Graphite is widely used in nuclear installations. It is used both in existing reactors and in the design of new Generation IV reactors. Despite its high resistance to extreme operating conditions, after some time, defects develop in it under the influence of radiation, which can alter its properties. Studies of the evolution of these defects have recently been carried out by scientists at the NOMATEN Centre of Excellence at NCBJ.

Graphite has a high resistance to radiation damage and is stable at high temperatures, while retaining its mechanical parameters. Thanks to these properties, graphite is used in the current generation of nuclear reactors as a moderator (decelerator) of neutrons. The application in next-generation nuclear reactors places higher demands on the materials, due to the extremely harsh operating conditions (temperatures reaching 1000°C, intense neutron beams, high pressure and a very long plant lifetime - on the order of several decades). They cause structural changes (defects), resulting in changes in functional properties. Understanding how radiation affects graphite, how its structure changes as it is irradiated and what type of defects appear in graphite depending on the type, energy and dose of radiation is crucial, because in the case of high-temperature gas-cooled reactors (HTGRs), graphite will not only act as a moderator, but also as a construction material. Work towards a detailed description of the kinetics of graphite degradation under irradiation has been carried out by researchers at the NCBJ's NOMATEN Centre of Excellence, who compared the defect formation mechanisms of several types of nuclear graphite: commercially available NBG-17 and IG-110 graphites and material from NCBJ resources (backup material for the EWA research reactor in operation from 1958 to 1995).

To simulate the irradiation that a material will be exposed to during years of operation in a nuclear reactor, beams of light or heavy ions can be used. Often noble gases are chosen that will not introduce significant chemical changes in the material, but will induce structural changes similar to those induced by neutrons in an actual reactor. For the graphite samples analysed, the researchers used argon Ar+ and helium He+ ion beams. The use of a helium ion beam was of additional importance in this case - it is with helium that the HTGR reactor core is planned to be cooled. In this way, the study could further illustrate the interaction of graphite with the coolant. The prepared samples of the three graphite types were irradiated with argon and helium ion beams at 400°C. "The elevated temperature is important here to better reflect the conditions to which graphite is exposed in real installations" - describes Magdalena Wilczopolska, M.Sc., first author of the paper. "As the temperature increases, the mobility of carbon atoms in the graphite structure increases, leading to changes in the types of defects and the mechanisms of their formation.’ Samples were bombarded with a stream of ions at energy of 150 keV at different doses (fluence from 10¹² to 2∙10¹⁷ ions per cm²) to simulate the generation of defects in the material over increasingly longer time intervals.

Graphite samples, both before and after irradiation, were subjected to Raman spectroscopy as well as scanning electron microscopy (SEM). Raman spectroscopy involves measuring the inelastic scattering of photons by the material under investigation. Depending on the structure of the sample and the chemical bonds present, substance-specific bands are visible in the resulting spectrum. In the case of irradiated samples, the degree and type of radiation defects can be determined by analysing the ratio of the intensity of the individual bands, their position on the spectrum and their half-width. "In the Raman spectra obtained, with increasing ion dose, we observe an evolution of the so-called D and G bands: a change in intensity ratios, a shift in position and broadening. These changes are visible faster, i.e. at lower fluences, when irradiating with argon ions than with helium ions" says Małgorzata Wilczopolska. "Argon ions are heavier, so they deposit more energy in the material, resulting in more defects in the graphite when bombarded with this beam".

The results obtained by the researchers show noticeable differences in the mechanism of defect formation in the different types of nuclear graphite. At the most intense ion beams, the accumulation of defects led to amorphisation of all samples. In the case of lower fluence beams, the amount and type of defects formed differs significantly between commercially available graphites and the NCBJ-derived material, in which the development of defects is more rapid, most likely due to differences in the manufacturing process. At the same time, NBG-17 material has been shown to retain the greatest structural stability and radiation resistance: especially at lower doses, it exhibits the least amount of defect formation and the slowest progressive loss of crystallinity.

The Raman spectroscopy results were further confirmed by scanning electron microscopy observations of the samples, before and after irradiation. Differences in microstructure were already apparent in the initial samples - commercial graphite was more homogeneous, made up of small flakes or larger, flat surfaces, NCBJ graphite showed a high inhomogeneity. As the ion flux dose increased, the SEM image showed increasing inhomogeneity of all surfaces, as well as the formation of pores. The microscope observations also confirmed the larger scale of the changes in the material induced by argon ions compared to helium ions. Based on all the results, it can be concluded that NBG-17 graphite has the highest stability at low ion fluence and, under these conditions, is the most resistant to radiation of all the materials tested. At higher irradiation levels, it has a similar level of defectivity as IG-110 graphite. The highest susceptibility to radiation damage was detected for NCBJ graphite. The higher quality of currently available commercial graphite demonstrates the significant technological advances and increased safety of material operation driving the development of nuclear technology.

Original article: Magdalena Wilczopolska, Kinga Suchorab, Magdalena Gawęda, Małgorzata Frelek-Kozak, Paweł Ciepielewski, Marcin Brykała, Wojciech Chmurzyński, Iwona Jóźwik, Evolution of radiation-induced damage in nuclear graphite – A comparative structural and microstructural study, Diamond and Related Materials, Volume 146, 2024, 111247, ISSN 0925-9635, https://doi.org/10.1016/j.diamond.2024.111247

Piotr Spinalski