Originally Posted By: pondering_it_all
The main problem is that hot lithium ions are very reactive, so they dissolve the uranium and thorium fuel, but they also combine with the reactor structure material, dissolving it into the molten salt.

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Lithium - World Nuclear Association

As a fluoride, Li-7 is used in the lithium fluoride (LiF) and lithium-beryllium fluorides (FLiBe) which comprise the coolant salt in most molten salt reactors (MSRs) now the focus of intensive development. FLiBe has about 14% lithium, so even higher levels of purity are required – 99.995% Li-7. In most cases the coolant salt also has the fuel dissolved in it. Such fluoride salts have very low vapour pressure even at red heat, carry more heat than the same volume of water, have good heat transfer properties, have low neutron absorbtion, are not damaged by radiation, do not react violently with air or water, and some are inert to some common structural metals.LiF is exceptionally stable chemically, and the LiF-BeF2 mix (‘FLiBe’) is eutectic (at 459°C it has a lower melting point than either ingredient – LiF is about 500°C). FLiBe is favoured in MSR primary cooling, and when uncontaminated has a low corrosion effect. The three nuclides (Li-7, Be, F) are among the few to have low enough thermal neutron capture cross-sections not to interfere with fission reactions. FLiNaK (LiF-NaF-KF) is also eutectic and solidifies at 454°C. It has a higher neutron cross-section than FLiBe or LiF but can be used intermediate cooling loops, without the toxic beryllium.
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