High-level nuclear waste produced from fuel reprocessing operations at the Savannah River Site (SRS) requires pretreatment to remove Pu. This paper provides a summary of data acquired to measure the performance of MST to remove strontium and actinides from simulated waste solutions. These tests evaluated the influence of ionic strength, temperature, solution composition and the oxidation state of plutonium.
SummaryPlutonium (Pu)-containing solutions currently stored in H-Canyon Tanks 12.1 and 16.3 do not meet acceptance criteria for conversion to a mixed oxide fuel. Therefore, the solutions will be neutralized and discarded to the Savannah River Site (SRS) high level waste (HLW) system. Prior to disposal, the addition of gadolinium nitrate (Gd(NO 3 ) 3 ) as a neutron poison is proposed to allow neutralization of quantities of Pu greater than a minimum critical mass per neutralization batch. This disposition strategy was previously studied and used to discard approximately 100 kg of Pu to the HLW system. However, the current solutions have a distinct difference in composition from that material. These current solutions contain slightly enriched uranium (U), 0.8% 235 U, at concentrations equivalent to a 3:1 ratio with Pu. The caustic precipitation behavior of Pu-U-gadolinium (Gd) mixtures had not been previously investigated.Before implementation, the effect of U on the precipitation would have to be evaluated to ensure that a sufficient quantity of Gd is always present in the precipitate slurry to ensure nuclear safety.A sample of the Tank 12.1 solution was obtained to evaluate the precipitation behavior of Pu-U-Gd mixtures during caustic neutralization. Experiments were also performed using surrogate solutions containing 3 g/L U or 3 g/L U with 1 g/L Pu. In each experiment, Gd was added to the acidic solutions as Gd(NO 3 ) 3 prior to neutralization with 50 wt% sodium hydroxide (NaOH). Samples from the Tank 12.1 solution were neutralized in a step-wise manner to a pH of 4.5 and 7 to measure the Pu/Gd ratio in the solids which formed prior to complete neutralization above pH 14. Subsequent experiments were performed in which samples from Tank 12.1 and surrogate solutions were neutralized to 1.2 and 3.6M excess hydroxide (OH -). Samples of the precipitate slurry and supernate were then analyzed to evaluate the effect of U on the precipitation.During the neutralization experiments, the initial solids formed at pH 4.5 in contrast to the previous studies on solutions in which solids were first observed at pH 3. The formation of solids at the higher pH is consistent with the behavior of U solutions. At pH 4.5, 6% of the Gd was found in the solids. This value is essentially the same as the 5% measured at pH 3 in the previous studies. At pH 7, at least 95% of the Gd, U, and Pu were removed from the solutions. Upon complete neutralization, greater than 99% of these elements were found in the precipitated solids. One week after neutralization, analysis of additional liquid and solid samples revealed no significant changes in composition. X-ray diffraction analysis confirmed the formation of sodium diuranate and gadolinium hydroxide in the solids after standing for one week. Scanning
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