Fission and Fusion

In this module, we look at how nuclear physics can be used for power generation. We analyse the UK's use of energy resources before finding out more about current nuclear power stations. We then explore cutting-edge research into fusion energy - making mini-Suns here on Earth - that may make clean energy for the future a possibility. 

Course requirements for Module 3: Fission and Fusion

Energy Resources

We all know that climate scientists are very worried about the amount of carbon dioxide being produced by the human race. Carbon dioxide is a so-called greenhouse gas, and such gases are causing our planet to heat up. Therefore, the way that electricity is generated in Britain is of vital importance. Balancing the increasing demand for power against the need to tackle climate change is an ongoing challenge.

The graphic below shows the live power generation data for Britain in GW (x109 W), the percentage of low-carbon electricity currently being generated, and the breakdown of the energy resources being utilised to generate power. The data updates every five minutes. 

In Activity 3.1 below, we are going to use this data to analyse how power is currently generated in Britain and how this may change in the future. 

Activity 3.1

Energy Resources

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The data and figures for this activity are largely taken from MyGridGB (set up by the Durham Energy Institute at Durham University and Advance Further Energy LTD) using data from ELEXON. 

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Having investigated our current energy usage, we are now going to find out more about how nuclear fission is used in current nuclear power stations, before exploring whether nuclear fusion might be an option for power generation in the future. 

Fission

Fission is when an unstable, heavy nucleus splits to form two lighter nuclei, emitting neutrons as part of the process.

A large nucleus splitting via fusion into two lighter nuclei and emitting three neutrons.

Figure 1: Example fission process. Protons are shown in red and neutrons in blue

This can occur spontaneously (i.e. it happens naturally) or it can be triggered by a neutron. It can also be triggered by other types of radiation, such as high-energy protons. However, low energy neutrons are particularly effective, and therefore this is what is used in nuclear power stations. In nuclear power plants, fission is used to convert nuclear energy into heat. Then, like typical power stations, this heat is used to boil water to produce steam, that then drives a turbine and generates electricity.

In the video below, Ashley Smith from the University of Manchester explains how we use nuclear fission to generate electricity.

Most nuclear power stations use uranium-235 as the fuel. The uranium-235 is bombarded by slow neutrons, which it can very easily absorb. It will, for a brief moment, turn into a highly energetic version of uranium-236, which immediately fissions (splits) into two parts, called fission fragments or daughter nuclei.

From this process, we get energy out because a little bit of the mass of the uranium is converted into energy, in line with Einstein's famous formula: E = mc2 (which we will use later in this module). The energy released is mainly carried as kinetic energy of the daughter nuclei, but the neutrons also have some kinetic energy.

A uranium-235 nucleus absorbing a neutron and undergoing fission. 200 MeV of energy is released, and a barium-139 and krypton-36 nucleus are produced, and two neutrons are emitted.

Figure 2: Fission of U-235, triggered by a neutron. 200MeV of energy is given out in the reaction.

(Original: Stefan-Xp (talk·contribs) / Vectorization: Wondigoma (talk·contribs), CC BY-SA 3.0, via Wikimedia Commons)

Figure 3: Chain Reaction of U-235 fission

(MikeRun, CC BY-SA 4.0, via Wikimedia Commons)

While the uranium-235 fission always results in two fission fragments and a number of neutrons, it is not always the same fission fragments or the same number of neutrons. Typically, we get around three neutrons out of every fission process. These neutrons are slowed down and then used to trigger the next fission, in a process called a chain reaction. 

If too many neutrons are captured by uranium-235 nuclei, the chain reaction accelerates, with more and more fission reactions happening over time. If too few are captured, the process will stall and eventually stop. The number of neutrons in a nuclear reactor is therefore carefully controlled. We will learn more about this in the Fission Power Station Simulator (activity 3.2).

The fission fragments also vary. There are always exactly 236 nucleons in the fission fragments and the released neutrons combined. If there are more individual neutrons released (we say that the neutrons are evaporated) in the fission of a specific uranium nucleus, there will be fewer left in the fragments. Conversely, there are no individual protons released in the fission process, so the 92 protons in uranium all stay within the fragments. 

If you look closely at figure 3, and check the element number for the two fragments, you will see that they always add up to the 92 protons, whereas for the number of neutrons (or total number of nucleons), you also have to count the evaporated neutrons for the numbers to match. 

The fission fragments are rarely equal in mass: usually one fragment is lighter (80 to 110 nucleons) and one is heavier (130 to 155 nucleons). The dynamics of the fission process is very complicated, and in some other cases the asymmetry is not even seen. However, for uranium-235 this asymmetry is very clear. The first person to realise why this was the case was Lise Meitner, a leading physicist also responsible for discovering the process of fission itself. Find out more about her in the 'Find Out More' section at the bottom of the page.

Activity 3.2

Fission Power Station

Instructions:

Radioactive Contamination

Radioactive contamination is the unwanted presence of radioactive atoms on other materials. The hazard from contamination is due to the decay of the contaminating atoms. The type of radiation emitted from this decay affects the level of hazard. Irradiation is the process of exposing an object to ionising radiation. The irradiated object itself does not become radioactive. Suitable precautions must be taken to protect against the hazards of the radioactive source used in irradiation. 

Nuclear Waste

Whilst fission power stations do not produce greenhouse gases and therefore don't contribute to climate change, they do produce nuclear waste. This is because the fission fragments are radioactive. The reason behind this can be seen in figure 5. 

Figure 5 shows the chart of all the known isotopes, with the number of protons (Z) on the x-axis and the number of neutrons (N) on the y-axis. The black line shows where the number of protons is equal to the number of neutrons (i.e. N=Z). The stable isotopes are shown in black and form a curve that is known as the 'line of stability'.

For low mass isotopes, the number of protons is approximately equal to the number of neutrons for the stable nuclei. However, as the mass increases, the line of stability moves away from the N=Z line. This is because, as more positive protons are packed together, more neutrons are needed to stop the positive charge from pushing the nucleus apart. 

Uranium-235 is stable and has 92 protons and 143 neutrons - that's 61% neutrons. The fission fragments will have a similar ratio. For example, barium-139 is 60% neutrons and krypton-95 is 62% neutrons. However, these are lower mass than the uranium. Therefore, to be stable, they need a lower ratio of neutrons. Hence, fission fragments are always radioactive.  

The half lives of these fission fragments vary, but radioisotopes caesium-137 and strontium-90 are the most dangerous in terms of their long-term effects. They are both highly radioactive and, with half lives of about 30 years, they will remain in the environment for hundreds of years. Such isotopes have to be stored securely to ensure that they do not leak into the environment, and cannot be used for purposes such as terrorism.  

Figure 5: Nuclide chart showing the line for N=Z

Table_isotopes.svg: Napy1kenobiderivative work: Sjlegg, CC BY-SA 3.0, via Wikimedia Commons

Nuclear Accidents

Although modern fission reactor designs are extremely safe, nuclear accidents have the potential to cause severe damage and release a significant amount of radiation. There have only been two major nuclear accidents: Chernobyl, Ukraine (1986) and Fukushima, Japan (2011). Both accidents occurred as a result of a nuclear meltdown, which is when the heat generated by the nuclear power plant exceeds the heat removed by the cooling systems. In both accidents, there was mass displacement, with the areas around the power plants evacuated. 

Chernobyl: Chernobyl was the most serious nuclear accident in history. The accident killed 30 people directly, and the World Health Organisation estimates that a further 4,000 people might eventually die as a result of radiation exposure from Chernobyl. Several factors, including reactor design issues and a poor safety culture, led to a failed safety test that caused two explosions, a fire that lasted for over a week, and the release of a large amount of radioactive material. Since the accident, there has been a continuous clean-up of the site and the neighbouring areas. A concrete shelter was built over the damaged reactor to stop further release of radioactive material (figure 6). Radioactive materials were emitted into the atmosphere, and prevailing air currents dispersed them over large parts of Europe and as far as the UK. Since 1986, the radioactive site has remained off limits to the public.

Figure 6: The new concrete shelter, built over the Chernobyl reactor. Omar David Sandoval Sida, CC BY-SA 4.0 

Fukushima: On 11 March 2011, the most powerful earthquake ever recorded in Japan triggered a massive tsunami. Reactors close to the earthquake, including those operating at Fukushima, shut down as designed. However, as a consequence of flooding, the backup generators at the Fukushima plant, which were designed to pump cooling water through the reactor, were destroyed. Three cores largely melted over the following three days and there were several hydrogen explosions, as well as the release of nuclear material into the environment. Only one death (from cancer) has thus far been attributed to the Fukushima accident, but a large area around the Fukushima nuclear power plant will be uninhabitable for at least 100 years. 

Activity 3.3

Fission Reactors

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Fusion

Fusion is the process that powers the Sun and stars. Small nuclei are joined together (fused) to make larger nuclei. In our Sun, hydrogen is fused into helium. Energy is released because a little bit of the mass of the hydrogen is converted into energy, and again this can be calculated using Einstein's famous formula: E = mc2 (more on this later!). 

On Earth, scientists are trying a similar process, fusing two isotopes of hydrogen (deuterium, hydrogen-2, and tritium, hydrogen-3) into helium-4. This is shown in Figure 7. The energy released is shown in MeV (where 1MeV ~ 10-13J). You can see that the energy is mainly carried as kinetic energy of the neutron, but that the helium nucleus also has some energy. 

Like fission, fusion does not produce any greenhouse gases. In addition, the products of fusion are not radioactive - helium is not only stable, but is also in short supply and is needed for important things such as MRI scanners in hospitals.  

In order to make fusion happen here on Earth, we have to first heat a gas until the electrons separate from the nucleus.  This creates a 'soup' of charged particles called plasma, often referred to as the fourth state of matter. We use magnetic fields in doughnut shaped devices called tokamaks to control the plasma. However, we cannot yet use fusion to generate electricity. Could this me an energy source for the future? Find out more in the short introductory video below.

A nucleus of deuterium (one proton and one neutron) and a nucleus of tritium (one proton and two neutrons) fusing and producing a helium-4 nucleus, (with a kinetic energy of 3.5 MeV) and emitting a neutron (carrying a kinetic energy of 14.1 MeV).

Figure 7: Deuterium-Tritium fusion. 

Energies are given in units of MeV. 

JET

The largest tokamak in the world is currently JET (Joint European Torus) at Culham Centre for Fusion Energy in Oxfordshire. In this ‘Born to Engineer’ video, Control Engineer Kim Cave-Ayland takes us inside this tokamak to find out more.

ITER

Construction of a new fusion reactor is underway in the South of France. ITER will be the world’s largest tokamak and is a collaboration between 35 nations. Experiments are scheduled to begin here in December 2025. The aim is to achieve steady state fusion and to produce net energy, (where the total power produced is greater than the thermal power input to heat the plasma). 

ITER has lots of information on its website: iter.org/. We particularly like exploring the ITER site in virtual reality (or just on a phone/computer screen). You can even choose what date to view the site - why not see how the building work has been progressing?

3D Virtual Tour of ITER (Optional)

Figure 8: ITER

STEP

The UK have also recently started a new programme called STEP: Spherical Tokamak for Energy Production. Find out more about STEP, ITER, and how much progress have we made towards using fusion to generate electricity in the video below. Professor Howard Wilson gives a brief history of fusion experiments to date, and gives some exciting insights as to what the future may hold.

Activity 3.4

Fusion

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Energy Produced in Fission and Fusion

We have mentioned that energy is produced in both fission and fusion reactions. This is because there is a small mass difference between the nuclei we start off with and those that are produced, and this mass difference is converted into energy. We can use Einstein's famous equation, E = mc2, to calculate how much energy is produced (where E is the energy, m is the mass difference, and c is the speed of light, 3.00 x 108 m/s). 

For example, an alternative fission reaction is:

3.968700 x 10-25 kg +  0.016749 x 10-25 kg = 3.985449 x 10-25 kg

2.223061 x 10-25 kg + 1.708773 x 10-25 kg + 0.050247 x 10-25 kg = 3.982081 x 10-25 kg

3.985449 x 10-25 kg - 3.982081 x 10-25 kg = 0.003368 x 10-25 kg

E = mc2 = 0.003368 x 10-25 x (3.00 x 108)2 = 3.0312 x 10-11 J


But why can energy be produced by both fusing nuclei together and splitting nuclei apart? The answer lies in the Nuclide Chart that we saw in Module 1. In the flyover of the 3D nuclide chart, below, the height of the towers shows the energy in each isotope. Energy is therefore released by going 'downhill' in this version of the chart. 

The flyover starts from hydrogen and other low-mass isotopes. You can see that the towers at this end of the chart are tall, and therefore energy can be released by moving towards heavier nuclei, (which have shorter towers). This is why energy is released by the fusion of lighter nuclei into heavier ones.  

At the far end of the chart, there is a gradual gradient up to uranium and the super heavy isotopes. Energy can be released by splitting these heavy nuclei (by fission) to make lighter nuclei (in the lower area in the middle of the chart). 

The lowest point on the nuclide chart is iron-56. Therefore, anything lighter than iron-56 will release energy through fusion, and anything heavier than iron-56 will release energy through fission. 

Activity 3.5

E = mc2

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Find out more (optional)

Lise Meitner: Discovery of fission

In atomic form, helium, neon, argon, krypton, xenon, radon (and probably the newly discovered oganesson, too) are noble gasses because their electrons are in a particularly favourable configuration, a so-called 'closed shell'. Meitner noticed that the intermediate-mass nuclei prefer groupings of 50 or 82 neutrons, and proposed that a nuclear shell model could explain the asymmetry seen in the fission process. She stated that:

"When uranium-235 suffers fission upon impact of a slow neutron, the two fragments formed contain altogether 144 neutrons, of which a few are afterwards evaporated. This makes it possible to give each fragment a complete shell, one of 50, the other of 82 neutrons." [Meitner, L. Fission and Nuclear Shell Model. Nature 165, 561 (1950)]

Uranium-235 therefore tends to split with around 50 neutrons in one fission fragment, and around 82 neutrons in the other fission fragment. If you distribute the protons in a similar ratio, you get exactly the fission asymmetry we observe in nature. So Meitner was right, both about fission and about the nuclear shell model.

Lise Meitner was nominated for a Nobel Prize no less than 48 times. Although she never won this award (her male colleague received the 1944 Nobel Prize for Chemistry for their joint discovery of nuclear fission), element 109 is named meitnerium in her honour.  

MEITNER, L. ,Fission and Nuclear Shell Model

Nature 165, 561 (1950)