Cyrus said:
vanesch, what is proposed in terms of nuclear waste disposal, and how long does it take before it becomes at a 'safe' level?
The standard solution (already studied by now for more than 20 years) is deep geological disposal (at depths from 200 - 700 meters underground). The idea is that the confinement of the geological layers is going to last for millions of years, but one studies eventual transport mechanisms that could bring some of the material to the biosphere before decay. Of course, you need to study carefully the geology and all the chemistry that goes with it, but there's by now a large body of knowledge on all this.
There are different "time scales" in nuclear waste, because of the different decay times of the different components. Also, the waste depends upon the kind of reactor we use. For the moment, we use "reactors with thermal spectrum" but "fast reactors" are the future.
I'll give you the numbers for a typical PWR. (pressured water reactor)
When used fuel is removed it is highly radioactive, because there are a lot of short-lived, highly active fission products. It is so active that it needs even some cooling or it gets hot. So the best thing to do is to keep it for a few years (typically 4 years) in a pool.
After that period, the activity has lowered quite a lot, and the material becomes more easily manageable (although still extremely radioactive).
After that, you can do several things. You can "can" it as "waste" (the open cycle). Or you can reprocess it, because only about 5% of enriched fuel has actually been "burned".
One considers that a component has become "safe" when it reaches the radiotoxicity level of the original uranium ore. That's a definition like any other, but it gives good orders of magnitude.
The spend fuel consists of 3 kinds of material:
- the fission products, the actual "ashes": they are by far the most active, but they decay fast: after about 300 - 400 years, this (essential) part is "safe".
- the minor actinides (americium, neptunium,...): unwanted side effects of a thermal spectrum: they are produced in small quantities, but they remain active on a scale of about 10 000 years, then they are "safe".
- the plutonium: partly unwanted, partly "new fuel", it is the nastiest component in the waste, and becomes "safe" only after 100 000 years.
Now, fuel reprocessing will take out the plutonium (that's what is done now) to re-use it as fuel. One COULD take out the minor actinides too, but doesn't know what to do with it for the moment, so one doesn't.
So as of now, unreprocessed fuel needs storage for at least 100 000 years, while reprocessed fuel from a thermal spectrum will only need 10 000 years.
However, in a fast reactor (which doesn't generate much, and can even burn, minor actinides), and can burn on plutonium, after reprocessing, the only thing that will remain is essentially the fission products. So there, the storage time needed is only 300-400 years. One can reprocess and reuse all of the plutonium, and the eventual small quantity of minor actinides, as fuel.
Reprocessed waste takes on the form of a solid solution in glass, put in a stainless steel container, of which the survival is estimated to be longer than 1000 years. In geological disposal, this means that the fission products don't matter, but that the actinides and the plutonium migration needs to be studied. However, it really seems that these things don't migrate a lot in the right geological layers. So even a dissolved container and glass wouldn't give a problem in the long run.
In the mean time, until permission is granted to do such actual disposal, the canisters are kept in temporary storage sites, where they can stay as long as one likes. In fact, the longer we keep them, the less active they are, and the closer we can pack them in the repository. So one usually thinks of keeping them 50 years (or longer) before putting them underground.
So:
- unprocessed fuel (still full of burnable material - so a real waste): 100 000 years
- reprocessed fuel from thermal power plant: 10 000 years
- reprocessed fuel from thermal power plant or from fast reactors, with removal of minor actinides, which can burn in fast reactors: 300 - 400 years.
After that, the waste is not "dangerous" anymore. Now, of course it is continuous decay: even after 200 years, the fission products are not very active anymore.
ANY of these schemes work out all right in deep storage. But of course, the shorter the needed period, the more sure we are of the predictions that nothing will go wrong.
