The science of nuclear energy
The science of nuclear energy

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The science of nuclear energy

2.2.2 Inside a nuclear reactor

The video shows Jem Stansfield looking around the Zwentendorf nuclear power plant in Austria.

This reactor was designed as a boiling water reactor (BWR). In this type of reactor, the reactor heats the water which produces steam that then turns the turbine. The steam is then cooled back to water and returned back to cool the core. The water is also used as a moderator.

The Zwentendorf reactor was built but never used; it was prevented by a vote within a referendum on the issue. Since 1978 Austria has banned using fission as an energy source in power stations.

Download this video clip.Video player: ou_futurelearn_nuclear_energy_vid_1037.mp4
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I'm about to do something almost no one ever gets to do - go right inside a nuclear reactor. Built in 1978, this one is almost identical to the Fukushima reactor, except it was never switched on. When you walk into a nuclear power plant, you can't help being slightly awestruck by the size and apparent complexity of the place.
But the truth is when you get to the heart of the operation, it's all surprisingly simple. All of this complex machinery is here just to monitor and control the nuclear reaction that then heats water and turns it into steam. Once the steam leaves the reactor, you're pretty much in the realms of conventional power. Hot, high pressure steam comes down pipes like this, and gets fed into a turbine like this.
There the technology is not so much nuclear as positively Victorian. The pressure of the steam pushes on the blades of the turbine, causing this to rotate. That then turns a generator, which produces the electricity that this whole plant was built for in the first place. A big problem I find with nuclear power stations is the sheer scale of them makes them a little confusing.
But honestly, it all just boils down to this. You've got yourself a nuclear reactor here. It's kind of like a kettle, except the water's not heated by electricity, it's heated by nuclear fuel rods. Boiling water produces steam. The steam comes down a pipe, and there it impacts on a turbine, which is essentially a bunch of spoons on a spindle. That produces electricity, and, hey presto, you've got yourself a happy town.
The thing that makes a nuclear power station different from a conventional one is how the water is heated to form steam. And to see that, I need to go into the reactor core itself. This is the heart of a nuclear reactor, and not many people get to stand here, because when active, all of this would be at around 300 degrees Celsius, and under a similar pressure to you'd find half a mile below the ocean, pushing these walls apart with a force of around 40,000 tonnes.
But where is all the energy coming from to do that? It's coming from down here. These are nuclear fuel assemblies. Now, if operational, this small space would be packed with over 100 of these, each giving out vast amounts of energy in the form of heat. And that's because every one of these square metal tubes would be packed with thousands of little pellets, like this.
The pellets are made of uranium oxide, and uranium is very special to us because it's an atom we can split. When things break apart, they tend to release the energy stored in whatever was holding them together. Now, it doesn't matter whether that's an atom or a stretched elastic band, like this one. So I'm going to come in, split it, and what I end up with is two smaller, high energy elements flying off in opposite directions.
Now, when that's an atom, those two smash into their surroundings, warming things up. Big difference is no matter how small your scissors, they're not the tool for splitting an atom. To do that you need a small particle called a neutron. Now, when this hits the very centre of a uranium atom, it can get absorbed, causing the atom to become unstable and split.
But as well as releasing all that energy, you also release two or three more neutrons that can then fly off into the surroundings, causing more trouble. Thing is, that's still not really enough to sustain a nuclear reaction, because uranium atoms don't absorb neutrons that easily. The neutrons need to be going at just the right speed, and for that, this whole reactor needs one more thing. Just add water.
The water plays a pivotal role, because it slows down those little neutrons to a speed where they're much more likely to be absorbed by nearby uranium atoms, causing them to become unstable and release more energy, and more neutrons in a continuous cascade. Now, if you can keep this sustainable, you've gone critical, which is a good thing, because then you're generating heat sufficiently quickly to run a power station.
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In the next section you’ll consider the distribution of nuclear power stations around the world.


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