A molten salt reactor (MSR) that uses molten salt mixtures as nuclear liquid fuel has recently received much attention due to its inherent safety. Various fluoride and chloride salt mixtures are considered as fluid fuel for MSRs. Among those, NaCl-MgCl2-UCl3 system is the one of the most promising candidates for molten salt fast reactor. The comprehensive information on thermo-physical properties such as density, viscosity, heat capacity and thermal conductivity are fundamental to MSR design development, but experimental data for NaCl-MgCl2-UCl3 system are unknown to the best of our knowledge. In this study, we estimated the thermophysical properties of NaCl-MgCl2-UCl3 system. The properties were calculated by mole fraction additive method using reliable experimental data from pure salt system. Other methods, such as rule of additivity of molar volume for density, modified Dulong-Petit method for heat capacity, and Rao-Turnbull prediction and Ignatieve-Khokolve correlation for thermal conductivity, have also been applied. Estimated values for the properties were compared with each other as well as available binary experimental data.

Spent nuclear fuel is a very complex material because various elements such as fission products, transuranium elements and activation products are produced from initial fresh UO2 fuel after irradiation. These elements exist in UO2 with various forms and can change the structure and of physicochemical properties of UO2. These changes could provide the surface activation site that could enhance chemical reactions and corrosion processes, and would significantly affect the storage environment for long-term disposal of spent nuclear fuel. Therefore, it can be important to understand the characteristics of spent nuclear fuel to design reliable and safe geological repositories. However, it is too hard to study the characteristics of spent nuclear fuel, because it is a very complex material by itself and not easy to handle due to its radioactivity, and it is also difficult to independently understand the effects of each element. Therefore, a simulated spent nuclear fuel containing an element that forms a solid solution and epsilon particle was manufactured to understand the change in characteristics of each element. Most of the elements that form solid solutions are lanthanides or actinides and can change the structure of the UO2 lattice itself. The epsilon particles exist as metals at the grain boundaries of UO2. In this study, structural changes were measured using XRD, SEM, and Raman spectroscopy, and physical and chemical properties were also identified by measuring electrical conductivity and electrochemical properties. The results were summarized, and the effects of solid solution elements and epsilon particles on the structure and properties of UO2 matrix were compared and discussed.

Polyoxometalates (POMs) are nanoclusters composed of transition metals with high oxidation states. Owing to their redox properties and structural diversity, POMs have been applied to broad fields, such as catalysis, materials, and medicine. Among various fields of application, POMs play an important role in radiochemistry. POMs can form complexes with tri- and tetravalent lanthanides and actinides (radioactive elements), which may be good sequestrators or agents for separating nuclear wastes. Among the most prominent POM structures, Anderson-type POMs with a general formula of [Hy(XO6)M6O18]n− (y=0–6, n=2–8, M=addenda atom, X=heteroatom) represent one of the basic topological structures of the POM family. An important feature of Anderson type POMs is incorporating a large number of various heteroatoms with different size and oxidation states, which can lead to tune chemical properties. Interestingly, no example of Anderson type POMs with early transition metal ions in the heteroatom site has been reported to date. Herein, we discovered that the Anderson POM Na2K6Ti0.92W6.08O24·12H2O, which consists of pure inorganic framework built from a central Ti core supported by six WO6 inorganic scaffold, and the crystal structure was confirmed and refined using single-crystal X-ray diffraction (SC-XRD). In addition, structural characterizations, including, Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and Inductively coupled plasma-optical emission spectroscopy (ICP-OES) were performed.

Efficient capture and storage of radioactive iodine (consisting of two isotopes: 129I and 131I), produced or released from nuclear activities, are of paramount importance for sustainable development of nuclear energy due to their volatility and long half-life. Therefore, it is very important to develop new adsorbents for efficient utilization of radioactive iodine from nuclear waste. Various methods and materials are used for I2 capturing and removing, including MOFs due to their high porosity and fast adsorption kinetics, which are rightfully considered effective sorbents for removing I2. Metal–organic frameworks (MOFs) are porous crystalline materials which have diverse pore geometry and unique physicochemical properties, have attracted enormous attention for use in gas storage, separation and catalysis. The ability of MOFs to adsorb volatile products at room temperature can significantly improve the cost-effectiveness of the utilization process. This work describes the synthesis and characterization of three new metal-organic frameworks based on pyrazine (pyz), 44’bipyridine (bpy), 1,2 -bis(4 - pyridyl) – ethane (bpe) and copper (II) hexafluorozironate, as potential adsorbents for I2 capture. All of these three MOFs exhibit a two - dimensional (2D) crystal structure consisting from infinity non-crossing linear chains. The crystal structure of [Cu(pyz)2(ZrF6)2(H2O)2], [Cu(bpy)4(H2O)2ZrF6] and [Cu(bpe)4(H2O)2ZrF6] were characterized using powder X-ray diffraction (PXRD), single crystal X-ray diffraction (SC-XRD). Comparative characteristics of synthesized MOFs, including Fourier-transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA) were also performed. The I2 sorption experiments were examined by UV-vis spectroscopy.

The lattice thermal expansion of zirconium-based samples containing tin, niobium, and iron elements at a temperature range of 30–870°C with intervals of 40°C was studied by in situ hightemperature X-ray diffraction (HT-XRD). The a- and c-axes lattice constants of the hexagonal Zr crystal structure for the zirconium-based samples were calculated by Pawley refinement using the in situ HT-XRD spectra. The a-axis lattice parameters for the zirconium-based samples with tin element overall decreased, whereas those for the samples containing niobium or iron elements are not declined, as compared to those for a pure zirconium sample. It suggests that the lattice thermal expansion along the a-axis direction of the hexagonal Zr crystal structure for zirconium-based samples was suppressed by the tin element. This effect is the greatest when the content of tin element added in zirconiumbased sample is 3wt%. On the other hand, the c-axis lattice parameters for all the zirconium-based samples overall increase as compared to the pure zirconium, indicating no suppression effect by tin, niobium, and iron elements, in contrast to the a-axis lattice constants.

Complexation of actinides and lanthanides with carboxylic organic ligands is known to facilitate migration of radionuclides from deep geological disposal systems of spent nuclear fuel. In order to examine the ligand-dependent structures of trivalent actinides and lanthanides, a series of Eu(III)-aliphatic dicarboxylate compounds, Eu2(oxalate)3(H2O)6, Eu2(malonate)3(H2O)6, and Eu2(succinate)3(H2O)2, were synthesized and characterized by using X-ray crystallography and time-resolved laser fluorescence spectroscopy. Powder X-ray diffraction results captured the transition of the coordination modes of aliphatic dicarboxylate ligands from side-on to end-on binding as the carbon chain length increases. This transition is illustrated in malonate bindings involving a combination of side-on and end-on modes. Strongly enhanced luminescence of these solid compounds, especially on the hypersensitive peak, indicates a low site symmetry of these solid compounds. Luminescence lifetimes of the compounds were measured to be increased, which is ascribed to the displacement of water molecules in the innersphere of Eu center upon bindings of the organic ligands. The numbers of remaining bound water molecules estimated from the increased luminescence lifetimes were in good agreement with crystal structures. The excitation-emission matrix spectra of these crystalline polymers suggest that oxalate ligands promote the sensitized luminescence of Eu(III), especially in the UV region. In the case of malonate and succinate ligands, charge transfer occurs in the opposite direction from Eu(III) to the ligands under UV excitation, resulting in weaker luminescence.

Density of chloride molten salts is an essential physical property in the reactor core design and thermal-hydraulic design simulation, especially in molten salt reactor (MSR) design currently under development in Korea. NaCl-MgCl2-UCl3 pseudo-ternary system is one of the various candidate chloride-based salt mixtures because it has relatively-low melting point, very low vapor pressure, high thermal conductivity, etc. However, to the best of our knowledge, the density data of NaCl-MgCl2- UCl3 have not yet been measured or published worldwide, and therefore the ballpark figures of the density should be given for the preliminary reactor design. In our present study, the density estimation of NaCl-MgCl2-UCl3 based on the pseudo-binary data, i.e., NaCl-MgCl2, MgCl2-UCl3, and NaCl- UCl3, reported in the literature previously were performed using the Redlich-Kister model. Binary interaction parameter for MgCl2-UCl3 was higher than that for NaCl-MgCl2 and lower than that for NaCl-UCl3. As an example, calculated density of 0.62 NaCl: 0.18 MgCl2: 0.20 UCl3 at 873 K was 2.578 g·cm−3. In our further study, the methodology using Redlich-Kister model will be applied to more complex multicomponent systems and to other physical properties such as viscosity, thermal conductivity, surface tension, etc.

The overestimation and underestimation of the radioactivity concentration of difficult-to-measure radionuclides can occur during the implementation of the scaling factor (SF) method because of the uncertainties associated with sampling, radiochemical analysis, and application of SFs. Strict regulations ensure that the SF method as an indirect method does not underestimate the radioactivity of nuclear wastes; however, there are no clear regulatory guidelines regarding the overestimation. This has been leading to the misuse of the SF methodology by stakeholders such as waste disposal licensees and regulatory bodies. Previous studies have reported instances of overestimation in statistical implementation of the SF methodology. The analysis of the two most popular linear models of the SF methodology showed that severe overestimation may occur and radioactivity concentration data must be dealt with care. Since one major source of overestimation is the use of minimum detectable activity (MDA) values as true activity values, a comparative study of instrumental techniques that could reduce the MDAs was also conducted. Thermal ionization mass spectrometry was recommended as a suitable candidate for the trace level analysis of long-lived beta-emitters such as iodine-129. Additionally, the current status of the United States and Korea was reviewed from the perspective of overestimation.