Nuclear energy is a vital contributor to sustainable energy generation, despite a level of skepticism among some of the general public and other stakeholders. As an energy source, it can provide clean energy with net zero-carbon emissions at a low environmental cost. Nuclear reactors come in different shapes and sizes. A nuclear reactor works by a process called fission where an atom is split into two smaller atoms and some additional neutrons. Some of these neutrons cause them to fission too and release more neutrons creating a chain reaction. The generated heat in this process is removed by circulating water which can then be used to generate steam and drive turbines for electricity production. Light water reactors operate at elevated pressures and can generate hydrogen, a source of explosion risk and great concern. Therefore, the development of “Generation IV” advanced nuclear power designs, such as molten salt reactors (MSRs), has attracted a great deal of commercial interest. MSR technology is attractive due to its inherently safe design principles, modularity, compatibility with sustainable fuel cycles, and other features that make it a leading candidate for future nuclear power installations and the next generation of nuclear power reactor designs developed in the U.S.
One major requirement for MSR deployment is that a deep understanding of the structure and properties of molten salt itself, the thermodynamics and the elements involved in the dissolution, is required to control the chemistry of the “elemental soup” inside an operating MSR. To take a “look” into the properties of nuclear materials, such as metal ions dissolved in molten salts, it is necessary to develop techniques that can yield information regarding the solubility, structure, and composition of this complex mixture. The Molten Salts in Extreme Environments (MSEE) EFRC has been a pioneer in developing advanced computational and experimental methods and multi-modal approaches for studying molten salt composition, structure, and properties, and how metal ions behave in them. The MSEE EFRC has developed various synchrotron-based techniques to investigate molten salt systems, including x-ray spectroscopy, scattering, and imaging methods.
Recent growth in using x-rays to investigate various energy-related materials—such as batteries, semiconductors, and nuclear materials—can be attributed to the high intensity and tunability of modern synchrotron user facilities. High x-ray intensity enables fast and high-resolution (micron to nm) data collection, while energy tunability allows experimenters to select the wavelength most appropriate for the study, enabling element-specific studies. For example, MSEE researchers utilized x-ray absorption spectroscopy to study local environments of metal ions in molten salts. In this study, x-rays penetrating through molten salt get absorbed if the incident photon energy matches characteristic binding energies of electrons in the K, L, M (etc.) shells of the absorbing metal ions. These energy levels are atom-specific, and hence, can yield information on the local structure of the metal ions (such as nickel and cobalt) dissolved in molten salts.
Synchrotrons are extremely bright x-ray sources. In a typical synchrotron, electrons are generated from an electron gun and accelerated to 99.9997% of the speed of light by a linear accelerator. After additional acceleration in a booster ring, the electrons are injected into the main storage ring, where they circulate through a series of magnets separated by straight sections. When the electrons are deflected through the magnetic fields they give off electromagnetic radiation and synchrotron light is produced. The synchrotron light is channeled down beamlines to experimental workstations where it is used for research.
Atomic-scale Science Enabling Advances in MSRs
High temperatures, exposure to radiation, mechanical stress, and corrosive fluids are some of the challenges that MSR reactors face. Chemistry plays an important role in the factors that stress MSRs, including corrosion and the behavior of corrosion products within the molten salt. MSEE scientists are pioneering new modeling and experimental tools to study atomic-scale metal ion speciation in high-temperature molten salt environments comparable to MSR operating conditions. Using in situ synchrotron-based methods, MSEE researchers have successfully characterized structural and conformational changes in nickel coordination as function of temperature and irradiation conditions in zinc chloride molten salt. It was observed that the local structure of nickel metal ions was heterogeneous where a mixture of coordination states exists, with the dominant state changing from octahedral to tetrahedral geometry with an increase in temperature.1 Further, in situ x-ray absorption spectroscopy was utilized for real-time characterization of radiolytic nickel metal ion reduction in molten zinc chloride solutions, where it was found that Ni nanoparticle formation is dependent on the irradiation dose and the concentration of nickel ion in the precursor solution.2
Experimental x-ray absorption and ultraviolet spectroscopy studies were combined with computational methods to investigate multiple coordination states of metal ions (such as Ni) in molten salts using theoretical models. The MSEE EFRC has developed a multi-modal approach combining x-ray-based methods, such as x-ray absorption spectroscopy, optical spectroscopy experiments, and ab initio molecular dynamics simulations, to explain strong dynamic heterogeneity in coordination environments and to understand the radiolytic reduction of metal ions in molten salts. The next step is to apply these same multi-modal techniques and principles to characterize metal ion speciation and chemistry in multi-component molten salt systems (such as LiCl-KCl, KCl-MgCl2, and LiCl-ZnCl2) to obtain a global understanding of solute–salt interactions.
The extensive COVID-19 research performed at synchrotron facilities worldwide has played a role in finding solutions to the pandemic, which highlights the impact of synchrotron science on society. It showcases that the value of synchrotron science extends far beyond nuclear energy applications. The high brightness of the x-rays produced allowed scientists to obtain ‘pictures’ from tiny crystal samples of the proteins in the virus, therefore, giving a better understanding of the processes in infected cells. This, in turn, expedited the process of drug discovery.
Conclusion
MSEE EFRC is a leading pioneer in developing advanced synchrotron methods and computational techniques for understanding the connections between molten salt composition, structure, and physical properties, including the interactions that control metal ion solubility and speciation. A better understanding of molten salt structure, dynamics, and interactions using the x-ray looking glass equips us with the necessary tools to investigate important aspects of reactivity in molten salts, such as corrosion leading to material degradation and radiation-driven chemistry.
