This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. Radioactive waste is the byproduct of activities that use natural or man-made radioactive materials. The main sources of radioactive waste are nuclear power stations and military activities. But while many of the radioactive isotopes present in radioactive waste have relatively short half-lives, measured in decades, some isotopes have much longer half-lives, measured in thousands, tens of thousands or even hundreds of thousands of years. So to dispose of such waste, we need to be thinking of time scales that approach geological time scales, which are orders of magnitude greater than any time scales associated with human civilizations. This has led us to create deep geological storage sites. What sort of geological context do such storage sites require, and what kinds of hazards do we need to guard against on such time scales?

Claire Corkhill is leading the UK's research efforts to understand and improve the safety of planned geological waste facilities. Her research focuses on how radioactive isotopes interact with minerals, and on how radioactive waste can dissolve in groundwater. She's Professor of Mineralogy and Radioactive Waste Management in the School of Earth Sciences at the University of Bristol. Claire Corkhill, welcome to Geology Bites.

Claire Corkhill: Thank you very much for having me.

Oliver Strimpel: Can you bring us up to speed on the nuclear processes that produce radioactive waste?

Claire Corkhill: So let's start with what happens inside a nuclear reactor. We start with uranium dioxide fuel. When uranium-235, a specific isotope of uranium, is bombarded by a neutron, that causes the uranium atom to split, and in doing so it creates a huge amount of energy. That energy is then used to heat water and turn it into steam, to turn turbines to create electricity. But the splitting of the atoms also creates smaller atoms, and these smaller atoms are themselves unstable, and their instability results in radiation. These are fission products that develop inside the nuclear fuel, and they can have half-lives of anything from seconds all the way through to millions of years.

Oliver Strimpel: So are those fission products, then, the main kind of radioactive waste that we're concerned with here?

Claire Corkhill: Yeah, exactly. When we've finished with the fuel inside a nuclear reactor, there are a number of different things that we can do with it. The fuel is removed from the reactor, and it's cooled for a while, because the radiogenic heat means it's actually quite hot to the touch. So it has to be cooled for a while in a wet pond, or maybe in dry cask storage. That fuel can then either be disposed of directly, which we call an open fuel cycle, or it can be reprocessed. That's because the fuel contains useful uranium and plutonium that could be turned into new nuclear fuel. Reprocessing is the operation by which you dissolve that nuclear fuel material and, through a chemical separation process, extract the uranium and plutonium, and then everything else that's left is, essentially, the fission products.

What we do is to dry those fission products out. It's difficult to think about what this looks like, but the dried-out fission products end up looking remarkably like instant coffee granules. This is not the kind of stuff you'd want to put into your hot water to drink, but they look just like coffee granules. These granules of fission products get mixed with a borosilicate glass, which is very similar to the kind of dish that you would put into the oven to make your dinner, and melted together and turned into a stable material. This is what we call high-level waste. So the spent fuel, together with the glass, we term high-level waste.

There are other items created through this process that are contaminated with radioactivity. For example, the fuel pins, which are small pipes in which the nuclear fuel sits, get split off from the fuel in the reprocessing operation, and they're contaminated with the fuel. We call this intermediate-level waste. It has a smaller amount of radioactivity than the high-level waste, but it's still quite radioactive. So there are lots of different types of waste that are generated through this process.

Oliver Strimpel: So some of this waste has fairly long half-lives. Do we try and extract the things that are likely to be problematic in the long term when we think about long-term storage, or do we just take the whole lot and treat it all together?

Claire Corkhill: We essentially take the whole lot and treat it together. There are shorter-half-life isotopes like strontium-90 and cesium-137, which have half-lives of the order of 30 years. What we'd ideally like to do is leave these isotopes for up to 10 half-lives before they're considered to be safe. So for the shorter-lived ones, like cesium and strontium, we only need to leave them a few hundred years, which means that we could store that waste in above-ground storage units for 300 years. That seems like a reasonable thing to do, to let the radioactivity decay. However, there are much longer-lived isotopes. We're talking about things like technetium-99, which is only made in a nuclear reactor. It has a half-life of 200,000 years, and it's extraordinarily mobile in the subsurface. So we really have to think about these isotopes that are going to take much, much longer to decay when we think about how we're going to store them safely over the time scales that are required.

Oliver Strimpel: OK, so that gets us to geological storage. You have mixed all this stuff together with this borosilicate glass. Do we just take all that, find a deep geological storage site, put it in there, lock the door and forget about it? How does that work?

Claire Corkhill: Well, really we have to try and find the best place for a geological disposal facility. What we're looking for is a location where we can safely isolate these wastes from future populations, and the time scales that we're looking at are longer than civilizations. So we really need to find a place that's going to be stable and secure over these really long time scales. The time scale that most organizations who are trying to dispose of this waste are looking at is a million years. So there's really no better place to put this waste than in a rock formation that has been stable for probably billions of years, if not hundreds or tens of millions of years.

When we think about the disposal of the waste, I don't want people's minds to jump straight to the Homer Simpson analogy, where you've got glowing green goo that gets poured down a hole. That is not what this is at all. This is a highly engineered facility somewhere deep below the ground. We're talking around 200 meters to 1,000 meters below the surface. You can imagine a series of large caverns and tunnels, and in these tunnels we'll be placing containers of waste in a series of layers of containment that will ultimately prevent groundwater from reaching the waste. Groundwater would ultimately start to dissolve the radioactive things from inside the waste and transport them outside, and that will inevitably happen over the long time scales we're talking about. So we use a series of layers of containment to try and prevent that from happening. At that point, once we've disposed of the waste and we've backfilled the tunnels, then yes, we're going to lock the door. But there's a lot of engineering and thought that has to go into all aspects of a disposal facility before we can even think about building one.

Oliver Strimpel: Let's go back to the selection of the site. You mentioned geological stability and billions of years. So are we really looking for these ancient cratons that have essentially been left alone since the Neoproterozoic, which are extraordinarily stable? Are we really trying to find something that's geologically stable on that kind of time scale?

Claire Corkhill: Well, I suppose in a way those kinds of formations would be ideal, but it's not necessary. What we're really looking for is any body of rock of the volume required to dispose of the inventory of a particular country, and it has to have certain properties. The most important point is that it has a very low permeability. I mentioned water interacting with the waste. If we can reduce the likelihood of water coming into contact with the waste materials, then we reduce the likelihood of it dissolving the waste and transporting those radioactive isotopes away into the geosphere, the subsurface. So the number one criterion is to find a low-permeability rock. Lots of rocks are very impermeable. Clays are very impermeable. If we have an unfractured granite, for example, that would have very, very low permeability. So it could really be any kind of rock.

We're also looking for somewhere that's very tectonically stable, somewhere where the rocks are not going to change significantly over that million years. We're not going to have intrusions of granite or dikes or whatever coming through the geological disposal facility. We're also interested in it not being close to any resources. The idea behind this is that a future population might find, for example, a coal deposit, if fossil fuels are not completely extracted in the future, as one example. Or maybe it's a metalliferous ore deposit. We don't want future populations to say, oh, let's dig there, and then accidentally stumble upon some really dangerous, hazardous radioactive waste materials. So there are a number of different criteria that would make us think that a site is potentially suitable.

Oliver Strimpel: So when we find an ideal site like this that has the right geological properties, do we use it for the waste from many different countries, or does each and every country have to have its own waste site?

Claire Corkhill: At the moment, and this is unlikely to change, all countries have to dispose of their own waste, when we're talking about the higher-activity waste that comes particularly from nuclear energy. Some countries are sharing other types of waste disposal, for example, the cesium medical isotopes that are used in hospital facilities. These are quite small sources, and some countries in Europe are starting to think about whether they could share a borehole disposal facility for those kinds of wastes. But generally speaking, once you've created waste in your own country, you're responsible for it and you have to dispose of it.

Oliver Strimpel: So what does a country like Japan do, which is, as I understand it, pretty much a volcanic island that's tectonically active and could have an earthquake anywhere?

Claire Corkhill: You're absolutely right. Japan is a really interesting case. They are planning to build a geological disposal facility for their radioactive waste. As we all know, Japan has a very long history of nuclear power, and even after the 2011 Fukushima accident they've started up their nuclear power stations again. So they are looking at a variety of different areas, in different types of host rock, to try and identify somewhere that has a very low risk of tectonic disruption. Again, this comes back to the point that I made earlier: really any rock is suitable for a geological disposal facility. It just needs to have a low risk of the radioactive waste being exposed to future populations. And if the organizations involved in disposing of the waste can make their case to the safety regulators that that's the case, then they should be able to get a permit to build their facility.

Oliver Strimpel: You just said that we want to minimize the risk of this waste coming into contact with human populations in the future. So it seems like the first thing we don't really want to have happen is for the radioactive isotopes to break free from the glass that we've put them into. So what exactly is the risk at that level? What do these radioactive isotopes do to their chemical and physical confinement inside this glass over long time scales?

Claire Corkhill: I'm going to take the example of alpha particles. If we can all think back to alpha, beta and gamma radiation, let's talk about alpha particles. An alpha particle, which is a helium nucleus, is quite big and quite heavy, and it moves with quite a lot of energy inside a material. So if we take, for example, a crystalline material (we'll come back to glass in a moment), where there are atoms arranged in a periodic structure, it has very nicely arranged long-range order. As soon as the alpha particle hits an atom in that crystal lattice, it will displace that atom, and there'll be a number of other displacements. You can think of it a bit like a set of billiard balls: you have the cue ball hitting the triangle of balls, and they just scatter everywhere. That's what happens when an alpha particle hits the crystal lattice inside a crystalline material. And obviously, the more bonds you break and the more lattice positions you move, the more the properties of that crystalline material break down over time, and they start to suffer a lot of damage, which may impact the durability. If you break that crystalline structure down, it might be easier to dissolve it in the groundwater, for example. So those radioactive decay processes inside the waste itself can be quite problematic. They can reduce the longevity, the durability, of those materials.

Now, this is one reason why we use glass. Glass doesn't have that nice crystalline long-range-order structure. It's a bit like a frozen liquid, where there's no real long-range order. It's got short-range order, but not long-range. So when an alpha particle smashes through that structure, it doesn't really matter, because it's got no structure in the first place. So glass is remarkably resistant to radiation damage. That's not to say that all crystalline materials behave in the same way. Some crystalline materials are very resistant to radiation, and they actually have internal defects and self-annealing repair mechanisms that can recover that damage. And therefore there are some crystalline materials that make excellent candidates for radioactive waste. But there are also some crystalline materials that would be terrible.

Oliver Strimpel: So do we actually use some crystalline materials, in fact, in addition to the glass?

Claire Corkhill: At the moment, no. But that brings us to something that I'm working on a lot in my research at the moment, which is exploring the use of certain titanate minerals. We're interested in a couple of key players, which are pyrochlore and also zirconolite. These mineral structures are relatively radiation-resistant. Over time, they do become completely radiation-damaged; they become amorphous. This is a process we call metamictization. But despite this metamictization, which happens over a very long period of time compared to other types of crystalline structures, they still retain the original uranium and thorium content that they started life with. Some of these minerals that I've been studying from the Canadian Shield are around a billion years old, and they still contain uranium and thorium that has been locked up in their structures for that amount of time. So we understand that even though these minerals have been subject to hydrothermal alteration, and even though they're completely metamict, they could be good host candidates for actinide wastes that come from nuclear power. The thing we're interested in at the moment is whether we could use these minerals as a template to create waste forms for plutonium. They could also potentially be used for actinides that come from future fuel cycles, so new types of nuclear reactor that haven't yet been invented. They're on paper at the moment, but they will generate types of waste for which you could potentially use these crystalline materials.

Oliver Strimpel: But why would we ever want to do that? Do these minerals hold out the prospect that they're actually better than glass in terms of how they respond to radiation damage over tens or hundreds of thousands, or millions, of years?

Claire Corkhill: These crystalline materials are not as good as glass at resisting radiation damage; they ultimately will become amorphous. The key factor here is that they are orders of magnitude more durable. When you try and dissolve these materials, they are completely different. Glass is a very durable material, super durable. You'll know this if you put glassware into your dishwasher, which is an extraordinarily harsh environment, if you think of all of the detergent and the soap and the temperature and whatever. You will sometimes see on the outside of your glass this cloudy white layer; that's a corrosion layer on the glass. But no matter how many times you put your glasses in the dishwasher, they don't completely dissolve. It's a very durable material. And it's just the same for radioactive waste glass. If we put a fist-sized lump of radioactive waste glass into a bucket of water, it would still be there after several hundreds of thousands of years. These are very durable materials, and we can look into the geological record to see glasses that are tens of millions of years old. There are obsidian glasses that are very old. But if we look at the titanate minerals, the zirconolites, the pyrochlores, they are several orders of magnitude more durable than glass, so they really win from the durability perspective. And durability is important, because when the groundwater comes into contact with these waste minerals, the process of dissolving out the radionuclides has to be as slow as possible for us to have confidence that those radionuclides aren't going to escape to the geosphere and to the biosphere. So for me, crystalline materials win hands down on durability.

Oliver Strimpel: I see. So you're saying that even if these materials are damaged, either the glass or the titanates like zirconolite, even in the presence of water or hydrothermal fluids, we've seen that the minerals will not release the radioactive isotopes?

Claire Corkhill: Yes, exactly. In some of the minerals that I've been studying, you see small hydrothermal veins. We think the temperature of those hydrothermal events was somewhere on the order of 300°C, which is much hotter than we expect any groundwater in the geological disposal facility to be. We do see slight leaching of the uranium, but it's only ever transported a very small distance. So we can have really strong confidence that these are very durable materials.

Oliver Strimpel: Are the uranium or the other radioactive isotopes located as part of the crystal structure in these minerals, or are they in interstitial sites?

Claire Corkhill: They're absolutely inside the main lattice structure. If we take zirconolite, its prototypical formula is calcium zirconium titanate, CaZrTi2O7, and the uranium will sit either on the calcium site or on the zirconium site, or both. So it's very flexible. Same with the thorium. And that's how we're using that crystal structure as a template for synthetic materials. We can look at those and say, all right, we can maybe put some plutonium on the calcium site and some on the zirconium site, but we might need to add some charge balance on the titanium site, so why don't we try using some iron? So we tinker with the chemical structure to try and recreate what nature has given us, to find the most radiation-stable, the most durable, the most long-lived crystal composition that we can possibly find.

Oliver Strimpel: You mentioned that permeability is the number one criterion that we look for in these deep geological storage sites. But over the time scales we're talking about, can rocks change in their permeability, or can the water level change, so that these materials would eventually be exposed to circulating fluids?

Claire Corkhill: Yeah, that's a really good question. When the organizations look at particular sites to find out whether they're suitable, future changes, for example sea level rise or various other climate changes, are something they have to take into consideration. Will these changes affect how water is entering the site, or how it's leaving the site? This is something that they'll have to ascertain, and it's something that's very difficult to do. Most climate prediction models at the moment only go out to the end of this century and a little bit beyond, but what we're talking about now are much longer time scales, to be able to say, well, this is where we think the sea level will be, and so on. So that is definitely something that will have to be looked at and taken into consideration when a site is being selected in the future.

Oliver Strimpel: Where are the existing deep geological storage sites? Are there quite a few that are already in service?

Claire Corkhill: Well, despite this being something that has been rumbling on for many decades now, there are no operating facilities for high-level radioactive waste in the world. But that doesn't mean that they're not being built. In Finland, the Onkalo facility is being constructed right now. This is a facility that's being built in a very old gneiss, a very stable, crystalline old rock, and the construction is ongoing. I've been down; I had the lucky opportunity to drive all the way down. It's 475 meters below ground. As you drive through the tunnels, there are two big fractures, and water is pouring through them, but the fractures only go through the entrance tunnel. They don't go anywhere near any of the storage vaults, and they're literally building tunnel after tunnel after tunnel at the moment, ready for when they put in the first spent nuclear fuel. This is fuel that's come straight from the reactor, not recycled. It will go into the facility at some point, we're hoping in the next decade or so. So it's really exciting that the Finnish facility will be the first.

Other countries are also planning facilities, and they're at various different stages, either in construction or planning. Sweden is building its facility in the same rock as the Finnish one. In Switzerland and France, they're using clay. That's a completely different type of concept, where you have super-impermeable rock. It requires different construction techniques, because the rock is perhaps not quite as strong. And other countries around the world are trying to find a site, or they're not thinking about it yet. The only other one I will raise is a facility in the US, in New Mexico, called WIPP, the Waste Isolation Pilot Plant. This is an underground facility built in an evaporite deposit, and they've been disposing of some military waste from the US military program there. It's been operational for a number of years. Some of you may have heard that there was an accident there a few years ago, and there was a small release of radioactivity. It's really a good lesson in how humans aren't infallible in this situation. The rocks are doing their job just fine; they're drilling vaults and doing everything that they want, but there was some human error that led to a mistake there. So that's something to think about. We can put our trust in the rocks, but we can't necessarily put our trust in ourselves.

Oliver Strimpel: I presume that nobody really wants this kind of thing in their backyard, so how do we actually go about selecting a site?

Claire Corkhill: Different countries have done different things. I can talk about my experience from the UK perspective. We have a voluntary site selection process, which is actually also how it was done in Finland, Sweden and Canada. What these countries do is find a community who volunteers to have it. So in the UK process, we need two important things: one is a good, suitable host rock, and the other is a willing community. If we don't have those two things, we can't move forward with any particular site. You might be thinking, well, why would a community want to have a radioactive waste disposal facility near them? And one of the major incentives is the financial boost that will come with it. Governments essentially offer communities money to support their local community. In Finland and Sweden, it actually came down to two communities, and they were fighting over it because they really wanted that investment. And particularly in the area in Finland where they have this facility, they get so much money in taxes from the radioactive waste disposal facility, plus other industry in the area, that they've got some of the best hospitals and the best schools in the country. So there really are incentives for local communities to accept this disruption. And I think the disruption is one of the biggest problems that local people find with this facility. It's not so much about the long-term safety, because there's a general trust that if we as scientists say it's going to be safe, then, OK, all right, it will be safe. What's more of a concern is the day-to-day disruption, digging all of that rock out from under the ground and having all the trucks going by and all this kind of stuff. That's more, I think, what people are concerned about.

Oliver Strimpel: So we're really talking about centralized facilities that provide enough space for the entire radioactive waste of a given country, with all that disruption. So roughly what kind of volumes of waste are we talking about?

Claire Corkhill: I can talk about the UK perspective, because that's what I really know. The UK has a volume of 750,000 cubic meters of waste, and that's a difficult volume to think of. So we tend to think of it in terms of the volume of Wembley Stadium, the home of English football. If you were to fill Wembley Stadium about two-thirds full, that's about the volume of waste that we have. Now, underground, it's difficult to think of what that translates into in terms of the volume of rock that has to be excavated. The waste management organization actually thinks about it in terms of area underground. We're thinking about an underground area of around 26 square kilometers. So it's really like the size of a large town that we're going to be building underground.

Oliver Strimpel: Wow, that's enormous.

Claire Corkhill: It is quite huge. It's important to say that the surface footprint will be a lot smaller. At the surface, the waste will probably be delivered by train or by boat, depending on where the facility is located, and then offloaded and sent on, probably via something like a railway, down into the facility. So you can think of the surface footprint as being a bit more like a football pitch in size. But underground, the extent is absolutely vast. That's a huge amount of rock that's going to be excavated to host all of that waste.

Oliver Strimpel: We're talking about periods of time here that are so much longer than any human organization or human structure has existed. Do we even know how to think about planning for millennia into the future?

Claire Corkhill: It is a really good question, and it's very difficult to foresee those really long time scales. What I would do is flip it on its head and ask, well, how good are we going to be at managing this waste above ground for the next 50 years, 100 years? These are time scales we can look to, and that's where we are at the moment. We have the waste in these above-ground facilities. Can we guarantee that they're going to be safe over the course of 100 years? Probably. I mean, probably our society is going to be more or less as it is now in 100 years' time, or better, I hope, thinking with my optimist hat on. But over 500 years, can we guarantee that people 500 years from now will be able to look after the waste in the same way? Will they have the same technology? Will we have the same kind of stability? So I think the question is: isn't it better to try and isolate it from future populations over time scales that we can't imagine, rather than leave it where it is, on time scales that we can? So while it is very difficult to think about those long time scales, I think the alternative is worse.

One other thing to say is that on a geological time scale, what we're talking about here isn't very long. You and I think of a million years as the blink of an eye in terms of the geological time scale. But for these materials, for engineers, for people who think about corrosion of the container materials, what's going to sit around the container, and the waste itself, a million years is a tremendously long time. We can only do the best we can as scientists in the lab to try and predict how those materials are going to behave over a long time period. I would love to have a time machine and be able to fast-forward a million years to say, yes, do you know what, that geological disposal facility worked; it did what it needed to do. But we don't, so we have to try and do what we can.

Oliver Strimpel: But do we worry about potential bad actors having access to this? Or are there much easier ways to wreak nuclear havoc than picking open some ancient radioactive waste facility?

Claire Corkhill: Well, I think the idea is, and countries are thinking about this now, that once the waste is disposed of underground and it's all sealed off and closed, potential future bad actors won't know it's there. The idea is that we bury it and completely forget it. This is, for example, what Finland has decided to do: there is nothing that points future populations to it and says, hey guys, there's this stuff here that you might want to get out of the ground. It's better that it's completely forgotten. That's not the opinion of everybody. Some people think that it should be marked, and there are some wonderful plans for putting up really scary statues to make it look like a very uninviting place, because we might not be able to rely upon language being the same over the million-year time scale. There are other, slightly crazier ideas. This is genuine, though I don't know how it would ever work: genetically engineering cats so that when they came within a particular perimeter around the geological disposal facility, their fur would change color. You can think of all of the weird and wonderful things; people are thinking about this. How do we mark the facility in the future? But making sure that it's completely forgotten is probably the goal, so that the waste can safely radioactively decay, completely undisturbed, for the million years that's required.

Oliver Strimpel: What are you working on at the moment?

Claire Corkhill: Aside from the question of plutonium, there's another set of projects that I like to call turning radioactive waste into rocks. This is the idea of using the titanate minerals as a template for future waste types. We also do a lot of work trying to develop new types of glass materials for types of waste that don't currently have a disposal route and need to be transformed in some way into a safer material. Glass is actually a brilliant material, as we've discussed. So we're using what we know about the Earth, the deep Earth and how rocks are melted, and we take all that knowledge and try and translate it into developing new types of glass materials. And when those glass materials are developed, we're also very much interested in how quickly they dissolve. So we do a lot of experiments to try and understand the rate at which the radionuclides that we're trying to lock up inside their structure might be released.

Oliver Strimpel: Claire Corkhill, thank you very much.

Claire Corkhill: Thank you very much.

Oliver Strimpel: To see pictures and illustrations that support this podcast, go to geologybites.com, where you'll also find transcripts and a subject matter index of all the episodes. There you can also give me feedback, which I welcome, as well as sign up to get my emails about new episodes.