A radionuclide is an atom with an unstable nucleus, prone to radioactive decay. As these materials break down, they emit subatomic particles and gamma rays.
Unfortunately (or fortunately?), these gamma rays won’t turn you into the Incredible Hulk. They’re highly radioactive and can be deadly if not properly managed. In fact, most radionuclides enter the environment (and subsequently, our drinking water) through accidental spills or poor disposal methods. These contaminants can cause radiation poisoning in humans if given long-term exposure.