Crystallization and Fouling Behaviour of Uranium and Sodium Aluminosilicate Based solids in High Level Nuclear Waste Evaporators.
During high level nuclear waste (HLNW) liquor processing, high ionic strength (6 - 8 M) sodium hydroxide solutions containing silicate, aluminate, nitrite and nitrate species and trace (< 300 mg dm-3) radionuclides (e.g. uranium-235, plutonium-238 and caesium-137) are concentrated in evaporators operating in the temperature range 40 - 140 °C. As the liquor becomes more concentrated, sodium aluminosilicate and radionuclide (e.g. uranium) species may exceed their solubility limits and precipitate, fouling the tube evaporator walls. If the fouling process is not effectively controlled or mitigated, critical radionuclide mass accumulation can proceed at an alarming rate, posing serious criticality concern and risk of intractable technological and environmental challenge. In the present work, crystallization studies have been carried out to unravel the mechanism of nucleation and growth and the concomitant fouling behaviour of sodium aluminosilicate and uranium based solids, under conditions reflecting HLNW processing in Westinghouse Savannah River (USA) evaporators, using simulant liquors. It is established that both thermodynamics and kinetic behaviour of the complex solid-liquor phase system play pivotal roles in determining the mechanisms of nucleation, growth, scale type and deposition. The implications of the findings with regards to the crystallographic destination of solution uranium and sodium aluminosilicate species in the scale deposited onto 316 stainless steel substrates over an extended periods of time are presented.