Science Interviews

Interview

Sun, 11th Feb 2007

Storing Nuclear Waste

Ian Farnan

Part of the show Nuclear Power

Chris - Now we were talking about Sizewell there. It's not a very big reactor: four metres by four metres they said. That sounds tiny, but they said there's a huge amount of radioactive waste. How much?

Ian - I'm not exactly sure how much is stored at Sizewell. In fact all the nuclear waste that's produced at Sizewell is currently on site there so the UK doesn't actually deal with that waste at the moment.

Chris - How much have we got stockpiled at the moment? We heard earlier how there are 19 functional power stations in this country and a number have been shut down.

Ian - The majority of our waste, the volume is 470 000 cubic metres. To put that in perspective, only a few per cent of that radioactivity is in the vast volumes there and I think 98 or 99% of the radioactivity is in a much smaller volume, and that's what we call high level waste. This is the really really dangerous stuff.

Chris - How dangerous and why?

Ian - It emits beta and gamma radiation. It's extremely harmful and you cannot approach it. It takes many years to cool down and so Anna at Sizewell will have seen the spent fuel ponds. When the fuel rods are removed from the nuclear reactor, they're placed underwater and left to cool for long periods of time. Other reactors leave the rods there for maybe 10 or 15 years and then pull them out into something called dry casks. But it takes about that period of time before you can store them out of the water.

Chris - But storing them: how long are we going to have to store them for before they're considered safe again?

Ian - In the UK we have actually taken many of our nuclear fuel rods and reprocessed them. And in that process you separate out into two main types of waste. You have what's called the fission products which is half of the heavy element or the two halves that have split apart, and then you get the remaining uranium, which is depleted uranium, and plutonium that is generated in the nuclear reactor. So you've got these two parts. The fission products themselves are dangerous for about three hundred years, so the principle elements would have half lives of about thirty years, so ten half lives would be three hundred years. There are a few other problematic elements in there; technicium is one which we will hear about and has medical uses but it exists there and is quite long-lived.

Chris - But some of this waste that we're talking about we say that it needs ten half lives to be safe and that's 300 000 years.

Ian - No, 300 000 years would be something like plutonium 239.

Chris - Which we're producing in these things.

Ian - Yes so there's two types of waste: there's the waste that we call the fission products and in the UK we've separated the plutonium and the uranium from the fission products so the fission products are ok in 300 years. The plutonium and uranium are on much longer time scales.

Chris - So at the moment we've got a considerable amount of material that could be radioactive for a third of a million years.

Ian - Exactly.

Chris - How do we store that?

Ian - In July the government commission called CoRWM reported and said that we should build what's called a geological repository. What they said was that we should dig a hole between 200 and 1000 metres deep. It will be a bit like a mine, so a shaft will go down and then you'll cut out drifts into the surrounding rock and then you'll place canisters of material.

Chris - So does this just mean you put some stuff in a barrel and bury it?

Ian - No. The fission products themselves are treated with some oxides, heated up, and formed into glass. Those are then poured into cans, extremely strong cans, and they're stored at Sellafield at the reprocessing site.

Chris - Now is that stuff stable for a third of a million years?

Ian - That stuff doesn't necessarily need to be stable for a third of a million years because it only contains the fission products.

Chris - But I'm talking about the ones that need storing for a third of a million years, Ian.

Ian - So that material is stored at Sellafield and we've not decided what to do with that yet. Well we've decided what we're going to do with 5%. 5% of that and 100 tonnes of plutonium has been set aside as not useful as a future nuclear. This really depends, Chris, on decisions on whether we build new nuclear power stations and whether those nuclear power stations will then be licensed to burn what we call mixed oxide fuel: uranium plus plutonium. So depleted uranium and then the fissile material will be plutonium.

Chris - But what I'm getting at Ian is how do we work out a safe way to store that stuff with this incredibly long half life that needs a long time in the ground to calm down?

Ian - Well what we're trying to do is to develop some mineral-based ceramics which in a similar way to forming the glass, we mix oxides and we form something which is like a mineral. There are certain minerals that occur on the earth that have been proven to hold uranium and thorium for billions of years in some cases, and so those are the kinds of models for the kinds of materials we want to use. So we would like to isolate these very long-lived isotopes into a mineral before storing it, and hopefully that mineral will be sufficiently durable that it would not decay or be damaged by the radioactive decay that occurs inside it.

Chris - Is that true? Does it?

Ian - We haven't yet found one that's going to last for 300 000 years.

Chris - How long does it last?

Ian - Well the particular case we looked at recently lasted for about 1400 years.

Chris - So that's a fraction of what you need. So at the moment what you're saying is that we have this high level waste and we've got nothing that we can actually do that's going to be a safe long-term prospect for it.

Ian - It's a question of confidence. We know that this material will degrade because of its own internal radiation. Whether that amorphous material will then be dissolved by water is not very well understood.

Chris - Why does it get damaged? Why does it fall apart over time?

Ian - What happens is that there's an alpha particle emitted from heavy nuclei like plutonium and that heavy nucleus then recoils a bit like a howitzer that's just fired a shell. That skittles into all the other atoms and knocks them all over the place, and basically you no longer have a very well-described very durable material at that point because the atoms are all in very high energy positions. They're all knocked out of their usual positions and so could be attacked more readily by water.

Kat - So the challenge is to find something stable. We've had a suggestion here from Keith in Watford who says that nuclear waste could be encased in glass and shot into space. Is there any other way of sensibly getting rid of nuclear waste without putting it somewhere on the planet?

Chris - Yeah David in Chelmsford says why don't we dump it on the moon?

Ian - I think the Committee on Radioactive Waste Management, which delivered its report last July did consider very quickly putting things in space; firing them into the sun is another option. A feasibility request went to NASA and they could only guarantee, I think, 1 in 35 launches wouldn't result in an explosion, so the idea is that it would be too dangerous to do that.

Chris - It would be similar to what happened with Chernobyl basically in terms of the amount of contamination.

Ian - You would deliver it into the stratosphere and it would be spread around the world.

Kat - That just sounds terrible. So more research is needed quite urgently.

Ian - I think that the research that's being done is to really research the materials we put these things in. We actually form these things into another material and that material is actually very very durable with respect to water. The main problem is that if you put this stuff down deep into the ground, the question is whether this material will come into contact with water. That's the only way that the radioactive isotopes will leak out if it comes into contact with water. So that can be controlled by the geology of where you put, or it can be controlled by the material itself. You obviously need a combination of those two, but the better you can make the material in the first place, the more certainty you have in the disposal process.

Chris - What's the situation in America, because they've invested quite a lot of money in deep burial.

Ian - That's right. They have a repository site in Nevada that's about ninety miles west of Las Vegas. All states in the US except one, which is Nevada, have agreed that nuclear waste should be sent there. At the moment I think there's one legal challenge that is preventing the go-ahead of that project. They will have lots of nuclear waste from their nuclear weapons programmes and from their commercial nuclear generation programme, which will be transported to Yucca Mountain and be set in the type of repository that I described earlier.

Kat - I was writing a blog last year for the Institute of Physics that looked into nuclear energy and those kinds of issues. And it did seem to be that there's a lot of discussion about the right way to build nuclear power stations and how environmentally friendly it is, but no-one seems to have solved what to do with the waste.

Ian - If you ask most scientists who work in the area, we have a gut feeling that it'll probably be ok, but that's not good enough.

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