Teen4Ideas said:
I just understand how there is nuclear waste, I've been taught that radioactive materials are radioactive because they are inherently UNSTABLE, unstable material is needed for a fission reaction, so why is there waste? Shouldn't what leaves the plant be stable, non radioactive material? I agree with theory process on the above comment, waste could be used as another fuel source right?
As far as it goes you are correct, but the difficulty is that you need to read up a lot on the nature of the
different types of instability involved. By way of analogy, think of nitroglycerine and of coal burning in air. Both release energy by oxidation and reduction of fuels, in other words, by exploiting instability of oxidants and reductants in contact. Both produce largely similar reaction products. And yet, coal would make a lousy material for blasting in mines, and nitroglycerine a lousy material for warming your stove.
OK?
Now, the nuclear instabilities in question are, as you realize, the source of energy, whether for bombs, power stations, or poisoning dissident Russians. Roughly they might be classed into two types:
- nuclear decay, which is what you get from unstable isotopes, and which proceeds in terms of half-life, each nucleus undergoing (usually) minor alterations at a random time while minding its own business and ignoring its neighbours and not doing anything on the lines of a chain reaction. Think radium, strontium 90, tritium and so on.
- nuclear burning, to which most radioactivity is only tangentially relevant, in which the nuclei undergo massive changes, either fission (splitting if you like) or fusion (joining, if you like) in which not only a lot of energy is given off, but particles that shoot off and split or fuse neighbouring nuclei that otherwise might have sat minding their own business. If this tickling up of your neighbour is effective enough, say every splitting/fusing atom causing two more to react, then we get a what we call positive feedback -- the faster it blows, the faster it grows. This is sometimes called a chain reaction and for serious power production, an effective, efficient chain reaction is crucial, either uncontrolled in a bomb, or carefully dripped out as in a power station.
Now, all the nuclear fuels in a fission reactor
happen to be radioactive, but that is simply because only very, very heavy nuclei are any good for splitting, and all nuclei more massive than lead (even bismuth it seems nowadays) are at least slightly radioactive anyway. Most of their fission products are incidentally radioactive, usually intensely so, meaning that they don't last long, but the point is not that they are active, which has nothing to do with chain reactions. What does matter is that they are too light to undergo any useful fission and therefore
support no chain reaction. In fact they tend to interfere with the main chain reaction and we speak of them as poisons, that we have to get rid of before we can use the remaining fuel.
At the light element end, where we can fuse hydrogen, lithium and similar nuclei,the process differs in detail, but the principle remains the same. Only the lightest nuclei are useful in fusion and the products ("ash") of the burn are either useless or a nuisance.
So I hope that helps you understand that it is not the instability of the "ash" that matters, but the fact that it is not suited for "burning" because
it cannot support a chain reaction. It does in fact give off heat and particles in quantities that continuously grow smaller (half-life) till we get an inert product, but the heat, though valuable in space vehicles etc, is too little to be valuable for bulk use in power stations.
