Several validation activities were dedicated for the High Flux Test Module (HFTM) of the International Fusion Materials Irradiation Facility (IFMIF) at KIT. The HFTM contains 24 capsules packed densely with Eurofer specimens to facilitate their irradiation in the high flux zone directly behind the IFMIF neutron source. The small gaps among the Eurofer specimens are filled by the sodium-potassium eutectic alloy NaK-78 to improve the thermal conduction among the specimens and achieve uniform and predictable temperature distribution. As a result of first trials, the filling of NaK-78 into the specimen stack had been identified as an issue worth further investigations. Therefore, the wettability of Eurofer (and for comparison 316L steel) specimens by NaK-78 is investigated by two experiments to evaluate the applicability of this concept and identify favorable conditions. In the first experiment, the capillary rise of NaK-78 in a twoparallel-plates channel was investigated by considering the following: (i) temperature of both NaK-78 and the parallel plates from 30 to 350°C, (ii) different machining techniques and surface roughness for the parallel plates, (iii) channel thickness, and (iv) parallel plates material (i.e. ... mehrEurofer and 316L steel). In the second experiment, a full scale prototype of the HFTM capsule packed with Eurofer specimens is filled with NaK-78 at a temperature of 350°C. The NaK-78 flow throughout the capsule and wettability of the specimens were observed by video recording and evaluated. The results of these experiments, which are considered to be relevant for IFMIF and other irradiation operations as well, are summarized and discussed in this paper.