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Yes, nuclear fission is reversible.  Actually, it's more like fission is the reverse of fusion.

Fusion is what stars do.  Mostly stars fuse hydrogen to make helium.  Fusing light elements releases energy, which is how we ultimately get sunshine.

It takes energy to result in heavy elements, like uranium.  In nature, that energy comes from the collapse of a large star as it blows itself to smithereens.  A few billion years later, some of this uranium ends up in a planet, and eventually the inhabitants of that planet figure out how to get a small fraction of that energy back by fissioning the uranium into lighter elements again.

So while in theory is it possible to store energy by creating heavy elements like uranium from lighter elements, the technology to realize that is way beyond our current capability.  We're only just now getting close to fusing hydrogen to helium in a sustained way to get net power out, and without blowing up the equipment, lab, and surrounding city in the process.