So you want moar POWER!!!!!
Maybe you have an excess of Uranium, or just need to run a shiny new machine!! Either way a sad realisation has come upon you, that solar array just ain’t cutting it anymore, your lava supplies are not sufficent, and no matter how green your thumb is Bio Fuel just aint there.
So whats left, Petro Generator (fuel) but that means relying on dead Dinosaurs, and there can’t have been many of them, of course there is Nuclear!
The Facts:
Nuclear power was once a thing of Danger, but with advancements in remote sensors, redstone etc the modern reactor is no more dangerous than a pet Creeper...err Rabbit!
The Design:
All Nuclear Reactor’s start life the same, they are 5 chambers surround a reactor, you can of course use anywhere from 1 to 5 chambers but remember that the chambers themselves offer you some cooling. So the more the better even if your going for a humble mk1 setup.
First make a casing, the casing has to be
7 Long
6 Wide,
5 Deep
The reason for this odd shape is to allow plenty of cooling around the reactor, while allowing access to one chamber face for the safety system. Reactors need loads of cooling, and while it can be done without water, the water does take the edge off.
To make sure you have water Source blocks all the way around the reactor, it is recommended you place layers of ice and break them, this will make sure no flowing water (which is useless for cooling in FTB) and is easier than using a bucket.
But before I get too far ahead, what to make the casing from?
Glass, stone and other basic materials are useless, Obsidian is good, but will only shrink an explosion by about 10 blocks, Reinforced stone or Glass is best, they will stop the explosion in its tracks, well kind of, a mk1 low yeild reactor will be stopped, but ramp up the power the explosion is greater, to stop a large explosion you need 3 thick reinforced stone!
Here is an example of a glass, stone and Obsidian Chambers (no cooling) and the respective Fall out, the stone one did not work cause it was destroyed by the glass explosion
Reinforced Glass and Stone on the other hand

(this second picture is post explosion,

yes thats right no damage!)
The Safety:
For this you need a remote sensor and a remote monitor. Using an inverted redstone Signal you can power the reactor, having the remote monitor set to 500 and wired into the reactor you will make sure that the reactor won’t go boom.
Once the reactor hits 500 heat units, the redstone signal will be cut and no more heat or power will be generated.
So To Build a Reactor
Step 1 - Reactor on Block
Step 2 - make the Chambers and attach them to the sides
Step 3 - Build the Box
Step 4 - Using Ice Fill it with Water
Step 5 -Remote Monitor and Wiring
Set up your remote Sensor system (just ask I can give more detailed instructions) you wiring, to power storage etc