Exactly right. The goal with nuclear waste is basically to get it back down to levels similar the original ore. At that point, it isn't especially dangerous.

Many radioactive products are either very short lived, and will decay down to minimal levels within a few hundred years, or are very long lived, causing very low radioactivity.

The problem are the elements that are in between which is mostly the other actinides. Those are the ones that would require tens of thousands of years or more to reach safe activity levels. Fast reactor designs don't have significant amounts of such elements in the waste. The waste will decay to uranium ore-ore like levels within only hundreds of years.

The biggest issue is that they need relatively enriched uranium to operate at first. After they are started, they can be used as a breeder reactor that can take in natural uranium and convert it to the enriched uranium it needs to continue running.

Fast reactors are also in principle subject to Chernobyl-like criticality accidents (only worse) in case of meltdowns.

I mean, any bad design is subject to failure. Aircraft are subject to catastrophic failure too, so we iterate and improve until they're super safe.

Fast reactor inherently have more fissionable material in a smaller volume, and don't have a moderator that can either keep things separated or remove itself in an accident to kill the chain reaction. If a supercriticality on prompt neutrons alone occurs in a fast reactor the neutron population will increase orders of magnitude faster than in a thermal reactor, because the neutrons are moving so much faster.

Edward Teller in 1967 famously expressed his skepticism about fast reactors because of this safety issue.

That's not the argument I was making. I said anything of sufficient scale carries significant risk, and we can in time reduce that risk to acceptable levels via engineering practices and principles. As with airplanes, which are metal tubes full of humans we load up with kerosene and send into the atmosphere. A bad plane will crash, a bad reactor will melt down. We can overcome the risks to reap the rewards. That's like our whole thing, as humans.

Chernobyl happened for a very specific reason - you know, the late-night tests and the positive void coefficient. Do you think we haven't learned? Please cite sources. The thing is, we have learned. That's why reactors are so expensive. You're double-dipping by citing the risks and the costs. It's really one or the other if you're not being disingenuous.

It's not the argument you were making, but it's a point your argument is compelled to address. Yes, any system of sufficient scale carries risks. In the case of breeder reactors, one of those risks is the possibility of something approaching an actual nuclear explosion, with fast neutron physics similar to what happens in actual fission bombs, not pitiful low yield moderated explosions as happened in Chernobyl.

> fast reactors yield waste products that principally live only a couple of hundred years.

Hundreds of years is not acceptable. Humanity is not able to handle projects spanning hundreds of years time. Look at the Zaporizhia atomic energy site for an extreme example of what I mean.

The problem is not just the time scale either. There are other areas humanity fails at time and again.

P.S. Thank you nevertheless for pointing out several advantages of fast breeder reactors.

> Humanity is not able to handle projects spanning hundreds of years time.

There's plenty of buildings that took centuries to make, this projects are comparatively simple/require less coordination once started.