Science Questions

Could nuclear waste be disposed of in subduction zones?

Wed, 31st Jul 2013

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Question

John Brockman asked:

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 Sizewell Bis 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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I don't think it's a practical because we can't get political agreement on where to put it.

We could get rid of it by burning it in nuclear reactors. We can process the waste and use plutonium (and other actinides) to power Gen IV breeder reactors. Such reactors are designed to breed their own fuel yet leave no significant long term nuclear waste. Gen IV reactors are fundamentally safer than today's pressurised reactors which use water as a coolant. For example: Well designed Gen IV

have passive systems designed to shutdown the reactor in an emergency.
will under no circumstances release radioactive caesium or iodine into the wider environment (as happened at Chernobyl)
will not melt down
because they don't use water coolant :

will not generate hydrogen explosions (as seen at Fukushima)
will not operate under pressure, with the consequent threat of loss of pressure leading to loss of cooling and release of steam

MarkPawelek, Thu, 14th Nov 2013

Deep boreholes (5 km or more) are one suggested solution. One elegant idea for high-level waste is to seal it in half-metre tungsten containers, put it at the bottom of a deep borehole so that the heat it generates will melt the rock below it, and it will sink into the depths, with the rock cooling and setting above it. dlorde, Thu, 14th Nov 2013

Ahh, just love this reply Chris. That's exacly what was being tried in the States, and found failing. There are no known containers that will contain radioactive waste from nuclear over longer time periods, as I know.

"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. "

Well done. yor_on, Fri, 15th Nov 2013

Ian Farnan was only about 50 years late in publishing his work. Radiation damage effects in structural ceramics have been extensively studied since about 1950 and indeed form the basis for archaeological dating of ceramic artefacts - though admittedly most of that work was done in Oxford in the 1960's so the news wouldn't have reached Cambridge just yet.

The good news is of course that everyone concerned (except possibly Ian Farnan) can calculate the future damage to a given container from a given source, and design the container accordingly. 

The real problem of nuclear waste, oddly, isn't the high level stuff. That can be concentrated and deposited deep in rocks or burned in another reactor. The real bugbear is thousands of tonnes of structural metal and concrete from decommisioned installations which is too active to recycle and too expensive to dismantle, move and bury. Plus the millions of tons of lowlevel garbage like test tubes, lab coats, radioactive Kleenex, and soiled bedlinen, from hospitals and industry. alancalverd, Fri, 15th Nov 2013


Use it to fill the next deepest sub-oceanic canyon to the Marianas Trench (which is a National Wildlife Refuge).

Only partly joking dlorde, Fri, 15th Nov 2013

I understand that we have amongst the best, stable old rock, in the world here in Sweden Alan, geologically speaking. and that we were thinking of trying to sink it into deep mines, 500 m down, filled with bentonit clay, radioactive waste clad in a copper and steel encasing. But we're still debating it, although waterfall (Vattenfall) in its infinite wisedom would love the project to go through. That's the same company now getting thrown out of Germany, more or less, as it bought up old nuclear facilities, now planned to getting dismantled, as well as 'brown coal power plants'. I really trust those guys :) and their approach to a greener world. The worst thing about them is that over fifty percent are owned by us all, aka 'the state'. Tells you something about our new political agenda, doesn't it :) A economical 'world power' in the making, well, at least Vattenfall thought so, until recently.

Anyway. I don't think it's so simple. If it was we would already have long term storage that worked. yor_on, Mon, 18th Nov 2013

by the way dlorde, check out the waters outside Africa for where some of that stuff gets illegally dumped, as outside Somalia. yor_on, Mon, 18th Nov 2013



"Working" is one thing, and quite easy to attain - it's just physics. The difficult adjectives are "affordable" (which is all about engineering) and "acceptable", which is all about sociology.

People seem quite happy to live on hurricane coasts, earthquake faults and active volcanoes, which unpredictably kill tens of thousands every year, but object to burying easily detectable materials that might  just begin to leach in a million years, a couple of miles underground in an uninhabitable desert.    alancalverd, Mon, 18th Nov 2013


Yeah; just look what happened to the Yucca Mountain Repository, which was technically very good and well under way - except for the public - and more damaging, political - opposition. dlorde, Mon, 18th Nov 2013

Wish I could be as sure as you guys, but I'm not. I know that the copper encasing is expected to leak in the Swedish idea for long term storage, one reason for the long term storage project being halted. Another is microscopic fractures even in this 'perfect old rock', as well as a question about the permeability of the clay surrounding those 'caskets'. The same problem as Yucca.

"The Department of Energy’s (DOE) own data as presented in the 1998 “Viability Assessment” shows that water moves quite rapidly through the rocks at Yucca Mountain. As soon as the containers begin to fail, radioactivity will also move rapidly – in centuries or less – to contaminate the ground water in the region. This is due to the same fractures in the rock that will allow the carbon-14 to escape.

Fingerprints demonstrating this fast flow pathway were left by fallout from the very industries that created the waste that would be sent to the site. Traces of chlorine-36 were found by DOE researchers deep in Yucca Mountain at the level where the waste would be dumped. This radionuclide is not found at these concentrations in nature. In fact, there is only one bulk source of chlorine-36: atmospheric nuclear weapons tests conducted in the Pacific. Salt in the seawater was activated, forming the radioactive chlorine isotope. This “fell out” all over the Northern Hemisphere; it is not unique to Yucca Mountain. But its presence at repository depth proves that water has traveled there within the past 50 years, and proves a “fast flow” path for ground water travel."

And then you have tectonic activity. We was going to another colder climate with a possible small ice age, coming about 5000 y, as I remember, and with it an expectation of earthquakes. Also it is so that this ice can press up mountain rocks from one km down, double the depth for our storage.

Actually I like it better when we can oversee it on a daily basis, the storage I mean. yor_on, Mon, 18th Nov 2013

Dave, it seems to me, a mere layman, that your first 2 paragraphs involve a difference in timescale of at least 5 orders of magnitude - that's a wide-open door that it should not be beyond the wit of man to walk through, especially for low-level bulk waste. A sub-oceanic active area of subduction, with a relatively high angle of flow, near to a relatively sparsely inhabited landmass of the most suitable geology would be ideal. D D Jones, Thu, 22nd May 2014

"...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."

So, this means 100% of EVERY subduction zone has a corresponding volcano?  There are NO subduction zones without a volcano?  I find that hard to believe. 

If it is the case that there are subduction zones without volcanoes, what is the problem with dumping waste there? Dickison.Richard@GMail.co, Sat, 22nd Nov 2014

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