The problem is that fission does not actually hybridize with solar/wind/water very well. There is one possible exception; on the question of fission, I've switched from "yes" to "no" to "maybe, but still probably no" due to that one exception.
Fission has extremely high up front capital costs, but relatively low ongoing costs. What that means is you really, really want to run it at 100% as close to 24/7 as you possibly can. You also can't stop your fuel from undergoing decay even when it's not at critical, though that's somewhat minor.
Solar and wind have the issue that the amount of power you get from them doesn't ever match the amount of power you want. They're also so dirt cheap that we want to use as much of it as we possibly can. To complement them, we need something that can vary its power output.
That is not fission. It fights against that for economic and physics reasons. You end up having to shut off solar and wind at otherwise viable times because fission wants to keep dumping power on the grid.
Can't seem to find the reference now, but there is one proposed type of plant that stores the superheated steam in the secondary loop, and can therefore vary its output as needed. This would hybridize much better with solar and wind. However, it hasn't been proven in practice, and doesn't address a number of other economic problems with fission.
All that said, grid upgrades, grid upgrades, grid upgrades. I can't emphasize them enough, even though they're really boring. When you have good long distance transmission, the wind is always blowing somewhere, and the sun is shining somewhere for much of the day. You end up not needing nearly as much storage as you'd think. In fact, we may already have enough pumped hydro in the US to make it work. If not, then it's fairly close.