Raman and X-ray absorption spectroscopy studies of chromium–phosphorus interactions in high-bismuth high-level waste glasses

High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated using Raman and X-ray absorption spectroscopy (XAS). The novel and practically important occurrence of foaming on cooling of these melts is associated with P and Cr from the HLW. In response, glasses were synthesized where Bi2O3 and P2O5 contents were varied independently. Relationships between P and Cr were found, where as P2O5-content increases, chromate Cr–O stretch Raman modes diminish intensity, while Cr XAS shows that Cr reduces, from 50% Cr6+ + 50% Cr3+ to nearly 20% Cr6+ + 80% Cr3+, explaining the chromate mode behavior. In the most P2O5-rich glass, the chromate Cr–O distance increases by approximately 0.10 Å, which may indicate bonding between CrO4 and PO4 tetrahedra, similar to that in chromo-phosphates. The presence of chromo-phosphate domains in HLW melts can be linked to oxygen generation as a source of the foaming.