Density and Thermal Expansivity of Molten 2LiF-BeF2 (FLiBe): Measurements and Uncertainty Quantification.

The variability among prior data for FLiBe is 11% for the liquid density and 61% for the thermal expansivity. New liquid density and thermal expansivity data are collected, with particular attention to uncertainty quantification. We discuss and quantify bounds for possible sources of variability in the measurements of liquid density: salt composition (<0.6% per 1 mol % BeF2), salt contaminants at 100 s ppm to <1 mol% (2%), Li isotopic composition (2%), sample isothermal conditions (0.2%), dissolved gases (<0.3%), and evolution of bubbles with temperature transients - depending on Ar or He cover gas (0.1 or 0.6% for dilatometry, 1 or 5% for hydrostatic measurements). To aid in quantifying thermal expansivity sensitivity to composition, we review and generalize the ideal molar volume prediction for FLiBe; to improve this model, measurements are needed for the thermal expansivity of BeF2. We collect new data on the density of liquid FLiBe using the hydrostatic method and 170 g of hydrofluorinated FLiBe with less than 0.13 mol % impurities (dominantly Al, K, Na, Mg, Ca), as determined by ICP-MS. We obtain the following: The dominant sources of uncertainty are the bobber volume uncertainty (0.15%), the mass measurement uncertainty (0.2%), and possibly the wetting angle of the salt on the wire (<0.3%). Occasional noise and <0.2% deviation from linearity may be due to the formation of gas bubbles on the bobber surface from the temperature-dependence of gas solubility; repeatable results for heating and cooling runs provide confidence that bubble effects are well managed in this experimental setup. These are the first measurements of the liquid density of FLiBe that report error analysis and that measure the liquid composition before and after density measurements.

Correcting thermal-emission-induced detector saturation in infrared spectroscopy.

We found that temperature-dependent infrared spectroscopy measurements (i.e., reflectance or transmittance) using a Fourier-transform spectrometer can have substantial errors, especially for elevated sample temperatures and collection using an objective lens. These errors can arise as a result of partial detector saturation due to thermal emission from the measured sample reaching the detector, resulting in nonphysical apparent reduction of reflectance or transmittance with increasing sample temperature. Here, we demonstrate that these temperature-dependent errors can be corrected by implementing several levels of optical attenuation that enable convergence testing of the measured reflectance or transmittance as the thermal-emission signal is reduced, or by applying correction factors that can be inferred by looking at the spectral regions where the sample is not expected to have a substantial temperature dependence.

Constant-potential molecular dynamics simulations of molten salt double layers for FLiBe and FLiNaK.

We report the results of constant-potential molecular dynamics simulations of the double layer interface between molten 2LiF-BeF2 (FLiBe) and 23LiF-6NaF-21KF (FLiNaK) fluoride mixtures and idealized solid electrodes. Employing methods similar to those used in studies of chloride double layers, we compute the structure and differential capacitance of molten fluoride electric double layers as a function of applied voltage. The role of molten salt structure is probed through comparisons between FLiBe and FLiNaK, which serve as models for strong and weak associate-forming salts, respectively. In FLiBe, screening involves changes in Be-F-Be angles and alignment of the oligomers parallel to the electrode, while in FLiNaK, the electric field is screened mainly by rearrangement of individual ions, predominantly the polarizable potassium cation.

Design and operation of a molten salt electrochemical cell.

Molten salts such as 2LiF-BeF2 (FLiBe) have been proposed as coolants for advanced nuclear fission and fusion reactors. Critical to the design, licensing and operation of these reactors is characterization and understanding of the chemical behavior and mass transport of activation and fission products, corrosion products, and other solutes in the coolant. Electrochemical techniques are a powerful suite of tools for probing these phenomena. The design of an experimental cell for molten salt electrochemistry is described herein. As a demonstration of this design, details of the experimental methods used to conduct electrochemical experiments with molten FLiBe with addition of LiH are provided. Decommissioning of the cell is considered from the point of view of decontamination and waste generated. Main features of the cell include:•Suitable for operation up to 800 °C; suitable for operation inside and outside of a glovebox.•Enables sweep gas, gas sampling and analysis; enables addition of solid and liquid materials during operation.•Supports a variety of electrode materials and arrangements.

Data analysis for characterization of IG110 and A3 by X-Ray diffraction and Raman spectroscopy.

This article contains data related to the research journal paper titled 'Comparative Analysis of Microstructure and Reactive Sites for Nuclear Graphite IG-110 and Graphite Matrix A3", Journal of Nuclear Materials 528 (2020) 151802. This article includes details of the calculation process of the crystallite edge area, additional tables and figures of XRD and Raman data, and additional summary of data reduction methods used in prior literature for the characterization of IG-110 nuclear graphite. Reduced data associated with this article is provided in the supplementary information. Raw data associated with this article is in the supplementary material of the companion article.

Datasets for elemental composition of 2LiF-BeF2 (FLiBe) salt purified by hydro-fluorination, analyzed by inductively coupled plasma mass spectrometry (ICP-MS) using two digestion methods.

This article shows the elemental analysis of a batch of FLiBe prepared from LiF and BeF2 and purified by hydro-fluorination, see "Batch-Scale Hydrofluorination of Li2BeF4 to Support Molten Salt Reactor Development" (Kelleher et al., 2015), which was performed by the method of inductively-coupled plasma mass spectrometry (ICP-MS), with analysis samples prepared by multi-acid microwave digestion with and without HF acid. Data shows quantification of a total of sixty-five elements and is reported for a total of eight digested samples. Quantification of 6Li/7Li isotopic ratio is reported for a total of eight digested samples.

Spectroscopy (Raman, XPS, and GDMS) and XRD analysis for studying the interaction between nuclear grade graphite and molten 2LiF-BeF2 (FLiBe) at 700 °C.

FLiBe-exposed IG-110 graphite and a control IG-110 sample were analyzed by Raman, XPS, GDMS, and XRD, and the complete raw data sets are provided in the Supplementary Information. These data sets enable full reproducibility and transparency of the data analysis we reported in the accompanying research paper titled "Fluorination of Nuclear Graphite IG-110 in Molten FLiBe salt at 700 °C", published in the Journal of Fluorine Chemistry, and facilitates quantitative comparison with future similar studies by other research groups. In this data article, we provide plots of the peak fitting for all Raman spectra from each sampling point on the graphite surface. We also provide the measured impurity concentrations of the IG-110 samples, as measured by GDMS; this data was not reported nor discussed in the accompanying research paper. The method and software used for peak fitting for the spectra from Raman, XPS, and XRD are listed separately.