Head to the elevator leading to the reactor, the one on the left as you approach from the VI core and the landlines. Assuming you already reconnected the landlines, a Rachni Soldier will emerge from a grate on the left hand side. Take it down and move to the elevator. Once you reach the main reactor, enter the first room and unless you have top-of-the-line Combat Scanners, your radar will be jammed. That's a sure sign it's time to re-equip your anti-armor rounds and other stuff you prefer against geth enemies. There are three main ways to approach this room: