Tungsten bronze-based nuclear waste form ceramics. Part 1. Conversion of microporous tungstates to leach resistant ceramics

The effective immobilization of Cs+ and/or Sr2+ sorbed on hexagonal tungsten oxide bronze (HTB) adsorbent materials has been achieved by heating in air at temperatures in the range 500–1000 °C. Crystalline powdered HTB materials formed by heating at 800 °C displayed leach characteristics comparable to Cs-containing hot-pressed hollandites in the pH range from 0 to 12. If the Cs-loaded HTB sorbents were pressed into pellets prior to calcination, ceramic monoliths could be prepared with negligible Cs volatilization losses. Heating to temperatures in excess of 1250 °C under dynamic air flow resulted in the melting of the sorbent to form phase assemblages consisting of millimetre-sized crystals of bronzoid phases. Up to 5 wt% mass loss was observed for small scale samples of melted materials under dynamic air flow. Both the calcined and melted bronzoid waste forms are multiphase ceramics in which Cs+ remains bound within, and appears to stabilize, the hexagonal bronze phase, even after complete melting at 1300 °C. The leachability of Sr from the phases prepared by heating appears to be somewhat worse than that of Cs. Saturation of the HTB adsorbents with lanthanide elements (Nd, La, Ce) gave rise to cubic bronze phases in which we propose that the lanthanides substitute at the tungsten or molybdenum sites rather than the tunnel positions. The lanthanides were rather easily leached from the calcined phases in 0.1 M HNO3 at 150 °C.