The radioactive isotopes of cesium (i.e. 137Cs (t1/2 = 30 years)) and strontium (i.e. 90Sr (t1/2 = 29 years)) are synthetic radioisotopes that are not found in nature. However, they can be found in spent nuclear fuel (SNF), where they are formed by nuclear fission of uranium-235 during nuclear power plant operation. Valorization or partitioning of these isotopes can be achieved by treating irradiated fuel using solvent extraction techniques or separation on ion exchange columns.
Radionuclides can be recovered from SNF assemblies by selectively extracting them. In general, a specifically designed solvent is used to extract specific ions to an organic phase. In the best case, molecules that have an affinity to only the ions of interest are used. In a second step, the ions are back-extracted (or stripped) to a fresh aqueous phase. A well-known example is the recovery of uranium and plutonium (and neptunium), where liquid-liquid extraction is used in the so-called plutonium uranium reduction extraction (PUREX) process. Here, SNF is dissolved in a concentrated nitric acid solution. Next, tributyl phosphate (TBP) in kerosene is used as a solvent to selectively extract uranium, in the form of UO2(NO3)2(TBP)2. Meanwhile, plutonium is co-extracted as Pu(NO3)4(TBP)2. In the next steps, the organic phase is treated with reducing agents to further separate U from Pu and uranium is then stripped from the kerosene using 0.2 M nitric acid. The aqueous phase, the so-called HAR (highly active raffinate), consists of a mixture of minor actinides (e.g. Np, Am, Cm,…), fission products, corrosion products, protons and nitrate anions. Several solvent extraction systems have been developed earlier to separate the heat-generating fission products, Cs and Sr.
Although the separation of 90Sr and 137Cs from High Level Liquid Wastes (HLLW) is one of the challenging subjects in the management of used nuclear fuel, reports concerning useful solvent extraction, column separation or precipitation methods for these fission products, especially from acidic HLLW which is typical for advanced partitioning scenarios, are rather limited. An overview of the existing processes is given in Figure 1 and the literature is described further in this manuscript.
|Number of pages
|Published - 6 Dec 2021
|SCK CEN Reports