|Being born in the early 60's, I grew up knowing that you could send a nuclear sub around the world on a chunk of uranium the size of a golf ball and that the half-life of plutonium was 38,000 years. So why does the world now have so much nuclear waste to get rid of? Why, if something has a half-life of many thousands of years, is it waste after only a few? — SG, Sydney, Australia
First, nuclear waste isn't 100% radioactive atoms. Much of it is radioactively contaminated material—normal materials that contain enough radioactive atoms to be considered hazardous. Second, nuclear reactors don't wait for radioactive materials to decay via spontaneous processes, the ones that are responsible for half-lives. Instead, they induce the radioactive decays using chain reactions. In a nuclear fission reactor, the spontaneous decay of one uranium or plutonium nucleus is used to induce decays in other uranium or plutonium nuclei. In this manner, huge fractions of the uranium or plutonium nuclei can be "used up" in only a few years. In fact, in a nuclear fission bomb, many or most of the uranium or plutonium nuclei are consumed in less than a millionth of a second because of these induced fissions. Half-life has almost nothing to do with a fission bomb. It becomes nuclear waste so fast you can't imagine it.