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So as far as I know, nuclear fission uses high weight atomic elements to manufacture power. If the risk of runaway reactions are a major reason for not expanding this technology, why don't we use elements with lower atomic weight (eg. less energy) or lower amounts of fuel (eg. less total mass)?

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The nuclei used are the ones which can be fissioned in a chain reaction!

The lighter ones You think of will not split. That's simply a result of experiments.

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Georg, yayu's answer is the correct one. It is the energy budget that decides whether one gains energy by fusion or fission, binding energy per nucleon. So fission happens after Fe in atomic numbers, and yes, then the appropriate chain decays of the fission have to be chosen so that a self sustained reactor can be made. –  anna v May 21 '11 at 11:20
    
""So fission happens after Fe in atomic numbers,"" No, wrong. Look up a table of isotopes about fission or not. Yayus answer is not wrong, but much too comlicated to someone as simple as the questioner. –  Georg May 21 '11 at 13:13
    
@anna v -- Energetically, it's favorable for something with an atomic mass of, say, 150 to fission into two daughters with atomic masses of 75 each, right? (The binding energy per nucleon peaks at iron-56.) Yet this is experimentally not seen to happen. –  Ted Bunn May 21 '11 at 20:22
    
@Ted Bunn Sure it is seen to happen in the decay chains from higher atomic masses, and in low energy neutron beams. But it is not sufficient to set up a self sustained chain reaction useful for a reactor or bombs. It is traditional to call reactions with atomic masses above iron "fission", and below, "fusion" . –  anna v May 23 '11 at 11:13
    
""It is traditional to call reactions with atomic masses above iron "fission", and below, "fusion" . "" ???? –  Georg May 23 '11 at 11:42
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This is a graph of Binding Energy per nucleon for the various nuclei.

enter image description here

The fission of heavier nuclei would lead to daughters with higher binding energy than the parent (graph slopes downward towards the high mass end). Daughters will be stable, hence energetics favors fission for heavy nuclei.

In Response to comment:

This answer address fissionable nuclei, i.e elements heavier than iron. It has received justified criticism that this actually does not explain why certain nuclei are "fissile" (i.e nuclei that undergo fission when struck by slow thermal neutrons). After reading the question, I realize that the OPs concern was really with fissile materials. So I apologize for the initial answer which concentrated on fissionable nuclei.

The answer, btw, is still energetics. Wikipedia is a good starting point to get an explanation.

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I don't think this really answers the question. Energetically, anything after an atomic weight of about 100 would be a possible fission candidate (daughters more tightly bound than parent). So why do we have to go all the way up to atomic weights above 200? The answer's not energetics, at least not in this sense. It's that only the very heavy ones are so weakly bound that neutron-induced fission has an appreciable reaction rate. –  Ted Bunn May 21 '11 at 20:18
    
@Ted Bunn Please see the clarificatory addition to my answer.It addressed why some nuclei are fissionable. You are right, of course. As for making a artificial fission reaction with slow neutrons, this explanation does not suffice. –  yayu May 22 '11 at 20:20
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Once again,it is a matter of necessary and sufficient. It is necessary to have a small binding energy in order to be fissible, so that leaves nuclei above Fe in the binding energy curve, but not sufficient for the choice. It is the reactions and decay chains themselves that determine the choice. If a chain reaction cannot be set up, it is no use as a reactor fuel. Small atomic masses, even if larger than Fe are not able to set up a chain reaction. The question was about using smaller atomic masses than the ones utilized by present reactors. –  anna v May 23 '11 at 3:51
    
@Anna that is a correction I accepted in edit and the comment above. By "this explanation" i meant my initial explanation. Sorry if it confused you, English is not my native language but I try my best. –  yayu May 23 '11 at 4:08
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Nuclear fission uses large atoms splitting apart to generate energy. Only large atomic nuclei are unstable enough for this to happen at any appreciable rate

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Can't we use atoms 20% smaller to take a hit in efficiency in exchange for safety? –  Eric Fossum May 21 '11 at 3:25
    
the safety factor doesn't really work like that. There are some possible new fuels such as Throium which have a lower spontaneous rate 9so when you turn the reactor off it goes off quicker) but they have other issues –  Martin Beckett May 21 '11 at 3:32
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@Eric Fossum look at my comment in Georg's answer. Both the binding energy and the decay chain have to be taken into account for a viable reactor design. –  anna v May 21 '11 at 11:23
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