The United States produces a large amount of cladding material from used nuclear fuel, which could approach 1,000 MT/year in the next 50 years. In nuclear fuel recycling, spent nuclear fuel assemblies are disassembled and the cladding hulls are separated from the U oxide spent fuel. Zr can be recovered from Zr cladding using the chlorination method. Zirconium alloys, including Zircaloy-4, Zircaloy-2 and Zr-Nb alloys are the most common fuel cladding materials in nuclear light water reactors due to their favorable mechanical properties, corrosion resistance, low thermal-neutron capture cross section and criticality. However, the presence of impurities in the recovered ZrCl4 was reported, including Sn, Cr and Fe. Thus, further efforts are necessary to improve the purification process significantly. As part of an effort to gain fundamental understanding of chlorination processes occurring in fuel cladding materials, we have investigated the adsorption of atomic and gaseous chlorine on Zr (0001) surface, using density functional theory (DFT). Our finding shows that chlorination of Zr(0001) is exothermic for dissociative adsorption of a single chlorine molecule, and that further chlorination to 1 ML can also occur exothermically under Cl-rich conditions. Dissolution of Zr compounds has been investigated in this study including three possible exfoliation processes. It is found that exfoliation of the top Cl−Zr−Cl sandwich layers is among the most energetically favorable degradation pathways leading to Zr(0001) surface dissolution. Further investigation to understand how the formation of ZrCl4 occurs experimentally is ongoing.

Ab initio study of the mechanisms and energetics of chlorination on Zr(0001)

255th National Meeting (2018)

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