Hello there! Welcome to lecture 34: fission and fusion.
In this last lecture video, we’ll talk about the energy that can be harnessed by the atomic nucleus in the processes of nuclear fission and fusion.
Each of the following concepts will be discussed in this video: fission, chain reactions, nuclear power, and fusion.
As we learned in lecture 33, radioactive decay occurs when an atomic nucleus is unstable due to the presence of enough protons to allow electromagnetic repulsion to become stronger than the strong nuclear force that otherwise keeps the nucleus of an atom glued together.
If instead, we artificially elongated an atom, the electromagnetic repulsion will cause the atom to split in two. It’s important to note that the process of nuclear fission does not happen spontaneously, something must provoke the process. This process is known as fission. Nuclear fission was observed in 1938 by scientists Otto Hahn, Fritz Strassmann, and Lise Meitner. Lise Meitner and her nephew Otto Frisch came up with the theoretical explanation of what was occurring in these observations.
The process of nuclear fission not only causes the generation of two different atoms from one original atom, it also generates a lot of energy in the process. Where does this energy come from? Let’s say we have an atom that will undergo fission. We carefully measure its mass before the fission process occurs. After bombarding the atom with a neutron, the atom splits into two new atoms. If we carefully measure the mass of all of the byproducts of the fission process, that mass will actually be smaller than the original mass… by a very small amount.
While we’ve talked about conservation of mass and conservation of energy, it turns out that the two are related. It’s really more correct to talk about the conservation of mass-energy. The energy generated in the fission process comes from the mass that is lost. The amount of energy created can be quantified using the famous equation E equals m c-squared. E is the energy, m is the mass of the particles at rest, and c is the speed of light. Because the speed of light is such a large number, even a small mass of particles can create an enormous amount of energy.
Can all atoms be used to generate energy by way of fission? The short answer is no. Very heavy atoms are capable of releasing energy. Very small atoms are enormously hard to split apart due to their small size and the power of the strong force. If we look at a graph of mass per nucleon in an atom, a hydrogen atom has a lot of mass in its one nucleon. On the other end of the spectrum is uranium. In between, with the least mass per nucleon is iron. Atoms heavier than iron can release energy when they undergo nuclear fission. But the most energy will be generated with the heaviest atoms, and uranium-235 is the source of most nuclear power generated in the United States.
Uranium-235 is a fissionable isotope of uranium. When shooting a neutron at uranium-235, the byproducts are krypton-92, barium-141, and three more neutrons. The total number of nucleons is conserved in this process, as is the charge. As mentioned, while mass is not conserved mass-energy is.
Usually, the fission process is started by shooting a neutron at an atom to cause the elongation. Why is a neutron used instead of a proton? The neutron, being neutral, will not undergo any electromagnetic repulsion forces that would cause it to deflect away from an atomic nucleus.
Some fission reactions generate even more neutrons. If enough fissionable material is present, the generation of new neutrons after each fission event will kick off something known as a chain reaction: a self-sustaining reaction that doesn’t require any intervention to keep it going.
When a large enough quantity, known as a critical mass, of fissionable material is present, an explosion can result, releasing a massive amount of energy. This is the principle behind nuclear weapons.
A chain reaction is a self-sustaining fission process. As mentioned, uranium-235 will generate three new neutrons after the fission process. If a sufficient amount of uranium-235 is present, the three new neutrons will cause more of the atoms to undergo fission, which will release more neutrons, create more fission, and so on.
Although three neutrons are generated, it is not necessarily true that each single fission process will kick off three more. It’s possible some of the excess neutrons may be absorbed, perhaps intentionally, to keep a nuclear reaction from generating too much energy.
The number of reactions that are provoked by a single fission event relates to criticality. A sub-critical process is one where, on average, less than one new fission reaction is generated after a single fission event. This would not lead to a self-sustaining nuclear reaction. Over time the reactions will stop on their own.
In a critical reaction, each fission process generates, on average, one additional fission event. A critical reaction is self-sustaining and will continue until all of the fissionable material is consumed. An analogy would be a line-up of dominos. When the first domino is knocked over, it knocks over a second, which knocks over a third, and so on. This process continues until there are no more dominos to knock over.
In a super-critical reaction, each fission process generates, on average, more than one additional fission event. If one fission event provokes two more, those two will provoke four, which will provoke eight, and so on. A super-critical reaction grows exponentially. That means the number of fission events becomes extremely large extremely quickly.
If we look at a graph of the number of fission events that occur after each process, we need to use a logarithmic scale in order to even compare critical and super-critical reactions. Assuming that one fission reaction started a chain reaction, after 10 fission events, a critical reaction has one fission event occurring; a super-critical reaction that generates two new events will have 512 fission events occurring; and a super-critical reaction that generates three new events will have nearly 20,000 fission events occurring!
Super-critical reactions are not tenable in nuclear power generation. To control the number of neutrons present in a chain reaction, control rods are used to absorb excess neutrons and keep a reaction at the critical stage.
Most natural sources of uranium consist of the isotope uranium-238, which is not fissionable. Samples of uranium-238 will naturally contain some uranium-235, which is fissionable. Enrichment is a process by which the proportion of uranium-235 is increased.
Nuclear power plants use enriched uranium as a fuel to create a critical chain reaction. A moderator is used to slow neutrons down until they travel at the right speed to cause fission in uranium-235. Control rods are used to absorb excess neutrons and prevent a super-critical reaction from occurring. Water runs through the reactor and heats up as it absorbs the energy generated by the fission reaction.
This water is sent through a heat exchanger, which creates steam, that spins a turbine, causing a relative rotation of a magnetic field and coil of wire, and as discussed in lecture 25, generates electricity. While the fuel is different in all turbine-powered generators, the actual mechanics of the power generation is the same for nuclear generators and fossil-fuel powered generators.
However, the major benefit of a nuclear reaction over fossil fuel is that there is no combustion during the actual fission process, and therefore there is no emission of greenhouse gases. In addition, uranium is a very energy dense material and per kilogram will generate much more energy than equivalent amounts of coal, oil, or gas.
Naturally, there are drawbacks to nuclear energy. While minimal, the nuclear reaction process does release small amounts of radioactivity into the environment. The uranium enrichment process can be used to create nuclear weapons. The byproducts of the fission process are themselves radioactive and have long half-lives. The safe storage of these radioactive wastes is an ongoing issue in the United States. Finally, there is always the risk of disaster in any nuclear power plant. Three Mile Island, Chernobyl, and Fukushima are all poignant examples of what can happen if things go wrong.
Fission is not the only way to generate energy from atomic nuclei. You may have noticed that light elements contain a lot of mass per nucleon. If we smash two hydrogen atoms together, the result, helium, contains less mass per nucleon. This means we would generate energy in the process of binding them together. This smashing of atoms together is known as nuclear fusion. All atoms lighter than iron can release energy in the fusion process, but without the same efficiency as hydrogen.
Fusion is perhaps the most prevalent source of energy in our lives. That’s because the sun generates energy using fusion, and that’s where the vast majority of our energy comes from! Our sun contains a massive amount of hydrogen, and converts that hydrogen to helium in the fusion process. Our sun has been generating energy in this manner for 4.6 billion years, and thankfully for us has enough fuel to keep going for about 5 billion more.
Fusion is a really great process for generating energy. Why don’t we use it on Earth? The problem is that hydrogen atoms will experience huge amounts of electromagnetic repulsion when we try to get them to come together. To overcome this, we need to get hydrogen atoms to come together at extraordinarily high speeds.
While some recent preliminary evidence indicates that it may be possible to break even on the energy required to speed these hydrogen atoms up to the point of fusion, versus the amount of energy they produce, we are far from capable of creating a sustaining fusion reaction, much less getting enough energy out of the fusion process to use it to light up and heat our homes.
Thanks for taking the time to learn about fission and fusion! Until next time, stay well.