The Nether Reactor was officially removed from 0.12.1 version as that's when the Nether was introduced. The Reactor Revival Mod adds the previously existing Nether Reactor feature to Minecraft Pocket Edition and it works just pretty much the same way. It also adds the Inferno Reactor Core. Basically it allows you to get many of the items which only exist in the Nether as well fighting mobs and a boss.
To build the necessary structure for the Nether Reactor you will need 14 cobblestones, 4 gold blocks and 1 Nether Reactor Core (ID: 247, 6 iron ingots + 3 diamonds). Place the blocks according to what's seen on the images.
When you've successfully built the structure go ahead and tap on the the Nether Reactor Core in the center of the structure to activate it.
In a few seconds the sky will turn to black and the next thing you know the world around you look much different from before. A big structure of Netherrack has been generated and the Nether Reactor will be active for the next 30 seconds or so.
Zombie pigmen will spawn and gradually items will be spawn around the structure. Among the items are such ones which only can be found in the Nether.
This is the structure which is generated after activating a Nether Reactor.
To build the Inferno Reactor you will need 18 nether bricks, 8 gold blocks, 9 cobblestones, 1 netherrack (and flint and steel) and 1 Inferno Reactor Core (ID: 200, recipe: 6 diamonds, 2 obsidians, 1 reactor core).
Use the following instructions to build the structure.
Tap on the Inferno Reactor Core in the center of the structure to activate it.
A few seconds later a new structure will be generated and loads of Nether related items and mobs spawned.
At the very top of the structure is a boss which you can fight.