Process Development for Bulk Separation of Trivalent Actinides and Lanthanides from Radioactive High-Level Liquid Waste

Abstract

In-house R&D studies have resulted in the development of processes for the bulk separation of trivalent actinides and lanthanides from radioactive high-level liquid waste. Synthesis of solvents, namely, n-octyl (phenyl)-N,N-di-isobutyl carbamoyl methyl phosphine oxide and diglycolamide-based tetra (2-ethylhexyl) diglycolamide (TEHDGA), at the required purity has been carried out, and a suitable process for their respective use in actual application has been developed. Inactive scale engineering runs comprised of simultaneous extraction and stripping operations were carried out to establish the process on an engineering scale, including reuse of the solvent system. The composition of surrogate high-level waste (HLW) used at engineering-scale studies corresponds to first-cycle raffinate from reprocessing of long-cooled pressurized heavy water reactor fuel with a burnup of 6500 MWd/tonne. Since trivalent lanthanides and actinides exhibits similar extraction behavior at higher acidity, cerium and lanthanum were only used in making surrogate HLW to represent all the trivalent lanthanides and actinides. Indigenously developed mixer-settlers using a passive system of mixing were used for these runs. Over a period of ˜10 h, ˜300 l of surrogate HLW solutions were contacted with solvent. The results of such repeated trials have shown near-total removal of cerium and lanthanum (>99.8% and 97%, respectively) at aqueous-to-organic ratio of 2.5:1 for a TEHDGA system. As the distribution coefficient values for trivalent actinide (²⁴¹Am) are found to be significantly higher than those for trivalent lanthanides for both of the solvent systems under consideration, it can be inferred that separation of trivalent actinides along with lanthanides could be feasible using these solvent systems.