Fusion
I worked in Fusion for 6.5 years. I can tell you: It doesn't have potential to replace fission.
(We have only one working fusion reactor, and its energy and power densities are approximately that of your average compost heap. But it is so massive, that it lights up the planets nevertheless. That's why any fusion power plant has to be even hotter and even higher pressure than the sun, which is hard to achieve. And the energy output of a fusion reactor is in terms of neutron radiation. It is much MORE radioactive, when operating, than a fusion reactor, the only difference is you can turn it off. It doesn't produce heavy isotopes with thousands of years of decay time. But it DOES irradiate your cobalt-steel with neutrons and make it radioactive, for example)
However, fusion may well have potential to augment fission though: e.g. hybrid fusion-fission reactors, you use a small "fusor" (i.e. a very small fusion reactor that doesn't care about reaching anywhere near unity output, these are cheap and compact, you could almost build one at home) as a source of fast neutrons with a fast "off" switch. These neutrons are then used as an "ignition" for a fission reaction using sub-critical Uranium, i.e. uranium that cannot sustain a reaction by itself. Now you have a nuclear reactor that you can turn off, just like that.
Large-scale battery storage is potentially as dangerous as Nuclear. Current battery storage systems are tiny, they are only able to compensate for frequency deviations in the grid, i.e. when the sun goes behind a cloud, they provide just enough juice to prop up the grid while we spin up a gas turbine and start burning more fossils.
If you wanted a battery that could power 10% of the UK grid for an hour, (never mind overnight on a still day), you're talking 4-5GWh. The biggest battery storage projects in the world are a fraction of that, and we have yet to see the effect of one of them having a 'meltdown', but we know that it involves a lot of extremely toxic fluorinated gases e.g. HF entering the atmosphere. On the other hand we have had a few meltdowns of nuclear plants over the last decades, and they have proved extremely rare, and surprisingly safe. For example, everyone seems to think of Fukushima as a disaster, but I see it as a triumph for nuclear safety: It was a very old design, it suffered pretty much the worst event that can happen to a nuclear reactor: A Tsunami that killed 50,000 people and made millions homeless flooded the plant and shut off all power to cooling systems. 50 people went in to stabilise it, all of them expecting to die from the radioactivity, but so far not one of them (afaik) has. The tsunami killed 50,000, the nuclear reactor killed approximately zero. So where is the disaster?
And without more reactors, we will burn more fossils, and we will have more typhoons. (I don't think we can blame global warming for tsunamis)
I think if the money we have spent on Fusion could have been spent instead on better Fission designs, then the world would be in a much better place right now.
We could have had molten salt fast (as in fast neutrons) reactors, which are able to consume the spent fuel that other reactors would consider to be "high level nuclear waste". They can even eat the Plutonium that we have stockpiled for weapons.
