Citadel Core

The Citadel Core was a huge radiactive source of energy, located within the City 17 Citadel. In normal circumstances it provided power for the many and varied needs of the Citadel; deliberately destabilized, as it was during the events of Half-Life 2:Episode One, it proved immensely destructive.

Ordinary Role
The Citadel contained two power sources of note: at the very top the Dark Energy Reactor, which Breen used to attempt to escape Gordon Freeman; and the Core, which sat almost at the bottom. It is not known how or to what extent they shared the role of powering the huge building, but the destruction of the Dark Energy Reactor at the end of Half-Life 2 seemed to place an untenable amount of stress on the Core. It was tended to by Stalkers, who unlike the other occupants of the Citadel did not seem to require safety equipment to operate within the Core itself.

Characteristics
The Core is never seen in normal circumstances. In a state of instability, it is shown to be a molten sphere of blue plasma, ebbing and receding in an agitated fashion. It atomises anything that comes into contact with it, including the metal ring which surrounds it.

It is protected by large blast doors carrying an insignia with the appearance of a stylized version of the international symbol for radioactivity. A large control room is situated overlooking the core with a central access lift leading down to its chamber. Three rooms are located across the chamber which are specifically designed to help stabilise the core in the event that its energy output becomes unstable. The machines in these rooms form a threefold containment system that ensures that the intensity of the dark energy reaction within the core stays within safe limits. Connected onto the core itself is a train station where it's likely that Razor Train cargo is emptied. This point in the Citadel lies approximately one kilometer below the surface and extends throughout City 17 and beyond.

Destabilization
After the destruction of the Dark Energy Reactor, the Combine deliberately destabilized the Core to facilitate an energy surge to send a message to its leadership through the portal rift above the Citadel, and to create a new Portal Storm. Although Gordon Freeman and Alyx Vance were able to temporarily stabilize the core in order to escape the city with most of its remaining citizens, the Combine were eventually able to set this plan into motion. It would come at a monstrous cost; once destabilized beyond the point of no return, the Core exploded, destroying the Citadel and City 17 with the force of an atom bomb.