This is Geology Bytes with Oliver Strumple.
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 wastes have
relatively short half lives measured in decades, some
isotopes have much longer half lives measured in thousands, 10s
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 kind of hazards do we
need to guard against on such time scales?
Claire Corkill 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 Corkill. Welcome to Geology Bites.
Thank you very much for having me.
Can you bring us up to speed on the nuclear processes that
produce radioactive waste? So let's start with what happens
inside a nuclear reactor. We start with uranium dioxide
fuel and the uranium. 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, and that energy is then used to heat
water, turn it into steam, to turn turbines to create
electricity. But the splitting process of the
atoms also creates smaller atoms, and these smaller atoms
are themselves unstable and their instability results in
radiation. So these are fishing products
that are developed inside the nuclear fuel and they can have
radioactivity of some seconds all the way through to millions
of years. So are those fission products,
then, the main kind of radioactive waste that we are
concerned with here? Yeah, exactly.
So when we've finished with the fuel inside a nuclear reactor,
there are a number of different things that we can do with that
fuel. So the fuel can either be
removed from the reactor, it's cooled for a while because of
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, we call this an open fuel cycle or it
could be reprocessed. And that's because that fuel
contains useful uranium and plutonium that could be turned
into new nuclear fuel. And so the reprocessing is the
operation by which you dissolve down that nuclear fuel material
through a liquid separation chemical separation process, you
extract the uranium and plutonium and then everything
else that's left, that is the fission product essentially.
And those fission products, what we do is to dry them out.
It's difficult to think about what this looks like.
The dried out fishing products ends 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. And these coffee granules are
fishing products. They 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.
And so the spent fuel together with the glass we term high
level waste. There are other items that are
created through this process that are contaminated with
radioactivity. So for example, the fuel pins,
which are small pipes in which the nuclear fuel sits, these get
split off from the fuel in the reprocessing operation and
they're contaminated with the fuel.
So 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. So some of this waste has fairly
long half lives. So do we try and extract the
things that are likely to be problematic on the long term
when we think about long term storage, or do we just take the
whole lot and treat it all together?
We essentially take the whole lot and treat it together so
there are shorter half life isotopes like strontium 90,
cesium 137 which have half lives around the order of 30 years.
What ideally we'd like to do is leave these isotopes for up to
10 half lives before they're considered to be safe.
So the shorter ones like cesium and strontium.
We only need to leave these a few 100 years, which means that
we could store that waste probably in above ground storage
units for 300 years. That seems like a reasonable
thing to do and let the radioactivity decay.
However, there are much longer lived isotopes and we're talking
about things here 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. OK.
So that gets us to the geological storage.
So you have mixed all this stuff together with this borosilicate
glass. Do we just take all that, find
deep geological storage, put it in there, lock the door and
forget about it? How does that work?
Well, really we have to try and find the best place to have a
geological disposal facility. So 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 really civilizations.
So we really need to find a place that's going to be stable
and secure, I guess over these really long time scales.
And the time scale that most organizations who are trying to
dispose of this waste are looking for is a million years.
So there's really no better place to put this waste than in
a stable rock formation that has been stable for probably
billions of years, if not hundreds or 10s of millions of
years. So when we think about the
disposal of the waste, I don't want people to jump their minds
straight to the kind of 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 1000 meters below the earth.
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.
That will ultimately start to dissolve the radioactive things
from inside the waste and transport them outside.
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.
So 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 that disposal
facility before we can even think about Building 1.
Let's go back to the selection of the site.
Now, 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 Neo protozoic, which
are extraordinary stable? Are we really trying to find
something that's geologically stable on that kind of time
scale? 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 anybody of rock of the right
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.
So 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 and the subsurface.
So number one key criteria is to find a low permeability rock.
Now lots of rocks are very non permeable, so clays are very non
permeable. If we have a non fractured
granite for example, that would be very, very low permeability.
It could really be in any kind of rock.
So we're also looking for somewhere that's very
tectonically stable, somewhere where the rocks are not going to
change significantly over a period of time of that million
years. We're not going to have
intrusions of granite or dykes or whatever coming through the
geological disposal facility. We're also interested in it not
being close to any resources. So 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 or
deposit. We don't want future populations
to go, 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.
So when we find an ideal site like this that has the right
geological properties, do we use that for the waste from many
different countries, or does each and every country have to
have its own waste site? So 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, and some countries are sharing in other types of waste
disposal, for example, the kind of 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
maybe they could share a borehole disposal facility for
those kinds of wastes. But generally speaking, once
you've created your own waste in your own country, you're
responsible for it and you have to dispose of it.
So what does the 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.
You're absolutely right. So Japan is a really interesting
case. They are planning to build
geological disposal facility for their radioactive waste.
As we all know, Japan have a very long history of nuclear
power and even past 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.
So again, this comes back to the point that I said earlier,
really any rock is suitable to be a geological disposal
facility. It just needs to have a low risk
of that radioactive waste being exposed to future populations.
And if the organizations involved in disposing 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. You just said that we want to
minimize the risk of this waste coming to contact with human
populations in the future. So it seems like the first thing
is for something we don't really want to have happen, 'cause 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?
Yeah, so I'm going to take the example of alpha particles.
So if we can all think back to alpha, beta, gamma radiation.
So let's talk about alpha particles.
If we think about an alpha particle, which is a helium
nucleus, it's quite big and it's quite heavy and it moves with
quite a lot of energy inside a material.
So if we take, for example, a crystalline material, so we'll
come back to glass in a moment. If we have a crystalline
material 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 it and it will displace that atom and
there'll be a number of other displacements.
So you can think of it a bit like a set of billion balls.
You have the cue ball hitting the the triangle of balls and
they just scatter everywhere. So that's what happens when an
alpha particle hits the crystal lattice inside a crystalline
material. And obviously the more the bonds
that you break, the more lattice positions that you move.
The properties of that crystalline material breakdown
over time, and they start to cause a lot of damage, which may
impact the durability. So 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. So glass doesn't have that
crystalline nice 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 1st place.
So glass is a remarkably resistant to radiation damage.
That's not to say that not 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.
So do we actually use some crystalline materials in fact,
in addition to the glass? At the moment, no.
But that brings us to something that I'm working a lot on in my
research at the moment is trying to explore the use of certain
titanate minerals. So we're interested in a couple
of key players who are pyroclore and also zucconolite.
These mineral structures are relatively radiation resistant.
They do overtime become completely radiation damaged,
they become amorphous overtime. This is a term we call
metamictization. But despite this
metamictization, which does happen 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. And some of these minerals that
I've been studying from the Canadian Shield, they're around
about 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, even though they're completely metamic, they could
be good host candidates for actinide wastes that come from
nuclear power. And the one that 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 be used potentially 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 that you could potentially
use these crystalline materials for disposing of the waste
products. 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 10s or hundreds of
thousands or millions of years? So these crystalline materials,
whilst they're not as good as glass at being resistant to
radiation damage, they ultimately will become
amorphous. The key factor here is that they
are orders of magnitude more durable.
So when you try and dissolve these materials, they are
completely different. So glass is a very durable
material, super durable. And 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, you know, the temperature and
whatever you will see sometimes on the outside of your glass,
this cloudy white layer, that's a corrosion layer on the glass.
But you know, no matter how many times you put your glasses in
the dishwasher, they don't completely dissolve, right, that
it's a very durable material. And that's just the same for
radioactive waste glass. If we put a fistful size 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 also glasses that are 10s of millions
of years old. There are Obsidian glasses that
a very old, but if we then go away and look at the titanate
minerals, the there's econolites, the pyroclaws.
They are several orders of magnitude more durable than
glass, So they really win on the durability perspective.
And the durability is important because when the groundwater
comes into contact with these waste minerals, the process of
dissolving out the radionuclides, it has to be as
slow as possible for us to have confidence that those
radionuclides aren't going to escape to the geosphere and
they're not going to escape to the biosphere.
And so crystalline materials for me, they win hands down on
durability. ICC are saying even if each of
these materials could be damaged, either the glass or the
titanite as a colite, even the presence of water or
hydrothermal fluids, we've seen that the minerals will not
release the radioactive isotopes.
Yes, exactly. So some of the minerals that
I've been studying where you see these 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 only ever is transported a very small
distance. So we can have really strong
confidence that these are really very durable materials.
Are the uranium or the radioactive isotopes located as
part of the crystal structure in these minerals or are they in
interstitial sites? They're absolutely inside the
main lattice structure. So if we take zirconalitis,
prototypical formula is a calcium zirconium titanate, so
CAZ RTI2O7 and so the uranium will sit either on the calcium
site or on the zirconium site or both.
And 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 where we can look at those and go, well,
all right, we can maybe put some plutonium on the calcium site
and we can put some on the zirconium site, but we might
need to add some charge balance on the titanium site.
So why don't we try and use some iron?
So we just 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.
So you mentioned that permeability is the number one
criterion that we look for in these deep geological storage
sites. But over the time scale 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? Yeah, that's a really good
question. I think when the organizations
who are looking at these particular sites to find out
whether they're suitable, future change in, for example, sea
level rise or other various climate changes, it's something
they have to take into consideration.
Will these changes affect how water is entering the site, how
it's leaving the site? This is something that they'll
have to ascertain. It's something that's very
difficult to do. Most climate prediction models
at the moment only go out to the end of the.
This century and a little bit beyond, but what we're talking
about now, a 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. Where are the existing deep
Geological Storage sites? Are there quite a few that have
already in service? 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 to say
that they're not being built in Finland.
The Oncolo facility is being constructed right now.
This is a facility that's being built in a very old nice, this
very stable crystal in 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 metres below ground.
As you drive through the tunnels, there are two big
fractures. Water is pouring through those
fractures, 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're going to put the first spent nuclear fuel.
This is fuel that's come straight from the reactor, not
recycled. This will go into the facility
at some point, we're hoping in the next decade or so.
So that'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.
So Sweden is building their facility in the same rock as the
Finnish one. In Switzerland and France,
they're using clay as a completely different type of
concept where you have super impermeable rock.
It requires different construction techniques because
the the rock's 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 WIP.
It's the Waste Isolation Pilot Plant and this is a facility
that is underground. It's built in an evaporite
deposit and they've been disposing their some military
waste from the US military program and that's been
operational for a number of years.
And 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 to see how humans aren't infallible in
this situation. The rocks are doing their job
just fine. There's a whole dog, they're
drilling vaults and they're doing everything that they want,
but there have been 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 a trust in ourselves.
I presume that nobody really wants this kind of thing in
their backyard, so how do we actually go about selecting a
site? 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 and Sweden and Canada. And So what these countries do
is to find the community who volunteer to have it.
So in the UK process we need two important things.
One is a good host rock, a suitable host rock and 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? Why would the 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 that. So governments offer the
communities money essentially to support their local community.
And in Finland and Sweden, actually it came down to two
communities and they were fighting over it because they
really wanted that investment. And the area, particularly in
Finland where they have this facility, they get so much money
and taxes from the radioactive waste disposal facility plus
other industry in the area that they've got some of the best
hospitals, the best schools in the country.
So there really are incentives for local communities to have
this disruption. And I think that's 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, Generally it will be safe.
What's more of 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.
So we're really talking about centralised facilities at all
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?
I can talk about the UK perspective because that's what
I really know. The UK has a volume of 750,000
cubic metres 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, so the home of English football, OK.
If you were to fill Wembley Stadium, about 2/3 full, that's
about the volume of waste that we have now the area
underground, it's difficult to think of what that translates
into in terms of a cubic meter volume of rock that has to be
excavated. The way that the Waste
management organization thinks about it is actually in square
footage underground. We're thinking about an area
underground with around 26 square kilometres.
So it's really like the size of a large town Underground is what
we're going to be building. Wow, that's enormous.
It is quite huge. It's important to say that the
surface footprint will be a lot smaller.
So the surface footprint will have probably the waste being
delivered on a train or by boat. Potentially, depending on where
it's located, the waste will be offloaded and then sent on.
Probably something like a railway down in into the
facility so that you can think of a bit more like a kind of
football pitch 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.
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 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 say well how good are we going to be at actually
managing this waste above ground for the next 50 years, 100
years, these 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 how it is now in 100 years time better.
I hope you're thinking with my optimist hat on, but over 500
years, can we guarantee that the people are going to be 500 years
hence from now? Will they 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 away from future
populations over time scales that we can't imagine, then
leave it where it is for now 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, We think of a million years as the blink of an eye in
terms of a 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? The waste itself, a million
years is a tremendously long amount of time.
We can only do as best as we can as scientists in the lab to be
able 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 to 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. But do we worry about potential
bad actors having access to this?
Or are there much easier ways to wreak nuclear havoc than picking
up some ancient radioactive waste facility?
Well, I think the idea is that once the waste is disposed and
countries are are thinking about this now, that when it's
disposed underground and it's all sealed off and closed, that
potential future bad actors won't know it's there.
The idea is that we bury it and we completely forget.
And this is, for example what Finland have decided to do, that
there is nothing that points to future populations to say, 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 more crazy ideas about trying to this
is genuine. I don't know how it would ever
work but genetically engineer cats so that when they came into
a particular perimeter near the geological disposal facility,
their fur would change colour. I mean you can think of all of
the weird and the wonderful things that you want to people
are thinking about this. How do we mark that facility in
the future? But to make sure that it's
completely forgotten is probably the goal, so that waste can
safely radioactively decay completely undisturbed for the
million years that's required. What are you working on at the
moment? So aside from the question about
plutonium, there's other sets of projects I like to call turning
radioactive waste into rocks. So this is the idea around 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 they need to be transformed in some way into
a safer material. And actually a glass is a
brilliant materials as we've discussed.
And 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
essentially. 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 to understand the rate at which the radio nuclides that
we're trying to lock up inside their structure, how quickly
they might be released as well. Clark Hawkill, thank you very
much. Thank you very much.
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