First-Principles Molecular Dynamics Simulations of UCl_n–NaCl (n = 3, 4) Molten Salts

Abstract

Molten chloride salts are one of the two main choices to dissolve nuclear fuels for fast-spectrum molten-salt reactors, as a generation IV reactor design. Both U3+ and U4+ can stably exist in molten chlorides, but the differences in their structure and chemical properties in molten NaCl are not well understood. In this work we use first-principles molecular dynamics simulations based on density functional theory to investigate the structure of UCl n (n = 3, 4) in molten NaCl at various mole fractions. We find that 7-fold- and 8-fold-coordinate structures of Cl around U become more populous in UCl3–NaCl when UCl3 concentration is higher than 25 mol %, while in UCl4–NaCl the 6-fold-coordinate structure is the leading species when the UCl4 concentration is higher than 32 mol %. Moreover, we find that the first solvation shell of Cl around U is more dynamic in UCl3–NaCl than UCl4–NaCl, evidenced by weaker U–Cl bonds and faster U–Cl dissociation in the former. Further analysis reveals that a network structure with sharing Cl– between U cations starts forming at the low U3+ concentrations, whereas the U4+–Cl––U4+ linkages become noticeable only after UCl4 concentration is larger than 32 mol %. This work sheds new light on the difference between U3+ and U4+ in molten NaCl.

Adding MgCl2 to Molten NaCl−UCln (n=3, 4): Insights from First‐Principles Molecular Dynamics

“…One interesting finding in our previous study [14] is the formation of U−Cl network structures, U m Cl x oligomers (m=2–4) and polymers (m>4), in the 1 : 1 NaCl−UCl n (n=3,4) molten salts. Figure 6a shows the change in the network structure from adding different amounts of MgCl 2 into NaCl−UCl 3 .…”

“…[8][9][10][11][12] NaCl is the main salt of choice to dissolve the fuel salt such as UCl 3 for fast-spectrum MSRs. [13] We have previously investigated the structural change and transport properties of the melts of NaCl with uranium chlorides using both first principles molecular dynamics [14] and classical molecular dynamics based on the polarizable-ion model. [15] Our studies revealed that the viscosity of the NaClÀ UCl 3 mixture increases with the uranium concentration as the network structure of UÀ ClÀ U is formed.…”

Complex Structure of Molten NaCl–CrCl3 Salt: Cr–Cl Octahedral Network and Intermediate-Range Order

“…Indeed, let us compare the octahedra formed in molten NaCl–CrCl 3 and NaCl–UCl 3 (78 mol % NaCl). In the uranium salt, the most likely distance between U 3+ and Cl – is 2.74 Å . This is larger than the Cr–Cl distance of 2.342 Å, as dictated by the larger Shannon radius of U 3+ (1.025 Å for U 3+ vs 0.615 Å for Cr 3+ ) .…”

“…We also found evidence for a mid-range order that is likely related to interchain correlations. Therefore, our model ternary salt shows the structure that is reminiscent of NaCl−UCl n , 35 the salt that is likely to be used in MSRs. We expect that Cr impurities will affect network formation of the abovementioned salts, thus affecting the diffusivity and viscosity, especially near salt−metal interfaces.…”

The Complex Structure of Molten NaCl-CrCl3 Salt: Cr-Cl Octahedra Network and Intermediate-Range Order

“…We also found evidence for mid-range order that is likely related to inter-chain correlations. Therefore, our model ternary salt shows the structure that is reminiscent of KCl-MgCl 2 17 , LiF-BeF 2 18 , and NaCl-UCl n 19 , the salts that are likely to be used in MSRs. We expect that Cr impurities will affect network formation of the above-mentioned salts, thus affecting the diffusivity and viscosity, especially near salt-metal interfaces.…”