On the subject of safety: if a prompt supercritical accident were to occur in a fast reactor, as happened at Chernobyl (which of course was not a fast reactor), the result could be even more extreme. If the rearrangement of fuel in the resulting event were to cause some small fraction of the core to compress into an even smaller volume (and this is difficult to rule out a priori), the reactor could explode like a nuclear bomb. Imagine an event where the entire contents of the core was ejected by a kiloton (or larger) explosion.

For this reason, I suspect only a homogenous fast reactor (molten salt reactor using chloride salts, for example) could be licensed.