Lanthanum and Neodymium Solubility in Simplified SiO2-B2O3-Na2O-Al2O3-CaO High Level Waste Glass

Lanthanum and neodymium incorporation in simplified high level waste glasses has been investigated for SiO 2 B 2 O 3 Na 2 O Al 2 O 3 CaO compositions quenched from 1200 degrees C, for varying La /( La + Nd ) (atomic) and increasing rare-earth oxides contents. In this system and beyond the solubility limit, rare-earths (RE) elements are reported to form apatite phases with the general formula Ca 2 RE 8( SiO 4)6 O 2. In the current study, speciation of these trivalent RE 3+ cations in both amorphous network and crystal phases was determined from X-ray diffraction, scanning electron microscopy, optical absorption at 10 K, Raman spectroscopy, and electron probe microanalysis. It appeared that RE 2 O 3 solubility was higher for La -rich formulations than for Nd -rich ones and that an increase in the RE oxide content reduces the connectivity of the network building units through formation of non-bridging oxygens at the expense of the oxygen bridges. This depolymerization of the glass network did not affect neodymium environment which consisted in silicate tetrahedra. The composition of the apatite crystals was found to be affected by the La /( La + Nd ) of the parent glass and deviation from the ideal composition ( Ca 2 RE 8( SiO 4)6 O 2) occurred in the neodymium end of the system. It thus appears that both RE 2 O 3 solubility and crystal composition are strongly dependent on the type and crystal chemistry of the RE elements.