Fia likes to hang out in a secret underground bunker that is guarded with a lot of automated security and plenty of people who patrol the interior and exterior. If you have visited this place and decided to go full Terminator and go in guns blazing, you will find that the reactor will eventually detonate, blowing up the entire island.
Fia has some secrets that she would rather take to the grave than let them fall into your hands. This might have you asking two different questions. How to stop Fia from triggering the reactor to blow and how can you stop the reactor from blowing up after Fia has triggered the meltdown. The handy thing for you is the answer is the same for both questions.
How To Disable The Reactor
In the workshop of the bunker, you will find a schematic of the back of the reactor that will show you how to cut the wires in order to shut the reactor down. I am unsure whether this order is randomly generated for players, if so, please leave a comment below letting me know. Here is the order in which the wires needed to be cut.
When the final wire has been snipped, the reactor will shut down, preventing a meltdown. If you have yet to break stealth, you can also shut this down in advance and then go guns blazing. When Fia tries to overload the reactor, nothing will happen.