Fusion Blankets and Fluoride-Salt-Cooled High-Temperature Reactors with Flibe Salt Coolant: Common Challenges, Tritium Control, and Opportunities for Synergistic Development Strategies Between Fission, Fusion, and Solar Salt Technologies

Developing power plant materials using the life cycle lens

The Spherical Tokamak for Energy Production (STEP) environment will include magnetic, thermal, mechanical and environmental loads far greater than those seen in the Joint European Torus campaigns of the past decade or currently contemplated for ITER. Greater still are the neutron peak dose rates of 10-6 displacements per atom, per second, which in-vessel materials in STEP are anticipated to be exposed to. Reduced activation and high-fluence resilience therefore dominate the materials strategy to support the STEP Programme. The latter covers the full life cycle from downselected compositions and new microstructural developments to irradiation-informed modelling and end-of-life strategies. This article discusses how the materials downselection is oriented in plant power trade-off space, outlines the development of an advanced ferritic-martensitic structural steel, describes the 'Design by Fundamentals' mesoscale modelling approach and reports some of the waste mitigation routes intended to make STEP operations as sustainable as possible.This article is part of the theme issue 'Delivering Fusion Energy - The Spherical Tokamak for Energy Production (STEP)'.

Surface Fluorination of Nuclear Graphite Exposed to Molten 2LiF-BeF2 (FLiBe) Salt and Its Cover Gas at 700 °C

This study demonstrates that the reaction of Li2BeF4 (FLiBe) with graphite both in the liquid phase and the gas phase of the molten salt leads to the formation of covalent and semi-ionic carbon-fluorine bonds at the graphite surface and is accompanied by surface microstructural changes, removal of C-O groups, and deposition of metallic beryllium, based on XPS, Raman, and glow discharge mass spectroscopy characterization. At 700 °C, the observed surface density of C-F is higher after 240 h than after 12 h of exposure to molten FLiBe salt; the kinetics of covalent C-F formation is slower than that of semi-ionic C-F formation, and the relative amount of semi-ionic C-F content increases with depth. The graphite sample exposed to the cover gas exhibits less surface fluorination than the salt-exposed sample, with predominantly semi-ionic C-F. Based on these observations and the observed LiF/BeF2 ratio by surface XPS, the hypotheses that fluorination of the salt-exposed graphite occurs via a gas-phase mechanism or that it requires salt intrusion are refuted; future studies are warranted on the transport of C-F semi-ionic and covalent species in graphite at high temperatures.

Density Measurements of Molten LiF-BeF2 and LiF-BeF2-LaF3 Salt Mixtures by Neutron Radiography

The densities of eutectic (LiF)2-BeF2 and mixtures of this salt (FLiBe) with LaF3 were measured by dilatometry and by neutron attenuation from 673 K to 1,073 K. Because LaF3 has a limited solubility in FLiBe, it was necessary to determine the amount of LaF3 in solution before the density could be determined. The FLiBe density determination was favorably benchmarked against the literature data. A simple comparison was not available for the LaF3-FLiBe mixtures, so extrapolation of published data was necessary based on analysis using the Molten Salt Thermal Properties Database-Thermochemistry, or MSTDB-TC, developed by the US Department of Energy. Solubilities for LaF3 in FLiBe ranged from 1 to 4 mol % over 673 to 1,073 K. The salt system was heated and cooled over 24 h to evaluate potential changes in composition and hysteresis during the measurement. Changes in the meniscus were observed, and these were included in the correction for density determinations. Salt surface tension may have led to supersaturation of LaF3 in the salt because the solubility curve was nonlinear with respect to the inverse temperature, as would be expected for an ideal system. Surface tension measurements are currently underway to test this hypothesis.

Are High-Temperature Molten Salts Reactive with Excess Electrons? Case of ZnCl2

New and exciting frontiers for the generation of safe and renewable energy have brought attention to molten inorganic salts of fluorides and chlorides. This is because high-temperature molten salts can act both as coolants and liquid fuel in next-generation nuclear reactors. Whereas research from a few decades ago suggests that salts are mostly unreactive to radiation, recent experiments hint at the fact that electrons generated in such extreme environments can react with the melt and form new species including nanoparticles. Our study probes the fate of an excess electron in molten ZnCl2 using first-principles molecular dynamics calculations. We find that on the time scale accessible to our study, an excess electron can be found in one of three states; the lowest-energy state can be characterized as a covalent Zn2Cl5•2- radical ion, the other two states are a solvated Zn•+ species (ZnCl3•2-) and a more delocalized species that still has some ZnCl3•2- character. Since for each of these, the singly occupied molecular orbital (SOMO) where the excess charge resides has a distinct and well-separated energy, the different species can in principle be characterized by their own electronic spectra. The study also sheds light onto what is commonly understood as the spectrum of a transient radical species which can be from the SOMO onto higher energy states or from the melt to pair with the excess electron leaving a hole in the liquid.

Method for the Determination of Oxygen in FLiBe via Inert Gas Fusion

In nuclear reactors that use molten fluoride salts, either as coolants or as the medium for the fuel, the purity of the salts is critical for controlling salt chemistry and mitigating corrosion. Water is a particularly important contaminant in this regard, as it participates in a number of important corrosion reactions, so the careful measurement of oxygen, which is principally present in the salts due to water contamination, is a critical step in salt characterization. Here, we present an analytical method for quantifying oxygen contamination in Li2BeF4 (FLiBe), a technologically important and suitably representative fluoride salt, with a detection limit of 22 μg of oxygen, or 110 ppm in a 200 mg sample. To test the method, four FLiBe samples from different batches were tested. Two of these showed oxygen concentrations below the method detection limit, while two showed concentrations above it. In particular, the difference in the oxygen concentration between purified and un-purified batches of material from Kairos Power showed the efficacy of this method in characterizing the degree of oxygen removal obtained from purification methods.

Temperature-Dependent Properties of Molten Li2BeF4 Salt Using Ab Initio Molecular Dynamics

Molten lithium tetrafluoroberyllate (Li₂BeF₄) salt, also known as FLiBe, with a 2:1 mixture of LiF and BeF₂ is being proposed as a coolant and solvent in advanced nuclear reactor designs, such as the molten salt reactor or the fluoride salt cooled high-temperature reactor. We present the results on the structure and properties of FLiBe over a wide range of temperatures, 0-2000 K, from high-throughput ab initio molecular dynamics simulation using a supercell model of 504 atoms. The variations in the local structures of solid and liquid FLiBe with temperature are discussed in terms of a pair distribution function, coordination number, and bond angle distribution. The temperature-dependent electronic structure and optical and mechanical properties of FLiBe are calculated. The optical and mechanical property results are reported for the first time. The results above and below the melting temperature (∼732 K) are compared with the experimental data and with data for crystalline FLiBe. The electronic structure and interatomic bonding results are discussed in correlation with the mechanical strength. A novel concept of total bond order density (TBOD), an important quantum mechanical parameter, is used to characterize the internal cohesion and strength in the simulated models. The results show a variation in the rate of change in properties in solid and liquid phases with anomalous behavior across the melting region. The observed trend is the decrease in mechanical strength, band gap, and TBOD in a nonlinear fashion as a function of temperature. The refractive index shows a surprising minimum at 850 K, among the tested temperatures, which lies above the melting point. These findings provide a new platform to understand the interplay between the temperature-dependent structures and properties of FLiBe salt.