Tungsten is the leading candidate for the plasma facing component material of fusion reactors. During fusion reactor operation, not only displacement damage but also transmutation elements such as rhenium and osmium are produced by a complex chain of transmutation reactions due to the neutron irradiation. The two grades of pure tungsten, single crystal and polycrystalline were irradiated over 208 days (1.6 dpa) in the HFR, Petten reactor at 900 °C to understand the irradiation response on mechanical properties and microstructure development. The both materials were investigated by transmission electron microscope to assess the microstructural changes after irradiation.
Investigations reveal the radiation induced formation of 4 nm - 14 nm large cavities as well as precipitation of rhenium and osmium around them in the both materials. The formation of rhenium-rich particles with a round and elongated shapes (needles) was detected only in the single crystal material. Rhenium rich layer was detected on the grain boundaries in polycrystalline material. These investigations allow conclusions concerning influence of neutron irradiation at high temperature on microstructure and mechanical properties of pure tungsten.