Most nuclear power plants in the US fleet are not variable-output for design reasons. Mostly, both the reactor (see my other comment about neutron poisoning) an

Most nuclear power plants in the US fleet are not variable-output for design reasons. Mostly, both the reactor (see my other comment about neutron poisoning) and turbines are designed for safe operation at a fixed output and cannot operate at "partial" power for extended periods (in the turbines I think the problem is vibration and how it interacts with the bearing designs). A lot of this basically comes from these reactors having been designed without the benefit of computer modeling (everything had to be hand-calculated around a specific target output), newer designs based on computational modeling and optimization techniques achieve much larger safe/sustainable power ranges. Unfortunately we have very few reactors of modern design in the US.