When enough energy is passed through lanthanum, it heats up, melts and/or vaporizes...
So, either the lanthanum hull grids are not element 57, or the physics of the OTU are different.
Or perhaps the Lanthanum is a crucial element that forms part of the alloy/compound/material that makes up a "Lanthanum" Hull-grid and/or inner jump-drive coils, whose material properties are diferent than the pure Lanthanum metal by itself.