Solubility of Uranium and Plutonium in Alkaline Savannah River Site High Level Waste Solutions

Abstract

Five actual Savannah River Site tank waste samples and three chemically-modified samples were tested to determine solubility limits for uranium and plutonium over a one year time period. Observed final uranium concentrations ranged from 7 mg U/L to 4.5 g U/L. Final plutonium concentrations ranged from 4 µg Pu/L to 12 mg Pu/L. Actinide carbonate complexation is believed to result in the dramatic solubility increases observed for one sample over long time periods. Clarkeite, NaUO 2 (O)OH · H 2 O, was found to be the dominant uranium solid phase in equilibrium with the waste supernate in most cases.

Evaluations of the Fates of Alkali Metals, Actinides, Mercury, and Iodine During DWPF Recycle Diversion

“…Clarkeite was observed to precipitate from a DWPF recycle stream sample containing 0.17 M free OH -(pH 13.2) and 0.5 M Na + following addition of excess U(VI). 20 Plutonium solids formed in this sample by the addition of excess plutonium were amorphous and were expected to be tetravalent Pu(OH)4 or PuO2•H2O. The formation of plutonium solids can be slow, as colloidal plutonium species are believed to form in solution and slowly agglomerate and precipitate.…”

“…Soluble uranium and plutonium from the acidic SMECT and OGCT streams which are exposed to caustic (current target pH: 13) in the RCT are expected to form hexavalent uranyl ion, UO2 2+ , and plutonium species with oxidation states ranging from 4 to 6. 20 If uranyl solubility limits are exceeded under these conditions (and a supersaturated state does not exist), uranium will likely precipitate as clarkeite, NaUO2(O)OH•H2O. Clarkeite was observed to precipitate from a DWPF recycle stream sample containing 0.17 M free OH -(pH 13.2) and 0.5 M Na + following addition of excess U(VI).…”

“…In the absence of significant nitrate ion, nitrite is expected to act as a mild reductant, increasing the proportion of tetravalent plutonium and decreasing the total plutonium solubility. 20 The addition of sodium nitrite to the RCT to minimize corrosion will produce a more reducing environment which should promote greater precipitation of tetravalent plutonium. Uranium is not expected to be reduced under these conditions.…”

“…Actinide solubility tests conducted with other more concentrated waste samples indicated high hydroxide concentrations result in increased actinide solubility. 20 However, literature reviews indicate that the relative amounts of carbonate and hydroxide anions may be the most important waste characteristic determining actinide solubility. 21 Increasing carbonate levels relative to hydroxide can result in significantly higher actinide solubility due to the formation of highly soluble carbonate complexes.…”

“…Additional solubility tests were conducted with a chemically modified sample of the DWPF recycle solution discussed above at a pH near 13. 20 The total sodium concentration of a sub-sample of recycle solution was increased to near 2 M from the original value of 0.5 M Na + by the addition of sodium carbonate. The final carbonate concentration in the chemically modified sample was 0.75 M, versus the free hydroxide concentration of 0.17 M (CO3 2-:OHmole ratio of 4.4 versus the original ratio of 0.24 corresponding to 0.044 M CO3 2-).…”