, materials are exposed to neutrons and gamma rays. However, the vast majority of defects in the irradiated material are not caused directly by neutrons or photons, but by the atoms that knock them out or the broken atomic bonds. In practice, material defects caused by neutrons do not differ substantially from those caused by ions. Instead of doing cumbersome research in the reactor, the NCBJ team could use its own prototype industrial ion implanter.
Insulating materials such as polyvinyl chloride (PVC), Teflon (PTFE) and various types of rubber (natural, EPDM, NBR, SBR) were irradiated. Researchers were interested in the chemical composition of the modified surface layer, the physical structure and the topography of the surface. The results of the measurements and their conclusions have just been presented in an extensive article published in the Journal of Applied Physics.
“Polymers are mainly composed of carbon and hydrogen,” explains Anna Kosińska, the first author of the above-mentioned paper, Ph.D. pupil. “The bonds between these elements are among the weakest and are broken during fast ion bombardment. The released hydrogen atom captures its colleague from the environment and escapes in molecular form from the surrounding material. What remains is amorphous carbon similar to adamantite, that can conduct electricity. All this together means that the polymer insulation of cables exposed to radiation will lose their insulating properties over time.”
The attention of NCBJ physicists was drawn to the fact that the surface layer of the polymer begins to shrink due to the release of hydrogen. As a result, it becomes denser than the original and up to ten times harder than the original. Close research has shown that there is a clear correlation between the changes in the mechanical properties of the cable insulation and its electrical resistance. To know whether the insulation is working properly, it is sufficient to measure the hardness of the cable insulation with a hand hardness tester.
“We realize that the method we propose to detect changes in the electrical resistance of polymer insulations is not perfectly accurate. However, it has very important functional advantages: it is simple, fast and allows you to determine almost immediately whether the tested cable becomes dangerous,” says Prof. Jagielski.
Researchers discover mechanism behind influence of radiation defects on tritium permeation barrier
Anna Kosińska et al, Structural and chemical changes in He+ bombarded polymers and related performance properties, Journal of Applied Physics (2022). DOI: 10.1063/5.0099137
Provided by National Center for Nuclear Research
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