The Impact of Hydrogen Valence on Its Bonding and Transport in Molten Fluoride Salts

<p>Interest in molten salts has increased significantly over the last decade due to their potential application in various clean-energy technologies including hydrogen generation, solar heat storage, and advanced nuclear power plants. In the development of new molten salt-based fission and fusion systems, controlling hydrogen poses a critical challenge due to its ability to corrode structural materials as <sup>3</sup>H<sup>+</sup>, and its potential to cause significant radioactive release as diffusive <sup>3</sup>H<sup>0</sup>. Yet, the chemistry and transport behavior of the hydrogen species remain poorly understood despite several decades of research. Using ab initio molecular dynamics, we present a coupled examination of hydrogen valence, speciation and transport in the prototypical salts 66.6%LiF-33.3¾F<sub>2</sub> (Flibe) and 46.5%LiF-11.5%NaF-42%KF (Flinak). We discovered significant differences between <sup>3</sup>H<sup>0</sup> and <sup>3</sup>H<sup>+</sup>transport behaviors. <sup>3</sup>H<sup>+</sup> diffuses 2-4 times slower than <sup>3</sup>H<sup>0</sup>, which can be ascribed to hydrogen bonding and complexation in solution. This work helps explain varying experimental results and provides useful species transport data for designing hydrogen control systems for molten salts. </p>

minimize oxidation in structural steels, the salt must be sufficiently reducing such that the vast majority

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of the tritium is converted to , with a to ratio less than 0.01. On the other hand, the potential H 2 HF H 2 71 must remain sufficiently oxidizing to prevent salt precipitation, resulting in a narrow operating 72 window ( for Flibe) at reactor operating temperatures. However, is highly diffusive and Δ~0. 1V H 2 73 must be continuously removed from the system and captured to minimize the potential for a significant 74 radiological release [19].

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The removal of tritium from salt is a coupled problem of redox chemistry and transport that 76 requires understanding and accurate prediction of the in-situ behavior of possible tritium species.

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Differences in diffusion and reaction rates can result in differential species removal rates. Depending 100 range [30] [31][32] [33]. For structural analysis, X-ray and neutron diffraction patterns are used. In 101 practice, these methods should be coupled to simulation and other experimental methods to fully 102 resolve features of multi-component systems [34][35] [36].

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In this work, tritium local structure, chemistry and transport are investigated using ab initio 104 simulation with realistic dynamics and thermochemistry [37] [38]. Dilute tritium as and are In Flibe, tritium as shows a distinct peak with a maximum at the radial distance of 0.

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The coordination number and peak distances are summarized in Table 1.

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The dissolution of is confirmed in Figure 2, which shows the interatomic distance between a    The hopping H + occurs in sequence 1-5 shown below.
Step 1 to 2 clearly show the existence of the 3 H 170 complex where the is bound to two fluorine atoms. b visualization of tritium hopping F - as an adjacency matrix and graph of nodes and vertices used to find chemical structures in system, c 175 relationship between chemical structures found across spatial and temporal dimensions to construct 176 reaction coordinates, d fraction of each molecule at 973K, 1173K, and 1373K

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Reactions with H + involving the most common complexes are shown in

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shown in Figure 3d. The last reaction shows the bonding of an HF molecule to two corner-sharing 186 tetrahedral complexes with . The 'other' reactions all involve less common BeF 2 - species, such as transition complexes swapping ions with solution, or breaking and joining with F -HF 188 larger Beryllium complexes: . The frequency of these reactions Be 3 F 10 HF 4 -↔BeF -3 +Be 2 F 7 HF 3 -

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increases with increasing temperature due to the increase in transient high-energy structures that are 190 discovered at higher temperatures. is able to exchange with the solution and the tritium becomes more dissociative. The most common F -

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states for are shown in Figure 4, of which the most common structure is ( ps at 973K) which existed for 80-90% of the 60-ps simulation, followed by ( ps), and NaF 4 HF 3 -= 0.14 NaF 5 HF 4 -

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( ps), which were found to be present for 1.5 -5% of the simulation. The 'other' category of

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In this section, the calculation of diffusivity and activation energy of hydrogen is discussed for clean

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Flibe and Flinak and compared to existing data. It should be noted that the form of tritium measured 223 in experiments is not explicitly known because metallic impurities and corroding materials can form 224 halides in the salt and change the chemical form of tritium being measured as described in Section 1.

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In this study, the isotopic effect of hydrogen is expected to be small, especially relative to the differences   Figure S7. Similarly, it was found that the introduction of

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shown in Figure S6. These results support the idea that impurity effects can impact experimental results 279 summarized in Figure 5 and provides further evidence that impurities can significantly impact chemical 280 and transport properties. However, many more atmospheric and fission-generated impurities can exist 281 simultaneously, which can have unpreditable effects in real systems. As such, the role of impurities in a variety of chemical forms, oxidation states, and conditions warrants a more detailed investigation in 283 future studies.

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The diffusivity of hydrogen isotopes in Flinak has been measured by only three experimental 286 studies, which all used as shown in Figure 6 [28] [29][27]. Similar to the relative behavior of tritium  highlighted region represents the 95% confidence interval from regression of the collected data 304 [27][28] [29].

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In this work, tritium chemistry and transport are examined for the two possible tritium 331 oxidation states and in prototypical salts Flibe and Flinak. Accurate prediction of these H 0 H + 332 properties is critical for the design of salt systems that can contain hydrogen from moisture ingress or 333 generated by irradiation. In many systemsm, the tritium transport in salt could limit overall transport 334 due to relatively slow diffusion through salt, particularly as ion. Further, quantifying large H + differences in chemistry and transport behavior of different chemical forms ( ) is H 0 : H + ≅2-5

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important in understanding and controlling the redox condition, and consequently, corrosiveness of 337 the salt. Conventionally, the differences between the diffusion of and in molten salt has been H 0 H + 338 ignored due to a lack of information. In some cases, it has even been assumed that , which H + > H 0

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could cause significant error in predicting transport behavior [50] [20]. From this work, it is clear that 340 experiments that controlled for speciation, impurities, and corrosion produced more reliable data than 341 those that did not, which was supported by simulation results. Further, the simulations provide insights 342 that allowed understanding of transport behavior at the level of local atomic structure, which would 343 otherwise be difficult to infer from experimental data alone.

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Through extensive first-principles molecular dynamics simulations, we discovered that hydrogen 5 Methods

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A range of fluoride systems were simulated and compared against experimental structure and transport 377 data over a range of temperatures from 775 -1373K. These systems include LiF, KF, NaF, BeF 2 , LiF-KF,

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LiF-BeF 2 and LiF-NaF-KF, which can be found in the supplemental information. In all of the salts, the system was initialized by randomizing the positions using Packmol software [51].

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The system was equilibrated under the canonical ensemble (NVT) for at least 10 picoseconds at least 200 391 K above the melting point. The production runs lasted for at least 60 picoseconds sampled at 1fs timestep.

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The temperature was controlled with a Nosé-Hoover thermostat using a Nosé mass corresponding to a Where, is the nearest neighbor coordination number, is the number density of species ,

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For each molecular subgraph found in the simulation, a binary time series is constructed ( ) 438 The transition probability of switching states, was set between , which is p 10 -6 -10 -7 439 markedly low to provide a stronger noise filter that removes false transitions in the observed signal.

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This is necessary due to the high-temperature liquid state causing high-frequency fluctuations in the 441 system. The Viterbi algorithm is used to find the likely sequence that produced observation .

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With the filtered sequences for each molecule, chemical reactions are then found. The    2016.