Such nuclear designs are being worked on. In fact they're essentially ready now, but one does not simply get approval to spool up a nuclear plant. There's no in

Such nuclear designs are being worked on. In fact they're essentially ready now, but one does not simply get approval to spool up a nuclear plant. There's no inherent reason why either type cannot supply variable power needs instead of baseload, but it tends to be more inefficient overall to use a design with a wide variance of generation capacity, which is likely why utilities try to stray away from that. I'm sure there's other problems though that an actual power distribution engineer could describe. E.g. the wide variance of wind/solar production from second to second would require a lot of energy storage and filtering to allow it to be placed on the grid (especially the rickety U.S. grid) without a series of transients accidentally opening circuit breakers and causing a blackout.