First of all, it's unclear that I had to build the reactor. I guess this should be one of the steps of the objective.
After placing the reactor, and adding a corridor, it gets added to the side instead of the port. When adding a second piece, it becomes unconnected again.
Initially when placing the reactor, it claims it's unconnected. After placing a single peace of corridor to the side, it claims all is ok and the reactor can't work, even when there is nothing connected in the end.