Nuclear Chemistry Nuclear Fission and Fusion Nuclear Fission Page [1 of 2]

Interested in destroying the planet? I can show you how to do it in one fell swoop. It’s the reaction of uranium-235 plus a neutron going to barium-141, krypton-92, plus 3 neutrons. And it’s an example of what we call a chain reaction. And to show you what a chain reaction is, let me show you this set of dominoes that I have in front of me, and I’m going to tip one over. This is the first neutron that’s going to come in and it’s going to impact on a uranium-235. And what happens is the product of that reaction makes more neutrons and those neutrons make more neutrons and those neutrons make more neutrons. And so, the reaction takes off. Let’s see if this works.

All right. So what we have here is a physical example of the reaction. Well, we had a uranium-235 and we hit it with a neutron, and it became a uranium-236, which then broke apart. And the reason why it broke apart is because when a heavy nucleus breaks apart, it moves up the binding energy per nuclei curve. In other words, it becomes more stable. And becoming more stable means that this is an exothermic process – gives off energy, gives off heat. Now that’s only half the story.

The other half of the story is it gives off 3 neutrons. And the reaction has to give off at least one neutron, and typically more than one neutron. Why? What these neutrons do is they go off and they hit other uranium-235’s. And if we make two or more neutrons from this process, we can get this thing to go in a chain, where it actually goes faster and faster and faster as the reaction proceeds. In other words, I tipped over one domino, and it tipped over two which tipped over each two. And it was going faster and faster and faster until we were tipping over six or seven at a time. That’s an example of a chain reaction.

Now, the way we make uranium-235 – remember the principal isotope of uranium is 238. We have to separate out the 235 from the 238. And it’s because 238 has a neutron capture cross-section. What it does is it gloms onto these

neutrons and takes them out of the reaction, essentially. So that’s a bad thing. So we have to separate out the 235U . And the way we do that, or a way it has been done, is to make uranium hexafluoride gases. And so uranium hexafluoride is a gas, so we make uranium hexafluoride with uranium-235. And we make uranium hexafluoride with uranium-238. And then we separate them based on the fact that their diffusion rates through a semi-permeable membrane are not the same. If you don’t remember what I’m talking about, go back and look at Graham’s law of diffusion and you can see that we can separate two gases based on their rates of diffusion, which depend inversely on their atomic masses. And so uranium-235 and uranium-238 hexafluoride, so each of those is hexafluoride gases. Those things have different masses, so they diffuse at different rates. At least in principle, you can separate 235 from 238, and it’s only 235 that is fissionable. That is, 235 – if you hit it with a neutron – will break apart into two roughly equal pieces plus some neutrons, and that’s what we call a fission reaction. It’s actually not necessary to have neutrons given off, but in the case of getting it to be useful as a source of energy to make electricity, we have to give off neutrons.

Now, the big problem with this reaction is that when you take a uranium-235 and you hit it with a neutron, it breaks apart every which way. It’s more like a car wreck than a chemical reaction, where so long as you balance for the mass and balance for the charge, you can make anything. You can make tellurium-137 and zirconium 97. You can make zinc-72 and samarium-162. You can make all these different things. And the key is, so long as we’re making a few neutrons – 2 or 3 neutrons – this reaction is going to be self-sustaining. It’s going to start chain reacting and keep going. How do we control this reaction? The way we control the reaction is we put something in the pot that can absorb these neutrons and essentially take them out of the loop. And the way we do that is with something that has a neutron capture cross-section – something that likes neutrons. And typically what people use is graphite. Graphite, carbon-12, has a reasonable neutron capture cross-section, so it just takes some of the neutrons out of the reactor and if you take neutrons out of the reactor, it slows the reaction down. So put the control rods in – they go by the name control rods – you put the control rod in. It reacts with the neutron, takes it out of the chain process, slows the reaction down. Pull the control rods out. More neutrons are going to react with uraniums and so the reaction proceeds faster. Now what do we do with all of this?

Well, it turns out it gives off a lot of heat and we do nothing more elegant than boiling water. A nuclear reactor – and there are a lot of fission reactors in the world. In fact, France generates more than half of its electricity with fission reactors. We do nothing more glamorous than boil water. We take these reactions. They get really hot, and then we use that heat to boil water. We superheat that water to make hot steam. And then that steam that’s really hot turns a turbine. And a turbine turning generates electricity. It generates alternating current. When we have water running over a dam, we’re also just turning a turbine and that generates electricity that fills your house. Well, instead of

Nuclear Chemistry Nuclear Fission and Fusion Nuclear Fission Page [2 of 2]

turning a turbine by water falling over a waterfall, it’s superheated steam, and that superheated steam turns a turbine that gives rise to electricity.

Now there’s another way we can do this reaction. We could take the common uranium isotope, which is 238, and if we hit it with a neutron then it forms neptunium-239 and then that neptunium can beta decay and it gives rise to a plutonium. And plutonium-239 is also fissionable. So we don’t have a lot of things that can do this, that can accept a neutron and then become something else that fissions and kicks off more neutrons and takes part in a chain reaction. But plutonium-239, which is something that doesn’t occur in nature, is another example of a fissionable material. So what we can do is take uranium-238 and, when we talk about a breeder reactor, what we mean is turning this into a useful fuel, 239. Well, the big problem with a fission reactor is that it makes all these different products – all this assortment of products. And all of these things, for the most part, are radioactive. Some of them are stable, but a lot of them are radioactive. And they have half-lives that range from microseconds, probably even shorter than microseconds, up to tens of thousands, maybe hundreds of thousands, of years. Plutonium, for instance, has a half-life of 24,000 years. Well, eventually, what happens is that the fuel rods, the things that contain the uranium-235, get contaminated with all of these other products. And then, because these other products might capture neutrons, the reaction slows down, and those fuel rods are no longer useful for running this reactor. So then, what you have to do is take the fuel and either reprocess it, do chemistry on it to separate out the uranium-235, or just take the whole mess and bury it in a hole. And that’s typically what we’ve done. We have done reprocessing, but eventually, you just bury it in a hole. The problem is that some of this stuff is going to stick around for tens or thousands – hundreds of thousands – of years, and that’s a problem. Where do you put something so that it’s going to be perfectly stable for hundreds of thousands of years? The best thing is to find some geological formation that we think hasn’t changed in millions of years and put it there. And so some of the places that we’re putting it are in salt mines near the border between New Mexico and Texas. The project goes by the name of WIPP – Waste Isolation Pilot Project. So that’s where this nuclear waste is going to go.

Well, we’ve also used this reaction in a less humane, more destructive, way. So we have harnessed this reaction, in contrast to the fusion reaction, what the sun does. We can’t harness that yet. But we’ve also used it in sort of totally random bad ways, and that is we’ve made bombs out of it. So before we figured out how to get useful energy for lighting up your television set and your refrigerator, what we did was we just took a bunch of uranium-235 and we blew it up. And it turns out that, if you think about it, you take the uranium-235 and there’s probably some low level of radiation that gives rise to neutrons. But if there isn’t enough – in other words, if the neutron doesn’t get captured by the uranium-235 – then the thing is going to start its fission. In other words, if we go back to the domino idea, if the dominos are too far apart, then you kick over the first domino, then you aren’t going to get the chain reaction going. So it turns out there’s something called the critical mass. And one thing that maybe you can explain to me, because I’ve never understood this, is why they call it a critical mass when it’s really a critical density. You have to have a certain amount of uranium-235 there in order to make the reaction go. And so what goes on in a nuclear bomb is – at least in one of the cases – you take two pieces of uranium-235 that are sub-critical, meaning there isn’t enough uranium-235 to get the reaction going, and you literally just smash them together. And when you smash them together, you get critical mass. And when you have critical mass, the reaction takes off. Well, this impacts things like storing of uranium-235. If you store too much of it in the same place, or if too much of it exists in the same place in nature, you’ll start a fission reaction. And it turns out that there’s a place in Africa where they think nature just accidentally created a fission reaction – a fission reactor that took the uranium-235 and it underwent reactions that decomposed it into these various mixtures of daughter products.

So what’s the bottom line? The bottom line is we really do use fission. We use fission to generate electricity – and some countries generate more of their electricity with fission than with other routes. It’s relatively dirty in the sense that it makes all of these things that we have to deal with – all of these radioactive isotopes, the radioactive wastes that we have to get rid of. But here’s something to think about. One of the things that it doesn’t create is it doesn’t create carbon dioxide. And carbon dioxide is giving rise to the greenhouse effect which is heating up our planet. And so you can ask the question is it more important to stop burning fossil fuels so we don’t make more , so we don’t heat up our planet, or is it more important to avoid having these radioactive wastes where we have to bury, or least figure out some way to treat, in order to continue doing fission reactions? In other words, so long as we make new fission reactions, we have to deal with the waste. But so long as we burn fossil fuels, we have to deal with the carbon dioxide. So that’s a quandary, something for you to think about. How do we decide the relative balance of burning fossil fuels versus doing nuclear reactions? Some countries have favored doing nuclear reactions and some countries are getting away from nuclear reactions and just continuing to burn fossil fuel. But it’s certainly something that informed citizens, like you, have to think about and we’re going to be debating over the next century.