(205e) Successes and Emerging Issues in Simulating the Processing Behavior of Liquid-Particle Nuclear Waste Slurries at the Savannah River Site

Legacy nuclear waste generated at the Savannah River Site (SRS) during production of enriched uranium and plutonium during the Cold War is currently being processed into a stable borosilicate glass waste form for long term storage. The majority of the waste is stored as a mixture of hydroxide and hydrous oxide insoluble solids in large cylindrical storage tanks at SRS. Over 90% of the waste solids are non-radioactive chemical byproducts derived from fuel rod targets and purification chemistry. The 3,500-4,500 cubic meter (~1,000,000 gallon) carbon steel waste storage tanks also contain 5-7M sodium solutions rich in hydroxide, nitrate, and nitrite anions that are contaminated with soluble radioactive isotopes of cesium and strontium. Insoluble solids have settled to the bottom of the tanks and have been aging for 25-50 years. Waste solids must be mobilized and transferred between tanks as the first step in waste treatment. Subsequent steps separate dissolved radionuclides from insoluble radionuclides for separate waste treatment and downstream processing. A partial summary of processing issues includes: 1) Rheological properties of suspended waste solids degrade with time under shear (the slurry becomes more viscous). 2) Slurry pump flow fields in the large, cooling coil filled, tanks have numerous stagnant zones where radioactive waste mounds form creating closure issues. 3) Settling times during tank washing operations are not predictable. Settling times also do not always mimic those of small scale radioactive settling test samples. 4) Radiolytic hydrogen generation and bubble accumulation during gravity settling constrains washing volumes. 5) Chemical adjustment of the waste solids results in various issues such as excessive foaming, catalytic production of hydrogen from formic acid via noble metal fission products, occasional problems with tackiness or excessive air entrainment, loss of mixing and heat transfer during concentration, etc. SRNL has a program that produces non-radioactive ?simulants? of the wastes in the tank farm. Simulant slurries are used to evaluate the potential feasibility of various proposed operations. Although simulant preparation chemistry generally follows that of the original radioactive wastes, the physical properties (rheology, particle size, cohesiveness, foaming tendency, etc.) often do not adequately match those of the actual wastes. The radioactive and simulant slurries generally behave like pseudo-homogeneous, thixotropic fluids while being mixed or pumped. Typical particles sizes for the precipitated solids are less than 40 micrometers. Significant progress has been made recently in producing rheologically similar simulants. Testing has shown that batch precipitated simulants tend to be more viscous than those produced using continuous stirred tank crystallizers. Shear mixers have been used to increase the apparent viscosity of simulants that were less viscous than their corresponding radioactive target slurries. This can come at the expense of creating a mismatch in particle size distribution. A fairly successful method for predicting the Bingham plastic yield stress and consistency of waste slurry blends was developed. Rheometric shear stress-shear rate data were obtained for two slurries. The Kendall-Monroe rule was used locally on the apparent viscosities (μ_i = shear stress/shear rate of slurry 1 or 2 at a selected shear rate): μ_m^1/3 = x₁μ₁^1/3 + x₂μ₂^1/3. The two x_i were taken as the mass fractions of the two slurries being blended. An example fit of the measured flow curve of a blend of two radioactive waste slurries to that predicted from adaptation of the Kendall-Monroe equation is shown below. Properties of the liquid-particle waste slurry change significantly in the final processing step upstream of the waste melter. Glass frit is added at a mass equivalent to 2-3 times the mass of the waste solids. The frit particles are essentially fine shards of ground up glass with particles in the 80-200 mesh range (0.07-0.20 mm). The impact of these particles on rheology is more comparable to adding 0.1 mm glass beads to water than to adding additional fine particles comparable to the precipitated wastes. Progress has been made on modeling the melter feed rheology as a Bingham plastic fluid (with the fine solids) in two-phase flow with the larger frit particles using a variation of the extended Einstein equation. The frit-free slurry data are converted to an apparent viscosity and used with the volume fraction, φ, of the frit particles to calculate the shift in viscosity from the entrained frit particles: μ_{melter feed} = μ_sludge*(1 + 2.5*f*φ + 10*φ²). The f term is a correction for non-spherical particles (f = 1 for spheres). The primary use for this equation is in predicting the impact of changing the ratio of frit to waste solids on rheological properties without having to repeat series of rheometer measurements on different compositions of melter feed at different total solids loadings. One on-going R&D program is investigating foaminess. Drs. Darsh Wasan and Alex Nikolov at the Illinois Institute of Technology first identified the nature of foam stabilization in SRS nuclear waste slurries. Micron-sized biphyllic particles were preferentially drawn to the vapor-liquid interface and formed ordered layered structures that stabilized the lamella. It has recently been discovered that the simulant can be boiled in its caustic state with no foaming tendencies whatsoever, while the comparable real waste has pronounced foaming tendencies. The current program is attempting to incorporate foaminess into the properties of the caustic waste simulant. Waste slurry is ultimately fed into a cylindrical, joule-heated melter. The water content of the feed impacts melter throughput, but it is often constrained by processing issues such as loss of mixing and heat transfer during the final concentration by boiling. Another on-going program is seeking to identify suitable rheology modifiers that would permit concentrating the melter to feed to a higher weight percent solids. Pre-screening and preliminary experiments are all being conducted with non-radioactive simulants. Ionic and non-ionic surfactants are being investigated.