Cesium Release From Tungstate and Titanate Waste Form Materials in Simulated Canister Corrosion Product-Containing Solutions

The leaching of two potential 137Cs waste form ceramics (Cs-containing hexagonal tungsten bronze (HTB) and hollandite) has been investigated in Fe(NO3)3 solutions of increasing concentration at 150°C over a period of 4 days. These ceramics contain within their structures reduced Mo5+/W5+ and Ti3+ species for the HTB and hollandite, respectively, which therefore might render them susceptible to oxidation-induced leaching. Elucidation of the extent and the mechanism of leaching of the Cs from these ceramics in the REDOX active iron nitrate medium has been investigated. Cesium (Cs) leached severely from both the Cs-loaded HTB and hollandite materials in iron nitrate solutions with virtually all of the immobilized Cs being extracted from both waste form materials in a period of 4 days at 150°C. In the case of hollandite, conversion to ilmenite and hematite was observed at low concentrations and was virtually complete in 0.5 mol/L Fe(NO3)3 over 4 days. In the case of the HTB, all of the Cs was extracted presumably by an ion-exchange mechanism because the structure of this oxide remained intact and iron was found in the composition. Iron oxide with a hematite structure was also easily observed in the reacted sample at high solution iron concentrations. It is shown that the leach resistance of the Cs-containing HTB can be improved by substitution of up to 20% Ti for W.