It is absurd to worry about the radiotoxicity of isolated (especially "depleted") Uranium; it borders on scientific illiteracy to do so. Chemical toxicity is of some concern though, although the chemical toxicity of uranium is much lower than the chemical toxicity of common elements like lead and mercury.
There are over three
billion tons of uranium in the ocean alone. Uranium is a significant part of my soils here in New Jersey. Actually uranium is a rather common element in earth's crust, about as common as tin. It has been used over the centuries to make beautiful pottery glazes and is found in most older churches in the stained glass. Most large plane crashes, including those at the WTC, have distributed depleted uranium widely, since uranium is used as ballast in many aircraft. (The alternative is the far more toxic element lead.)
One of the largest sources for the wide atmospheric distribution of Uranium in the environment are coal fired powerplants, not tank shells. Uranium rains down continuously on people whenever they live near coal fired plants, particularly those fueled by Wyoming coals.
http://www.ornl.gov/info/ornlreview/rev26-34/text/colmain.html Also, the statement that uranium "persists" for 4,500,000,000 years is wrong. It will persist much longer than that if not fissioned. 4,500,000,000 is the
half-life, not the lifetime. This means only one half of the Uranium now existing will be gone around the time the sun moves into a red giant phase and fries the earth. There is four times as much Thorium as there is uranium. Thorium, which was widely used in the gas mantles for camp lanterns and other gas stoves, is also radioactive. It's half life is 14,000,000,000 years, meaning that one half of it will not have decayed in the entire history of the earth.
The only way to eliminate uranium and thorium is to fission it. When a uranium atom or daughter atom is fissioned, it generally produces two radioactive nuclei which decay comparatively rapidly, some in a matter of minutes or even seconds, others, in a few hundred years. It takes millions of years for uranium to come into radioactive equilibrium with its decay daughters; this is why "depleted" uranium is much, much less radiotoxic than natural uranium. All of the decay daughters and much of the more radioactive isotope U-235 have been removed from depleted uranium. This compares with around 20 radioactive nuclei produced in a decay chain of either Uranium and Thorium. Even if all the Uranium and Thorium on earth were fissioned however, the world would still be radioactive because of potassium, rubidium and other elements.
A nuclear program lasting one thousand years will begin to precipitously reduce the radioactivity of the planet. This may not be a good thing, since the existence and health of life on the planet may depend in subtle ways on radioactivity. Life on earth evolved in the presence of radioactivity, indeed, much higher radioactivity than we have now, since so much, in particular that of the highly radioactive potassium-40, has decayed. Quite a bit potassium-40 is still present though, and the radioactivity from that remaining fraction dwarfs the "problem" of so called "nuclear waste." Indeed, although all people contain traces of uranium and always have, most of the internal radioactivity that people exhibit derives from the also naturally occurring radioisotopes of potassium and rubidium. Rubidium, because of its similarity to potassium, is freely taken up by tissues. It is a "spectator element." This is to say that it is the element found in highest concentration in human flesh that has no known physiological role. About one quarter of the rubidium in flesh is radioactive Rb-87.