Salting Out of Americium-241 during Sorption Using a Solid-Phase Extractant Based on TODGA

Abstract

The Proryv project is developing effective ways of reprocessing irradiated nuclear fuel (SNF) to return long- lived radionuclides to the fuel cycle and close it. One of the challenges of closed fuel cycle development is the reprocessing of highly active nitric acid raffinates from the PUREX process. To do so, americium-241 must be separated from liquid radioactive waste. Technologies for the extraction, sorption, purification, and concentrating of radionuclides are widely used when processing and fractionating liquid radioactive waste. The highest efficiency and selectivity in the extraction of actinoids (III) and lanthanides (III) with rare earth elements (REE) and transplutonium elements (TPE) from nitric acid solutions of spent nuclear material reprocessing are shown by extractants based on N, N, N', N'-tetraoctyldiglycolamide (TODGA). Before using a solid-phase extractant based on TODGA, ions of the substance in the solution must be converted to neutral complexes or other nondissociated compounds. This can be done by adding neutral salts to the solution that reduce the solubility of the elements to be separated, shift the distribution of extraction, and greatly improve the efficiency of extraction. The extracted substance is taken in the form of a new phase: a solid precipitate, or a liquid or a gas phase. (With liquid extraction, there is an increase in the capacity of the extractant for the target component). Adding salts or salting agents to an aqueous phase to improve the ionic strength of a solution raises the coefficients of distribution of extracted substances, which in turn increases the capacity of sorbents. The aim of this work is to study the salting out of americium-241 during sorption using an experimental modified sample of solid-phase extractant based on TODGA and model solutions of liquid radioactive waste with a uranium macrocomponent for different contents of NaNO₃. It is found that the highest coefficients of distribution for the sorption of americium-241 and uranium are obtained in a solution containing 100 gL of NaNO₃, but this effect is much less pronounced for uranium than for americium-241. Studying the sorption kinetics of americium-241 and uranium reveals the salting effect, which is confirmed by the equilibrium concentrations of americium-241 and uranium in the solution at the same time but with different concentrations of NaNO₃. The difference between the equilibrium concentrations for americium-241 is an order of magnitude, in favor of a drop when the concentration of NaNO₃ is raised to 100 gL. The use of this effect allows the maximum capacity for americium-241 to be obtained in a system with uranium macrocomponents.