I have, perhaps, the privilege, to have grown up in Germany in the 80s and at that time there was a really big societal discussion around nuclear energy in general and breeder technology in particular. A focus point was the breeder project in Kalkar, the SNR-300and many physicists and technicians chimed in into that discussion. I say privilege because it was a discussion unlike anything I've seen in this decade, people went very much into the details how such a technology works and what as a society we want from it.

https://en.wikipedia.org/wiki/SNR-300

One main point of the discussion was that such a breeder reactor uses fast neutrons and this has the effect that it is far less controllable. Normal fission power plants use slow neutrons, that is neutrons which come from a nuclear reaction, and are slowed down by a moderator, which is water. Breeder reactors do use fast neutrons and need a far higher energy density. One consequence this has is that they need really exotic and difficult to handle coolants, such as liquid sodium. I therefore call bullshit in that there is nothing complicated in them.

Also, such reactors have a different safety behavior. A key task of any nuclear technology is to keep the chain reaction of atoms and neutrons under control. This happens at the microsecond scale, and some chain reaction needs to be going for the reactor to not fizzle out, so it is anything but trivial.

If the reaction in a normal pressured-water fission reactor becomes too strong, this has the effect that immediately the water heats up. This makes the water less dense, and this reduces the effect of its moderation, which makes the neutrons faster, which reduces their potential to react with uranium atoms, and therefore it slows down the reaction. This achieves self-control. What happens when this self-control is lost, is perhaps best illustrated by the Chernobyl accident.

A fast breeder reactor does not have such an inherent control via the moderator and coolant. It was therefore judged by the public to be more complicated, and more dangerous.

A funny thing is that during that discussion, the proponents of nuclear energy said again and again that nuclear power plant's are safe and that they in particular can't explode.

I still remember the twitter post with these pictures:

https://www.youtube.com/watch?v=7lrJCqNpeZ8

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.

I was like 200-300mi/km from that thing when that happened. That morning the plant was nice sky blue, on evening it was like archival photos of Dresden bombing, that weekend many of news shows were explaining how concrete walls worked as blowoff panel using mock-ups and what could be going on up there in Unit 4’s spent fuel pool, and by the end of that decade the Japanese society as a whole showed deterioration by equivalent of that amount on a national scale.

I’m still pro-nuke though! In fact the whole situation made me so. Before I was naively thinking we better seek alternate fuels.