The Reactor
On the other side of the large door, you will find a truck sitting on a lift. Activate the lift by hitting the button in the booth and step onto the lift. At the bottom, there are several headcrabs scattered among breakable boxes and a bullsquid in a tunnel around the corner. Break the boxes for supplies, then go through the door and climb down the ladder. Downstairs, for the second time in the game, you will run into assassins. The next room is somewhat open, filled with stacks of green boxes and some small explosive boxes and barrels. There are also catwalks above that assassins will constantly use to pick at you from above. Equip the magnum or shotgun before entering this room, exploring it cautiously, killing any assassins that you encounter. Remember that snarks work rather well against assassins, as do weapons that deliver a lot of damage in one blow. On one side of the room, there are several breakable boxes blocking a doorway. This leads up to the catwalks above where more assassins may be lurking. On the opposite side of the room is another stairway leading down to a health station and an HEV station. When you've recharged, go back up to the top and look for an elevator and take it down.
At the bottom, you will be in a large, semi-circular tunnel. As you approach the door, an alien grunt forces it open and an alien grunt teleports in behind you. Give the alien in front you a good shotgun blast or two to the chest, then switch to the pistol, turn around, and shoot the explosive barrels by the alien behind you. Two more alien grunts will teleport in and you must kill them to proceed. When the last alien grunt is dead, a scientist will unlock the door for you. He will tell you that you need to restart the reactor and flood the core to get to the teleportation labs. Gather the ammunition and use the health and HEV stations here before taking the lift up.
Go down the hallway and take a right at the fork to find a scientist an interesting weapon. Save the game if you want to test it out without wasting ammo. Otherwise, just take the weapon and ammo and leave. Go back to the fork and go the other direction. This hallway will take you to the lift that leads to the nuclear reactor's core.