Modern nuclear plants can do this with control rods, but the fixed and sunk costs makes it prohibiting expensive. It is simply more economically to keep them running, even when wind/solar is producing. Changes in fuel costs could incentivize a more economical use, through that would only work if nuclear were made economical competitive compared to natural gas in the first place.

If we look at a map like https://www.electricitymap.org and imagine the carbon intensity to be a real economical costs then it would be a relative simple matter to determine how high those costs would need to be to make nuclear competitive against burning fossil fuels.