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.
Evaporation of High Level and Low Activity (HLW & LAW) radioactive wastes for the purposes of radionuclide separation and volume reduction has been conducted at the Savannah River and Hanford Sites for more than forty years. Additionally, the Savannah River Site (SRS) has used evaporators in preparing HLW for immobilization into a borosilicate glass matrix. The Hanford River Protection Project (RPP) is in the process of building the world's largest radioactive waste treatment facility, Waste Treatment Plant (WTP), which will use evaporators to concentrate the liquid waste and plant recycles prior to immobilization into a borosilicate glass matrix. Radioactive waste is evaporated at each site using various evaporator designs (e.g., forced circulation, horizontal bent tube). While the equipment used to evaporate radioactive waste is relatively simple in design, the complexity in the evaporator processes in current service and in those currently in the design stages stems from the heterogeneous nature of the waste and the effects of seemingly minor components (e.g., Si) on the process.Aqueous electrolyte thermodynamic modeling and experiments have been conducted by the SRS Savannah River Technology Center (SRTC) in support of the SRS HLW and Defense Waste Processing Facility (DWPF) Evaporators and the Hanford RPP WTP. After 40 years of successful operation, accumulation of two solid phases (a nitrated aluminosilicate, Na 8 Al 6 Si 6 O 24 (NO 3 ) 2 •4H 2 O and sodium diuranate, Na 2 U 2 O 7 ) developed as an insoluble phase in the Savannah River Site (SRS) 2H evaporator in 1996. The aluminosilicate scale deposit caused the SRS 2-H evaporator to become completely inoperable by October 1999. Accumulation of the sodium diuranate phase on the aluminosilicate scale has caused criticality concerns. Modeling and experiments were conducted to develop a method to control the process chemistry in order to prevent the formation of aluminosilicate deposits in the future.The lessons learned from the development, design, and operation of the SRS waste treatment facilities and the currently operating 242-A Hanford HLW evaporators were applied by SRTC in support of the development and design of the Hanford WTP evaporators. Thermodynamic equilibrium modeling along with solubility and physical property experiments are being conducted to develop process control and flow sheet models. Additionally, lessons learned from the development of an advanced antifoam agent for the SRS vitrification process evaporators are being applied to the testing and development of an antifoam agent for the Hanford WTP evaporators. This paper will discuss the methodologies, results, and achievements of the SRTC evaporator development program that was conducted in support of the SRS and Hanford WTP evaporator processes. The "crosspollination" and application of waste treatment technologies and methods between the Savannah River and Hanford Sites will be highlighted. The "crosspollination" of technologies and methods is expected to benefit the Departm...
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