# Leaching Behavior of Cesium, Strontium, Cobalt, and Europium from Immobilized Cement Matrix
## 1 Introduction  
Many facilities that use nuclear and other radioactive materials are expected to shut down in the near future as they reach the ends of their operating lifetimes [1]. For example, in South Korea, the first nuclear power plant, Kori Unit 1, was shut down permanently in June 2017. Radioactive wastes are produced mostly from the operation or decommissioning of nuclear facilities and unexpected leakages from nuclear power plants [2, 3]. These radioactive wastes are classified as High-Level Waste (HLW), Intermediate-Level Waste (ILW), Low-Level Waste (LLW), and Very-Low-Level Waste (VLLW) [4, 5]. Radioactive wastes, such as ILW and LLW, produced from decommissioning processes are disposed in near-surface facilities and landfills to isolate these wastes from the natural ecosystem for sufficient periods of time [6, 7, 8]. To ensure safe disposal, fluidizable radioactive wastes such as concentrated wastes, spent resins, sludge, and dry active wastes must be solidified or immobilized homogeneously to limit the release of radionuclides and maintain the structural integrity of the disposal environment [9]. Cementation is most commonly practiced to solidify and immobilize low- and intermediate-level radioactive wastes [10, 11, 12]. Cement is used for the encapsulation of radioactive wastes due to its chemical, thermal, and radioactive stability; corrosion resistance; and beneficial compatibility with most varieties of waste [13, 14, 15, 16]. Among the numerous types of cement, Portland cement is the most commonly used for the solidification of LLW/ILW and has high retention potential resulting from mechanisms such as sorption, ion exchange, characteristic phase formation, oxy/hydroxyl precipitation, and combinations as cement-waste interactions [7, 17, 18]. The physical and chemical immobilization of radioactive wastes minimizes radionuclide migration and provides sufficient structural stability [17, 18]. The retention capacities of solidified matrixes for the immobilization of radioactive wastes are evaluated using leaching tests. Numerous leaching-test protocols for evaluating the performance of immobilization were used in previous studies, e.g., the American Nuclear Society ANSI/ANS-16.1 [19; 20; 21; 22; 23; 24], the International Atomic Energy Agency static test [18; 25], ASTMC1285 [26; 27], and the Chinese GB7023-86 standard [28; 29]. The leachability index derived from these leaching tests is generally used for evaluating the immobilization performance of solidified decommissioning wastes; the result should be greater than 6 as the threshold value for waste-acceptance criteria [30]. In addition, the effective diffusion coefficients for each radionuclide can be determined by leaching tests. Various radioactive elements, such as cesium (Cs), strontium (Sr), cobalt (Co), and europium (Eu), are contained in decommissioning wastes from nuclear power plants [31; 32; 33]. These radioactive elements should be monitored for their impacts on ecosystems, particularly around nuclear facilities, due to their risk of harmful damage to the human body and long half-lives [34]. Cs is one of the most problematic and hazardous radioactive elements for the environment and human health due to its high radioactivity and long half-life [2; 35]. Sr, which emits harmful radiation over a long period of time and generates heat for a short period of time, widely coexists with Cs in radioactive wastes [36]. The contamination of soils and groundwater with representative fission-product radionuclides such as Cs, Sr, and Co produces long-term radiation, which then spreads to humans through the food chain and represents an environmental hazard to human health [15; 37]. Eu(III) is commonly used as a homolog for trivalent actinides due to its ionic radii, which are almost the same as those of all trivalent actinides and lanthanides, resulting in similar physical and chemical behaviors between Eu(III) and other trivalent actinides and lanthanides [38].