Ceramic materials are proposed for the immobilisation of actinide bearing materials, including separated uranium and plutonium oxides. While stocks of uranium and plutonium oxide are not currently considered as a waste within the UK nuclear materials inventory, the possibility of this changing in the future cannot be eliminated. Uranium and plutonium oxides from reprocessing spend nuclear fuel (SNF) have trace quantities of fission products still associated with them, these are a by-product of the irradiation of the fuel within the reactors. Fission products vary depending on the reactor design and irradiation conditions. The aim of this project was to assess the chemical flexibility of the ceramic materials, which adopted either the pyrochlore or brannerite phase. Both phases have been identified within natural minerals to retain radioactive species on time scales, and in conditions, indicative of those required for long term disposal within a geological disposal facility (GDF). Studies include: • The incorporation of uranium within lanthanum zirconate pyrochlore La2Zr2O7 to form materials of the series La2Zr2-xUxO7. Synthesis occurred within both air and hydrogen/nitrogen atmospheres in order to understand the variation in the average uranium oxidation state by XANES analysis. • Analysis of the structural flexibility of the pyrochlore phase through minor variations in composition, characterised by the rA/rB, ionic radius ratio, of the general composition CaLn3+M3+ 0.5Nb1.5O7, where Ln = La, Nd, Sm, Gd, Ho or Yb and M = Al, Fe, Cr, Sc or Y. • Analysis of the chemical composition dependence to radiation induced damage via insitu irradiation studies, at the IVEM-Tandem facility, of materials with target composition CaLn3+Fe0.5Nb1.5O7, where Ln = La, Nd, Sm, Gd, Ho or Yb. • Analysis of the structural flexibility of uranium brannerite with target composition U1- xMxTi2O6 with M= Cr, Fe, Pr, Y or Yb.