Nuclear Waste Processing: Supported Liquid Membranes

Abstract

Supported liquid membranes (SLM), also called immobilized liquid membranes, have found a number of applications in nuclear industry – from uranium mining to reprocessing of spent nuclear fuel. In supported liquid membrane, two phases, feed and receiving solutions, are separated by porous barrier with organic phase inside the pores. In order to increase the mass transport rate between the phases, the carrier, which forms complex with one of the separated species, is introduced to the organic phase. Such a facilitated transport causes not only fast transport rates but also results in higher selectivity of the process. SLM were studied for radioactive waste treatment and for removal of actinides and fission products from the waste after reprocessing of nuclear fuel. Radionuclides, such as cesium, strontium, cerium, and europium, were separated from radioactive solutions to reduce the radiotoxicity of the waste. Uranium plutonium and americium removal from the wastes produced in the back-end of fuel cycle, as well as the effective methods for actinides partitioning, were broadly described in the literature. Example applications are collected in the Table 1. The main drawback of immobilized liquid membranes is their low stability associated with loss of extractant via volatilization or dissolution into contacting phase (Koltuniewicz and Drioli 2008). The advantages are high selectivity of the method allowing a wide range of applications and easy optimization of the membrane-extractant system. Although the industrial application of this process is far from implementation, ongoing research on new extracting agents for actinides takes place in many laboratories around the world. A solution to the problem of loss of extracting agent may be development of inclusion membranes. Polymer inclusion membranes (PIM) in some cases incorporate base polymer and a suitable extractant, while in other cases a plasticizer may be needed to be added (Bayou et al. 2010). PIM have been successfully developed for extraction of radionuclides from radioactive waste using various extracting agents like TOPO, Cyanex 272, Cyanex 301, Cyanex 302, CMPO, TODGA, D2EHPA, and crown ethers (Kusumocahyo et al. 2004; Kozlowski et al. 2006; Bhattacharyya et al. 2011). Another approach for the extraction of radionuclides with selective organic reagents is application of membrane solvent extraction carried out in membrane contactors (Drioli et al. 2005).