Radioactive wastes are dangerous because unstable elements are too concentrated. Originally radioactive elements come from nature where they were very diluted and that's why they were secure. So why dilution of radioactive waste back in to the nature is never mentioned as waste treatment ?
6$\begingroup$ @borilla Nuclear reactors create massive amounts of radioactivity. Before a load of new fuel goes into a reactor, it is no more dangerous than an equivalent amount of sand so long as you don't get any of it inside your body. When it's "spent", it is so radioactive that just standing next to it for a minute or two would kill you. $\endgroup$– Solomon SlowNov 6, 2015 at 21:01
$\begingroup$ It seems the OP has invented a principle of conservation of radioactivity :) Regardless, the premise of the question (and thus the assertions contained therein) is just fundamentally wrong so I'm afraid I must downvote. $\endgroup$– Lightness Races in OrbitNov 7, 2015 at 12:53
1$\begingroup$ 1984 some people did an experiment and diluted a fraction of their core into the atmosphere. We can still measure the results today... $\endgroup$– PlasmaHHNov 7, 2015 at 15:49
$\begingroup$ @plasmaHH do you have a reference for the specific event you are referring to (I tried a quick google)? $\endgroup$– TylerNov 7, 2015 at 16:07
$\begingroup$ @TylerBailey: I meant 1986... Weird mobile phone autocorrection... Anyways, there is about only one event of massive release of radioactive material in that decade $\endgroup$– PlasmaHHNov 7, 2015 at 16:11
Originally radioactive elements come from nature where they were very diluted and that's why they were secure.
When these naturally radioactive materials like Uranium are used in processes like civilian nuclear energy production the resulting waste becomes many, many times more radioactive than the raw materials one started off with. Even after the cooling off period the waste contains many dangerously radioactive materials that would be very toxic for human and other life forms, even in very low quantities.
[...] dilution of radioactive waste back in to the nature is never mentioned as waste treatment
... because it would constitute an unprecedented ecological disaster.
$\begingroup$ This. Even if you extract the useful isotopes it's still 10,000 years before the waste decays to the same radioactivity as the original uranium. $\endgroup$ Nov 7, 2015 at 2:19
$\begingroup$ @LorenPechtel Well, hopefully, we'll find a use for the other "waste" over time. It happened with pretty much all the waste we've ever produced, including many kinds of toxic industrial waste. As you said, some of the radioactive waste is already useful (though often still heavily restricted by law), the rest is just waiting for a good use. I'm pretty sure that if it were possible to ignore the terrorist potential of the stuff, it would be used already (even for simple stuff like RTGs). $\endgroup$– LuaanNov 7, 2015 at 16:09
$\begingroup$ Basically, the dilution level needed is on the order of "mix with the entire contents of the Atlantic Ocean". $\endgroup$– MarkNov 7, 2015 at 21:01
$\begingroup$ Isn't this being done (but with the Pacific) in Japan? $\endgroup$– JehanNov 7, 2015 at 21:20
Probably too expensive and disruptive to try to deal with nuclear waste that way. You're talking about processing through an enormous amount of earth and/or seawater. Note that nuclear waste includes not just material that was initially radioactive when it came out of the ground (e.g., uranium ore), but a lot more material as well. If a nuclear plant worker touches some rods filled with uranium, for example, those gloves also become (low level) nuclear waste since they can't very well throw such gloves into a regular trash can, can they? Also with the advent of breeder reactors it's been possible to make more highly radioactive material than was initially mined out of the Earth, so you're talking about not only putting back a lot of radioactive material that we initially got from the Earth but a tremendous amount more besides. Bottom line is that trying to grind or pulverize all that material and then apply massive dilution with lots or soil and/or seawater is probably not a very convenient or cost-effective option.
One on the problem is re-concentration, by the help of water circulation in the soil (possibly up to water sources) or by the help of small animals (then to food chain up to us).
The stability of geological layers is not so easy to predict.
Beside, the radio-activity of wastes can be a lot higher, and spreaded through a huge variety of chimical species, with each further change might react differently to environmental conditions.
Also there is a responsability issue: it's more easy to ensure quarantaine in a limited volume. Nobody would like to hold the garantee that spreading diluted waste everywhere will have absolutely no consequence. Populations accept artifical risks a lot less than natural risks !