Foaming and Antifoaming in Radioactive Waste Pretreatment and Immobilization

Radioactive waste treatment processes usually involve concentration of radionuclides before waste can be immobilized by storing it in stable solid form. Foaming is observed at various stages of waste processing like sludge chemical processing and melter operations. Hence, the objective of this research was to study the mechanisms that produce foaming during nuclear waste treatment, to identify key parameters which aggravate foaming, and to identify effective ways to eliminate or mitigate foaming. Experimental and theoretical investigations of the surface phenomenon, suspension rheology, and bubble generation and interactions that lead to the formation of foam during waste processing were pursued under this EMSP project. Advanced experimental techniques including a novel capillary force balance in conjunction with the combined differential and common interferometry were developed to characterize particle-particle interactions at the foam lamella surfaces as well as inside the foam lamella. Laboratory tests were conducted using a non-radioactive simulant slurry containing high levels of noble metals and mercury similar to the High-Level Waste. We concluded that foaminess of the simulant sludge was due to the presence of colloidal particles such as aluminum, iron, and manganese. We have established the two major mechanisms of formation and stabilization of foams containing such colloidal particles: (1) structural and depletion forces; and (2) steric stabilization due to the adsorbed particles at the surfaces of the foam lamella. Based on this mechanistic understanding of foam generation and stability, an improved antifoam agent was developed by us, since commercial antifoam agents were found to be ineffective in the aggressive physical and chemical environment present in the sludge processing. The improved antifoamer was subsequently tested in a pilot plant at the Savannah River Site (SRS) and was found to be effective. Also, in the SRTC experiment, the irradiated antifoamer appeared to be as effective as nonirradiated antifoamers. Therefore, the results of this research have led to the successful development, demonstration and deployment of the new antifoam in the Defense Waste Processing Facility chemical processing