Uranium is a naturally occuring element in the Earth's crust. It is already ratioactive and is continually degrading spontaneously to form all sorts of radioactive byproducts such as radon. Uranium exists in two main forms or isotopes: 99% U238 and <1% U235. U238 is a stable isotope and not considered radioactive. U238 is denser than Lead and just slightly less dense than gold and is used by the millitary as "depleted uranium" as tank armor and in armor peircing shells.
U235, on the other hand, is the highly radioactive fissionable isotope. If you just get enough U235 together in one place at one time the spontaneously splitting atoms of U235 will give off extra neutrons which in-turn cause more U235 to split, resulting in an uncontrolled chain reaction giving off enormous amounts of heat and light. After reaching "critical mass" the U235 would create a blinding blue flash of light and then vaporize and explode into the familiar mushroom shaped cloud. However, this reaction would only be about 1% efficent because the vaporized metal would quickly dissipate; breaking the chain reaction.
Nuclear bombs are designed to be more efficent by using a polonium/berylium core to add extra neutrons to the reaction and U238 shielding. In a more controlled setting, if you simply purify U235 into a 6-8 sub-critical fuel rods and just get them close together they will get hot, make steam, which is used to make electricity. It's just like how the Shankarah Stones in Indiana Jones and the Temple of Doom glow when they are next to each other. It really is that easy, but you wouldn't want to hold hot U235 fuel rods.
Neutron absorbing control rods are placed in-between the U235 rods to absorb extra neutrons and prevent the core from getting so hot the fuel rods melt down, flow together into a super-critical mass and explode into a hiroshima-like mushroom cloud, or vaporize all the tritiated heavy water and blow the roof off the building that is housing the reactor (Chernobyl).
A new design of nuclear reactor called the pebble-bed reactor uses spheres of U235 encased in a shell of neutron shielding/slowing material (boron). The reactor is run by simply adding more or less spheres to the bottom of the heavy water tank. The shielded spheres are designed such that even when surrounded by other spheres, they can never get hot enough to melt down. They will heat up and make steam, but thats it. More pebbles makes more steam, and more electricity, but the U235 spheres will never get so hot that they melt.
U235 should be considered a clean way of producing electricity bacause it does not create more ratioactivity. U235 is already in the ground and highly radioactive. Using it to generate electricity requires processing it and concentrating it and then having to dispose of the same amount of radioactive material that was originally mined out of the Earth. Using heavy water (D2O) to cool and shield the reactor does create some tritium but its is reliatively short-lived.
U235 is very difficult to separate from U238. It requires very expensive and sophisticated gas-diffusion ultracentrigation techniques. However, at the same time, the process of separating the U235 isotope from U238 is very clean and generates almost no waste. The leftover U238 is used as tank armor. On the other hand, if you take that same non-radioactive U238, and put it in a reactor next to U235, it will capture a neutron and become the unstable and highly radioactive element plutonium (Pu239). This is much easier to separate from U238 with solvents but much more problematic. If you use solvents to separate U238 and Pu239, then you are left with hundreds of thosands of gallons of solvent with traces of Pu239 in it.
The Savannah River Site in CSRA Central Savannah River Area on the border of South Carolina and Georgia is one of the country's centers of plutonium production. Since the cold war, the site has been innundated with thousands of barrels of left-over solvent with radioactive residue known as "mixed waste." The site has pioneered a process of vitrification or "glassification'" in which the solvent is slowly and gently evaporated off and the remaining radioactive sludge, residue, and/or precipitate is encased in glass which is then encased in concrete which is then encased in a stainless steel jacket.
Although innovative, the production of plutonium from U238 is creating additional radioactive material from matter that was not originally radioactive. However, Pu239 will continue to be produced because it is the preferred fissile material used in US atomic warheads. It is my opinion that Atomic energy from processed, concentrated U235 using the new pebble-bed reactor design is both safe and enviromentally friendly. Absolutely no carbon dioxide or other polutant (except tritium) is produced in the the reaction. I encourage the US to focus more on Uranium-based reactors over Plutonium-based atomic energy.