Radiation is related to the making of the space. Even now, a tiny amount of a type of radiation called “cosmic rays” is always falling on the Earth. These cosmic rays then react with the air to create radioactive materials. Therefore, earth and atmosphere are both emitting radiation.

Uranium fission generates a variety of radioactive materials, such as iodine-131, cesium-137, etc. Inside a nuclear reactor, materials such as iron and cobalt become radioactive.
Let's take iodine as an example. Iodine is an essential element of the human body, but some kinds of iodine isotopes are radioactive.

When atoms have the same atomic number with a different number of neutrons, they are “isotopes” of that element. Radioactive isotopes, such as iodine-131, are called “radioisotopes”. They are generated artificially through uranium and plutonium fission.

In some cases, things that are normally not radioactive get “radioactivated” by radiation exposure and start to emit radiation. However, their radioactivity lessens as the time passes, and they eventually stabilize themselves again. The time needed for this stabilization differs. For example, iodine-131 loses half of its radioactivity in eight days