The reactor is located in Phia’s fortress, in the back room, and she initiates a self-destruct process whenever she realizes that you are following her. That is, if you get noticed.
Go to the Phii Fortress through the large gates and go to the room on the right, through the white doors, opposite the cage with one of the pictograms… There is also a camera hanging there and there are two enemies. In this room there is a charging station, and in the far left corner, on the table, is a diagram of the reactor. Examine it carefully and remember the order of disconnecting the wires. You may have it different. Remember? One of the wires will be unsigned, but I think it will not be difficult for you to determine its serial number (by elimination). Then, when Phia raises the alarm, or even just like that, go to the bottom of the reactor, go around it, standing between it and the nearest wall, and with a burglar open the grate on the control panel. Cut the wires in the correct order. If you are not mistaken, the sequence will be permanently saved in your journal. Now you know how to shut down the reactor in the Phii fortress Deathloop!