16.4 Fission and Fusion

All right, today is our last topic for Chapter 16, and it's on fission and fusion. Both of these are nuclear reactions that occur to produce energy. Over the course of history, nuclear power and nuclear power plants have scared quite a few people away. But in general, it's typically quite safe and probably a lot cleaner than our typical power plants that we use around here, where we burn coal. So, what we're gonna do is we're gonna figure out first what they are, and then we're gonna look at one in each of them independently to kind of see how they work and how the reactions would work for that. So first off, fission is where we take a large atomic element. So, we have something like uranium or something like that, and then we're going to split it into smaller elements, smaller nucleuses. And in that process, we release some energy. Typically, what happens is we start with something like uranium-238 and we add a neutron, which would be 0 1. So, if we add a neutron to something like uranium, it becomes really unstable. And once it's unstable, it then undergoes a reaction — alpha, gamma, whatever — it undergoes some sort of reaction which releases power. In general, fission is fairly easy to do. We can make these reactions quite simple, and we actually get quite a bit of power out of it. The problem is the byproducts, the elements that are created after the reaction tend to be quite toxic. So, we get lots of nuclear waste that has to be stored for thousands of years before it decays enough to be safe. So that's the biggest issue. Fusion, on the other hand, is sort of the opposite. We're taking two smaller elements, so that'd be like taking hydrogen atoms and adding them together, and they would form something like helium. Or we could take say hydrogen and oxygen, and maybe we get something like water or whatever. So, it's taking smaller nucleus of items and merging them together. And by fusing them together, that's going to give off a certain amount of energy as well. This is the process that our Sun does. So, when we get heat and light from our Sun, it's through fusion. And so far in history, we can actually do this okay in a lab situation. We could actually create energy by fusion. The problem is, at this point, it takes more energy to create the heat and the power that's needed to make these things fuse together than what we actually get out of it. So the problem is, it's not feasible to put more energy than what you're getting up, and the pro is definitely obviously we get nice, safe, easy-to-handle chemicals and the byproducts. So we'll start with fission because it's the most common, and this is what all of our power plants and things use. We'll first of all understand how this works, and then we'll look at fusion briefly afterwards. So, fission typically, the most common chemical we're using to start with is uranium-235. And it just comes from some Germans in 1939. They found out that if you take uranium-235 and have a neutron to it, you make uranium-236. That uranium-236 is extremely unstable, and it almost instantly splits into smaller atoms. What happens is we actually get two different types of reactions, and they happen about 50% each. So, uranium-235, you can see in this first one we add one neutron to it, it becomes uranium-236, and then instantly it splits into 141 barium and 92 Krypton. And if we balance our numbers, we have 141 plus 92 that gives us 233, and if we balance the bottom numbers, they do add up to 92. So that's good, but basically, we're short three neutrons. What happens is we get extra three neutrons ejected. So, we take uranium-235, add one neutron, we get two different chemicals, and three extra neutrons.Neutrons get created and then it also releases a certain amount of energy that gets used. The other reaction is almost the same, except it changes into xenon and strontium and then it only releases two neutrons. So, out of the two reactions, we both get extra neutrons and we both get extra energy created. So, at any point in any of these reactions, you're gonna get one or the other. And, like I said, they're roughly about fifty percent of the time each. So, quite often what we do is we just call it element X&Y. It doesn't really matter which two we get. And, we get, on average, two and a half neutrons emitted. So, the question then is, if we had one neutron to the initial reaction and we'd get these other chemicals plus, on average, two and a half neutrons, what do we do with those extra two and a half? Obviously, we need to do more reactions like this. Only for one nucleus we need to be more and more. So, where do we get that extra neutron that we need to do the reactions? Well, we can either add it again, which is gonna require energy. Or, what ends up happening is we take some of those two and a half extra neutrons and put them back into the reaction chamber to start all over again. So, you can see here we call this a critical reaction. So, we start with one neutron and the uranium-235 that produces our extras. You then take one of those, add it to another nucleus, that creates another reaction. And, we take another one of those, add it again, and another one of those, add it again, and so on. The cycle goes on and it'll basically continue forever as long as we keep adding one extra neutron and as long as we have enough uranium to react with. So, you might just have to keep adding uranium to the nuclear reactor. We call this critical, so critical is where the numbers are balancing. We're adding one, we're creating, where we're adding one, we create 2.5. We take one to add another one to create more, take another one, create more, and so on. If we added all two and a half, what's going to happen is we're gonna get an exponential reaction. So, we took two neutrons, you can see in this diagram if we took two and put them both back in, they would create two more, and so on. So, we'd end up getting this exponential growth which we call supercritical. So, that means our reactions are getting bigger and bigger, we get more and more reactions exponentially. Sometimes, this is good. So, in a nuclear bomb, that's what we wanted. We want one reaction to kick off millions of them within a split second and that's when the bomb then explodes. When a nuclear power plant first starts, we wanted it to be supercritical to get it up to speed, to get it towards producing enough energy. But, we don't want it to happen when a nuclear power plant is already at its peak. If we want it to keep going, then we just want to make it back to critical, want to keep it stable. So, if it keeps going supercritical, then eventually it's going to explode like a bomb would as well. And that's what happened in Chernobyl when the power plant exploded. And, it almost happened in Tokyo a few years ago with the tsunami. They had trouble getting it back to a critical level. It was going supercritical and they couldn't slow it down. So, all of a sudden, if what would happen if we didn't send the neutrons, so that one extra neutron didn't go back to the reaction? Then we're gonna basically cancel it. We will create the extra neutrons, but if we don't send them back, and we would call it subcritical. So, that means that the reactions are gonna start to slow down, eventually they would stop. And that's what we do, just shut down a reactor. So, if they say we're going to go under shutdown, they basically cancel those reactions so that eventually it stops. So, the question then is how do we control whether it's critical, supercritical, subcritical, and so on? And the idea is, well, we just have to control how many neutrons go back in. So, if we take those two or three neutrons, we need to get rid of the leftover. So, let's suppose we only want one. To come out of a reaction that produces three, we'll send one back in. We need to get rid of the other two, so they use a control rod, which is basically just a long rod of material that absorbs a neutron. So if we have a neutron coming along that hits the rod, it absorbs it and it goes away. So quite often, we use things like boron and cadmium because they're pretty good at absorbing neutrons. And all they do is they just basically raise or lower the rods to hit more or fewer neutrons. So if we don't want any neutrons to be collected, you just raise the rods right out so the neutrons just keep on going by and they miss the rods. If you want to slow down more of them, we've dropped them down so that these neutrons come across more of them get absorbed by those control rods. So it's just sort of like think of like fingers going up and down to collect all those extra neutrons that they want to get rid of. But there are still a couple of problems that they have to figure out. The first of all, these neutrons that are being ejected or coming out at extremely fast rates and they're actually too fast to be absorbed by the uranium and they're too fast to be absorbed by the rods. So what we have to do is we have to slow them down somehow. So what they do basically is we need to put some sort of moderator in front of it to slow it down. We want to basically create a collision to get that neutron to slow down. So the best way to slow something down is put something pretty big in front of it. So we can't get neutrons very easily from other sources or without adding a lot of energy. So that's not a good idea. So we want to pick something that's about the same size as a neutron, which is that which is the proton. And a cheap source of protons is water. So basically what we do then is we just shoot the neutrons into a water tank so that'll slow them down. And then we have our control rods that go inside the water tank to absorb the ones that we want to get rid of. And then the ones that make it through the water are going to slow down enough that they can go back into the reactor to start all over again. So that seems to work. That is, we can use water to slow it down. But now the problem is sometimes the water will gain these extra neutrons, and it ends up changing it to hydrogen too, which we call deuterium. And basically what ends up happening is those water molecules have an extra neutron. And if they have an extra neutron, they end up becoming heavier. They double the mass. So we often call it heavy water. And the problem with heavy water is it can be toxic. So if we just dump that back into a river or whatever, it causes some environmental problems. So then the issue is if we have heavy water, then what do we do with it afterwards? So there are basically three main solutions that solve this issue. The first is in the United States, what they do is they use enriched uranium. So that just means they find and get more carefully so that the critical reactions have a better chance and the neutrons don't make as much heavy water. So they use a different uranium to start with. There are fewer neutrons being ejected. Therefore, the water stays fairly clean. It still heats up, which when they dump it into a river, that causes problems too. But in general, it's safer. The other option is what Canada typically does. They use heavy water in the reactor. So instead of starting with regular water, they intentionally put in heavy water, which basically won't absorb anymore. So they'll take heavy water and just keep reusing it. The neutrons don't get absorbed by the water and they just go to the control rods, and that's it. So that's why in Canada, we tend to be leading in this industry because we're not putting as much pollution back into the environment. And the last option is sort of the old-fashioned version. And what they found out was if they use graphite rods instead of boron or whatever, the graphite tends to absorb all the neutrons, so the water stays clean, but the problem is graphite is very reactive when exposed to oxygen. So, if the graphite absorbs the neutrons, that's great. But, if any oxygen gets into the system or there's an explosion, these graphite rods become bombs themselves, causing lots of problems. That's exactly what happened in Chernobyl. The carbon rods they were using ignited. So, that's the downside of fission. It works well in terms of control and power generation, but the leftover byproducts and heavy water cause issues. Ideally, we would switch to fusion. Fusion, as mentioned before, involves small elements merging into a safe substance, releasing energy in the process. We start with two protons merging into a new element that gives off energy and an anti-neutrino. We continue this process to generate more and more energy. Why don't we use fusion all the time? Well, we haven't figured out how to make it efficient yet. Building fusion reactors requires a lot of heat and pressure, which ends up costing more energy than what we produce. A current focus in physics is to achieve cold fusion, which would eliminate the need for intense heat and hopefully make fusion more efficient. So, that's the basic difference between fission and fusion. Fusion produces more energy, for example, a typical reaction can yield around 200 mega electron volts as a byproduct, whereas fusion typically yields around 17. This significant difference presents a challenge. If we can reduce the energy requirement for fusion to, let's say, five or six mega electron volts, then it would be profitable and worthwhile. Currently, with fission, we don't need to invest much to get 200 units of energy. That's why most nuclear power plants around the world use fission. And that's all for today.