Besides maybe coal

They've had to keep upgrading them - the percentage of nuclear is the same, but no new plants have been built, so that extra power has come from research on how close to the red line they can actually run.

This is the trap. The fossil fuel industry has co-opted wind and PV solar by way of filling in the gaps and transitioning to net zero emissions. Of course, the gaps will always be there and the transition will never complete and "net zero" seems to just leave the door open on fossil fuels forever.

Nuclear power, on the other hand, has the reliability that @FireTower@lemmy.world mentioned and it closes any of the gaps from wind and solar right up. You don't have to quickly cut the power on a reactor if it's sunny or windy, just divert it to hydrogen and ammonia production. Even if the efficient high temperature electrolysis tech isn't ready yet, it doesn't really matter since it's emissions free. Furthermore, nuclear power produces good heat/steam to support cogeneration and various industrial processes.

The problem is that there are currently no good (cheap, scalable) technologies to store these large amounts of electrical energy.

I totally agree with this. A lot of places have cheap electricity in off-peak hours, as a workaround to this limitation (steady output).

I think that this obsession about intermittent power comes partially from the idea that any new sources of power must be drop-in replacements for the systems that we've had for so many decades. However those systems run the way they do as an accident of technology, not because of a careful analysis and design to match optimal usage patterns.

Consistent output is certainly useful when you break down demand into a constant demand + variable demand. For instance, if demand is typically 200-350 kWh, you could build a nuclear plant to generate 200 kWh and constantly run while you meet the varying 0-150 kWh demand with wind and solar.

I will agree though that we need to run numbers on this to determine the best approach. I don't have a feel for what wins out if we make larger solar and wind farms -- increased cost for the additional capacity, or increased efficiency from economies of scale.

im assuming by "winding down" you mean production of power? Not shutting down the plants, nuclear plants operate the most efficiently at high capacity factor, when they aren't producing power the fuel is still decaying, thus you should be producing power for AS LONG as possible. This is why if you ever look at capacity factor >80% is really common, i've even seen >100% a couple of times, as well as the term "baseload plant" being used almost always in reference to nuclear.

That wouldn't make sense for an existing nuclear plant, the nuclear plant should stay running in place of solar/wind. As you would be burning money actively otherwise, or you could just shut it down permanently, thats the other option.