Inside the main hall, the reactor structure is about 10 m across, most of that is shielding made from heavy concrete, steel and water. The core itself, deep inside the shield, is about 3 m wide.
Shielding is there to stop gamma radiation and neutrons from shining out of the reactor into the hall. Gamma rays are in the same family as ultra-violet light and x-rays. We know that ultra-violet light can damage living tissue, so we shield our skin with sun-block or a shirt. Gamma rays have more energy than ultra-violet light, so the shielding used is thicker. Outside the shield, in the main hall, people go about their normal work every day, conducting research, or operating the reactor.
In NRU there are holes through the shielding with gates on them. These holes are used to let neutrons shine out from the reactor into instruments called neutron spectrometers. That is how neutrons are used in materials research; in the spectrometer, neutrons are used as a very penetrating probe that diffracts and reflects of atoms in a sample.