But you can measure how much of the power of a grid is generated with fossil fuels at a particular place and time. For example, if they have more data centers where energy is cheap like from hydro or geothermal, then the carbon footprint will be less than if they were just using average power statistics.
Or at least not decommissioning old ones. A dollar invested into new solar or wind goes further than new nuclear right now, but we’ll see if it tips more towards nuclear once the grid is a higher percentage intermittent and needs a lot more energy storage with it.
Modular nuclear reactors seem really cool though for replacing large long term generators like at construction or excavation sites.