## 3 Results and Discussion
The regions near the interface between the substrate and the inner layer and that between the inner and outer layers (denoted by \"A\" and \"B\", respectively) are enlarged in Figure 3. In addition, the spatial compositional distribution was revealed by EDS mapping at interface \"A\". At both \"A\" and \"B\", relatively higher Ta content was observed from the irregular shape with bright contrast. On the other hand, the dark phase contained a larger amount of Hf than Ta. Considering the morphology and the compositional distribution together with the previous analysis results [12], the inner layer is regarded as a \"reaction layer\", which is composed of hafnium oxides and \(\beta\)-Ta(Hf) phases, whereas the outer layer is an \"oxide layer\" of Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure. An EDS point scan revealed the local composition of the hafnium oxides to be 28.58 Hf-3.21 Ta-68.21O, that of \(\beta\)-Ta(Hf) to be 19.57 Hf-33.93 Ta-46.50O, and that of the Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure to be 22.85 Hf-7.08 Ta-70.07O. It is noted that the atomic ratio of the oxide layer closely matches the previous observation [12]. In the reaction layer, from the area near the substrate to the oxide layer, the size of the \(\beta\)-Ta(Hf) phase and its connectivity decreased simultaneously. Thus, the Ta content gradually decreased outwards, resulting in the compositional profile shown in Figure 2. However, the chemical composition was similar regardless of the position; i.e., a similar composition was observed at the \"A\" interface to that at the \"B\" interface. All these observations are quite similar to those in a previous study that revealed the same \(\beta\)-Ta(Hf) phase in the reaction layer by XRD [12]. Figure 2: SEM and EDS results on the reaction layer of the HT-1 sample after oxidation. The dashed lines indicate the interface between the substrate and surface layer formed during oxidation. Figure 4 shows the thermal profile of the sintering process in the head-end of pyroprocessing [8], where it is speculated that the Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure can be applied as a crucible material. The crucible material should endure both oxidative and reducing atmospheres during repeated thermal cycles. At the initial stage, during ramping, the uranium oxide contained in a crucible starts to be reduced, giving off oxygen gases resulting in the oxidation of metallic crucibles. Reduction of U\({}_{3}\)O\({}_{8}\) in a 4% H\({}_{2}\)-Ar balanced atmosphere is known to start and finish at about 500 \({}^{\circ}\)C and 900 \({}^{\circ}\)C, respectively [9]. During ramping through this temperature range, the crucible material must remain intact against the oxidation attack. The oxygen generated from the reduction of uranium oxide creates a locally oxidative atmosphere despite abundant hydrogen gas in the atmosphere. Although the oxidation attack would initially focus on the surface, it can be widespread, and moreover, the degree of damage would be severe as the processing scale is enlarged [9]. Consequently, during the initial stage of sintering (denoted as \"O\({}_{2}\)(g) emit\" in Figure 4), the crucible material should be oxidation resistant. Soon after, the crucible material is surrounded by a H\({}_{2}\)-containing reducing atmosphere, and it should endure the reducing attack up to 1450 \({}^{\circ}\)C, which is the highest temperature during sintering in the head-end process. From a thermodynamic viewpoint, the oxygen partial pressure (\(P_{O_{2}}\)) increases as the temperature increases; thus, the reduction attack rate would be mitigated, whereas its kinetics can be thermally stimulated. A candidate material for the crucible should thus be reduction resistant in addition to being oxidation resistant. Figure 5 shows SEM micrographs of the HT-1 and HT-3 samples after the thermal cycle shown in Figure 4. From both macroscopic- and microscopic-scale observations, consistent results were obtained.