Since a nuclear reactor is ‘highly radio-active,’ especially when operating at high power levels, producing a lot of penetrating gamma levels, producing a lot of penetrating gamma rays and neutrons, radiation workers need to be protected from the radiation.
So, ‘shielding’ materials of some kind are used, and these depend on the design of the reactor and the power levels. Most commercial nuclear plants can rely on concrete and distance the farther away you are, the less damaging the radiation-to reduce the radiation levels where people work.
Another good shielding material is lead. Water is also a good shield of neutrons, since the neutrons bounce off the hydrogen, and give up a lot of their energy with each collision.