Iron phosphate based glasses are currently being developed as candidates for the immobilization of special radioactive wastes which contain both actinide elements and metal chlorides. These glasses are of particular interest due to a combination of useful properties which include relatively high actinide and chloride solubility and excellent chemical durability. Glass compositions are currently being assessed both as a direct solvent for these wastes, and as an encapsulating phase in which waste-containing particles are encapsulated in a compatible glass matrix to provide a solid monolithic wasteform suitable for storage. Whether employed as a solvent or an encapsulating phase, it is important that the glasses remain stable during processing with no reactions or crystallization products being formed that would adversely affect their durability or long term stability. A thorough understanding of the factors affecting thermal stability and crystallization behaviour of these glasses is therefore essential. In the present contribution, a brief summary is given outlining previous work reported in the literature on iron phosphate based glasses and their properties. This is followed by details of our own study on the thermal stability and crystallization behaviour of a sodium aluminium phosphate glass and the effect that iron additions have on the resultant properties. The thermal properties of the glasses have been measured using DSC, and an attempt made to correlate the findings with differences in glass composition, in particular variation in iron content. The implications of these findings in relation to the long term stability and durability of the glasses are highlighted and discussed.
