Ionizing radiation
Nuclear reactors are used to make electricity. They make a lot of radiation, but the reactors are built carefully to keep the radiation inside the reactor. But many people are afraid that if there were a problem with the reactor, radioactive material could escape into the environment, harming or killing many animals and people. Also, the parts of the reactor stay radioactive, and can kill people, for hundreds or thousands of years, so people are not sure where they can keep parts of old reactors safely away from people.