Could nuclear waste be disposed of in subduction zones?

01 August 2013

Question

I have wondered if the geological, tectonic subduction of nuclear waste materials might be a practical way to permanently remove these dangerous substances from the earth's surface and at the same time to feed the Earth's magnetic field. Is anyone working on the development of such notions?

Thank you.

Answer

Dave - So, this sounds like a lovely idea, this subduction zone. It's where stuff is being sucked down deep into the earth and you'd expect then it not to come back out again and it will be a nice comfortable place to put all the nasty stuff we want to get rid of. Actually, that's not quite how the geology works. Where a subduction plate gets pulled down under earth, it gets a huge amount of friction and that surface layer get very, very hot. That surface layer tends to melt and then come back up to the surface and form a volcano. And so, you get a lot of volcanism related to subduction zones and it's this top layer heating up, melting and floating up to the surface through the surrounding rock and creating volcanoes. This means that actually, if you put all your nuclear waste on that surface zone, there's a big possibility it would shoot out a volcano, probably in a few hundred thousand million years later. And also, there's an awful lot of water, hot fluids flooding through that so there's a really, really aggressive environment. Chris - What about John's point about the magnetic field?

Dave - So, magnetic field is all to do with the very, very centre of the earth, right down in the core. In there, there's some complicated system involving liquid metal flowing around because parts of the core is metal and to do with the earth spinning and a convection current in there which to be honest I understand and I think scientists have only recently understood it at all. Creates this magnetic field and so, doing anything near the surface probably isn't going to affect that very much. It's got to go down through the whole mantle which is thousands of kilometres of really thick gooey rock. I think at the moment, the best thing for the nuclear waste is probably just to bury it somewhere where nothing is going to happen. So, a really dull geological place, ideally in some clay because it will sit there for a hundred thousand, few million years. Nothing is going to happen to it and you can just sit there until it calms down and isn't dangerous anymore.

Chris - A lot of people say, "Well, we'll just embed it in concrete or glass or something" but then there was this paper which was published by Ian Farnan who's a researcher at Cambridge University about 7 or 8 years ago. He found that if you look at the ceramics that you put these radioactive chemicals into, because of the radioactive decay, when a uranium atom decays, it fires almost like a recoil as it fires out a radioactive particle. It's like a gun recoiling into your shoulder when you fire a shotgun for example. This has the effect of knocking all of the other atoms off kilter in the substance. The result of that is that over time, with all these atoms being knocked off kilter, you end up with the material becoming amorphous as it's called and it's basically riddled with holes. It's leaky. So, after just 5,000 years, you'd go from something which was a solid concrete or piece of glass which would be something analogous to a sieve.

Dave - Yeah, this is why you want to put it in some rock which is naturally waterproof and actually doesn't have cracks in it which is why I think the ideal solution is a big lump of clay. I think East Anglia is meant to be especially good for it. But possibly not popular.

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