Robert Hargraves is fairly down on LiF as a salt for LFTRs in his recent book Thorium: Energy cheaper than coal because the Li produces radioactive tritium inside a LFTR, in quantities higher than can be legally released into the air (in the US, anyway). That means you need a chemical process to capture and sequester the tritium, hence more expense. Instead, Hargraves suggests another fluoride salt be subsituted for LiF, such as NaF for example. Not as good a neutron moderator, but that can be finessed by putting graphite into the core.