Immobilisation of radioactive waste solutions with ceramic precursor processing

Abstract

Australian Nuclear Science and Technology Organisation (ANSTO) developed a method to immobilize the Intermediate Level Liquid Waste (ILLW) arising from its Mo-99 production process. The immobilisation process involves impregnation of waste solution into ceramic precursor powders, drying, calcining and consolidation (Hot Isostatic Pressing, HIP) to produce final ceramic wasteform. Ceramic precursor powder is produced by spray drying of a sol-gel based colloidal dispersion. These free-flowing, microspherical, 20-80 microns, precursors have porosity of 4050%.

An in-house custom designed and manufactured microwave-heated and mechanically fluidised mixer-drier was used for impregnation of the precursor powder with the simulated waste (Depleted Uranyl Nitrate Hexahydrate, DUNH, and inactive Cs, Sr nitrates as fission products) and drying. During impregnation run evaporation rate of 1 1/h water per kW microwave energy in steady state was achieved by matching the feed rate of DUNH to produce equivalent of 35% IJ02 loading. It was demonstrated that the tuned microwave energy can be delivered to the mixer-drier during the entire impregnation process within very low reflection values. The samples of the waste loaded free-flowing powder were subsequently calcined at 750C under reducing atmosphere for thermal denitration and mineral phase nucleation. Calcined powders were filled into cans. After evacuation and sealing, the cans were isostatically pressed at 1260C. The consolidated ceramic wasteform produced from the DUNH run has been assessed by durability and material characterization tests. Confirmation of each processing step at pilot and/or plant scale, has led to the design and construction of the overall process at full scale (equivalent of processing 8 kg U per batch) in a simulated hot-cell mock-up plant. The constructed plant mainly consists of a MicroWave-heated Mechanical Fluidised Bed (MVWIFB) mixer-drier a fluidised bed calciner, an off-gas unit, material transfer/holding and can filling units. Performance of the overall process by integrating each of the processing steps and material transfer operations are currently being tested with inactive from the point of remote operated plant design perspective. Definitive design of a hot-cell production system has been initiated in parallel to the mock-up plant tests.

This paper describes the results of both full-scale DUNH impregnation run and inactive mock-up plant tests in developing unique Mo-99 waste immobilisation technology. simulants