Perivayne wrote...
According to the lore, the Citadel has mass effect fields. We can be fairly safe in assuming that if there's an explosion, those fields are likely compromised in regards to keeping the Citadel in stationary orbit. Hate to trump the space magic, but Einstein/Newton rule that scenario. The Citadel would break up under its own inertia and fall to the nearest gravity well, which in this case, is Earth.
Either it's in orbit or it isn't. If it's not in orbit and just being held above the Earth then it will fall (and not break at all until it hits something, ignoring for a second that it blows up anyway), although what the point in putting it in such a position would be I don't know.
If it is in orbit the explosion will change the orbits of all the pieces. It's possible that some of those will then have orbits that take them into the atmosphere; how likely that is depends upon the height of the original orbit (lower == more likely), the size of the explosions (bigger == more likely) and the size of the piece (the smaller bits are more likely to undergo a greater change of orbit). An eventual ring of debris around the Earth, with the smallest bits coming down every now and then, would be the most likely result.