Quote from GadzooksThePaperCreeper
Yes, what is a Nether reactor?
It is a special PE feature. Its basically a building that has a core in the center, and when you activate it the area around it is covered by obsidian, with zombie pigmen spawning and special nether items and special items randomly spawning. The objective is to survive as long as you can, and grab items that may have not generated in your world (like cacti, sugar cane, and mushrooms). When you die the reactor “explodes", and a large chunk of the obsidian remains, so it also marks yout world.
There are already Youtube videos showing it, they uploaded Jbernhardsson stream.