The recovery of metals from simulated high-level radioactive waste (HLW) in a glass form using the phase separation of borosilicate glass was studied in order to satisfy possible future demands such as the adoption of some new treatments for the waste in the glass or use of the vitrified elements as resources. The simulated HLW glass was separated into SiO 2 -rich and B 2 O 3 -rich phases at 973 and 873 K when the ternary mass ratio of SiO 2 :B 2 O 3 :Na 2 O was adjusted to 68:27:5 by the addition of SiO 2 and B 2 O 3 to the simulated HLW glass. Annealing at the lower temperature of 878 K promoted the distribution of the elements in the B 2 O 3 -rich phase. Approximately 90% of the Ni, Zn, Fe, Nd, Te, Zr, and Mo were distributed in the B 2 O 3 -rich phase, and these elements, except Zr, were almost completely leached into 1 mol dm À3 nitric acid at 363 K. The leaching of Zr was also achieved using 1.5 mol dm À3 sulfuric acid after the nitric acid leaching. The leached fractions of the glass-network components, such as B and Al, were lower than the other elements at approximately 70-80% due to the existence in the SiO 2 -rich phase. An increase in the concentration of CaO in the glass in the range of 2-5 wt% inhibited the distribution of elements into the B 2 O 3 -rich phase. The increase in the concentration of CaO also changed the structure of the B 2 O 3 -rich phase from the continuous unit shape to the discontinuous spherical shape. Consequently, the leaching fraction of every element dramatically decreased. The structure of the B 2 O 3 -rich phase returned to continuous when the concentration of CaO was more than 10 wt%. From this glass, the leaching of 69% Zr was possible using only nitric acid without any sulfuric acid.
The stoichiometry of UO 2 dissolution with a tri-n-butylphosphate (TBP) complex with HNO 3 and H 2 O was experimentally determined in supercritical carbon dioxide (SF-CO 2) at 25 MPa, 323 K to estimate the required amount of the complex for a process design calculation. The molecular ratio of TBP:HNO 3 :H 2 O was determined as 1.0:1.8:0.6 when prepared as an organic phase after vigorous mixing of concentrated HNO 3 and TBP. The HNO 3 consumption was approximately 4 times the amount of extracted uranium. In designing a supercritical fluid extraction process, the following equation can be used to describe the overall dissolution of UO 2 using the TBP complex with HNO 3 and H 2 O in SF-CO 2 : UO 2 þ 4HNO 3 þ 2TBP ! UO 2 ðNO 3 Þ 2Á 2TBP þ 2NO 2 þ 2H 2 O:
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