Sounds like we could solve this problem for for nuclear the same way we look to solve problems for renewables: storage.
Note that even some of the OPERATING costs of a nuclear plant are fixed. You still need about as many staff to run the plant even if you cycle it up and down.
The renewables will be wind and PV. Biomass uses too much land area, and would likely be reserved for specialty markets like chemical feedstocks and perhaps aviation fuel.
?? What renewable isnt given that?
Nuclear works when you want it to. Solar and wind work when they want to. That's a very big difference when you're producing the electricity people rely on to live their daily lives.
Hydrogen can be much cheaper for (say) the last 10%, because (1) hydrogen has very low capital cost per unit of storage capacity, and (2) the efficiency hit of going through hydrogen vs. batteries is less important when it's just 10% of the total.
Think of batteries and hydrogen as analogous to cache memory and main memory in a computer. They have different performance and economic characteristics and compensate for each others weak points.
To see this in operation, go to https://model.energy/ and try turning hydrogen off and on in the settings. If you simulate for Germany, for example, turning off hydrogen can double the cost of achieving a certain level of constant grid power. Hydrogen can be particularly valuable for places with large seasonal variation or lots of wind (which has a long timescale component in how it varies.)
I will add that China is already selling electrolysers for < $300/kW, less than half that simulation's 2030 cost assumption.
Basically all hydrogen comes from fossil fuels. It's just natural gas with extra steps.
When describing hydrogen for energy storage in a 100% renewable grid, the hydrogen would be produced by electrolysis using renewable energy.
I once compared a proposed pumped hydro system in Arizona near Phoenix vs. the water evaporated by the Palos Verde nuclear generating station. Per unit of levelized power output, the pumped hydro system used at least an order of magnitude less water than the nuclear plant.
If the panels don't mind settling on the banks, or if they are far enough offshore not to, that will not be a problem. It is already common to float panels on regular hydroelectric generation reservoirs, so the event is anyway familiar to operators.
If the floats are bottom-heavy and attached to cables spanning the reservoir, the water dropping out from under just leaves them suspended. You need cable attachments, anyway, to extract power and maintain spacing.