Reactor

The very heart of the Nuclear Power Plant. Following the instructions of the site's manager, Mark will attempt to repair the reactor pumps in order to bring back water on top of the reactor and buy some time before the meltdown, before attempting to properly shut down the reactor itself. All throughout the level, radiation level screens are seen, giving the player a sense of how much time they have left. That system is completed with flashing lights and various alarms.

Disengaging the Reactor (SPOILERS)

Depending on his previous actions during the game, Mark will have one or two ways to go about shutting down the reactor safely.

The first way is to follow the manager's instructions and walk right into the reactor's core chamber. There, the player will have to solve a timed puzzle to push back all the control rods into the reactor, effectively slowing down then stopping the nuclear reaction entirely.

The other way requires (possibly ?) Mark to have fixed the Hammer Valley Dam and gained access to all parts of the Bunker level. Near the Reactor Core doors, a locked door with a keypad, similar to Door B2 in the Bergmann Tunnels is hidden behind boxes. Mark will ask the manager about that door, and depending on what Mark found in the bunker, the manager will either avoid the question or Mark will confront the manager, revealing what he knows about the whole operation, and gain access to this part of the power plant.

Here some offices, living quarters and a second control room will allow Mark to safely shut down the reactor without exposing himself directly to the radiations. An audio log will give some more insight on the bunker's purpose and its connection to the NPP.

A note pinned on the wall also mentions a mysterious code "0451", very famous sequence of numbers in video games.