Here’s the generation statistics of the BN-800 reactor I mentioned before: https://pris.iaea.org/PRIS/CountryStatistics/ReactorDetails.aspx?current=451 It’s been operating at about 70% of it’s rated capacity basically since it was first turned on, that’s large scale power generation. Breeder reactors have been in commercial use for decades (see also: Phenix and Superphenix).

The simple reason why breeder reactors aren’t the default is because most reactors don’t need to be breeders. The two main upsides of a breeder reactor is a) breeding of nuclear material, which as I said before was only ever a concern in the very early days of nuclear power. We have thousands of years’ worth of fuel available now. b) The reuse of nuclear waste for additional power generation. Of course you have to have nuclear waste to reuse first, which necessitates many other, non-breeder reactors already being in use, so breeder reactors are usually restricted to countries that already have significant investment into nuclear power, like France, Russia, China, etc… If you don’t need to breed more nuclear fuel, and you don’t have waste to reprocess you might as well keep it simple and build a regular LWR reactor.

The Wikipedia page for breeder reactors has a whole list you can even sort by output capacity. For example, the BN-800.

There have been plenty. For example, the CANDU series of reactors developed in the 1950s and 60s. Breeder reactors were quite popular during the early days of nuclear power, as it was initially thought that there was maybe only 100 years’ worth of (easily accessible) nuclear material on earth, rather than the thousands (or tens of thousands) of years’ worth we know of now, due to both more reserves being discovered and also easier methods of fuel enrichment being developed. The fact that breeder reactors have fallen out of favour due to abundant fuel reserves certainly says something.

Breeder reactors produce more fissile material than they consume.

The MSP430 is just the chip I happen to use at work, if you’re not convinced you could try looking for an actual ultra low power chip, I found the STM32U0 at 70uA/MHz and the STM32U5 at 16uA/MHz in the first result.

Even ignoring selecting a more efficient micro, a smattering of tiny ceramic caps will buy you a few hundred microjoules for bursts. If you’re already operating at 2V you can get a 6V rated 100uF cap in a 1210 package - and that’s after considering the capacitance drop with DC biasing. Each one of those would buy you 200 microjoules, even just one ought to be plenty to wake up for a few tens of milliseconds every second to get a reading from some onboard peripheral (as an example) then go to sleep again.

For sure, you’re not going to be doing any heavy lifting and external peripherals could be tricky, but there are certainly embedded sensor use cases where this could be sufficient.

It’s more than you think. I work with the MSP430 microcontroller, which is capable of a sleep current of 40nA @ 2V, full active mode at 140uA/MHz with all onboard peripherals turned on. With this you could achieve almost a 20% on-off ratio with a 1MHz clock, or keep it in active mode all the time at ~150kHz, which is sufficient for many embedded sensor applications.