Why is uranium highly pollutant?

Well it's radioactive, meaning it's atomic structure decays and emits radiation and particles like neutrons that destroy the atomic structure of other matter. If enough pure uranium is in close quarters, it can cause a chain reaction and a huge explosion called a atom bomb. So much radiation, heat, light and particles are released it causes destruction on a widespread scale. The atomic structure of people, animals and plants alters their DNA and they can become mutated or die of radiation poisoning (if they survive the initial explosion) as their normal functions are no longer working correctly anymore. To find out more, you really need to research a good book on the subject, or Wikipedia.