I don't know what experts you are listening to, but the idea of shutting down a nuclear reactor by lowering the control rods when the sun shines or the wind blo

I don't know what experts you are listening to, but the idea of shutting down a nuclear reactor by lowering the control rods when the sun shines or the wind blows is just plain silly, for economic and safety reasons. Maybe for some future Gen IV design without fuel rods, but what you are talking about would require doubling the staffing and would defeat any cost savings in fuel use. Better to curtail the wind and solar or find ways to store the excess energy in batteries, hydro, or fuel. EDIT: Perhaps you mean something else by "spin up and spin down" but the usual sense of the words implies a complete shut-down. Nuclear reactors do have a normal operating range which can safely be reduced to half-power on a diurnal cycle (typically at night, but could just as easily be during the day.)