Spectroscopic and first-principles calculation studies of the chemical forms of palladium ion in nitric acid solution for development of disposal of high-level radioactive nuclear wastes

Shinta Watanabe,Yusuke Inaba,Tomoko Yoshida,Jun Onoe,Masato Nakaya,Kenji Takeshita,Toshikazu Sato,Masahito Yoshino,Takanori Nagasaki

doi:10.1063/1.5025778

Abstract

We have investigated the chemical forms of palladium (Pd) ion in nitric acid solution, using XAFS/UV-vis spectroscopic and first-principles methods in order to develop the disposal of high-level radioactive nuclear liquid wastes (HLLW: radioactive metal ions in 2 M nitric acid solution). The results of theoretical calculations and XAFS/UV-vis spectroscopy indicate that Pd is a divalent ion and forms a square-planar complex structure coordinated with four nitrate ions, [Pd(NO3)4]2-, in nitric acid solution. This complex structure is also thermodynamically predicted to be most stable among complexes [Pd(H2O)x(NO3)4-x]x-2 (x = 0-4). Since the overall feature of UV-vis spectra of the Pd complex was independent of nitric acid concentration in the range 1–6 M, the structure of the Pd complex remains unchanged in this range. Furthermore, we examined the influence of γ-ray radiation on the [Pd(NO3)4]2- complex, using UV-vis spectroscopy, and found that UV-vis spectra seemed not to be changed even after 1.0 MGy irradiation. This implies that the Pd complex structure will be still stable in actual HLLW. These findings obtained above are useful information to develop the vitrification processes for disposal of HLLW.

Highlights

It is a world common issue to safely dispose high-level radioactive nuclear wastes
We have investigated the chemical forms of Pd ion in nitric acid solution by examining the UV-vis absorption and X-ray absorption fine structure (XAFS) spectra of Pd complex in the solution in combination with theoretical calculations based on density-functional theory (DFT),[12,13] time-dependent (TD) DFT,[14] and first-principles many-electron methods.[15,16]
We have investigated the chemical form of Pd ion in nitric acid solution, using XAFS/UV-vis spectroscopy and first-principles calculation methods

Summary

Introduction

It is a world common issue to safely dispose high-level radioactive nuclear wastes. At the present stage, a deep geological treatment of the vitrified objects containing the wastes is a better way to dispose them stably and safely until the clearance radioactive level of the wastes is achieved (it will take an order of 105 years).High-level radioactive nuclear liquid wastes (HLLW: radioactive metal ions in 2 M nitric acid solution) are generated in the reprocessing of spent nuclear fuels or in the decommissioning of nuclear reactors. It is a world common issue to safely dispose high-level radioactive nuclear wastes. A deep geological treatment of the vitrified objects containing the wastes is a better way to dispose them stably and safely until the clearance radioactive level of the wastes is achieved (it will take an order of 105 years). High-level radioactive nuclear liquid wastes (HLLW: radioactive metal ions in 2 M nitric acid solution) are generated in the reprocessing of spent nuclear fuels or in the decommissioning of nuclear reactors. In the process of producing the vitrified objects containing the wastes by using a glass melter, platinum-group metals (PGMs) [especially, ruthenium (Ru), rhodium (Rh), and palladium (Pd)] and molybdenum (Mo) cause serious problems: PGMs tend to be accumulated on the sidewall surface of the melter, whereas Mo compounds, so called yellow phase, are formed in the vitrified object.[1,2] These issues result in (i) degradation of vitrified objects due to heterogenization and (ii) an increase.

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Conclusion