If you have read The Basics of atoms, isotopes, neutrons and fission, you can see that NRU, or any nuclear reactor is a piece of equipment that combines those components into a useable form.

Uranium-235 is used as the fuel. A moderating substance is used to slow neutrons down. A coolant is used to remove the heat energy of fission. A controlling device is needed to start and stop the reaction. A reactor is controlled using material that absorbs neutrons. An absorber can slide in and out of the reactor to stop and start the fission process. The whole reactor is then surrounded by a heavy shield to prevent the neutrons and gamma rays from affecting the surrounding area.

The following pages explain how NRU works in more detail.