> Fusion-fission hybrids combine the worst features of both.

Maybe, but maybe not.

Worst features for fusion reactors: 1. they don't exit now and they won't exist for the next 50 years; 2. they produce lots of neutrons, which make the surroundings radioactive

Worst features for fission reactors: 3. they can go Chernobyl, 4. they produce long-living radioactive waste, 5. they are horribly expensive 6. proliferation concerns

How do these things look for a fusion-fission hybrid:

1. fusion reactors don't exist. Well, they do exist but they are well below the breakeven point. For a hybrid, the fusion part has (a very) negative energy balance, but it's more than made up for by the fission part, so being above breakeven is not a concern. The technology to manufacture the fusion part of a hybrid exists today (and has existed for decades)

2. fusion reactors produce lots of neutrons. For a hybrid, this is actually the point of the fusion half

3. fission reactors can go Chernobyl. This is so because the current fission reactors are powered by a chain reaction. This chain reaction threads the very fine line between subcritical and supercritical, in other words a classical fission reactor sits in a very narrow region between a bomb and a fizzle. The fission reactor in a hybrid gets its neutrons from its fusion partner, not via a chain reaction. The beauty of not having a chain reaction is that you can't have a supercritical chain reaction, or a Chernobyl event

4. fission reactors produce long-lived nuclear waste. I agree with you that this is not the big deal that's made up to be by environmental groups, but the fact that you can burn it via a fusion-fission hybrid is a nice bonus point

5. fission reactors are expensive. this is fundamentally a consequence of 3, that they present the danger of going boom. And as long as the fission reactors get their energy from a chain reaction, this danger exists. If you have a design that cannot go supercritical because it does not rely on a chain reaction, this is going to be inherently passively safe.

6. proliferation concerns. Here I simply have no idea how fussion-fission hybrids compare with classical fission reactors. That's why I mentioned the Department of Energy. If they develop and run these new reactors, then proliferation concerns become moot.

Besides all these points, the fusion-fission hybrids have another advantage: they can burn U-238 [1], which makes up 99% of the uranium on Earth. This means not only you have more fuel available, but you don't have to go through the stupendously expensive process of enrichment. Or it can burn Thorium-232, which is 3 times more abundant than uranium. In other words, not only the construction costs would be much lower, but the operation costs too.

Oh, and here's another advantage. Because classical fission reactors are based on a chain reaction that has to be very narrowly confined between supercritical and subcritical, at any given point only a very tiny fraction of the fuel is burning. Nuclear advocates don't like to dwell on that, but they like to point to the flip side of this coin, that the fuel lasts for a very long time (years). However, if you could burn the fuel faster, you can get the same power from a smaller reactor. We could be talking a factor of 100. Since construction costs don't scale linearly with size, a reactor that's 100 times smaller could easily be 1000 or 10000 times cheaper. And we could end up being able to send gigawatt-size reactors to Mars, rather than the kilowatt-size currently envisioned by NASA [2]

[1] https://en.wikipedia.org/wiki/Nuclear_fusion%E2%80%93fission...

[2] https://en.wikipedia.org/wiki/Kilopower