Phy6explorer said:
What about constructing factories for producing energy from spend fuel side-by-side the nuclear industries. In that way we can avoid too much transportation of the spend fuel and of course increase the amount of energy produced.
In fact, the US had such a pilot plant, with integrated fuel reprocessing (which was moreover much more proliferation resistent, because it didn't isolate the plutonium specifically, using pyro-processing). It was the IFR project (Integral Fast Reactor).
http://en.wikipedia.org/wiki/Integral_Fast_Reactor
For totally incomprehensible reasons, this has been abandoned when it was almost finished (sounds like the Superphenix and Kalkar debacles).
But actually who said it is being thrown away?According to a UN census in 1997, in the 20 countries which account for most of the world's nuclear power generation, spent fuel storage capacity at the reactors was 148,000 tonnes, with 59% of this utilized. Away-from-reactor storage capacity was 78,000 tonnes, with 44% utilised.
Yes, temporary storage is OK with me. But when one talks about the open cycle, one means: geological (irreversible) storage for good of the fuel elements "as is".
But the fact is that even after using up the spend fuel, that is after the reprocessing and vitrification of the 25-30 tons of spent fuel produced per year by a typical large nuclear reactor, waste is produced which amounts to about three cubic meter per year. I read that it has been accepted that this final waste will be disposed of in a deep geological repository.But my question is, what will happen to the final waste there?
It will decay, and its radioactivity will decrease. The radioactive components of spend fuel are of 3 orders. You also have to know that the shorter the half life, the higher the activity, but the faster it decays, while the longer the half life, the lower the activity, but the longer it takes.
There are 3 components to the spend fuel:
- fission products: the most active, but after a few hundred years, they have decayed. It is the "essential waste" because it is the "ashes" of the fission process.
A ton of spend fuel contains about 50 kg of fission products.
- minor actinides (americium, neptunium, curium). They are undesired products produced in thermal spectrum, and they remain active for a few thousand years. They are much less active than the fission products (except for the curium, but which has a half life of 18 years, so decays quickly), but are nevertheless sufficiently active to "consider them a hasard" for several thousand years (although, as I said, much less so than the fission products).
A ton of spend fuel only contains a few kilogram of minor actinides.
- the plutonium. Similar to the minor actinides, but we have an activity for about 100 000 years. A ton of spend fuel contains about 10 kg of plutonium.
So, the fission products need to be contained for a few hundred years, the minor actinides for say 10 000 years, and the plutonium for 100 000 years. These are in fact the times it takes for the radio-toxicity to decrease to the level of natural uranium ore, at which one considers that geological presence is not much more of a problem than actual natural uranium ore.
So the problem of geological disposal is to ensure that no significant amounts of the material can get back to the biosphere and ground water before stated times.
This is partly accomplished by the human structure (canisters, fillings, ... ) and partly by the geology itself (for the longer times). The point is also that the longer one waits for a leak, the less severe are the consequences.
If we take out the plutonium (PUREX or another technique), then the last component is not part of the waste. This diminishes the necessary "containment time" from 100 000 years to 10 000 years. If we take out also the minor actinides, we arrive at a few hundred years.
The last point is important, because it is possible to design canisters that will last that long. However, it is difficult to design canisters (although the Swedish did so) that are supposed to remain intact for 10 000 years or longer.
But "taking out" is only part of the story: what do you do with it next ? With plutonium, that's easy: its a good fuel for fast reactors. So you burn it in a fast reactor. You can burn it partially in thermal reactors (MOX), but that's limited. You will always remain with a certain fraction of unusable plutonium that way.
The minor actinides are part of a discussion. You can burn them (in small amounts) in fast reactors, or you can build ADS systems which try to burn them on purpose. The discussion is whether this is worth the effort - we'll come to that.
Fast reactors don't produce minor actinides (or in very very very small quantities, that is). So this problem is purely a difficulty of thermal reactors.
Now, if you reprocess fuel, then you vitrify the essential waste (let's say, minor actinides and fission products), and these go into a stainless steel canister, which will go into a bigger "repository" canister. Around that, you put some filling material, mainly clay or concrete. Normally, that's it.
But people study what's going to happen if there is some ground water flow through the repository. The stainless steel will rust away over about 1000 years, but in doing so, it will generate iron oxide, which is a strong reducing agent. The glass will very slowly dissolve in the ground water, which will also take a few thousand years. At that point, the waste is now "free" but the fission products are gone by now. We only have the minor actinides to worry about. Turns out that minor actinides don't migrate easily through a reducing atmosphere, and the dissolved glass also forms chemical migration barriers. After that, the clay sorbs actinides very easily. So it will take several thousands of years for the actinides to even be able to migrate outside of the "human structure" in small quantities. After that, they are confronted with the actual geological barrier.
One studies several possible scenarios and tries to estimate what will be the release of radioactive material after several tens of thousands of years. By then, most has decayed to very low activity levels. From this results then the final potential "contamination" of the repository. It is usually orders of magnitude below the natural background radiation level.
EDIT:
In fact, in the case of (tiny) releases in the far future, it turns out that the culprit is mostly Sn-126, with a half life of more than 100 000 years. This is one of the few fission products which live longer than a few hundred years, are produced in very tiny quantities, and ARE able to migrate. But, as said, the doses they can deliver are orders of magnitude below background doses. The actinides never seem to be able to migrate out, as they are chemically bound so easily to the local material (clay or other).
It is this observation which makes one put a question mark on the utility of getting rid of minor actinides, as visibly it are not these which get out after a long time. With current knowledge, it wouldn't make any difference in most if not all scenarios whether or not the minor actinides were removed or not.