Here's how to start up the reactor:

Coolant Systems:

Through the entrance, head down the the control station (pd_c3a2c)
then follow the same startup procedures as the post-disaster version.
This opens up the Aux Tank, so just swim down that, then make your way up, press the button in the control room (pd_c3a2f).

Assuming you did this right, one of the monitors in pd_c3a2h should display something instead of static. Pick up your suit and weapons, and the scientist in the room starts up the portal for you.

(going to the portal without a suit or weapons allows you to roam freely)