1.Introduction

Research, mainly published in the United States, has studied the technologies used in solidification processing by melting spent nuclear fuel assembly materials and hulls for metal waste reduction that occurs during the pyro-processing [1-7]. Cladding hulls in spent fuel assemblies from a domestic PWR are composed of a metal alloy such as Zircaloy-4, Zirlo, stainless steel, or Inconel and account for the highest volume and weight fraction in pyro-processing wastes [8-10]. In particular, the contaminated cladding hulls are classified as low- and intermediate-level waste in the Korean domestic standard, but they are classified as GTCC waste in the US standard owing to residual nuclear fuel, fission product permeation, and activation of the cladding hull materials [1]. In other words, decontamination is essential to the development of volume reduction and recycling technology for cladding hulls and for a significant proportion of the waste generated by pyro-processing to be classified as low- or intermediate-level waste. Currently, France has been operating a commercial facility where high-pressure compaction processes are used for volume reduction of the cladding hulls after wet reprocessing. In the United States, a demonstration facility for melting and solidifying stainless steel cladding hulls after pyro-processing has been recently developed and operated [11]. Recently, at KAERI, we have been conducting research on using Zirlo cladding hulls as a host matrix to solidify and handle highly radioactive anode sludge residue that includes noble metal (NM) fission products from electro-refining in pyro-processing. If we produce a Metallic Waste Form (MWF) by adding alloying elements and anode sludge to the cladding hulls, the overall waste volume can be minimized because the cladding hulls can be used as the solidification host matrix for fuel wastes in the MWF. Although stable waste forms can be made with the SS-15Zr alloy developed for steel cladding, this is disadvantageous for domestic cladding hull disposal efficiency because the Zr content is low [2-6]. In addition, the mechanical stability and corrosion resistance of the Zr-8SS alloy is relatively low [12]. As a result of the new composition with Zr-Cr in KAERI, a stable MWF was formed without cracks. The chemical durabilities of representative Zr-Cr alloys are being evaluated as possible waste forms for the cladding hull and anode sludges generated from the pyro-processing.

2.Experimental Procedures

The chemical compositions of the Zr-Cr based alloys used in this study are shown in Table 1. The Zr-22Cr composition has a eutectic point composition and the Zr-27Cr and Zr-17Cr compositions vary the Cr concentration by ±5 wt%, respectively. Three additional alloys were prepared by adding 8 wt% total NM to each of the three Zr-Cr based MWF alloy compositions. For convenience of descrip- tion, the materials are referred to as Z17C, Z22C, Z27C, Z17C8N, Z22C8N, and Z27C8N according to the composition. All the specimens were fabricated by high frequency induction heating in a silica crucible in an Ar atmosphere after a vacuum of 5 × 10^-5 torr was formed. To ensure the uniformity of the specimen, it was held at 1650 to 1750°C for 5 minutes. After the melt was completely formed, it was kept at 1350°C for about 15 minutes, furnace cooling was then applied to reduce the solidification shrinkage cracks and porosity. All the prepared specimens were cut and polished, and XRD, SEM, and EDS analyses were performed for a microstructure and phase analysis. To evaluate the corrosion properties, potentiodynamic (PD) and potentiostatic (PS) tests were carried out in an acidic brine aqueous solution (0.0001 mol·kg^-1 H₂SO₄ + 0.01 mol·kg^-1 NaCl in demineralized water, adjusted to pH 4) [7]. The solutions from the PS tests were analyzed using ICP-MS.

3.Results and Discussion

To relate the corrosion properties to the microstructure, the phases and microstructure of the six Zr-Cr-based metal alloys were analyzed. As a result of the XRD analyses, which are shown in Fig. 1, the main phases of all the specimens were α-Zr, ZrCr₂, and Zr₂Si, and the secondary phases were CrSi₂, Zr₃Si₂, ZrSi, and Cr and Si single phases. This indicates that the Si from the crucible surface reacted with the melt. The NM elements were mostly dissolved in the main phases. Fig. 2 shows the microstructure of the Z22C specimen by SEM image and EDS mapping, and the results of EDS composition analysis of each phase are shown in Table 2. In the SEM observation, the α-Zr phase of the first region is white, the Zr₂Si phase of the second region is gray, and the ZrCr₂ phase of the third region is black. The α-Zr, Zr₂Si, and ZrCr₂ phases coexist as the eutectic structure of the fourth region (Fig. 2(b)). Fig. 3 shows the microstructures of the Z17C, Z22C, and Z27C specimens with a solidification host matrix composition observed using SEM, and the fraction of each phase in the microstructures is shown in Table 3. As shown in Fig. 3, the Z22C specimen has a large eutectic structure (yellow arrows) with a unit size of about 30 μm. The Z17C and Z27C specimens have a high fraction of α-Zr (white arrow), and a ZrCr₂ (black arrows) phase was formed with a coarse size of over 50 μm. Zr₂Si phases (gray arrows) with various sizes (5 to 20 μm) and shapes were formed in the α-Zr region in all the specimens. Based on the volume fraction analysis shown in Table 3, the Z22C specimen made with the eutectic composition showed the highest eutectic structure fraction of 45 vol%, and the Z17C specimen had the highest fraction of α-Zr owing to the high Zr weight fraction. The Z27C specimen exhibited a relatively high volume fraction with eutectic structure and the highest ZrCr₂ phase volume fraction owing to the high Cr weight fraction.

Fig. 4 shows the EDS mapping results for the Z22C8N specimen. The Zr is uniformly distributed in the whole area, and Cr and Si form intermetallics with Zr. The NM elements were distributed differently: Mo was uniformly distributed in the whole area, Pd and Ru were concentrated in α-Zr, Ru was partially detected in the ZrCr₂ phase, and Re, which is a surrogate for Tc, was mostly concentrated in Zr₂Si.

PD and PS tests were carried out to evaluate the corrosion properties of the specimens and the microstructures were observed before and after the test to identify active phases. Fig. 5 shows the I-V curves obtained from the PD tests with each specimen. The voltage range was set to -0.5 to 1.5 V. The E_corr and I_corr values were determined by Tafel extrapolation from the curves of each specimen, and the currents (I_meas) were measured at 200 mV and 500 mV. Table 4 shows the results of these corrosion tests and results of the tests performed with other alloys using the same test conditions in this study, including pure Zr, Zircaloy-4, Zr-8SS-2NM, and a NM-only alloy (EWF: Epsilon-metal Waste Form), respectively [7,13,14]. Fig. 6 shows a graph comparing E_corr values measured in the PD tests. In the Zr- Cr alloys, the E_corr value increased about 300 to 400 mV due to the addition of NM. The same tendency of E_corr to increase as the NM composition increased has been observed in other research results [13,14]. The Z17C8N-27C8N alloy had a higher E_corr than Zr-8SS-2NM and the Z27C8N alloy had a higher E_corr than EWF. Fig. 7 shows a graph comparing current values measured at E_corr, 200 mV, and 500 mV in the PD test. The I_corr values at E_corr are similar for all the specimens including the comparative materials, but relatively large differences are seen at 200 mV and 500 mV. Pure Zr, Zircaloy-4, and Zr-8SS-2NM materials have I_meas values of about 0.01 A·cm^-2 at 500 mV, which are about 1000 times higher than currents measured for the Zr-Cr alloy and NM alloy. The Zr-Cr-NM alloy specimen had a higher E_corr value and a lower I_corr value than the Zircaloy-4 cladding material or the Zr-SS-NM MWF alloy. This means that, under these conditions, the Zr-Cr-NM alloys have lower corrosion rates than other Zr-based alloys. Fig. 8 shows the microstructure and element distribution of the Zr-22Cr-8NM specimen in the same area before and after the PD test. When the applied voltage was increased to 1.5 V, the α-Zr region containing Mo, Pd, and Ru was largely corroded, but the ZrCr₂ phase containing Mo and Ru and the Zr₂Si phase containing Mo and Re mostly remained. Most of the corrosion was concentrated at the phase interfaces, but corrosion rarely occurred in the eutectic structure, which had a large amount of phase interfaces between various phases.

The PS tests were carried out at 200 mV, 500 mV, and 800 mV for 3 hours in the same electrolyte solution composition, volume and working electrode surface area as the PD test, and the analysis results of the composition of the leachate are shown in Fig 9. In the case of NM elements, leaching occurred only at 800 mV, while leaching of elements forming the host matrix tended to increase with an increase in voltage. Most of the Pd was leached from α-Zr, which was highly corrosive, and the amount of Ru leached mostly from ZrCr₂ was small, while Mo was leached at a medium level. In particular, the Re enriched in Zr₂Si did not leach under any voltage conditions. The Zr₂Si phase showed high corrosion resistance and effectively immobilized Re despite the absence of Cr, which contributed highly to the improvement in corrosion resistance of other phases. The Zr₂Si phase formed from Si released from the crucible during alloy production is a very stable phase. The addition of Si as a trim chemical should be investigated.

4.Conclusion

In this study, we evaluated the use of spent cladding hulls and added Cr as a solidification host matrix for NM waste in the anode sludge generated in the electro-refining process. Three Zr-Cr alloys simulating the solidification host matrix and three Zr-Cr-NM alloys simulating the final metal waste form alloy were prepared using an induction melting method. The microstructure and corrosion characteristics of the different Zr-Cr alloys were compared and analyzed. The main phases composing the Zr-Cr alloy consisted of α-Zr, ZrCr₂, and Zr₂Si. The eutectic structure and ZrCr₂ phase increased with an increasing Cr content. Pd was concentrated in α-Zr; Ru in α-Zr and ZrCr₂; Re in Zr₂Si; and Re in the Zr₂Si phase. Electrochemical corrosion tests were carried out on the Zr-Cr and Zr-Cr-NM alloys prepared in this study, and the results of the PD tests were compared with results for other Zr-based alloys. The Zr-Cr-NM alloys exhibited high corrosion resistances and low corrosion rates compared to conventional materials. As a result of the analysis of the composition of the leachate after the PS test, the NM elements were not leached in tests conducted at potentials under 500 mV, and Re, which is a surrogate Tc, was not leached in the test at 800 mV. In particular, the Zr₂Si phase, which is the encapsulating phase of Re, was very stable and showed no shape change or leaching of Re after the corrosion tests, suggesting that the immobilization of Tc will be excellent. Through this study, we concluded that it is likely that Zr-Cr alloys will be effective waste forms for spent cladding hulls generated by pyro-processing. The corrosion properties appear to be suitable for long-term disposal of high radioactive metallic waste.