Incorporation of chlorine in nuclear waste glasses using high-pressure vitrification: Solubility, speciation, and local environment of chlorine

Abstract

The solubility, speciation and local atomic environment of chlorine have been determined for aluminoborosilicate glasses quenched from high-pressure (0.5-1.5 GPa) and high-temperature (1350-1400°C) equilibrated with various sources of chlorine (NaCl and PdCl 2 ). The Cl solubility reaches up to 11 mol.% in borosilicate glass and appears to be strongly influenced by the concentration of network modifying cations (Ca and Na) and increases with increasing CaO + Na 2 O content. The Cl solubility is enhanced in Ca-bearing rather than Na-bearing borosilicate glass, suggesting a higher affinity of chlorine for alkaline-earth cations. Cl Kedge XANES and Cl 2p XPS spectra reveal that chlorine dissolves in glasses only as chloride species (Cl -) and no evidence of oxidized species is observed. Using PdCl 2 as a chlorine starting source leads to pre-edge signal for PdCl 2 in the XANES spectra. The EXAFS simulations show that Cllocal environment is charge compensated by Na + or Ca 2+ at a distance to first neighbor on the order of 2.7 Å that is comparable to the observed distances in chlorine crystalline compounds. The coordination to charge compensating cation is lower in the case of Ca 2+ (~1.1) than Na + (~4.3).