## 1 Introduction  
Zr alloy has been extensively used in nuclear fuel cladding, since Zr has corrosion resistance and a low neutron capture cross-section [1, 2]. The CANDU pressure tube is made of Zr-2.5 Nb alloy to endure high temperature, pressure, and radioactivity during its long-term operation. However, Nb-93, the most stable niobium, can be activated into Nb-94, which emits beta radiation with a long half-life of 20,300 years under high neutron flux. In the Republic of Korea (ROK), Nb-94 is one of the most crucial isotopes in the geological disposal of irradiated Zr-Nb alloy. The activity concentration limit of Nb-94 for the Cyengo-ju low- and intermediate-level waste (LILW) disposal facility is much smaller than the radioactivity caused by Nb-94 in several types of irradiated Zr-Nb components [3, 4]. Moreover, Wolsong unit 1, a CANDU-6-type nuclear power plant (NPP) in the ROK, was permanently shut down from 2019. Hence, the irradiated CANDU pressure tube, which can be classified as intermediate-level waste (ILW), should be decontaminated by separating Zr from the alloy to reduce the final radioactive waste volume as well as its waste level. Since Nb-94 is uniformly distributed in the irradiated pressure tube volume, volumetric decontamination is required to separate Zr from Zr-Nb alloy, leaving behind Nb. [PERSON] et al. performed the surface treatment on irradiated Zircaloy cladding by using HF, but it was revealed that the surface decontamination could not eliminate the actinides and fission products that deeply penetrated the cladding [5]. Halogenation methods have been investigated on Zr alloys in the nuclear energy industry since Zr refinement was commercially developed [6; 7; 8; 9; 10]. In terms of Zr-2.5 Nb alloy, however, they can accompany secondary radioactive waste generation, such as chloride gas related to C-14. Besides, corrosion issues may occur in industrial-scale reactor structural materials due to Cl\({}_{2}\) and HCl. For this reason, electrorefining using molten halide electrolytes has been studied for the volumetric treatment of metallic radioactive waste [11; 12; 13; 14; 15; 16; 17; 18; 19]. Previous electrorefining tests in fluorides have shown that it is possible to recover metallic Zr deposits as a dendrite from on the cathode with high purity due to the single-step Zr reduction reaction in fluorides [11; 12; 13]. [PERSON] et al. conducted the electrorefining of Zirlo, containing Nb of 1 wt. % in Zr alloy, in molten LiF-KF-ZrF\({}_{4}\) salt at 973 K, and they achieved an Nb concentration in the cathode deposit of less than 30 ppm [11]. [PERSON] et al. conducted the electrorefining of a BWR channel box in molten LiCl-KCl-ZrCl\({}_{4}\)-ZrF salt at 923 K to separate Co-60 [12]. [PERSON] et al. conducted the electrorefining of Zirlo in fluoride-added chloride salt to recover pure Zr, with a coarse deposit formed at the cathode [13]. Despite this advantage, fluorides have high corrosivity to structural materials, and it is required that a high operating temperature is used due to its melting point, which can potentially lead to scale-up issues. In terms of the chloride process, it has been reported that Zr shows multiple redox steps in chlorides which can affect process throughput [14; 15; 16; 17]. However, experimental and computational studies on Zircaloy-4 electrorefining showed that a high separation efficiency for Zr can be achieved by potentiostatic methods [18; 19; 20]. In addition, chlorides have less corrosivity and lower melting points than fluorides. Herein, in this study, we performed the Zr separation from Zr-2.5 Nb alloy by electrorefining in molten LiCl-KCl-ZrCl\({}_{4}\) salt with a variation in cell design in terms of cathode surface area and presence of the cathode basket. Three electrorefining tests were carried out to investigate the change in the cathode. The chemical form of the cathode deposit mixture was identified by X-ray diffractometry (XRD).