AFAIK nuclear output can be controlled only in a certain range, but if you go below a certain threshold it shuts off and you then have to restart it, which takes a lot of time.
Coal has a similar issue, these plants also can't be turned on and off quickly. In fact, the term "base load" originally was driven more by the minimum amount of power that has to be consumed to keep a plant running. Only now with the move to renewables with varying output the meaning has shifted to the consumption side of things.
Gas plants however can be turned on and off and easily, they work similar to aircraft engines. In Germany, the heat produced is then used for district heating, which gives these plants almost perfect efficiency. It's not a coincidence that Germany picked gas as supplement for renewables during the transition to 100% green energy. Gas (in the form of hydrogen directly ~ methane made from hydrogen) is also a key contender for very long term energy storage (in Germany, you have to save energy produced in the summer for use in winter, the batteries needed for that would be roughly in the order of ~50 million Tesla Model S (100kWh) batteries). The cost of overbuilding renewables and storing the energy in gas, with a lower round-trip efficiency, seems to be lower than building the big battery that would be needed.
Wind is also better in winter than solar, which is why wind is such an important and hot topic in Germany. Solar doesn't bother anyone, but wind turbines are huge and people can see them in the landscape. Solar does not have this problem, but during the winter, the output goes down to almost nothing.
Coal has a similar issue, these plants also can't be turned on and off quickly. In fact, the term "base load" originally was driven more by the minimum amount of power that has to be consumed to keep a plant running. Only now with the move to renewables with varying output the meaning has shifted to the consumption side of things.
Gas plants however can be turned on and off and easily, they work similar to aircraft engines. In Germany, the heat produced is then used for district heating, which gives these plants almost perfect efficiency. It's not a coincidence that Germany picked gas as supplement for renewables during the transition to 100% green energy. Gas (in the form of hydrogen directly ~ methane made from hydrogen) is also a key contender for very long term energy storage (in Germany, you have to save energy produced in the summer for use in winter, the batteries needed for that would be roughly in the order of ~50 million Tesla Model S (100kWh) batteries). The cost of overbuilding renewables and storing the energy in gas, with a lower round-trip efficiency, seems to be lower than building the big battery that would be needed. Wind is also better in winter than solar, which is why wind is such an important and hot topic in Germany. Solar doesn't bother anyone, but wind turbines are huge and people can see them in the landscape. Solar does not have this problem, but during the winter, the output goes down to almost nothing.