## 1 Introduction  
Since its first development in the 1960s in the U.S. as a promising solution to problems regarding accumulated spent fuel, pyroprocessing, which recycles spent fuel as a fuel material and reduces the amount and toxicity of the spent fuel, has also been studied at KAERI (Korea Atomic Energy Research Institute) [1; 2; 3; 4]. Pyroprocessing begins with the head-end process, which recovers fuel oxide from the spent fuel and converts it into the form of a UO\({}_{2}\) porous pellet. As a starting process of the pyroprocessing, the head-end consists of sequential subprocesses: de-cladding, mixing, pelletizing, and sintering [5; 6; 7; 8]. As noted in a previous paper [9], a reduction process has been adopted between the pelletizing and sintering processes to protect metallic crucibles from oxidation attack during sintering. Molybdenum has been used as the metallic crucible material due to its excellent chemical and physical integrity, in addition to its outstanding thermal shock resistance up to high temperature. However, unless reduction heat treatment is preceded, the metallic crucible will be oxidized during sintering. The uranium oxide in the form of UO\({}_{2+\mathrm{x}}\) gives off oxygen accompanied by a reduction to the UO\({}_{2}\) phase during sintering in a reducing atmosphere. The oxygen forms an oxidative atmosphere temporarily in the vicinity of the pellets and thus attacks the metallic crucible, even under the reducing atmosphere [9]. To inhibit this oxidation attack, U\({}_{3}\)O\({}_{8}\) pellets are pre-reduced to UO\({}_{2}\) by high-temperature heat treatment to eliminate the oxygen source before sintering. This allows the use of a commercial metallic sintering crucible without any other treatment. In other words, without an appropriate alternative material for the sintering crucible, reduction heat treatment is inevitably required. However, such heat treatment requires high temperature, \(\sim\)1000 \({}^{\circ}\)C [9], and a long processing time, detrimentally affecting processing economics and safety. Therefore, once a successful alternative material is developed for the crucible, the reduction process should be excluded. Requirements for the alternative material are very strict: it should possess oxidation and reduction resistance, as well as excellent thermal shock resistance up to high temperature. This is why studies for the development of an alternative crucible material should be carried out in parallel to related research. Modifications of existing materials, such as surface coatings, may be promising routes. As a general approach, a coating of a promising material satisfying the requirements for a metallic crucible could be a possibility. However, it would be strictly limited when the scale is enlarged under a severe temperature gradient [10]. In addition, if the shape of the coated material is irregular or complex, the coating method will hardly form a uniform passivation layer over the surface. Unfortunately, the sintering crucible in pyroprocessing presents difficulties in terms of size and shape. To meet processing yield requirements, the size of the crucible reaches about 300 mm in diameter [9], and penetration holes facilitating the flow of atmospheric gas should be made in its bottom surface. In other words, a promising method of surface passivation of the metallic crucible is required to overcome the problems related to size and shape. Previous studies highlighted the formation of an oxidation-resistant Hf\({}_{6}\)Ta\({}_{2}\)O\({}_{17}\) superstructure in Hf-Ta alloy systems as a favorable method for passivation [11; 12]. Basically, the Hf-Ta alloys have sufficiently higher melting temperature, \(\sim\)2000 \({}^{\circ}\)C, than the sintering temperature used in pyroprocessing, \(\sim\)1450 \({}^{\circ}\)C. Moreover, a uniform oxide can be formed because its formation is due to solid/vapor reaction-induced oxidation of the metallic surface. By securing solid/vapor contact, a uniform superstructure layer can be formed even on the inner surface of penetration holes.