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I was reading a question about why not to drop nuclear waste into volcanoes; the short answer is it would come back out and not be rendered safe.

Ignoring the cost and energy requirements, why can't waste be assimilated into artificially melted rocks?

If conditions could be controlled, it as if one could allow rocks to cool very hard subsequent to an appropriate melting process; the gradual weathering of these rocks could allow the integrated nuclear waste to disperse harmlessly over huge periods of time or be contained long enough to realize the biologically meaningful portion of their half-lives.

The rocks could perhaps be located to a geologically stable environment (for example polar ice)

What I'm asking is, what would prevent a society with access to massive amounts of energy (such as fusion or advanced solar power) from simply melting their old nuclear waste into rocks under controlled conditions?

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Please search for Vitrification –  Deer Hunter Apr 17 '13 at 3:18
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We already store waste by combining it with molten glass. It doesn't take very much energy - obviously no more than making glass!

The main reason for not doing it is political, it cost a lot to make that plutonium and it might come in handy to build bombs one day - so we don't want to waste it by reprocessing it.

(We also have a small problem of persuading a representative that voters in his district want to be the repository)

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+1 especially for mentioning the NIMBY effect –  Chris White Apr 17 '13 at 3:37
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The artificial rocks would be just as radioactive. By melting the waste into the rock, you only add some of the rock's matter to the shielding. So you invested energy to create a shape that's handy, but equally dangerous.

This is what's actually done. The spent fuel is molten into glass which is then cast into a concrete shell (see Wiki: http://en.wikipedia.org/wiki/Spent_nuclear_fuel_shipping_cask). The issue is the place to store such containers.

The polar ice is not as stable as you might think. Especially with global warming on our backs, I'd consider it one of the last places to put nuclear waste. Besides the high cost to put it there, one would have even higher costs retrieving it, should the storage threaten to spill into the ocean.

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