Impurities in Primary Coolant Salt of FHRs: Chemistry, Impact, and Removal Methods

Abstract

The fluoride salt‐cooled high‐temperature reactor (FHR) is one of the Gen. IV reactor concepts. Considering the complexity of salt chemistry, radiation risk, impurity‐induced material corrosion, and impurity‐induced solid precipitation, the salt chemistry should be well controlled. Therefore, molten salt cleanup systems are needed in operation. Based on the experience of molten salt reactors (MSRs) and available data, the present study identifies the types of impurities, their potential sources, and their impact in FHR primary coolants. Cleanup methods for each type of impurity and future research activities are also discussed.

The impact of hydrogen valence on its bonding and transport in molten fluoride salts

“…While previous studies have found that impurities can signicantly increase in corrosion and change the thermochemical properties of salt, the specic impact of different impurities on tritium is not well understood. 8,9 Here, additional simulations were performed for tritium and Flibe in the presence of a single unit of Cr (dominant corrosion product) and H 2 O (atmospheric moisture) at 973 K. In these cases, tritium diffusivity decreased by 25% and 15% respectively. From structure analysis, tritium was found to coordinate with the impurities (Cr-3 H and 3 H-O bonding), which likely resulted in the observed differences in diffusivity relative to pure Flibe.…”

“…O ¼ 0:6 0:4 0:4 0:6 ; T ¼ 1 À t p t p t p 1 À t p (8) The transition probability of switching states, t p was set between 10 À6 to 10 À7 , which is markedly low to provide a stronger noise lter that removes false transitions in the observed signal. This is necessary due to the high-temperature liquid state causing high-frequency uctuations in the system.…”

“…In halide salts, atmospheric impurities such as hydrogen, oxygen, moisture and hydroxides are thermodynamically unstable and are consequently chemically reduced by the corrosion of metallic components of structural materials. 7,8 The presence of impurities has been shown to cause high rates of initial corrosion, resulting in signicant mass loss and material degradation. 9 Fluoride molten salts are of particular interest in the development of next-generation nuclear reactors, since they offer a combination of high heat capacity, high boiling point, and low neutron absorption.…”

“…Furthermore, corrosion products like CrF 2 have the potential to precipitate in the system, which leads to heat exchanger fouling and the presence of these dissolved species can also change the physical, thermodynamic and chemical properties of a molten salt mixture. 8 In order to minimize oxidation in structural steels, the salt must be sufficiently reducing such that the vast majority of the tritium is converted to H 2 , with a HF to H 2 ratio less than 0.01. On the other hand, the potential must remain sufficiently high (oxidizing) to prevent precious metal precipitation, resulting in a narrow operating window [U lower , U upper ] (U upper À U lower ) $ 0.1 V for Flibe) at reactor operating temperatures.…”

Recovery and separation of rare earth elements by molten salt electrolysis

“…A eutectic mixture LiF-NaF-KF (FLiNaK, 46.5LiF-11.5NaF-42KF, mol%) was proposed as a promising coolant in molten salt reactor applications [131]. Some studies focused on the corrosion of materials and reprocessing fission products in FLiNaK melts.…”