"There is arguably no other material that holds more potential for advancing future nuclear power applications than molten salts. Their advantageous thermodynamic and chemical properties could help close the fuel cycle and revolutionize the way in which nuclear reactors operate. However, important technical challenges and proliferation concerns must be solved before molten salts can reach commercial utility. Perhaps most urgently needed is a reliable technique for real-time quantification of molten salt composition. Many future MSR designs require in situ chemical processing to continuously remove neutron-absorbing fission products and corrosion-causing impurities (i.e., oxygen and water). Complementing chemical removal, many MSR designs also require some level of periodic refueling, which will change both the quantity of fissile material and its isotopic concentration. Salt-cooled reactors will also require process monitoring to alert operators in the event of a nuclear fuel breach. Commercial scale pyroprocessing of used nuclear fuel must also have adequate process monitoring for controlling the rate of actinide removal. Furthermore, the unprecedented mobility of nuclear material within flowing molten salts poses serious problems for material accounting. Unlike solid fuel rods, actinide-containing molten salts are continuously mixed and transported in a liquid state, which makes them fundamentally more susceptible to unauthorized extraction. Therefore, future molten salt accounting will require a remote chemical-isotopic sensor that can operate in a high radiation environment, thus greatly limiting direct human involvement. Although the leading candidate method for real-time material accounting (laser-induced breakdown spectroscopy, LIBS) has progressed in recent years, its inherent technical challenges may limit widespread commercialization. The overarching objective of the proposed research is to investigate a new method for quantifying nuclear materials in molten salts using a technique we call ����������������plasma-bubble spectroscopy��������������� (PBS). The strategy of our proposed technique (Figure 1) is to transform small quantities of bulk molten salt into a low-density gaseous bubble using an impulsive spark discharge or ultrafast laser breakdown. This molten salt bubble is then converted into a dilute plasma by means of a glow discharge, whose sharp atomic emission lines can be spectrally analyzed with sub-angstrom resolution. The proposed technique addresses several critical challenges facing materials accounting in molten salts: online monitoring capability, shot-to-shot stability, optical clarity, and the possibility of uranium isotopic quantification. Our long-term vision is to enable a low-cost, low-maintenance, high throughput device that can operate in the extreme conditions found in MSRs and advanced fuel reprocessing."

Our increasing need for safe, abundant, reliable, and carbon-free energy sources is stimulating renewed interest in nuclear energy. On Aug. 28, 2019, the U.S. Department of Energy (DoE) announced that it is ����������������all in��������������� with new nuclear energy technology1. Among these efforts is the DoE������������������s commitment to investigate the utilization of molten salts for advanced nuclear reactors. As stated in the DoE������������������s 2017 Basic Research Needs for Future Nuclear Energy (BRN-FNE), the first Primary Research Direction is to enable the design of revolutionary molten salt coolants and liquid fuels2. A critical requirement for designing these advanced nuclear reactors is to understand fluid behavior of molten salts over a wide temperature range and varying chemical composition. This is especially important (and daunting) for predicting molten salt reactor (MSR) behavior because of the virtually unlimited and evolving chemical parameter space featuring different eutectic and fissile compositions, fission products, corrosion products, and impurities.

It is remarkable that in the 21st century a natural phenomenon experienced by all and known since antiquity remains unexplained at its elementary level. The exchange of charge resulting from contact and separation of two dissimilar materials, known as triboelectrification, has been described at length by the scientific community but never explained1. And yet the importance of triboelectrification cannot be overstated, as the phenomenon is pervasive throughout nature and in our daily lives (e.g. planetary, atmospheric, and clothing electrostatic charging). The technological impact of triboelectrification is equally widespread (e.g. electrostatic discharging and generators). Recent academic and industrial interest for energy-harvesting triboelectric nanogenerators and microelectromechanical systems has reinvigorated the study of triboelectrification. However, advancing triboelectric theory has been problematic as it requires an atomistic description over large spatial and temporal scales. In addition, the interfacial physical processes span classical and quantum domains and exhibit strong off-equilibrium behavior. For these reasons, there is not even a leading ���������������ab initio������������������-order theory of why charge should move at all from one insulating material to another and remain on the surface2. Although the exact mechanisms are unknown, our empirical understanding of triboelectrification is well-developed. Published over 260 years ago, the first triboelectric series ranked the propensity for a material to become positively or negatively charged after contact with another material, thus providing a connection to surface chemistry (by the work function). Since then, researchers have identified other parameters that strongly influence triboelectric charge transfer: surface morphology, applied force (magnitude and direction), and atmospheric conditions (humidity)13. Until recently, much of this knowledge was obtained through electrical probe measurements and electrostatic induction, which cannot access ultrafast timescales nor buried interfaces. A recent enabling study has introduced a new way of measuring tribocharged surfaces using a non-invasive, non-resonant optical method (SHG)8. The proposed research will extend this method into the ultrafast regime to make the first operando triboelectrification study. Doing so will allow for the determination of which charged species is responsible for the macroscopic fields produced by contact electrification, which is the overall goal of this proposed research.