Having toyed around with breeders for a bit now, I'm severely questioning their utility. For instance, the "easy breeder" which is a 6 chamber reactor that is filled with dumb Reactor Plates except for 5 empty spots (so a neutral breeder) so you can actually heat it up with lava (only 5 buckets!) has a big problem in that you can only breed 4 depleted cells per rod used, and even that can only be done if you're paying attention and swap the depleted cells out at the right time. Bigger breeders are near impossible to manage if you want to do something other than babysit your reactor. It just doesn't seem worth it.
The big problems (for me):
1. Heating up a reactor takes too long for anything that has any active cooling at all. A simple spreader/cooling cell flower adds 60,000 base heat capacity to the reactor (30 buckets!), which makes it impractical to heat up with buckets, added to the fact that you have a very narrow window between "breeds at a reasonable speed" and "smoking crater" makes it risky to use regular reactions to heat the reactor up.
2. Reactor cooling components melt at 10,000 heat, even if your reactor hull is good to 16,000, making that extra hull mostly worthless (if you hit it and start melting down your cooling components, the hull will rocket up to explosion in no time flat).
3. Because you're down at 1eff for the rod while it is breeding, you have to factor that into the equation. If you breed 2 rods using 1 when your normal reactor has a 2.2 eff rating, then you're losing 1.2 eff right off the top.
4. Mostly, it just takes too damn long. Almost an hour and a half at maximum speed. At best you're looking at two breeding runs per rod. If you spent your time mining instead of babysitting the reactor, you would almost certainly run across more rods than you would get out of your breeding program, and not have to spend hundreds of copper/iron/etc... making the reactor in the first place. It's far too much effort for too little payoff. If the rods bred in like 15 minutes or so (at max speed), then you could hang around your house building stuff and breeding rods one day to replenish the stock, but the current system basically requires you to run the breeder 24/7 to make it pay off. Nuke plants already have a disadvantage of being fuel based compared to the more common solution of just installing a ton of solar these days, why make breeding so uneconomical to boot?
Would it have really been so bad to have a little thermometer built into the side of the reactor window similar to the energy level on energy storage units?