How a Nuclear Power Station Works

Kat - Now we've just been hearing about radiation in your own home, but what about the stuff that creates radioactivity: nuclear reactions? Earlier this week we sent the Naked Scientists Anna Lacey and Dave Ansell to Sizewell B nuclear power station in Suffolk, which is one of the nineteen nuclear reactors in the UK and currently supplies around 3% of the UK's electricity needs. They went along in their sturdy protective gear to find out what's inside a power station and how it actually works.

Anna - I've managed to get through security and I'm now fully kitted out in blue overalls and a nice hard hat. But before I go and have a look around the power plant, the first questions should really be: what is nuclear energy? Well in the case of the power plant here at Sizewell, it's all about a fission reaction. To do that, you have to split the nucleus of an atom like uranium in two, and it's this splitting it in half gives off loads of energy. But to find out a little bit more about how that really works, I'm here with Dave. So Dave, how do we go about splitting an atom in two?

Dave - Well as the name suggests, nuclear energy is all about the nucleus of an atom. Now the nucleus of an atom is made up of two kinds of particles: you've got protons which are positively charged and repel each other quite strongly, and you've got neutrons that don't have any charge at all and kind of sit there. Both neutrons and protons are attracted together over very short distances by something called the strong nuclear force. This acts like glue and sticks neighbouring ones together. This means that a very big nucleus, something like uranium which is the heaviest nucleus you can find naturally, you've got 92 protons that are repelling each other. That's almost overcoming the attraction of the glue. All you've got to do is give it a bit more energy by throwing in a slow neutron into the middle of that to give it enough energy to split it into two and break that glue connection. Once that's broken, you've got two halves of a nucleus, which are repelling each other really strongly. They fly apart and release an immense amount of energy. Luckily, it also releases two or three neutrons which can fly off and hit another uranium atom which will split and release others. This gives you a chain reaction and you can release an immense amount of energy.

Anna - Ok thanks for that Dave. Well now we've heard about some of the science behind splitting atoms, let's talk to somebody who works with this kind of stuff every single day. So I'm here with Colin Tucker and he's a physicist and nuclear safety engineer, I think that's right isn't it Colin, here at Sizewell. So Colin, what is it that we're actually looking at here?

Colin - You're stood outside the reactor building, which is the big white dome that people see when they walk up and down the coast here in Suffolk. That's 72 metres high and it makes people think that maybe we've got a big reactor here; we haven't. The reactor's only four metres across and it's buried right down in the basement of that building.

Anna - So what kind of energies are we talking about being produced here at Sizewell B?

Colin - In that reactor, which as I say is only about four metres across, we're generating 3500 million Watts of heat. So it's about a million electric kettles-worth in that small volume. We use that heat to heat water, about 20 tonnes of water a second. We heat it up to more than 300 degrees Celcius. What do we do that for? Well we can then use that water to heat up some more water at a lower pressure and make steam, about 2 tonnes of steam a second. That travels from the big white building across to the right here into the turbine hall where it spins our turbines at 3000 times per minute. At the back end of those turbines are the generators, and that's what we're here for. Nuclear power stations exist to generate electricity and that's what we do, day in day out.

Anna - That sounds fantastic. Can we go and see it?

Colin - We can go and have a look at the turbine hall, certainly.

Anna - We are now in the turbine hall and as you can probably hear it's insanely noisy. What we have is an enormous open building with lots of metal floors round the edge, one of which I'm standing on. And down in the middle is a whole series of turbines, and this is what Colin was talking about. The steam comes into here and it turns the turbines that generate electricity. But we can't really talk in here so we're going to go outside now and take out our earplugs... Ok then Colin, so we're now outside the turbine hall because it's a bit too noisy to be talking to you in there. How is this different to what's going on inside a coal-fired power station? Is there a difference?

Colin - Very little difference between this and a coal-fired power station. The steam conditions are a little bit different and some of the bits of the turbines are a little bit different, but if you walked into a coal-fired station such as something like Drax, you'll just see a row of turbines that looks just like these.

Anna - So the only difference really is what you're putting in at the beginning. Is it coal or is it going to be uranium?

Colin - Absolutely. We just use the uranium as a heat source. That's all it's there for.

Anna - So are you pumping in uranium fuel all the time? Are you always constantly having to stoke up the turbines so to speak?

Colin - No we replace about a third of the fuel with new fuel every 18 months. So we run for sixteen and a half months continuously at Sizewell B at full power continuously 24 hours a day. At the end of that we shut it down, do a lot of maintenance, testing and a lot of inspections and we replace about a third of the fuel.

Anna - So I suppose the next question is what are we going to do with all the waste and it's a question a lot of people are very very concerned about. So we've got another person here called Matt Lunn. Matt can you tell me, what do you do here at Sizewell?

Matt - My job is to advise the management on the safety of the use of ionising radiation and also to advise on the protection of the environment.

Anna - Well it sounds like you're the person we need to be around to make sure we're nice and safe. We're going to go off now I believe, to see what happens to the waste and where it all goes.

Matt - We're currently in the spent fuel building, which is next to the reactor building and what you're looking at here is a deep pool of water about the size of a five-a-side football pitch. It's about fourteen metres deep and in that pool we've got our spent fuel.

Anna - So you just said about spent fuel there. What is spent fuel?

Matt - Spent fuel is basically a fuel assembly where we've burnt a certain proportion of the fuel. However we can't burn the rest because of a build up of what are called fission product poisons. During the fission process, the uranium splits and it splits into roughly two halves, and they're called fission products. However those fission products are actually much better at absorbing neutrons than the actual uranium itself and therefore what happens is that the nuclear reaction dies out. Effectively 96% of the actual useable fuel is actually unburned.

Anna - Ok well we've got all this fuel left over and it's in a big pool of water. Why is it that you're storing the radioactive waste under water?

Matt - The fission products inside the fuel give off intense radiation, and water is a good shield, it's cheap and it also has a cooling effect as well.

Anna - So how does the water shield you from the radiation?

Matt - Basically the gamma rays just bounce off the water molecules and eventually dissipate their energy in the water.

Anna - What are you going to do with it after that?

Matt - Unlike the earlier power stations, our spent fuel is designed to be stored underwater, and we can actually store it here until the end of the station life and beyond, and that's the current strategy. The options for dealing with it in the long term are either reprocessing like we do at Sellafield or you can bury the fuel in casks deep underground. Unfortunately the United Kingdom doesn't have a final repository for spent fuel at the moment. Therefore we'll just continue to store this either underwater or eventually in special casks above ground.

Anna - Well we've now come to the end of our tour of the nuclear power station. Thanks very much to Matt and to Colin for showing us round and getting us past security. But Colin finally, we're standing now next to the National Grid building, which is where all the power from the power station goes into fuel our homes. Do you think that nuclear power is going to be a big contributor to our electricity needs in the future?

Colin - Absolutely. At the moment we generate about 20% of the electricity used in the UK. We're a very very low carbon emmiter in terms of the generation that we produce and on a large scale. What are we going to do as these power stations get older? We're going to have to replace them with something. To replace them with renewables on such a large scale is going to wreck the landscape. We're going to have to have some renewables and some nuclear. It's a very exciting time to be working in the industry because everyone now is seriously looking at new build again and I'm sure that in a few years' time we'll see more of these being built.