Treatment of high level nuclear waste

A process is claimed for immobilising high level radioactive waste calcine. It comprises: mixing the HLW calcine in a minor proportion with a mixture of oxides. When the mixture is heated and then cooled it crystallises to produce a mineral assemblage containing crystals capable of providing lattice sites in which elements of the HLW are securely bound. The crystals possess structures closely related to crystals belonging to mineral classes which are resistant to leaching and also closely related to at least two of the titanate mineral classes selected from perovskite, zirconolite and hollandite-type mineral classes