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

• Eurofer SSTT specimens were immersed in static NaK-78 for 6 months at 480–500 °C.
• The Eurofer steel did not suffer any degradation in its mechanical properties.
• The chemical interaction between Eurofer specimens and NaK-78 was minimal.
• Using NaK-78, with low oxygen content, is feasible for the IFMIF HFTM capsules.

In the high flux area of the International Fusion Materials Irradiation Facility (IFMIF) neutron source, the capsules of the High Flux Test Module (HFTM) contain SSTT Eurofer specimens for fusion relevant irradiation at temperature up to 550 °C. Using the sodium potassium eutectic alloy NaK-78 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 temperature (cycling between 480 °C and 500 °C) and duration of six months. Following the experiment, mechanical tests (tensile and Charpy impact) of the Eurofer specimens were performed in addition to surface and microstructure analyses to detect any relevant corrosion or degradation. The mechanical tests revealed that the Eurofer specimens did not show any degradation in their mechanical properties. Also, the surface and microstructure analyses showed that the chemical interaction between the Eurofer steel and NaK-78 was minimal after six month of exposure at cyclic temperature between 480 and 500 °C.