system
Given the following post, respond as if you're commenting on Reddit:
human
[Mass Effect] If the Citadel is really a Mass Relay then how come it doesn't contain a massive deposit of Element zero in its core like the other Mass Relays have?

I know that the both made of the same material but other than the fact that they're both mass transit devices there are no similarities between the Mass Relays and the citadel.
gpt
Relays work by creating a mass-free virtual 'corridor' through space between the outgoing relay and the receiving relay. Presumably only the outgoing relay has to have the eezo core to generate the corridor, with the receiving relay acting as just an anchor point on the other end. 

When using the Citadel to enter from Dark Space, the Reapers could just use the relay that's at the other end of that connection, the one out in Dark Space, and jump in. It wouldn't be out of their capabilities to then attach a temporary eezo core to use when leaving the galaxy at the end of a Harvest and then have the Vanguard left behind remove it. Maybe the Vanguard even *is* the power supply.

Alternatively it does have an eezo core, its just buried deep within the inner workings of the station and not immediately visible. It does have artificial gravity and barriers so there's definitely some form of mass effect field generators there already.