Tungsten bronze-based nuclear waste form ceramics. Part 2: Conversion of granular microporous tungstate–polyacrylonitrile (PAN) composite adsorbents to leach resistant ceramics

Abstract

Conversion of a granular molybdenum-doped, hexagonal tungsten bronze (MoW–HTB)–polyacrylonitrile (PAN) composite adsorbent to a leach resistant ceramic waste form capable of immobilizing adsorbed Cs + and Sr 2+ has been achieved by heating in air at temperatures in the range 600–1200 °C. Thermal treatment of the Cs- and Sr-loaded composite material at 1000 °C was sufficient to invoke a 60% reduction in volume of the composite while still retaining its spherical morphology. Cs-133 MAS NMR studies of this sample suite at 9.4 T and 14.1 T showed that multiple Cs sites are present throughout the entire thermal treatment range. Scanning electron microscopy investigations of the phase assemblages resulting from thermal treatment demonstrated that the full complement of Cs, and the majority of Sr, partitions into HTB phases (A 0.16–0.3MO 3; A = Cs +, Sr 2+ and Na +; M = Mo, W). The potentially reducing conditions resulting from the removal of the PAN matrix or the presence of high concentrations of Na + relative to either Cs + or Sr 2+ does not retard the formation of the high temperature HTB phases. The fraction of Cs + and Sr 2+ leached from the tungstate phase assemblages was superior or comparable with cesium hollandite (Cs 0.8Ba 0.4Ti 8O 18; f = ≈8 × 10 −5; rate = <1.2 × 10 −4 g/m 2/day) and strontium titanate (SrTiO 3; f = 3.1 × 10 −3; rate = 2.63 × 10 −4 g/m 2/day), respectively, using a modified PCT test in Millipore water at 90 °C. Furthermore, where aggressive leaching conditions were employed (0.1 M HNO 3; 150 °C; 4 days), the tungstate phase assemblages displayed leach resistance almost two orders of magnitude greater than the reference phases.