Electrical property and redox state in iron phosphate melts

The electrical resistivity of iron phosphate melts and high level radioactive waste (HLW) melts simulated by iron phosphate was measured at 1223-1723 K. The electrical resistivity cell was set up according to the standard 4 electrodes method. The Mössbauer spectra of the corresponding glass were obtained by ASA600 spectrometer at room temperature. The redox ratio of iron ions was calculated from the absorption envelope area. It is shown that in the iron phosphate melt with low soda content, the temperature dependent electrical resistivity displays the irreversibility during the heating and cooling cycle. In the iron phosphate melts with high soda concentration, the temperature dependent electrical resistivity is found reversible during the first heating and cooling cycle. The irreversibility decreases with the increase of the soda content. The electrical resistivity of the binary iron phosphate melt tends to decrease with time. The electrical resistivity of iron phosphate melts containing simulated HLW is similar to that of the iron phosphate melts with corresponding soda content. The redox ratio increases greatly with the melting temperature and slightly increases with the holding time. The activation energy of these melts is discussed using the Mott theory. It is found that the melts with low soda content are electronic conductors and the irreversibility is correlated to the redox ratio.