CRYSTALLIZATION FOULING BEHAVIOUR OF URANIUM OXIDE AND ALUMNOSILICATE SCALE IN HIGH LEVEL NUCLEAR WASTE MEDIUM

During high level nuclear waste (HLNW) liquor processing in evaporators operating in the temperature range 30- 140 °C, dissolved silica, alumina, sodium hydroxide, uranium-235 and transuranic species (e.g. plutonium-238) invariably become concentrated. As the liquor evaporation proceeds, the sodium aluminosilicate (SAS) and radionuclides may exceed their solubility limits and co-precipitate, fouling the tubes and walls of the evaporator. If the fouling process is not effectively controlled or mitigated, radionuclide scale accumulation exceeding the critical mass necessary for self-sustaining nuclear fission reaction may proceed at an alarming rate, posing a serious criticality concern. To probe the mechanisms underpinning uranium oxide-sodium aluminosilicate co-crystallization fouling, fundamental studies simulating the process were undertaken. New knowledge and greater understanding gleaned from the present work comprise crystallo-chemical structure characteristics, solubility and the fouling mechanisms involved in the mixed oxides scale deposition. The implications of the findings with regards to uranium-based scale formation in HLNW plants are highlighted