Radiation is scary because no one can tell it’s there without special detectors. Animals like you and me have senses which means we can see, hear, touch, taste, smell -- but we can’t tell if something is radioactive (gives off radiation). Waste products from making nuclear bombs and from nuclear power plants are very radioactive indeed. If you happened to fall into a nuclear reactor, you would die almost instantly. Lower radiation levels can also make people die -- over a matter of days or, by causing illnesses like cancer, over a period of years.
Radiation comes from new elements (uranium fuel is an element; so is carbon and so is oxygen) which get made during nuclear reactions. These elements, called isotopes, often only exist for a few weeks or years. But some last for hundreds of thousands of years which is why no one really knows what to do about getting rid of them. Isotopes have something called a ‘half-life’. One dangerous one which got spewed out from the wrecked Chernobyl reactor was an iodine isotope. This has a half-life of 8 days which means that if you start with a chunk the size of a can of beans, in 8 days its size will have halved; a half can of beans. Eight more days later and you have one quarter of what you started with... and so on. Where does all that missing stuff go? That’s the nasty bit: the radiation. And it’s dangerous to life like you and me because it’s made of tiny particles which travel very fast. So if you stand near something radioactive, you’re being hit by trillions of tiny ‘bullets’ all the time. You can’t feel it but these ‘bullets’ damage the cells in your body. A lot of damage breaks them and you die. Less damage messes up their genes and causes cancer and kills you slowly. Like I said, nasty stuff.