Thermodynamic Assessment of the NaF-KF-UF4 System

Abstract

In the Molten Salt Reactor (MSR) concept, metal fluorides are key components of possible fuel and coolant salts. The fast reactor option opens the possibility for alternatives to the Li based matrix salts, avoiding the costly 7Li enrichment and the tritium production from residual 6Li. Such alternatives can be based on NaF and KF as matrix components. In this study, two pseudo-binary phase diagrams of NaF-UF4 and KF-UF4, and the NaF-KF-UF4 pseudo-ternary system were experimentally investigated using Differential Scanning Calorimetry (DSC). The obtained data were used to perform a full thermodynamic assessment of the NaF-KF-UF4 system. The calculated pseudo-ternary eutectic was found at 807 K and a 68.9-7.6-23.5 mol% NaF-KF-UF4 composition. The comprehensive experimental and modelling data obtained in this work provide further extension of the JRCMSD thermodynamic database describing thermodynamic properties of key fuel and coolant salts for the MSR technology.

Thermophysical Properties of FUNaK (NaF-KF-UF4) Eutectics

“…In a recent study by Ocádiz et al [ 11 ] the thermodynamic assessment of the NaF-KF-UF 4 system was further improved and the compositions of two of FUNaK’s low melting eutectic points were experimentally determined based on a systematic DSC investigation of 25 measurements of various ternary compositions [ 11 ]. The two found eutectic compositions are NaF-KF-UF 4 (55.6-18.7-25.7 mol%) and NaF-KF-UF 4 (50.4-23.2-26.4 mol%), at melting points of 809 and 810 ± 5 K, similar to what was indicated by Schacherl et al [ 9 ]. Notably, the relatively high content of UF 4 in these compositions, as opposed to mixtures such as LiF-BeF 2 -ZrF 4 -UF 4 used in the Molten Salt Reactor Experiment (MSRE) (65.0-29.1-5.0-0.9 mol%), with a 33% U enrichment [ 3 ], permits a lower U enrichment to obtain to the same amount of fissile material in the fuel, facilitating the fuel acquisition of more available low-enriched U and regulatory approval.…”

“…Both compositions exhibit similar melting behavior. A first transition identified at 468.9 ± 2.0 °C, related to Na 3 UF 7 [ 9 ], and a consecutive peak referring to a transition related to NaKUF 6 [ 11 ], i.e., the 1:1:1 phase of FUNaK, are visible [ 18 ]. The last transition and, at the same time, the major peak on the DSC heat flow curve correspond to the eutectic melting suggested by the single peak at 2 K/min for both compositions (experience shows that, for certain systems, a slow heating rate better reveals the liquidus shoulder on the heat flow peak).…”

“…However, due to the elevated toxicity associated with beryllium, there is an escalating exploration of alternative salts and their properties [ 4 , 5 , 6 , 7 , 8 ]. One such candidate is NaF-KF-UF 4 , commonly denoted as FUNaK, and recent studies have delved into its properties [ 9 , 10 ]. The noteworthy characteristics of FUNaK include the accessibility of all its components, favorable neutronic properties, the absence of the typically required Li enrichment in FLiBe-based salts, and the lower toxicity of Na and K in comparison to Be salts, thereby simplifying research and development as well as waste handling [ 11 ].…”

“…However, details regarding the procedure for establishing the phase diagram and the potential measurement of the experimental data were not specified. Subsequently, Schacherl et al thermodynamically assessed the NaF-KF-UF 4 phase diagram by optimizing the system’s Gibbs energy functions against the phase transition data of 12 novelly measured ternary compositions [ 9 ]. This assessment identified the lowest eutectic melting point at 808 K, a result corroborated by experimental DSC data; however, the composition of the eutectic was not experimentally verified.…”

Static corrosion of stainless steel 316H in chemically purified molten NaF-KF-UF4 salt

“…SS316H was selected for specimens of corrosion tests, which is one of the materials certified by the American Society of Mechanical Engineers (ASME) Boiler & Pressure Vessel Code (BPVC) for application in high-temperature reactors 21 . FUNaK was selected, as it has been recognized as a liquid fuel salt for molten salt reactors 22,23 . Two different batches of FUNaK were synthesized for the static corrosion tests, chemically purified FUNaK with (Test B) and without UF 3 (Test A), to investigate the impact of UF 3 on mitigating corrosion of SS316H.…”