[pfdietz]

The ARC design immerses the vacuum vessel in a bath of molten salt (which is where the Be is, in lithium beryllium fluoride (FLiBe) salt). That salt is where the neutrons deposit their heat. Replacing the Be with lead means the heat is getting deposited in that lead (or, more likely, molten lead-lithium alloy).

Even though ARC uses salt, it would also have to worry about voltages induced by flow across magnetic field lines -- not because of currents, but because if the voltage becomes high enough it can induce electrochemical reactions, like production of elemental fluorine (or corrosion of metal where the fluorine would have been evolved.) I think they keep the velocity x coolant channel diameter low enough to avoid that, but it's still a consideration they have to address.