Building a Reactor in Big Reactors

A reactor is a hollow multiblock box with fuel rods inside. The smallest reactor is a 3x3x3 with a 1x1x1 interior, but a 5x5x5 gives room for real fuel columns.

1. Build the outer shell from Reactor Casing (and Reactor Glass where you want windows). Every block of the outer surface - all six faces, including corners and edges - must be a reactor block. Leave the inside hollow.
2. Place at least one Reactor Controller somewhere on the outer wall. You need exactly one controller.
3. Add the working parts to the wall: at least one Access Port (for fuel in / waste out) and at least one Power Tap (for power out). Add a Control Rod for each fuel-rod column.
4. Fill the interior with Fuel Rods. Fuel rods are placed as full-height vertical columns from the floor to the ceiling of the interior, each topped by a Control Rod on the ceiling.
5. Complete the structure. When the box is valid the multiblock assembles into a working reactor; right-click the Controller to open its GUI. If it will not form, check for gaps, a missing controller, or fuel rods that do not run the full interior height.