Rare events like that can be covered by combustion turbines. First with natural gas (not used often, so little CO2 emission), ultimately with hydrogen.

Because a simple cycle combustion turbine power plant is maybe 5% of the cost per watt of capacity of a nuclear power plant, we could cover the entire grid with CT backup and not come anywhere close to the cost of powering the grid with nuclear. And because these would be used so infrequently the fuel cost would be negligible.