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The Citadel Core was a huge radioactive 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. Its meltdown released a massive amount of portal energies, which were focused and channeled into the creation of the Superportal.
 Ordinary role
The Citadel contained two power sources of note: at the very top the Dark Energy Reactor, used to power the Combine teleport (which needed huge amounts of power and therefore its own reactor), and the Core, which was located near 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. Alternatively, the Combine may have considered the Stalkers assigned to the Core to be expendable.
The Core is never seen in normal circumstances until Gordon returns to the main control room. In a state of instability, it is shown to be a blue sphere of unknown composition (most likely a sort of forcefield, due to its light-bending properties), encompassing a pitch-black volume of space , ebbing and receding in an agitated fashion, and occasionally giving off flashes of yellow light. It atomizes anything that comes into contact with it, and the metal ring which surrounds it was partially melted off. The stabilized version of the Dark Energy core is basically a non-ebbing blue ball, the black volume inside it having disappeared (or, more likely, shrunken to a microscopic size), with its four rotating containment pincers above retracted to contain it. If any object is thrown into it, it expands momentarily.
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 stabilize 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. After each of these energy tethers is activated, a section of the reddish symbol on nearby console screens change its color to blue. Connected onto the core itself is a train station where it is 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.
 Half-Life 2: Episode One
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 Superportal. 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 released a dark energy flare, destroying the Citadel and City 17 with force equal to or, perhaps, even greater than an atom bomb, as it easily blows apart the entirety of the Combine Citadel within a few seconds.
 Related Achievements
|Half-Life 2: Episode One|
|Contain the Citadel Core.|
|Contain the Citadel Core without killing any Stalkers.|
 Behind the scenes
- As seen in the WC mappack map "
proto_core002.vmf" (last edited February 2001), the Citadel Core was originally to appear in Half-Life 2. Just a prototype, it is mostly made of concrete only.
- The Episode One subtitles file contains several Alyx and Breen lines that were to be heard in the Citadel Core but were removed from the final game.
 List of appearances
- Half-Life 2: Episode One Trailer (First appearance)
- Half-Life 2: Episode One
- Source Particle Benchmark (Game files only) (Non-canonical appearance)
 See also
|Half-Life 2 story arc journey||Succeeded by|
City 17 Underground