Evaluation of Process That Might Lead to Separation of Actinides in Waste Storage Tanks Under Alkaline Conditions

Abstract

This study addresses the physical-chemical processes that might naturally or inadvertently occur and that would lead to a separation of the poisoning nonfissionable actinides (²³²Th, ²³⁸U) from the fissionable ones (²³⁹Pu, ²³⁵U) by selective dissolution and redeposition over a prolonged storage of the waste. Of the various chemistries that were evaluated, carbonate complexation reaction is the most plausible means of achieving the separation of these actinides. Carbonate ions (formed by the dissolution and hydrolysis of atmospheric CO₂) can selectively dissolve the actinide oxides through the formation of soluble carbonate complexes, which could result in the separation of poisoning actinides from the fissionable ones. The concentrations of these soluble carbonate species are dependent on the pH, temperature, and other ions; therefore, changes in any of these parameters over time—especially cyclic changes (daily or seasonal)—could cause a selective dissolution and redeposition of the more soluble species away from the less soluble ones. Detailed calculations using the stability constants for the carbonates have shown that the most likely pH range for this process to occur is pH = 10–11. Increased solubility through reaction with organic complexants such as EDTA was also considered, and while it presents a situation similar to carbonate complexation and similar potential for autoseparation of the actinides in the waste tanks, it would require the uncontrolled dumping of large amounts of complexants into the storage tanks.

Acknowledgments

Notes

† TC as used here for total carbonate concentration should not be confused with a similar notation used by other authors to represent total carbon (which includes organic carbon as well).

†† Where, according to the notation used by Palmer,³ CO₂′(aq) is the total dissolved CO₂, which is the sum of CO₂(aq) and H₂CO₃ but does not include the ionized forms HCO₃ ⁻ and CO₃ ⁼.