Nuclear power plants can increase output approximately 40MW per minute. So going from minimum output to maximum for 1.5GW plant takes hours.

Load spikes is like everyone comes home and turns the kettle on (plugs their hybrid car, turns the TV on, goes to shower, etc). You need to ramp up in minutes, not hours. Hydro and fossil fuel plants can do that. Others can't by design and because of the physical processes involved.