In NRU, as in most reactors, the fuel is uranium-235. It is the special properties of uranium discovered in the 1930s that enable a nuclear reactor to work. If u-235 captures an extra neutron it becomes unstable and splits into two parts, emitting two or three more neutrons. NRU fuel is in the form of long rods. These are lowered into the reactor from above. The lower section of each rod contains uranium. The long upper section is just used to handle the fuel.

A fuelling machine moves over the top of the deck plate of NRU. It can be connected to any of the vertical tubes that pass down into the core of the reactor. The fuelling machine loads and unloads fuel rods while the reactor operates. NRU was the first reactor in the world with that capability. The fuelling machine also takes used fuel rods away to a storage area.

With all the fuel rods inserted into the reactor, the lower sections form a core surrounded by heavy water. The core of NRU is about 2.5 m high and 3 m across.