"Radioactivity is the energy that heats the water in a nuclear reactor." That statement, quoted directly from your last post, is incorrect. Here's why.

"Heat" is nothing more than molecular motion, the atoms and molecules vibrate more vigorously and we call it "heat". The energy to cause this mechanical motion can come from either chemical reactions (the coal combining with oxygen in the air to form carbon dioxide and water, plus excess heat) or it can come from nuclear fission (a uranium nucleus splitting to form lighter nuclei, plus excess heat. The vibrations of the products of combustion as well as the products of fission are transferred to the surroundings by mechanical contact! Simple collisions.

The energy from combustion comes from the rearranging of the electrons in the carbon and oxygen atoms before and after the combustion. The energy in the uranium comes from the rearrangement of protons and neutrons in the uranium nuclei. In both cases, you get motion in the products, motion of the water molecules of your boiling pot, and motion of the potato molecules in your food. There is no difference between the motion (heat)caused by combustion and the motion caused by fission. The only distinction is that you get a lot more energy (per unit mass of fuel) in fission than you do in combustion. And the reason for this is that there is much more energy locked up in the atomic nucleus than there is in the electron shells surrounding those nuclei.

You seem to be confusing radioactivity with fission. The two are NOT the same. A radioactive atom will occasionally emit a high energy particle plus energy. Radioactive substances have unstable nuclei which periodically give off these particles, plus energy. This is a slow and gradual process, and occurs quite intermittently, the exact speed determined by the half-life of that isotope, or the amount of time it takes for half the atoms of that isotope to undergo radioactive decay. Most radioactive materials have half lives that range from months to milennia. Those with shorter half-lives all disappeared a long time ago! The few isotopes with short half-lives, from seconds to weeks, don't stick around for very long, and are usually produced as a by-product of nuclear reactions. Consequently, they are quite rare.

SOME, (but not all) radio isotopes are fissionable, and they can be coaxed in a chain reaction to undergo wholesale disintegration in a relatively short time. The energy releases can be enormous, which is why we use them as power sources. By "enormous" I mean that the energy released is millions of times greater than the slow release of energy we get from natural radiactivity.

So for example, a pound of uranium may be radioactive, but you can handle it without even feeling any warmth, and the rate of radioactivity is so low (the half-life is so long) that it will take thousands of years before it all naturally decays to lead. By the way, I have actually held a pound of uranium foil in my hand. It was cold to the touch, and I was not burned by radiation. The only protection I had was a thin layer of paint on the foil, which protected me from being contaminated by that chemically toxic heavy metal. On the other hand, that same pound of uranium, in a controlled fission reaction, can push an aircraft carrier at flank speed for months!

So you need not worry that the radioactive decay of our uranium reactor fuels will cause the earth to overheat. The amount of energy available from radioactive decay is insignificant compared to that available from fission of the same uranium. It simply doesn't matter.

Besides, what would have happened if we had never made nuclear reactors or weapons in the first place? All that uranium would still be in the ground, as naturally occurring uranium ore, and it would still be radioactive and happily giving off heat! Its going to do that whether we mine it or not. Even if we never had discovered atomic energy, all the uranium in the world would still be in the world, locked up in minerals in the ground, slowly disintegrating. You can stop worrying about it. Fission power plants have a lot of drawbacks and problems, but global warming is not one of them.

Uranium is slightly radioactive (another way of saying it has a long half life). If you can induce that uranium to undergo a chain reaction, the fission products (pieces of split uranium nuclei) may also be radioactive (perhaps even highly radioactive). This is what that website you quoted meant about fission by-products becoming more radioactive as time passes. Radioactive byproducts (daughter nuclei) may have shorter half-lives than their parents. But the dynamic mix of radioactive species in spent nuclear fuel eventually reaches equilibrium, with new species replacing those which become inert. And eventually, all radioactive nuclei decay to stable isotopes.

However, the amount of energy (in the form of heat) due to that radioactivity is insignificant compared to the energy released during fission, and it is released over a very long time. You can convince yourself by looking up the amount of MeV (millions of electron volts of energy) due to radioactive emission from each nucleus and comparing it to the energy released by each individual fission event. Trust me, Johannes, compared to the latter, the former simply doesn't matter.

If you're really looking for a cause of global warming, don't blame nuclear reactors. Blame the fossil fuel industry, their greenhouse gases, and the reactionary, greedy politicians and businessmen determined at all costs to protect their profits from regulation and taxes.