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Fair warning, I have a bachelors in CS and have chemistry 211/212 under my belt. My understanding of the atom consists of a proton, neutron, and electron quasi-orbiting it in some sort of strange probability cloud. If everything I say past this paragraph is completely wrong, you can be comfortable in the fact that I will never actually get the chance to put this limited knowledge (mis)use.

So, I was reading about nuclear waste and vitrification, and a thought occurred to me, why don't we attempt to subject the unstable fission products to further fission until we "boil" them down to stable elements which don't undergo such quick nuclear decay? To my understanding, to force an atom to split you only need to bombard it with enough neutrons to add enough energy to overcome the binding energy of the neutron/proton nucleus.

I understand that there would be a law of diminishing returns because there is less binding energy in a smaller nucleus (In my mind this is analogous to there being less "binding" energy in the moon than there is the earth), but wouldn't that also mean that less energy would be required to force it to split?

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    $\begingroup$ You are right, storage is only one solution. There have been plenty of suggestions to reduce the long lived radioactive elements by spallation reactions using a combination of accelerators and reactors. The problem is funding and the risk of proliferation, which plagues the politics of the issue. From a purely theoretical perspective we could get rid of much of the problem while producing energy in the process, but that's not enough. The economics and the politics have to work out, too, before such facilities can be built. $\endgroup$
    – CuriousOne
    Commented Oct 15, 2014 at 19:06
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    $\begingroup$ See e.g. large.stanford.edu/courses/2013/ph241/baxevanis1 for some details on accelerator driven subcritical reactors that could also be used to transform the nuclear waste. Also, in Belgium, there will be such a system in operation in a couple of years that is also designed to test these applications: myrrha.sckcen.be/en/MYRRHA/Applications $\endgroup$
    – Martin
    Commented Oct 15, 2014 at 19:25
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    $\begingroup$ There's a big difference between radioactive, fissionable, and fissile. Many of the waste products produced in fusion reactors are radioactive but only a small portion are fissionable, and an even smaller portion are fissile. The direct byproducts of fission is just waste. It isn't fissionable (and hence not fissile). Some indirect byproducts are fissionable. For example, the non-fissile U238 present in nuclear fuel can capture neutrons and become fissile Pu239. Breeder reactors take advantage of this. $\endgroup$ Commented Oct 15, 2014 at 19:53
  • $\begingroup$ Of course almost any sufficiently active material can be used in a RTG, but ... (a) RTGs are scary, (b) while they are reasonable high in energy density RTGs are quite low in power density (c) aside from being scary, most isotopes do either produces some ionization outside the case or require heavy shielding exacerbating item b, (d) the chemistry of a decaying mixture of various isotopes is a mess which makes engineering the enclosure hard, and (e) if you want to avoid d by purifying first you get into issues of containment while you do that. So the cost engineering is tough. $\endgroup$ Commented Oct 15, 2014 at 20:46
  • $\begingroup$ @dmckee RTGs aren't scary, they're just misunderstood. Sure, I wouldn't want to have one over for dinner, but I'm quite happy and unconcerned with continuing to use them on spacecraft or submarines $\endgroup$
    – Jim
    Commented Oct 16, 2014 at 13:44

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Apparently, the CANDU reactor can accept a variety of fuels, including what would be considered "waste" from other reactor types, although some amount of reprocessing is involved.

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It might be possible, but it's uneconomical and probably also uses more electric energy than you get from the power plant.

Do you really mean fisson? Fission is splitting a nucleus in two approximately equal parts. This is done with certain isotopes e.g. uranium 235 which are only barely stable and sometimes fission spontaneously. Other isotopes such as 238-U are more stable can be fissioned with fast/energetic neutrons. The nuclear waste nuclei are much more stable and fissioning them requires very high energy impacts which could be achieved with particle accelerators, but this would be very slow, expensive and consume more energy than produced by the nuclear power plant.

OTOH you could just transmute long lived nuclear isotopes by irradiating them with neutrons into isotopes that undergo radioactive decay (alpha, beta, etc, which is different than fission) in a short time into stable isotopes. It would have to be researched which isotopes are best suited for trarnsmutation as there is only a limited amount of excess neutrons available in a nuclear reactor, as some neutrons are required to keep up the chain reaction.

Actually, the fission products are relatively short lived compared to actinides produced by neutron capture of reactor fuel without fissioning them. These could be bred to fissile material.

So, the quick decaying isotopes are less of a concern as they decay quick, though not always, into stable or short lived isotopes, but sometimes into long lived isotopes. And with neutron transmutation you usually get short lived isotopes anyways. Bigger atoms have a higher neutron/proton ratio, and even when some neutrons are released during fission the fission products have excess neutrons. You can't usually make them stable by adding neutrons.

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