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. Wait for the lift to start descending, then jump on top of the truck. 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. Alternatively, just fire the hivehand into any new room before you see any hostiles. 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 somewhere in the alcoves here 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 with 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. If you choose, open the gates to the alien's cells and leave the scientist to his fate. 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.
At the bottom of the lift, you will find yourself on a balcony overlooking pools of water. Here, unlike just about everywhere else in this place, the areas are clearly marked. Thank goodness. Follow the blue signs and stripes that say "Coolant System 01". Just before the door to the pump room, a vortigaunt will teleport in behind you, so be wary. The signs will take you to the pump station, with a large pool in the center, guarded by alien grunts. When they're down, enter the room and climb up a ladder. A catwalk up here leads to the pump room infested with headcrabs and vortigaunts. Kill them using whatever means you please, but I think you'll find that your recently acquired Hivehand is curiously efficient at clearing headcrabs. When you've cleared the room, find and flip the switch marked "Pump 01". This will cause the reservoir to fill with water as well as several more aliens to teleport in. Clear them out, then leave the room.
Follow the blue signs back to the lift, then follow the orangey yellow signs leading to "Pump 02". The hallway to the second pump will collapse as you go down it, and may seem blocked off, but if you approach the rubble, you'll find that there's still a way through. This hallway leads to a pool guarded by alien grunts very similar to the first one except that there are barnacles on the ceiling here. As you walk on the upper catwalk, an alien grunt will teleport in and break it in half. Back up as he teleports in and drop a grenade or two on his head. Further on, there is a steam leak blocking the path. Turn the valve wheel to your right to stop it and crawl through the gap. Down the corridor and up the stairs, you will find another pump room infested with headcrabs. Drop down and pull the lever to flood pools. When you've pulled the lever, check the chart on the wall. It should show that both Pump 01 and Pump 02 are operational.
Flooding the Reactor
Once the two pumps are operational, find your way back to the elevator where you entered the core. Take the passage to the side, following signs marked "Aux. Tank Reactor Access" and "Maintenance Area". There are two vortigaunts getting a bite to eat. Disregard Will Smith's advice and blast them while they're grabbing a snack. The tunnel to the right, marked "Maintenance Area" leads to a storage room guarded by a security guard. Run past him and down the stairs. Put up with the griping scientist here while you heal and grab supplies. When you're ready, run back down the other corridor marked "Aux. Tank Reactor Access".
At the end of this hallway is a door with a couple of alien grunts behind it. Open the door and immediately run forward, ignoring the grunts, and jump into the pool. Duck under the water to avoid hornets shot at you, then look for the tunnel underwater leading out of this room. The flooded tunnel takes you to the reactor's core. Underwater at the core, you will find two valves for each coolant tank. Turning each one raises the water more until it goes to the surface. Swim up and shoot the vortigaunt prowling around up here before you open the second valve.
At the surface, you'll see that the core is leaking electricity just a little bit. Swim over to the ladder and climb up halfway. Watch the bolts of electricity and run just after they pass. Do this all the way up to the top to avoid taking electrical damage. Save the game every time you go up a ladder just in case you bump into a particular nasty surge of electricity. At the top, wouldn't you know it, there is a bolt of electricity aimed right at the door. Time your run so you go through when the beam temporarily subsides.
Follow the corridor outside the reactor to find a supply station with lots of goodies. The lift nearby is broken, but there's a ladder going up. Climb the ladder to the top of the elevator shaft and jump to the piece of door protruding into the gap. There's another supply station up here with health packs and shotgun shells if you need either.
After you supply up, head down the hall till you get to a security station with a health station. The door is locked but you can easily duck-jump through the window. From here you can go left to a short hall. You will see the G-Man through a window but he will disappear followed by a headcrab.
As you walk down the hall, a Grunt will spawn in front of you. A vortigaunt will spawn behind you, then a second Grunt in front. After downing each of these, head to the wheel to turn off the steam pipes. Continue down and yet another Grunt and vortigaunt will spawn. Be careful after turning the corner because 2 more grunts will be waiting. Most of the doors will be locked.
One lab to the left of the steam pipes you turned off earlier will have a pair of portals. Walking into one will send you to the other. There is also another room linked to the lab but there is really nothing of note in here.
At the end of the hall, you will find a ladder that will take you to a dead scientist and a few supplies. Finishing here, head back up the ladder and into the last door. This will take you to core 2.
You will find a bunch of portals and a few more inside the core itself. Time the spinning platforms just right and make your jump inside. Looking up, you will see your next destination. Time your jump again and go on up. Save frequently. This one will have a pipe that you will have to duck under. Do this one more time to get to the top of the core. Now jump in the last portal and you will be teleported.
This room will have two rings of spinning platforms. The outer ring has walls that will knock you into the toxic pit. It will also have a track that will move up and down. time your jump just right to get to the center ring of platforms. As you spin around their will be 2 buttons marked 1 and 2. Work your way around and push each of these buttons. Again, make sure you save often as the toxic pit is a death sentence.
After releasing the two buttons, a door will open inside the core with another portal. Duck-jump into this portal and you will find yourself back in the earlier core but in safety. Head through the door. On your left, you will see 3 headcrabs feasting on a dead scientist. You will also find a couple battery packs here as well. The path will be blocked both ways so the only way to go is up.
Path to Xen
When you get to the top, you will find the Lambda survey team's supply depot being guarded by a shotgun wielding scientist and a security guard. The Scientist will open the door for you and give you some instructions about going into Xen. Stock up, this place has everything and you won't be back for a while. The scientist will also give you a long-jump pack. Since this is the first place you will have this, it might be a good idea to save and take a quick refresher course in the Training Room.
After stocking up and saving, head to the portal room. Another scientist will be up high getting the beam warmed up for you. No sooner does he start the machine, you will get ambushed by Xen Masters. Fend them off the best you can with your Magnum by shooting and running. When the scientist gives you the go-ahead find the platform on the ground level and jump on in to Xen.