Rheology Modifiers for Radioactive Waste Slurries

One factor limiting the production rate of radioactive waste immobilization processes is the rheological limitations imposed by the design of remotely maintained slurry process equipment (i.e. pumps, piping). Rheology modifiers (dispersants/flocculants) that could potentially decrease the yield stress and/or plastic viscosity of radioactive waste slurries were tested on simulated waste to determine which provided the largest decrease in yield stress and plastic viscosity. The goals of this study were to: 1) determine if trace levels of chemical additives could be used to reduce the rheological characteristics of radioactive waste slurries, 2) identify potential chemical additives for this work and future testing, 3) test a limited set of chemical additive candidates on simulated radioactive wastes, and 4) develop advanced techniques to visualize the internal slurry structure and particle-particle interaction within the slurry. Radioactive wastes slurries generated from the production of plutonium and tritium during the Cold War are being (and will be) immobilized in a borosilicate glass matrix using joule heated glass melters at various Department of Energy (DOE) facilities located across the United States. The maximum insoluble solids content of the waste slurries is limited by the design-basis rheological properties (e.g. the Bingham plastic yield stress and plastic viscosity) used to design the slurry handling systems. It is possible to modify the equipment used to mix, sample, and transport the waste slurry. However, the design and construction cost for any such modifications is very high due to the constraints (radiation, non-visible remote operation) imposed on the design and operation of radioactive waste processes. The rheology of two slurries with various rheology modifiers was evaluated using a conventional concentric cylinder rheometer (Haake Rheometer RS150). Only one rheology modifier of those tested was found to decrease the apparent viscosity of the waste slurry by any significant amount and several of the modifiers tested produced the opposite effect. Duramax D-3005 was found to decrease the Bingham Plastic yield stress of simulated radioactive waste slurries by approximately 18%. Selected slurries were further analyzed by a laser scanning confocal microscope. This technique allows the slurry to be analyzed in an unaltered condition. The microscope has the ability to make both two-dimensional pictures and three-dimensional representations of the slurry’s internal structure. The microscope allows the user to understand how particles are flocculated or dispersed throughout a concentrated suspension of heterogeneous simulated nuclear waste slurries.