Nanofiltration fractionation of radioactive solution components as a method for reducing the volume of wastes intended for permanent disposal

Baromembrane purification methods as part of liquid radioactive media processing complexes are increasingly included in the practice of radioactive waste management. The paper presents the results of a comparative study of the performance of commercially available hyper- and nanofiltration elements when a simulated solution is continuously phosphatized. The study revealed the influence of changes in the feed solution salinity on the permeability, working pressure in the brine chamber of the hyper- and nanofiltration apparatus and the permeate salinity. It is shown that, in a closed loop of liquid radioactive waste, the introduction of polyphosphates to stabilize the truly dissolved forms of multivalent metals on the ULP reverse-osmotic membrane leads, as expected, to a systematic performance degradation, first of all, in the membrane permeability at a fixed pressure in the apparatus. The permeate of the system with a nanofiltration membrane, VNF (Vontron NanoFiltration), contains a sufficiently high salt concentration indicating that sodium salts formed during complexation are removed from the circuit, thereby reducing the solution osmotic pressure which critically affects the yield of the purified solution. Thus, nanofiltration in combination with chelating agents can be an effective tool for fractionating components of radioactive solutions, ensuring the achievement of standard indicators for wastewater and biologically hazardous substances that are subject to permanent disposal.