Chemical compatibility of Eurofer steel with sodium-potassium NaK-78 eutectic alloy

In the high flux area of the IFMIF (International Fusion Materials Irradiation Facility) neutron source, the capsules of the High Flux Test Module (HFTM) contain SSTT Eurofer specimens for fusion-relevant irradiation at temperatures 250–550°C. Using the liquid metal NaK-78 eutectic alloy to fill the gaps among the Eurofer specimens stacked inside the HFTM capsules was introduced in order to improve the thermal conduction among all specimens and have uniform and predictable temperature distribution. Therefore the objective of this study is to investigate the chemical compatibility between Eurofer steel and NaK-78 to evaluate the applicability of this concept. In the present experiment, the SSTT Eurofer specimens were immersed in static NaK-78 inside a capsule made of Eurofer and kept under IFMIF HFTM-relevant conditions including high temperatures (cycling between 480 and 500°C) and duration of 6 months. Following the experiment, mechanical tests (i.e. tensile, fatigue crack growth and Charpy impact) of the Eurofer specimens were performed in addition to surface and microstructure analyses to detect any possible degradation. The results of these mechanical tests revealed that Eurofer did not suffer any significant degradation in its mechanical properties. The surface and microstructure analyses showed minimum chemical interaction at some spots with depth less than 5 µm.