Phy6explorer said:
So ultimately, deep geological storage is the only decent choice? Artificial transmutation is out of the question?
The MAIN source of radioactivity in the waste, for the first few hundred years, is a large array of fission products. They are not transformable. Well, you could pick out a few of them, and apply a specific transformation technique, but for the bulk of it, there's no transformation going to be possible: there are hundreds of them! Just putting things undifferentiated into a neutron flux will maybe improve some, and activate others. So the main source of radioativity is in ANY CASE there. Transmutation only applies to a few long-living nuclides that we might want to get rid of, and after a lot of separation work, get them concentrated and do something about it.
The obvious target are the minor actinides, such as Am-241, which are still pretty active, and have relatively long half life times. There are also a few trace fission products who live long (and hence are only a little bit active), like I-129 and Sn-126. Point is, they are only a little bit radioactive. For instance, the La Hague reprocessing plant in France just dissolves the extracted I-129 directly in the seawater, because in that way, it is immediately isotopically diluted and the dose one can obtain from it is hundreds of times smaller than the natural background dose.
The whole "problem" with the actinides is that they only decay significantly after 10 000 years, a time span which is too large for an engineer to build something that he can guarantee will survive that time span. On the other hand, we know that actinides migrate very slowly through most geological layers, and in any case, their activity 10 000 years from now is about 10000 times less than the activity of the waste NOW. But there remains the (psychological ?) problem of having it confined by geology, and not by human engineering.
The long-lived radioactive products are only a few, and THEY can be selectively transmuted. Transuranics undergo fission in a fast spectrum (and hence are reduced to fission products), and there are also a few solutions for these few small long-living fission products like I-129 and Sn-126. The only question is: does it solve a genuine problem ?
THIS is what transmutation is about.
But, again, there's no hope to get rid of the (high) radioactivity of the bulk of the fission products during the first few hundred years. That remains the essential waste. BUT, one can engineer canisters which will withstand time for a few centuries.
But fast reactors sounds appealing, every thing has its own dis-advantages, but as you say instead of making a reactor which produces less energy than the thermal ones and its main purpose is to produce fissile material to run thermal reactors, what's the point? It is just as good as what's happening now.
No no, you have it wrong there. A fast reactor is just as well energy producing as a LWR or a BWR, while it is breeding. But the main advantage of a fast reactor is that it can use U-238 as fuel (well, first breed it into plutonium, and then consume it), while a LWR (or any thermal reactor) can do this only marginally. This means that a fast reactor can use mined uranium about 100 times more efficiently than a LWR. THAT is the main advantage of a fast reactor.
A LWR reactor uses only the U-235 component of uranium (0.7%), and "breeds" only about 60%, of which it consumes 30% on the spot, meaning, overall, about 1% of the actual natural uranium is used as a fuel. A fast reactor can use most of it, once it has some plutonium to get started with.
In fact, all the current "waste fuel elements" still contain 95% of their energy, and all the depleted uranium (5 to 10 times more, used to make the slightly enriched fuel of LWR in the first place) is still 100% "energetic". It's a big waste to throw all that away. We could still extract in principle about 100 times more energy from the uranium than we did up to now. If we've been running for 30 years, that means that, JUST USING THE "WASTE", we can still run for about 3000 years at the same power level if we were to use everything in breeder reactors (producing electricity).
I thought of it as a choice because of the cleaner waste thing.But now that ther's no problems with deep geological storage...But isn't artificial transmutation like bombarding the wastes with neutrons and stuff.What about that choice?Instead of waiting for centuries, why not do it in hours.Why not take all the radioactivity of the actinides and stuff and finish it off right away?Nice link!
As I said, the bulk of the short term activity, the fission products, cannot be transmuted. We're only talking about the relatively low activity on long times, caused by just a few components.
EDIT: a fast reactor has two "waste advantages" over a LWR. The first is that it produces way less minor actinides than does a LWR. As such, one of the longest-living (but by far not the most active) components in the waste are seriously diminished. But the main advantage of a fast reactor is that it can burn plutonium efficiently. LWR can only burn Pu-239 and Pu-241, and produce Pu-240 and Pu-242 in the process. That means that only a certain fraction of the plutonium could eventually be used up in a thermal reactor, and we would still end up with a lot of Pu-240 and Pu-242 as waste, together with a lot of Americium and curium (and if one pushes, even californium). And that's far worse waste than the original minor actinides, because now the decay periods run into the 100 000 years, and moreover the activity and the quantities are larger than in the case of the minor actinides. On the other hand, a fast reactor can burn about any plutonium, so there's no build-up of "bad plutonium".
