It's not slow: the turbine water is can be cooled more aggressively immediately, and will start reducing output with one circuit of the generation turbine. Also, modulation only needs to vary 20-30% over the span of entire days not of tens of seconds. And no, dispatchable generation does not ramp in 10s of seconds. Natural gas plants - the most popular peaking generation plant - still takes an hour to activate. But this isn't an issue because electricity use doesn't fluctuate by 20% in the matter of tens of seconds.
> And it's still not dispatchable if not using it costs you anyway.
No? This just plain wrong. A dispatchable source is a dispatchable source, regardless of any associated costs. And with nuclear there isn't even any direct cost with running the plant at a reduced capacity. There's only the opportunity cost of lost electricity sales, which would happen anyway because there isn't enough demand.
If there's 100 GW of peak demand and 80 GW of minimum demand, building 100 GW of nuclear plants and reducing output during periods of non-consumption does not have any increase of costs.
If there's noone to sell your $150/MWh electricity to because they took one look at the price and put a solar panel on their roof, then you're not selling $150/MWh electricity, you're selling $500/MWh electricity for the 20% of power they must buy. Then when they take a look at the new price, they go buy a battery. The only way to pay it off is a government enforced utility connection fee for a product nobody wants.
The only way to sell it for $150/MWh is to underprovision or to build storage or to find dispatchable loads. Just like renewables.
None of this has anything to do with dispatchablity. Nuclear power is indeed dispatchable, which is why you're pivoting to this strawman about pricing. If we had a primarily nuclear grid, there's be no need for solar panels anyway.
> Then when they take a look at the new price, they go buy a battery
You're making the same error a lot of renewable activists do: assuming that household electricity use is all there is. How do you power the turbopumps that make our sewage and plumbing systems? How about our telecommunications systems? We'll just deal with cell phones shutting off after dark?
Energy storage requirements are staggering. The world uses 60,000 GWh of electricity every day. Storage requirements are at least 12 hours for diurnal storage, and several days for seasonal storage. Just going out and buying a hundred terawatt hours worth of batteries is a lot easier said than done.
The mines and aluminium smelters and arc furnaces and polysilicon plants are all building their own renewables. They're not going to buy your daytime energy either when they can make their own DC power at $10-30/MWh. The industries which require hydrogen or derivatives will just make it on site and store a few weeks worth. The industries that need heat or steam will store it in a lump of iron ore wrapped in some fire bricks and rockwool.
Then you might want to just stop and think about how you might go about storing energy if you have a pump and a reservoir on a hill or a water tower. Just ponder that one for a few seconds.