Dr Ian Farnan, Cambridge University
Ian - Well when we’re talking about nuclear energy to generate electricity, what we’re talking about is controlling the reaction of a neutron with a heavy element such as Uranium 235. What we do is we divide up the uranium into pellets and put those pellets into fuel rods, and those fuel rods are then clad in a zircaloy, an alloy cladding. What that allows us to do is to have, between those rods of fuel, water and control rods which are made of neutron absorbing materials. The way that the water and the control rods slow down the neutrons, is that it controls the rate at which that uranium atom splits. It’s the fission of that uranium atom that we’re trying to control with nuclear energy.
Meera - So essentially, you're bombarding Uranium atoms with neutrons and breaking the uranium apart to release energy?
Ian - That’s right and the neutrons come from the uranium itself, so each fission of the uranium has on average about 2 ½ neutrons produced during that fission event and those neutrons go on to split further uranium atoms. And so what happens is that then you split the uranium atoms and you'll get different fragments produced of other elements. So you're transmuting the uranium into two roughly equally sized smaller elements.
Meera - But how much energy does that release say, in comparison to fossil fuels?
Ian - Well typically, you might expect a tonne of uranium produces as much energy as a million tonnes of coal, for example.
Meera - So that’s quite a big difference.
Ian - Yes, that’s a very important difference because when we’re talking about waste products, for example carbon dioxide from the burning of coal, there’s an enormous amount of carbon produced from that million tonnes of coal whereas the waste products from the nuclear energy is much, much smaller.
Meera - Well what are the waste products? So you split this uranium and then what’s produced?
Ian - You get various products from the uranium fission. So typically, you might have barium and zirconium, then there is some distribution of different elements below those. The neutrons which are in the reactor also activate other elements, but it’s within the fuel and within the steels which make up the reactor vessel. Those elements might be Cobalt 60 or Iron 55 for example.
Meera - Now I guess the important difference though, when it comes to nuclear energy, is the risk associated with these waste products. Which of these are a concern and are they different as well?
Ian - Well, when we take the fuel out of the reactor, it can be separated basically into three types of waste products. There’s the fission products that I've already mentioned, there’s neutron activation products, and then there are what’s called the actinides; uranium which is still the vast majority of the nuclear fuel, there still uranium dioxide, then you have plutonium and some other heavier elements; americium, and curium. So you need to handle these materials in different ways and a standard way in this country has been to reprocess the nuclear fuel rods and put the fission products and the neutron activation products into a glass. This is a process called vitrification. You can dissolve up the nuclear fuel rod in acid and then separate the heavier elements like uranium or plutonium, leaving you with an acidic, highly radioactive liquid which is then dried and made into a boro-silicate glass.
Meera - And are these radioactive elements quite stable within this glass?
Ian - As far as we can tell, they seem to be pretty stable in the glass. The glass itself is not often tested because it’s extremely radioactive and it’s very hot. Typical centreline temperatures of a glass canister will be over 400 degrees centigrade. So the glass is immediately poured into this canister. It is molten glass at 1100 degrees C but the radiogenic heat keeps the temperature of the glass very high. So the glasses themselves are not really examined but they seem to be quite stable.
Meera - But when it comes to radioactive elements, the important thing to consider is their half-life. So ‘how fast are they going to decay?’ and I guess the faster something decays, the more risk it is immediately to you. So these fission products actually differ quite a lot in say, their half life and their radioactivity to the other heavier actinide elements, don't they?
Ian - That’s right. One of the advantages of separating the fuel by the reprocessing process is to separate the fission products which have shorter half lives, typically 30 years for Strontium 90 or Caesium 137 after say, 300 years which will be 10 half lives, the radioactive load of those materials would be low enough that we wouldn’t need to worry about it too much. That means that a lifetime or the guarantee that the repository has to provide for the safety of the materials is much shorter, typically a thousand years or so.
Meera - The big concern then or the main concern when it comes to the waste are then these heavier actinide elements because they've got quite a long half life. So they're a concern, long into the future as well.
Ian - That's exactly right. So the elements like uranium, plutonium, americium – actinide elements – are very heavy. They decay by what’s called alpha process and their half lives are typically say, for plutonium, 24000 years, so you'd have to wait 10 half lives, so say, 240,000 years for that to decay to some sort of safe level. There, we run into problems. If we want to guarantee the integrity of the geological repository site over those sorts of timescales, we’re less certain about how the geology of the site will change, we’re less certain whether the metals of the canisters will last, then start to get concerned about how water will progress through the repository, interact with these packages of nuclear waste, and water is the only way really that that material can come back up to the surface and enter the biosphere where it would be dangerous to humans and animals.
But it’s a very interesting question because we’re looking at timescales now where Homo sapiens are only about a quarter of a million years old ourselves and now, we’re looking that far into the future. So, what will the human race look like at that point in the future when we want to guarantee the lifetimes of these repositories? It’s very, very difficult to provide that guarantee. We can probably make a pretty good shot at it, but people want guarantees and that's a very, very big challenge for scientists.