Properties and structure of vitrified iron phosphate nuclear wasteforms

The structure of vitreous iron phosphates containing up to 35 wt% of one or more common nuclear waste components such as Na2O, Cs2O, SrO, UO2 or Bi2O3 have been investigated using Fe-57 Mössbauer, X-ray absorption, X-ray photoelectron and Raman spectroscopies and high energy X-ray and neutron diffraction techniques. The glass samples investigated were prepared by melting appropriate chemicals in air at approximately 1200°C for 2 h in alumina crucibles. The chemical durability of the glasses was investigated by measuring their dissolution rates in distilled water at 90°C using the product consistency test (PCT) methodology. The chemical durability of these glasses is characterized by dissolution rates of 10−10–10−11 g/cm2/min. The Mössbauer spectra indicate the presence of both Fe(II) and Fe(III). However, the maximum Fe(II) fraction, Fe(II)/[Fe(II) + Fe(III)], was 0.34. X-ray absorption data show that, on average, both species of iron ions are coordinated with approximately 5 near neighbor oxygens and that the iron–oxygen coordination is only slightly (<10%) influenced by the type or the concentration of the waste elements. Iron ions are also coordinated with 4–6 phosphorous ions in the second shell. Only a minority of the oxygen ions, about 22%, take part in bridging –P–O–P– bonds. Raman spectra show that the phosphorous–oxygen network, which is dominated by (P2O7)4− dimer units, in different samples are similar.