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[Jessica H]

You can find charts listing the relative abundance of elements in the universe(i.e. the most common elements in the universe are Hydrogen and Helium).  Since some of these are radioactive, is the composition of our universe slowly changing? 

I'm a biologist, so I know that C-14 (radioactive carbon) is constantly being regenerated by the breakdown of nitrogen gas in the atmosphere, making carbon dating possible.

On the other hand, I assume that radioative Uranium on Earth is relatively finite, since it has to be made in the lab to get enough for bombs and energy???

Do we eventually "run out" of some radioative elements as they decay, or are all of them being constantly regenerated by some process?


Jessica

[Bored chemist]

On our timescale we don't need to worry about uranium decaying before we use it. On the other hand, from a geological point of view the change can be significant.
When this happened
http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor
the proportion of the more easilly fissioned U235 was higher- otherwise the "reactor" would probably never have worked.
With "modern" uranium it wouldn't work- too much of the U235 has decayed so now we usually need to enrich the uranium to get fission reactors to work (there are other ways to get round it too).

On an even longer timescale things like supernovae produce "fresh" uranium- but you don't want to be there at the time.

Also there's the posibility of making radioactive materials in nuclear reactors.

[JimBob]

Thorium is also being developed as a new nuclear fuel because of its relative abundance, low cost and short half-life.

Any question, ask Bass.