New reactor designs promise to throttle, but I don’t know if any have been deployed.

This is a bit more complicated than a gas turbine. With a gas turbine, you just inject more gas as needed to keep the turbine turning at a constant speed. The combustion chamber is a violently small part of the whole.

The heat producing part of a reactor, by contrast, is massive. As a result you have thermal gradients, and these gradients are different at different power. They’re also larger at larger power.

A large thermal gradient isn’t a huge problem by itself (engineers have been accounting for this since the steam engine). The problem is the change of gradient itself. As the gradient changes, the materials inside change shape and this causes large mechanical stresses.

The situation is pretty much the same as when you turn off an ICE car: ever hear those ringing or popping sounds it makes? That is the various hot parts cooling and changing shape abruptly.

The problem for a reactor is made worse because material selection is very difficult: you cant just pick any alloy that has good thermal expansion, or good fatigue characteristics. Every alloy must be from a very narrow set of isotopes (not elements!!) that are compatible with the reactor’s neutron environment (balance, distribution, energy spectrum, etc).