Investigation of Plutonium and Uranium Precipitation Behavior with Gadolinium as a Neutron Poison

Abstract

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

Characterization of Uranium in Archived 2H Evaporator Scale

“…3 Clarkeite has also been identified to form during the caustic precipitation of U-rich solutions. 72 In summary, the above information indicates that the existence of clarkeite within caustic solutions or evaporated HLW salt solutions that are high in Na and U is to be anticipated. However, the presence of U-rich silica material in the scale is considerably less expectedmainly because the techniques that have been traditionally used to characterize the scale and other materials in the HLW at SRS are not highly sensitive to amorphous U forms.…”

Neutralization of Plutonium and Enriched Uranium Solutions Containing Gadolinium as a Neutron Poison

“…The concentration of Al is based on a measured value for the previous H-Canyon solution discarded to the HLW system. [2] The HB-Line flowsheet uses a small amount of fluoride to catalyze the dissolution of plutonium oxide which is subsequently complexed with Al to prevent excessive corrosion. No other elements were added to the synthetic solutions since the characterization data indicated no individual concentration exceeded 1/10 of the Pu concentration.…”

“…This value is consistent with the pH measured during the partial neutralization of the 3:1 U:Pu solution tested during previous studies. [2] While stirring, four 1.5 mL aliquots of the precipitate slurry were removed from each beaker and transferred into four 1.5 mL conical centrifuge tubes; the 16 samples were then centrifuged at 5000 g for 5 minutes. A 1 mL aliquot of supernate was removed from each of two centrifuge tubes for Pu, U, Gd, and Al analyses for each neutralization test.…”

“…[1,2] Sufficient Gd precipitated with the Pu and U/Pu mixture and enough water was associated with the solids during transfer accident scenarios such that no nuclear criticality event could occur in those cases. However, the materials currently being dissolved in the HB-Line Phase I Facility will result in an accumulated solution containing an estimated U:Pu ratio of 4.3:1 which is outside the evaluated concentration range.…”

“…Following recent dissolution campaigns, similar solutions were poisoned with Gd, neutralized, and discarded to the SRS HLW system. [1,2] A neutron poison is required to allow the neutralization of solution batches containing greater than a fissile mass of Pu. Gadolinium is very effective in capturing thermal neutrons; therefore, the amount of Gd added to a solution is much less compared to other poisons.…”