How does nuclear energy work?

28 March 2017

Interview with

Paddy Regan, University of Surrey (and National Physical Laboratory)

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It wasn’t long after its discovery before scientists realised that atoms lock away enormous amounts of energy, which is what holds these particles together.This enormous power was demonstrated all too plainly during World War Two with the creation of the atom bomb. Today physicists more commonly use the energy of atoms in the nuclear energy sector, which generates one sixth of all the energy in the UK; in France, three quarters of the electricity generated is nuclear in origin. But what’s the principle behind it? Paddy Regan, is from the University of Surrey and the National Physical Laboratory, and he explained to Chris Smith how we can harness energy from atoms...

Paddy - The main thing to remember, I think it was mentioned in the previous section, is that the atomic nucleus is very, very, very small. And the reason it’s very small is because the fundamental force that holds together the particles that make up that nucleus - we call it the strong nuclear force - acts over a very, very short range, and that range is something like 100 million millionth of a centimetre. So what that means is that force is very, very strong and that extra binding or strong force, if you like, the sticking glue, between the protons and neutrons in the nucleus adds a little bit of extra energy to the system.

Listeners will be familiar with the only equation that sort of matters in physics which is the famous Einstein one e=mc2, and that idea is that energy and mass are sort of two interchangeable products - two sides of the same coin. And the idea is if you’ve got a very, very tightly bound stuck together nuclear system, some of the mass is converted into what we call “binding energy.” Not very much, only about probably less than one percent of the total mass of the nuclear system is binding energy but, if we can release that and that’s what happens in nuclear fission, we can release an enormous amount of energy that’s held together and compact in there. A little bit like bursting a balloon by pricking it with a needle.

Chris - And in this instance, what’s the nuclear prick that you use to burst that nuclear balloon then?

Paddy - Well, it’s the particle that was just mentioned earlier; it’s the neutron. And it’s a very interesting particle the neutron. So it wasn’t discovered as the previous gentleman said until 1932. That was almost 30 years after the discovery of the nucleus itself. The reason it’s hard to discover or to find it is because it has no charge.

There’s another interesting thing about the neutron in that if you have a neutron on its own, a sort of lone, standalone neutron, it only survives as a neutron for about ten minutes; it naturally radioactively decays. But when it’s bound inside an atomic nucleus, it can basically live forever. So the little magic bullet that tickles the specific chemical element that we use in most nuclear reactors to cause nuclear fission, which is uranium, is processing a very, very slow bullet of this neutron material. And it’s just captured by the uranium nucleus, and that tiny capture causes the nucleus to be unstable, to wobble a little bit, and split up into two smaller fragments releasing some of that binding energy. The amount of binding energy that’s released is about a million times more per energy release than you would have in a chemical reaction like coal burning, and that’s why nuclear power is so efficient.

Chris - So we have a nuclear reactor; it has something fissionable, something capable of doing this in it, in this case: uranium. Neutrons from that uranium hit the nuclei of other uranium atoms; they destabilise the nucleus; it falls apart and in the process releases some of this energy and want more neutrons so it can then do this again.

Paddy - That’s the idea of the chain reaction.

Chris -. So why doesn’t the power station meltdown or explode?

Paddy - There’s a very precise amount of neutrons that are produced in each nuclear fission. And what you need for a sustainable chain reaction is exactly one of those neutrons that’s released to produce fission in the next generation. In order to do that you basically control the amount of neutrons that are in the reactor and that’s done by materials called “control rods.” These are special elements, special lumps of material. They’re made of things like boron, or hafnium, or cadmium, and they’re material that basically drops into the reactor core and, for want of a better word, gobbles up neutrons. Takes the neutrons away from causing fission on uranium and the amount of control rods you put in will determine how many neutrons are still available to go on and cause fission. If you put in too much control rod, you basically don’t have any nuclear fission; that happens, they steal all the neutrons.

If you take all of the control rods out, then you would have an increased amount of fission. Most reactors wouldn’t be able to make a bomb just because of the nature of the uranium fuel that’s in there.

Chris - Why does the uranium respond in this feedback loop by fissioning and breaking apart and producing neutrons, but the other chemicals that you mentioned that are used to control the numbers of neutrons; they do not?

Paddy - There is something very special about the element uranium (element 92 in the periodic table), and that is it’s the heaviest element that occurs naturally on the planet. That means it’s got the most number of protons in it for a naturally occurring element. And what causes fission is that basically the repulsion between the protons in the atomic nucleus can be rearranged to give you a release of binding energy. So if you’ve got those 92 protons that form uranium, if you can split that uranium nucleus into two lighter elements, where those 92 protons are divided into two separate types of chemicals or different chemical species, you get a big release in binding energy. And the biggest release in binding energy you’ll get is from the heaviest occurring element, and that is uranium.

Chris - Where do we get all of the uranium that we’re using in our power stations from?

Paddy - Uranium is ubiquitous over the Earth. About one atom in a million in the Earth’s crust is uranium. It’s all over the UK; it’s in every bit of the ground here. There’s plenty of deposits of it in the west coast of Ireland, around Galway for example. But most of the mining is done in big countries like Australia, some bits of Russia, Kazakhstan, and South Africa, Canada. There are big geological deposits of concentrated types of mineral rich in uranium and that’s where most of the uranium comes from that would be used in nuclear power stations.

Chris - How do we get energy out of this to turn it into electricity?

Paddy - Well it’s a very simple idea. It’s the same way that all power stations work basically. It translates the energy that’s released off these atomic nuclei as they’re exploding, if you like, and it just turns that into heat, and it turns that into heat by interacting by slowing them down in so-called fuel rods. Heats up metal and that metal is then used to either heat up and boil water, or as in the earlier type British reactor, to heat up carbon dioxide and subsequently boil water. Boiling water turns to steam; steam turns turbines and you produce electricity in the way that any other power station would do.

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