Not that fossils/natural gas are required per se but their capabilities. Some places like Norway and Quebec are geographically blessed with distributed hydro that can fill a lot of that need. The variable load for a nuclear in that case could be many times larger than the generator itself but I'm not aware of any studies on that. Kinetic storage with massive flywheels is maybe the closest thing to that, or even batteries. You can ramp nukes by venting steam but that heat can cause environmental issues. Similar to hydro how their capabilites are reduced based on environmental factors like handling spring runoff.

There are some very recent reports out of the Ontario regulator who are dealing with this exact issue right now. Long term demand increasing for the first time vs carbon legislation, and the mandate to have a reliable grid.