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Fission Reactor

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Fission reactors use fissile materials to initiate fission reactions. A moderator is used to slow the reaction. If the reaction gets too hot, the fuel melts and the reactor no longer functions. Control rods absorb neutrons from within the fuel to keep the fissile material from overheating. The thermal medium is either water or sodium and is used to harness the energy. The hot thermal medium boils and in its gaseous state is used to run a turbine and generate electricity.

So let's talk about nuclear fission reactors. So these are the standard types of nuclear reactors that you hear about that generate a good proportion of the power that we use in the United States. How do they work?

Well, let's talk about what we need to build a nuclear fission reactor. First, we need fuel. Now the important thing about this fuel is that it is a fissile material. What that means is that if you shoot a neutron at it, there's a good chance that it will split in a half and send other neutrons. So what you're trying to do is initiate this reaction by having some sort of a natural fission process. So somebody just breaks in half naturally and some neutrons come out. Then you want these neutrons to go and split other nuclei. Every time that a nucleus is split, it releases energy and it also releases more neutrons to go split more atoms. So that just gives you more and more and more energy called the chain reaction. Alright.

Now there's a couple of things we need to work for this thing to take place properly. So first off we need our fissile material. The standard choices for fissile fuel are uranium 235 which is called enriched uranium. Plutonium 239 which is just called plutonium because we don't usually deal with the other isotopes and uranium 233. Uranium 233 is a real special one. It's used in something called the reactor which not only gives you energy but it also gives you more fuel, so you get everything. But anyway, so we've got these fissile materials, that's our fuel. Now the fuel doesn't just work on its own. Here's the idea. When the uranium 235 nucleus splits in half, it sends off those neutrons and we want those neutrons to split other nuclei. But these neutrons are going too fast. They're just going too fast. It's not very likely that they're going to split other uranium 235 nuclei. It's more like they just go right by. Not even communicate. So what we need to do is we need to slow these neutrons down. We need to make them from fast neutrons to what are called thermal or slow neutrons. So the way that we do this is we introduce something called a moderator. Now what a moderator does, is it will just kind of bounce off of these neutrons and slow them down so that they're going slow enough that it's likely that they'll go and break other nuclei and give us more energy. Alright. So we need the moderator to do that.

Now, what about safety? Here's the issue. If the reaction goes on too fast, then the fuel which is a metal, will just get too hot and it will melt and that's called the nuclear meltdown. That's literally what that means. The fuel melts and then just kind of goes to the bottom of the container and your reactor doesn't work anymore and in fact has become a radiation hazard. So you don't want that to happen. So how do you do that? How do you stop the process? Well, what you use are something called control rods and these are rods that you literally stick down into the fuel and their purpose is to absorb the neutrons. So they just take them out of commission, no more splitting nuclei for you. Alright? So that's the purpose of the control rods and you'll put those down in there when the temperature of the fuel gets too high and you got to cool it down. Alright. So those are control rods they're generally made of about 80 per cent silver, 15 per cent indium, and 5 per cent cadmium basically because these metals all absorb neutrons fairly well, and this specific alloy is easy to work with. So you can machine it and you can make it into rods. Alright.

Then we need a thermal medium. And the purpose of the thermal medium is to take the energy that's released in these fission reactions And turn it into energy. So we're going to use, most of the time, people use water or sodium, alright? A lot of the nuclear submarines use sodium, liquid sodium but most of the reactors up here on the ground level use water, alright? So the purpose is to heat up the water and then send it somewhere. Then we're going to boil off the water and use it to run a turbine. And that turbine runs the generator and that's where we get the power from. So let's look at this process, I've got it outlined here in this wonderful diagram form the US nuclear regulatory commission.

So what we've got is the reactor over here. We've got our control rods that we can stick down into the fuel, and we've got our mediator or moderator that is just kind of flowing around here. Now, this is a pressurized reactor and so what's happening is there's water or sodium in here that's all pressurized. Now, when the fissions happen over here, that heats up the fuel and then the water takes that energy and it's circulated through here.

Now, what we're going to do is that we're going to separate our turbine action from our thermal mediator because what, this stuff is radioactive and we don't really want to, we don't really want to use it in the external environment. So we're going to keep all this pressurized and then we just got a part here where it overlaps and this real hot superheated water is going to heat this water and boil it off. And then we've got steam going through here, the steam runs through and turns this turbine and then that is what runs the generator. So the turbine's going around like this and that runs the generator which has a magnet in it and that's what generates the electricity over here. So then we've got a condenser that takes this steam and condenses it back into liquid water and we've got some sort of an external water supply here running through cold water that will condense the steam. So basically we've got two enclosed systems. We've got the radioactive system here, the less radioactive system here and we're cooling it using the external environment.

And that's a nuclear reactor.