The Reactor Room is an unused room in Metroid Prime. It was designed by James Dargie and survives in renders shared on his website (seen to the right). The reactor was meant to open and close as some type of gameplay mechanic.
It is not clear whether it was meant for the Frigate Orpheon or some part of Tallon IV (likely either Glacier One or the Phazon Mines), although the final game contains a Reactor Core aboard the Orpheon, which is the setting of the game's first boss battle. Unlike this room, the reactor in that room is not interacted with in gameplay, although it overloads after the boss falls into it.