Evaluation of process that might lead to separation of actinides in waste storage tanks under alkaline conditions

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