Earlier quoted context omitted.
Fusion and Fission systems tend to run 24/7, unlike Wind and Solar. There is far less need to store energy when using the former types of generation.
Fission: Nuclear power stations have relatively low capacity factors (a measurement of how much of time they're actually connected to the grid and producing power). This is typically 70% over the last few decades for Western countries (i.e. a third of the time, a nuclear power station is not producing anything). As well as unintended maintenance shut down periods, most reactor designs require a multi-month shut down…
[1]: https://en.wikipedia.org/wiki/Illusion_of_transparency
Edit: +1, much clearer what you mean now.