You will be able to enter a new room very soon. There's a single guard inside and you must kill him. As you've probably noticed, there's also a locker here. Open here if you need to. Keep moving towards your current destination. Once you're close to a new corridor, wait for enemy soldiers to get here. Kill them by aiming at their heads. You should also know that more enemy units will appear below you. Kill them or try ignoring them by moving faster.
Keep pushing forward. You will have an opportunity to enter a new room. Just as before, you'll have to take out several guards here. There should be three of them (screen). Make sure to check your surroundings, because you'll find an ammo crate here. Rearm yourself and head on to the next door. You should also consider saving your current progress here.
Open this new door. As you've probably noticed by now, you're near the main reactor. Don't enter this area right away. Instead, wait for enemy soldiers to reveal themselves. Most of them will be using jetpacks (screen). Obviously you must kill them in the first place. Stay near the door all the time, so you will be able to retreat if you suffer some major injuries. Once you've killed all soldiers, enter this area and wait for the game to display a new objective for you.
New mission objective: Advance to the coolant control room beneath the reactor
Now you will have to go to your right. You shouldn't encounter any additional soldiers along the way. Your goal here is a small elevator (screen). Enter it and you'll be automatically taken to a lower floor. Wait for the elevator to stop and save your game before you decide to enter this new area of the map.
Once you've left the elevator, you should consider turning right. You will find a new locker here (screen). I'd recommend opening it and picking up one of the available weapons, especially if you don't have any grenades with you.
Make a few steps forward. You will probably notice that soldiers equipped with energy shields are heading your way. The easiest way to get rid of them would be to use a grenade launcher or a rocket launcher. You could also using the weapons from the locker, however it would be risky. Either way, they have to die. Save your game once again.
Start moving towards a nearby corridor. You shouldn't encounter any additional troops there. You will have to turn right in just a few seconds. There's a small tunnel in this area (screen). Sadly, it's being protected by laser beams. You will have to avoid these traps in order to be able to reach your destination. Obviously you should be using a time stop feature. Each time you must wait for the laser beam to be in a vertical position. I wouldn't recommend trying to avoid more than one laser beam at a time.