Controlled fission is a reality, whereas controlled fusion is a hope for the future. Hundreds of nuclear fission power plants around the world attest to the fact that controlled fission is practical and, at least in the short term, economical, as seen in Figure 1. Whereas nuclear power was of little interest for decades following TMI and Chernobyl and now Fukushima Daiichi , growing concerns over global warming has brought nuclear power back on the table as a viable energy alternative.
Australia and New Zealand have none. China is building nuclear power plants at the rate of one start every month. Figure 1. The people living near this nuclear power plant have no measurable exposure to radiation that is traceable to the plant.
The cooling towers are the most prominent features but are not unique to nuclear power. The reactor is in the small domed building to the left of the towers. Fission is the opposite of fusion and releases energy only when heavy nuclei are split.
The amount of energy per fission reaction can be large, even by nuclear standards. Thus, if a heavy nucleus splits in half, then about 1 MeV per nucleon, or approximately MeV per fission, is released.
As always, the energy released is equal to the mass destroyed times c 2 , so we must find the difference in mass between the parent U and the fission products.
A number of important things arise in this example. The MeV energy released is large, but a little less than the earlier estimated MeV. This is because this fission reaction produces neutrons and does not split the nucleus into two equal parts. Fission of a given nuclide, such as U , does not always produce the same products. Fission is a statistical process in which an entire range of products are produced with various probabilities.
Most fission produces neutrons, although the number varies with each fission. This is an extremely important aspect of fission, because neutrons can induce more fission , enabling self-sustaining chain reactions. Spontaneous fission can occur, but this is usually not the most common decay mode for a given nuclide. Neutron-induced fission is crucial as seen in Figure 2. Being chargeless, even low-energy neutrons can strike a nucleus and be absorbed once they feel the attractive nuclear force.
Large nuclei are described by a liquid drop model with surface tension and oscillation modes, because the large number of nucleons act like atoms in a drop. The neutron is attracted and thus, deposits energy, causing the nucleus to deform as a liquid drop. If stretched enough, the nucleus narrows in the middle. The number of nucleons in contact and the strength of the nuclear force binding the nucleus together are reduced.
Coulomb repulsion between the two ends then succeeds in fissioning the nucleus, which pops like a water drop into two large pieces and a few neutrons. Neutron-induced fission can be written as.
Most often, the masses of the fission fragments are not the same. Most of the released energy goes into the kinetic energy of the fission fragments, with the remainder going into the neutrons and excited states of the fragments.
This can also be seen in Figure 3. An example of a typical neutron-induced fission reaction is. This is not true when we consider the masses out to 6 or 7 significant places, as in the previous example. Figure 2. Neutron-induced fission is shown. First, energy is put into this large nucleus when it absorbs a neutron.
Acting like a struck liquid drop, the nucleus deforms and begins to narrow in the middle. Since fewer nucleons are in contact, the repulsive Coulomb force is able to break the nucleus into two parts with some neutrons also flying away. Figure 3. A chain reaction can produce self-sustained fission if each fission produces enough neutrons to induce at least one more fission.
This depends on several factors, including how many neutrons are produced in an average fission and how easy it is to make a particular type of nuclide fission. Not every neutron produced by fission induces fission. Some neutrons escape the fissionable material, while others interact with a nucleus without making it fission.
We can enhance the number of fissions produced by neutrons by having a large amount of fissionable material. The minimum amount necessary for self-sustained fission of a given nuclide is called its critical mass. Some nuclides, such as Pu , produce more neutrons per fission than others, such as U.
In particular, forces between nucleons at the surface of the nucleus result in a surface tension similar to that of a water droplet. A neutron fired into a uranium nucleus can set the nucleus into vibration.
If this vibration is violent enough, the nucleus divides into smaller nuclei and also emits two or three individual neutrons. U fission can produce a nuclear chain reaction. This chain reaction can proceed in a controlled manner, as in a nuclear reactor at a power plant, or proceed uncontrollably, as in an explosion.
View a simulation on nuclear fission to start a chain reaction, or introduce nonradioactive isotopes to prevent one. Control energy production in a nuclear reactor. The possibility of a chain reaction in uranium, with its extremely large energy release, led nuclear scientists to conceive of making a bomb—an atomic bomb.
These discoveries were taking place in the years just prior to the Second World War and many of the European physicists involved in these discoveries came from countries that were being overrun. In addition, the uranium sample must be massive enough so a typical neutron is more likely to induce fission than it is to escape. The minimum mass needed for the chain reaction to occur is called the critical mass. When the critical mass reaches a point at which the chain reaction becomes self-sustaining, this is a condition known as criticality.
The original design required two pieces of U below the critical mass. When one piece in the form of a bullet is fired into the second piece, the critical mass is exceeded and a chain reaction is produced. An important obstacle to the U bomb is the production of a critical mass of fissionable material. Therefore, scientists developed a plutonium bomb because Pu is more fissionable than U and thus requires a smaller critical mass. The bomb was made in the form of a sphere with pieces of plutonium, each below the critical mass, at the edge of the sphere.
A series of chemical explosions fired the plutonium pieces toward the center of the sphere simultaneously. When all these pieces of plutonium came together, the combination exceeded the critical mass and produced a chain reaction. Whether to develop and use atomic weapons remain two of the most important questions faced by human civilization.
Calculate the energy released in the following rare spontaneous fission reaction:. Several important things arise in this example. The energy release is large but less than it would be if the nucleus split into two equal parts, since energy is carried away by neutrons.
However, this fission reaction produces neutrons and does not split the nucleus into two equal parts. Fission is a statistical process in which an entire range of products are produced with various probabilities. Most fission produces neutrons, although the number varies. This is an extremely important aspect of fission, because neutrons can induce more fission , enabling self-sustaining chain reactions.
The first nuclear reactor was built by Enrico Fermi on a squash court on the campus of the University of Chicago on December 2, This is an incredibly huge force for such small particles.
This huge force over a small distance leads to a fair amount of released energy which is large enough to cause a measurable reduction in mass. This means that the total mass of each of the fission fragments is less than the mass of the starting nucleus. This missing mass is known as the mass defect. It is convenient to talk about the amount of energy that binds the nuclei together. All nuclei having this binding energy except hydrogen which has just 1 proton and no neutrons.
It's helpful to think about the binding energy available to each nucleon and this is called the binding energy per nucleon. This is essentially how much energy is required per nucleon to separate a nucleus. The products of fission are more stable, meaning that it is more difficult to split them apart.
Since the binding energy per nucleon for fission products is higher, their total nucleonic mass is lower. The result of this higher binding energy and lower mass results in the production of energy. Fission of heavier elements is an exothermic reaction. Fission can release up to million eV compared to burning coal which only gives a few eV. From this number alone it is apparent why nuclear fission is used in electricity generation.
Additionally, the amount of energy released is much more efficient per mass than that of coal. This then creates a sustained nuclear chain reaction , which releases fairly continuous amounts of energy.
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