The tooltip on the nuclear reactor says its accepted fuel is either the cube or uranium fuel cell, but it's not. It's only the cube. Wasted almost a couple of hours setting up a nuclear power setup and automating uranium fuel cells just for them not to work.

You need to start the reactor by feeding it the cube. Once it burns through the cube and is hot (I think), then it will accept uranium fuel cells. If it ever runs out of fuel, you'll need to bring the cube back to get it going again.

This is regrettably not very clearly telegraphed in the game, but the description text on the nuclear reactor does explain it.

Perhaps Ultracube could add a side panel onto the right-hand side of the reactor window, like Filter Helper adds one onto some containers/inserter windows.
The panel could state what phase/mode the reactor was in, and if cold, say that it needs the cube to warm it up before it can accept fuel cells.