## 5 Discussion
### Zr Recovery Yield  
All electrorefining tests showed that the Zr deposit was detached from the cathode into the molten salt electrolyte. For this reason, it was difficult to clarify the Zr recovery yield by measuring the mass of the deposit at the cathode directly. If the salt ingot, containing the dropped Zr deposit at the bottom, was dissolved in water, Zr precipitates could be easily filtered. However, the Gibbs free energy of formation and the equilibrium constant for Equation (5) were \(-565.44\) kJ/mol and \(1.178\times 10^{99}\) at 293 K based on the HSC Chemistry 8.0 calculation. This indicates that Zr metal was highly oxidized in aqueous media. Since the oxidation occurs at the surface of the Zr metal particle, it would be not appropriate to specify that all Zr metal is converted into ZrO\({}_{2}\). Material loss might occur during the dissolution and filtration as well. Therefore, this method is far from a direct filtration of Zr in the inert atmosphere at high temperature, which is much more accurate for estimating Zr recovery yield.  
\[\text{Zr}+2\text{H}_{2}\text{O}=\text{ZrO}_{2}+2\text{H}_{2} \tag{5}\]  
In order to identify the recovery yield indirectly, a passed charge during the electrorefining was utilized to calculate the amount of reducing metal with an assumption that the current efficiency was 100%. The passed charge was 2520 C for 24 h and that was equivalent to 0.5956 g of Zr metal in test 1. In tests 2 and 3, however, 230 and 410 C passed for 24 and 72 h, respectively. These were equivalent to 0.0544 and 0.0969 g. These small charge values were attributed to poor mass transfer during the electrorefining because the cathode basket was a closed structure that did not have any holes. Despite these results, the cathode basket was required to prevent dropped deposits from disturbing electrodes and other components in the electrorefiner, such as in test 1. In the case of tests 2 and 3, which employed the cathode basket, it was shown that only chloride salt and detached deposits were clearly observed inside the basket. Therefore, it is important to investigate methods to enhance the mass transfer rate when a cathode basket is installed to the electrorefiner. For example, a cathode basket can be manufactured to have a perforated structure, and electrolyte mixing systems can be applied to the electrorefiner.  
Assuming that the weight ratio of Nb-94 to Nb did not change due to its long half-life, the radioactivity concentration of Nb-94 was proportional to the weight concentration in the irradiated CANDU pressure tube and the cathode deposit. Based on the ORIGEN-2 code calculation, the weight concentration of Nb and the radioactivity concentration of Nb-94 in the irradiated CANDU pressure tube after 10 years of cooling were shown as 2.364 wt. % and 8.345 \(\times\) 10\({}^{6}\) Bq/g. Therefore, the radioactivity concentrations of Nb-94 in the cathode deposit obtained by electrorefining tests 2 and 3 were estimated as 85 and 448 Bq/g. These low concentrations could be attributed to the low amount of recovered Zr. Meanwhile, the radioactivity concentrations of Nb-94 could not be determined due to the very low Nb content in the deposit from test 1, which had a much higher recovery yield than tests 2 and 3. Further investigations with a cathode basket are required to increase the cathode yield.