The problem of nuclear waste

07 April 2020

Interview with 

Claire Corkhill, University of Sheffield

NUCLEAR-WASTE

Yellow barrels of nuclear waste photoshopped in front of a nuclear power plant

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Our issues with radioactivity though are obviously not behind us. A major headache today is how to handle and safely store nuclear waste. Here in the UK, we’ve got 650,000 cubic metres of the stuff - enough to fill Wembley Stadium - and it’ll be radioactive and dangerous for 100,000 years. Claire Corkhill is at the University of Sheffield where she works on ways to store this stuff safely, and she joined Chris Smith and Adam Murphy...

Claire - It starts with the nuclear fuel from a nuclear reactor, which is uranium; and much like the bombs that we just heard about, the atoms inside it split to create energy. They release that in the form of heat, which we then use to boil water, which gives us electricity. But in a nuclear reactor we have some way of controlling the splitting, so we've got some brakes where we can slow it down and control it so it doesn't end up like a bomb; but that material, when it's been finished with in the reactor, we recycle it. So it contains a lot of uranium and plutonium which we can then turn into new fuel. And the byproducts of the recycling are nuclear waste. These byproducts are smaller atoms, so as the uranium atom for example splits, it makes smaller atoms which are also highly radioactive, we call these fission products. They end up being mixed with a type of glass which is not too dissimilar from the Pyrex that you have, Pyrex dishes that you have in your kitchen. And we turn this waste into a very durable glass. Everything else that's leftover from that recycling process is usually encapsulated inside a cement material. And then there are a whole range of other materials that we don't yet class as a waste, but they might be a waste in the future; for example, the plutonium that we've been recycling, we don't actually have a plan to turn that into new fuel yet.

Chris - So what do we do with all this stuff? We end up with these barrels of what looks like glass or concrete; that sounds fine. What do we do, just bury them?

Claire - Well, they're currently packaged in specially-engineered containers and stored in over 20 different secure nuclear sites around the country, and most of it is at Sellafield in Cumbria. And these stores are designed to withstand extreme weather and earthquakes. But the problem that we have is that the waste is so radioactive, we can't actually go anywhere near it. If you were to touch the outside of one of the glass waste containers, the radiation dose that you'd receive is 200,000 times more than a fatal dose of radiation. So whilst it's okay to store the waste securely for the time being, it's clear that we need a more permanent solution that requires less security. So remember, these wastes will be radioactive for over a hundred thousand years and they'll be highly radioactive for several thousands of years, so we can't just leave them in their warehouses and hope that future civilizations will know what to do with them.

Chris - Now will they remain intact though? Because the key question... today it looks like glass, but with that much radioactivity spitting out that much energy, will it remain like that and in a stable form for the next a hundred thousand years, if we store it ideally, or is there a danger it might not?

Claire - That's a really good question, and that's really what we've been doing a lot of work on at the University of Sheffield. These nuclear waste materials will change over the hundred thousand years that they'll be radioactive. And there are some different ways that this might occur. One would be corrosion, so the natural corrosion of the materials once they're buried deep under the ground, which is their final disposal route; if they slowly corrode in groundwater they may release their radioactivity. But the other issue is, as you rightly noted, that the radioactivity inside the waste might actually cause the waste itself to break down. And you can think of this as a highly energetic particle, a bit like was described before with breaking DNA; instead of breaking DNA we're actually breaking the intrinsic chemical bonds inside our nuclear waste material, and this will essentially cause the waste to disintegrate. And this is something that we have to understand.

Chris - So what you're saying is, if we've got say something that looks like glass, because it's spitting out all these energetic particles of radiation all the time, it's slowly going to shatter the glass. It's almost like shaking the glass very, very hard for hundreds of thousands of years; it's eventually going to fall to pieces and it will no longer be any good at retaining and constraining the radioactive products inside.

Claire - Essentially yes. I mean, glass is a very good material because it has a very flexible structure, and if you break one bond it's not going to affect the material. If you were to use something that was a bit more of a crystalline structure, this might cause you a problem. But actually what we're doing is trying to look at designing new types of wasteful materials that are based on natural uranium-containing minerals that we know are millions of years old. And while their chemical bonds have been broken by radioactivity, they haven't disintegrated and they haven't released their radioactive elements. And so these are the kinds of materials that we're now trying to develop for nuclear waste.

Chris - And are these... effectively mineral cages, that you're talking about making, are they any good? What studies have we done to say that if we bake this kind of cake and we put these materials into this molecular architecture, that it will actually perform better than what we can do with our present generation of things like glass and cement and so on.

Claire - So one really good example is a mineral called zirconalite. And these are some of the oldest minerals known on earth - there are some that are slightly older - but they contain uranium. And we know that they contain uranium because they today contain lead, which is the stable isotope that uranium decays to at the end of its decay chain. Which means that over the millions and millions of years that these minerals have existed on the earth's surface, they've managed to contain their radioactivity safely. They've gone amorphous, which means their crystal structure has broken, but they're still intact. And what's more, we know that despite the fact they've been on the surface for such a long time and there's been lots of weathering and lots of geological events, that they haven't changed and they haven't been weathered, and as I said they still contain their uranium. So we base our design on minerals likes zirconalite. And so one of the pieces of research we're doing at the moment is looking at how to dispose of the huge stockpile of plutonium that we have in the UK. And actually is zirconalite is the ideal mineral material that we can develop to immobilise plutonium safely so that it doesn't get into the wrong hands in the future.

Adam - How do we design something in the future so that this stuff stays where it is, and isn't archeologist bait, and they suddenly dig up a radioactive cube of glass?

Claire - At the present time we are thinking that we will not mark when nuclear waste is kept. It's going to be buried deep below the ground and nobody will know it's there. The worry about putting a marker on the surface is that it will automatically draw, humans particularly because we're very inquisitive, to that site to find out what's going on. And the example that I like to use is of the hieroglyphs on the pyramids, which clearly said, in a language that nobody understood at the time, don't enter or you will be cursed. But the archaeologists of the time were fascinated and thought, "well we must go inside and see what's in there." And so sometimes leaving a marker may give you the result that you didn't want, and people might then use that as a reason to go down and start looking for the waste. So the plan at the moment is to not mark the waste and hope that people forget about it; and that if in the future they decide to dig there, they have the technology to dig that deep - so we're talking between 500 metres and a kilometre below the ground - and if they have that technology, then they will also have some technology to be able to detect the radiation and know that they shouldn't go there.

Comments

Would it be feasible to carry this waste far into outer space and dispose of it there?

It would cause an international pollution disaster if the rocket carrying that lot aloft exploded. So this is not an option.

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