Partly true. The bulk of the waste is just U238, which isn't really dangerous. However, instead of the U235 there are unstable fission products, left over afte

Partly true. The bulk of the waste is just U238, which isn't really dangerous. However, instead of the U235 there are unstable fission products, left over after heavy atoms split. They provide most of the radioactivity for the first century or two. There's also plutonium, formed when U238 absorbs a neutron without fissioning, and other transuranics formed in similar ways. The transuranics make up most of the long-term radioactivity. (This is an appealing thing about fast reactors; they fission more of the transuranics so only the fission products are left, and overall you're back to the radioactivity of the original ore in a couple centuries.)