In addition to what the sibling poster said, https://info.ornl.gov/sites/publications/files/Pub29596.pdf https://info.ornl.gov/sites/publications/files/Pub29596

In addition to what the sibling poster said, https://info.ornl.gov/sites/publications/files/Pub29596.pdf https://info.ornl.gov/sites/publications/files/Pub29596.pdf and http://moltensalt.org/references/static/downloads/pdf/MCFR_BATR.pdf http://moltensalt.org/references/static/downloads/pdf/MCFR_B... contain some discussion of the salt options, particularly for fast reactors. For thermal reactors, particularly if you want to breed, then you have to be very careful wrt losing neutrons (since you're barely with your nose above water to begin with), so there parasitic neutron capture becomes a very important parameter. And yes, in that regime FLiBe turns out to be a pretty good choice. For fast reactors, you have more excess neutrons so you have a bit more freedom. Cl, being heavier, allows a slightly harder spectrum, which is good, though not that critical. That being said, there are many fast spectrum paper reactors using flouride salts as well (not FLiBe, but FLiNaK, or some ZrF salts etc.).