Nuclear Fission Reactor Design

Hello. Today we're continuing with our series on nuclear physics looking at uh nuclear fishision reactor design. In the last video we looked at the principles of the design. Now we can look at more detail. Just to remind you the principle is that you have nuclear fuel rods um which will contain uranium 238 and uranium 235. The uranium 235 is usually only about. 7% of the total natural uranium. The rest of it is uranium 238. But you can enhance that or enrich it to produce about 3% uranium 235. But it's still a small proportion. But that is what is fishing. producing the energy. The water is used for two purposes. Firstly, it's the coolant. You pump it round and that takes the heat away and the heat can be used to produce steam. Steam can drive turbines that can drive generators and produce electricity. But the water is also used as a moderator. It slows down the neutrons produced by this reaction to produce further thermal neutrons that can cause the reaction to happen again. All covered in the last video. These are cadmium control rods. Usually cadmium cadmium. They are placed in position to restrict the number of neutrons so you don't get the thing getting out of hand and blowing up. You just get a nice steady reaction. So what are the options? Well, one option is to pump this water in under pressure. And if you do that, the pressure would be something of the order of 100 atmospheres. And if you do that, then the boiling point of water which is under pressure will go up from 100° centigrade to about 300° centigrade. So you pump in high pressure water which gets heated to say 250° centigrade. So it's not boiling because the boiling point now is 300° because it's under pressure. And so you've got boiling water or rather water coming out at 250° which you put through a heat exchanger. And of course anything that's at 250° will boil ordinary water that isn't under a pressure at all and produce steam. And that steam of course can be used to drive the turbines. And that is called a pressurized water reactor, PWR. I said in the last video that some people use heavy water instead of water because the hydrogen atoms can absorb neutrons to produce neutrons. So you get ordinary water converted into heavy water and that takes some of your neutrons away. But if you put heavy water in here in the first place, then even though there is a probability that dut dutyium heavy water dutyium will absorb a neutron to produce tritium, another isotope of hydrogen, the probability is much less. So some people put heavy water in their reactor and then it becomes a pressurized heavy water reactor, PHWR. A further option of course is just simply to put the water in not under pressure at all but just put it in, let it boil and take steam out at the top and the steam immediately goes and drives the turbines. And that is called a boiling water reactor. It's just ordinary water, not under pressure. You allow it to boil. you take the steam out at the top and you let that drive the uh turbines. There's a further option um which is essentially to take the same principle. Here is our uh lead lined concrete lined container. We've going to have our u uranium fuel rods. We're going to obviously have our control rods coming down. We're going to have um this time instead of putting water in we're going to put gas coming in, usually carbon dioxide. So the gas comes in is heated up and hot gas comes out at the top and that can be used to boil water to produce steam. And usually as I've said you use say carbon dioxide as your gas but the gas is now only being used to take the heat away. You need something else to do the moderation. And what is often used for a moderator in this system is graphite which is a form of carbon. And that is called an advanced gas cooled reactor. A GCR The earlier version of which was called a Magnox. So there are some Magnox reactors left which are

early versions of this advanced gas cooled reactor. Now we've shown how uranium 235 is what's called file. In the presence of a thermal neutron it will produce these fish materials more neutrons and energy. Uranium 235 is not the only thing that will do that. Plutonium 239 is also file as is uranium 233. But these are rare commodities. You cannot go out and dig them up. But you can make them. And here's the way We know that a neutron can indeed be absorbed by a 238 uranium atom. And that will produce uranium 239 in an excited state. But that will not fish. That's 92 down here. That will not fish for reasons we've told. The ex the energy of the excited state is not enough to get over the activation energy. So what does this do? Well, it deexites to start with and gives you off a gamma ray. Well, that's no use. But then it will beta decay because it's unstable. So it will bet to decay to 239 93 neptunium and that two is unstable. That will be to decay to 239 plutonium. In each case a beta decay beta minus decay changes one neutron to a pro proton. So the total number of nucleons doesn't change but protons increases by one. So inside our ordinary reactor where of course we had nuclear fuel rods which contained uranium 238 but that wasn't doing much good because it's the 235 which is file. Nonetheless, when neutrons are absorbed by uranium 238, they can go through a process which will deliver you another file material, plutonium 239. And that is called a breeder reactor. Sometimes called a fast breeder reactor because the advantage is these neutrons can be fast neutrons. You don't have to slow them down. So you can have fast neutrons producing plutonium fuel. In other parts of the world, there is a prepoundonderance of thorium. Thorium is thorium 232 and it has 90 protons. And once again, that can absorb a neutron to produce an excited state of thorium. That would be 233 thorium with 90 protons. Um, which will and let's put the excited state. And the first thing that will happen of course is it will deexite. It will give off gamma ray. that's no use to us. But it will then go through beta decay again. And beta decay doesn't change the number of neutrons. It just changes a neutron to a proton. So that goes up to protactinium. Protactinium which has 91 protons and that will decay by beta decay to 233 92 uranium. So that is another file material that can be produced. So if in your reactor you put some thorium just sitting there then the neutrons that are being produced as part of the nuclear reaction uranium 235 reaction some of those neutrons can be absorbed by the thorium to produce a process that will eventually give you another file material uranium 233. How are nuclear power stations used around the world? In the United Kingdom, we have 16 reactors that produce 16th of the UK's electricity supply. 14 of them are advanced gas cooled reactors. One is a pressurized water reactor and one is a Magnox reactor which is an early version as I told you before of the advanced gas cooled reactor. So here in this country you can see overwhelmingly we've gone for the advanced gas cooled reactor types with just one pressurized water reactor. In the United States, they have 100 reactors, of which 65 are pressurized water reactors and 35 are boiling water reactors. So you can see how different countries have selected different types of technology. Different parts of the world of course in other countries choose different things again. And uh but that just gives you an idea of where we are

um at this stage. Of course, the absolute dream is not a nuclear fishision reactor, but a nuclear fusion reactor. And we shall learn about that next time.