Pyroprocessing is one of the options for the effective treatment and recycling of spent nuclear fuel (SNF) owing to its economic advantage, environmental safety, and proliferation resistance of the nuclear fuel cycle. [1,2] The head-end process of the pyroprocessing under development at KAERI includes disassembly and oxidative decladding steps, where cladding hull wastes are generated from the fuel rods. The hull wastes are presumably categorized into high-level wastes (HLW) due to traces of SNF residue and fission products that are implemented into the inner surface of the hulls. Thus, the reduction of the amount of HLW will be one of the key issues for the waste management of the pyroprocess.This study demonstrates the electrorefining process, which is able to recover Zr, which is a major component of the hull wastes such as Zircaloy-4 or Zirlo. Electrochemical behaviors of Zr in LiCl-KCl molten salts are examined using cyclic voltammetries and chronoamperometries.An anhydrous LiCl-KCl eutectic (99.99 % purity, Sigma-Aldrich) was used as the electrolyte at 500 °C by adding 4 wt% ZrCl 4 for Zr electrorefining. All the experiments were performed in an Ar-purging glove box, where the oxygen and moisture are controlled below 5 ppm. To examine the electrochemical behavior of Zr, cyclic voltammetries and chronoamperometries were performed using a pure Zr rod as an anode, a tungsten wire cathode, and a Ag/AgCl reference electrode. Subsequently, the Zr anode was replaced with Zircaloy-4 hulls to recover Zr by electrorefining. The Zr deposits were characterized by SEM-EDX, XRD, and ICP-AES. Figure 1 shows cyclic voltammograms of tungsten wire cathodes in LiCl-KCl molten salts at 500 °C. For Zr rod anodes in the absence of ZrCl 4 , the residual current was less than 0.4 mA with no apparent anodic and cathodic reactions associated with salt components within the potential range of -0.2 V to -1.4 V. However, in the presence of 4 wt% ZrCl 4 (red dashed line), several peaks When the Zr rod was replaced with Zircaloy-4 cladding hull samples, additional reduction peaks followed by a shift of the oxidation peaks are revealed owing to the alloying components such as Sn, Fe, and Cr (blue dotted line). To examine the purity of Zr in the recovered Zr deposit, chronoamperometric experiments were performed for 1 hr at a constant potential of -1.15 V, where the reduction of Zr occurs. Assuming that the total amount of Zr ions dissolved from Zircaloy-4 anode was deposited, the number of electrons involved with the electrorefining process can be estimated from the following Faraday equation by measuring the weight difference (Δw) of the Zircaloy-4 hull before and after the deposition.M w is the molecular weight of Zr, and Q is the total charge flown for 1 hr. The calculated n value was found to be 3.99, which reflects the fact that the reduction process may include a two-step reduction of Zr
Zr electrorefining from Zircaloy-4 cladding hulls is demonstrated in LiCl-KCl-ZrCl4 molten salts at 500 oC. Cyclic voltammetric measurement for pure Zr reveals that multiple reduction/oxidation reaction processes are involved due to various valence states of Zr in chloride baths. An insoluble reduction reaction is found to occur at -1.15 V vs. Ag/AgCl, where the both ZrCl and Zr metallic phases are produced. Almost similar voltammetric response is evident for Zircaloy-4 tube anode with additional reduction peaks associated with alloy elements. The purity of Zr recovered at -1.15 V from Zircaloy-4 tube appears to be 99.94 wt% by ICP-AES.
The effect of the Zr oxide layer on the electrochemical dissolution of Zircaloy-4 cladding tubes is examined in LiCl-KCl-ZrCl 4 molten salts at 500 • C. The cyclic voltammetry of Zircaloy-4 cladding tubes oxidized at 400 • C reveals an evident oxidation peak associated with an immediate dissolution of Zr, as also observed in the current transient. In contrast, the Zr oxide layer formed on Zircaloy-4 tubes at 500 • C and 600 • C significantly inhibits both Zr reduction/oxidation processes on the voltammetric scans in a given potential range. However, the current transients at a constant potential of −0.78 V, where the oxidation of Zr is involved, reveal a dissolution process that is strongly suppressed for an initial period of time proportional to the oxidation temperature, followed by a gradual increase in the dissolution current over time. After the dissolution at −0.78 V for 1 hr, metallic Zr is exposed on a Zircaloy-4 tube oxidized at 500 • C, which leads to a recovered current response for Zr reduction/oxidation in the subsequent voltammetric experiments. The XPS results show that the oxide passivation layer is found to be dissolved or removed under high-temperature molten salts.
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