It's neither as far as I can make out.

It's just another huge chasm (the first Death Star had them so why should this be any different) which runs down the centre of the tower, along side the lift shaft (just like the one in the picture Angel just posted from ANH).

Haven't a clue what it's for or why it opens up in the room (the Sith seem to hire computer game designers for architects, Palpatine is from Naboo and just look at their powerstations) but it's there and for the reasons I described before it's doubtful in the extreme that Palpatine was chucked into the reactor.

He was merely chucked down the centre of a really tall tower.