Reactions during melting of low-activity waste glasses and their effects on the retention of rhenium as a surrogate for technetium-99

Abstract

a b s t r a c tVolatile loss of radioactive technetium-99 ( 99 Tc) to off-gas is a major challenge when vitrifying low-activity waste (LAW) at the U.S. Department of Energy's Hanford Site in Washington State. We investigated the partitioning and incorporation of rhenium (Re) (a nonradioactive surrogate for 99 Tc) into the glass melt during crucible melting of two simulated LAW feeds that have exhibited a large difference in 99m Tc/Re retention in glass from small-scale melter tests. Each feed was prepared from a simulated liquid LAW and additives (boric acid, silica sand, etc.). The as-mixed slurry feeds were dried at 105°C and heated to 600-1100°C at 5 K/min. The dried feeds and heat-treated samples were leached with deionized water for 10 min at room temperature followed by 24-h leaching at 80°C. Chemical compositions of the resulting solutions and insoluble solids were analyzed. Volume expansion measurements and X-ray diffraction (XRD) analyses were performed on dried feeds and heat-treated samples to characterize the progress of feed-to-glass conversion reactions. We found that incorporation of Re into the glass melt was virtually completed during the major feed-to-glass conversion reactions that occurred at ≤700°C. The results of our study suggest that the different compositions of the salt phases formed during early stages of melting at ≤700°C are responsible for the large difference in Re incorporation into the glass melt in these two feeds.

A crucible salt saturation method for determining sulfur solubility in glass melt

“…The chemical form of the extracted boron in the salt phase is unknown; it could be borate ions such as , , etc. was more likely based on the charge balance calculation similar to Jin et al; if is used instead, the mole ratio of total positive charge to negative charge would increase slightly to 1.04‐1.10. Uncertainties associated with the chemical analyses are estimated based on duplicate tests.…”

A Method for Determining Bulk Density, Material Density, and Porosity of Melter Feed During Nuclear Waste Vitrification

“…The simple inexpensive volume expulsion test is a handy technique that can minimize the effort needed for execution of melter experiments. Experiments performed with pellets from feeds other than A0, a high‐alumina feed chosen for this work because of the availability of data needed for density and porosity evaluation, show that volume changes in response to heating can widely differ depending on the gas evolution rate and melt formation rate as functions of temperature . This diversity of responses may challenge the formulation of ρ b ( T ) and ϕ( T ) approximation functions that are essential for cold cap modeling.…”

“…Experiments performed with pellets from feeds other than A0, a high-alumina feed chosen for this work because of the availability of data needed for density and porosity evaluation, show that volume changes in response to heating can widely differ depending on the gas evolution rate and melt formation rate as functions of temperature. [30][31][32][33] This diversity of responses may challenge the formulation of q b (T) and /(T) approximation functions that are essential for cold cap modeling. These functions will also be useful for quantifying the effects of feed makeup variations, such as quartz particle size or the chemical and mineralogical form of the feed additives, on the feed foaming extent and, hence, on the impact of feed formulation on cold cap behavior, including the rate of melting.…”

Rheology of simulated radioactive waste slurry and cold cap during vitrification

“…To determine the rheological properties of the slurry feed, water was slowly evaporated from the initial slurry while aliquots of ~5 mL and ~40 mL were sampled at regular intervals. The 5 mL aliquots were used to measure the water content ( w p ) by determining weight loss in a 24‐hour period at 105°C . The 40 mL aliquot was used to perform rheometry in an Anton Paar M301 rheometer at 80°C.…”