Critical Fact: Tailings contain approximately 85% of the radioactivity present in the original, unprocessed uranium ore.
This may seem counterintuitive — after all, the uranium has been removed. The explanation is that the uranium itself is only weakly radioactive (due to its very long half-life). The majority of the radioactivity in uranium ore comes from the uranium daughter products (such as radium-226, radon-222, bismuth-214, and lead-210) that have built up over geological timescales through the natural decay chain. When the uranium is chemically extracted, these daughter products remain in the waste stream.
The uranium product from the mill (yellowcake) therefore has a very low specific radioactivity — all the problematic daughter nuclides have been left behind in the tailings.