DT fusion produces a neutron and a helium nucleus. In the process, it also releases much more energy than most fusion reactions. In a potential future fusion power plant such as a tokamak or stellarator , neutrons from DT reactions would generate power for our use. Researchers focus on DT reactions both because they produce large amounts of energy and they occur at lower temperatures than other elements.
They work with the Advanced Scientific Computing Research program to use scientific computing to advance fusion science as well as the Nuclear Physics program on nuclear reaction databases, generation of nuclear isotopes, and research in nucleosynthesis. 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. Additionally, some nuclides are easier to make fission than others.
In particular, U and Pu , are easier to fission than the much more abundant U. Both factors affect critical mass, which is smallest for Pu. The reason U and Pu are easier to fission than U is that the nuclear force is more attractive for an even number of neutrons in a nucleus than for an odd number.
When a neutron encounters a nucleus with an odd number of neutrons, the nuclear force is more attractive, because the additional neutron will make the number even. About 2-MeV more energy is deposited in the resulting nucleus than would be the case if the number of neutrons was already even.
This extra energy produces greater deformation, making fission more likely. Thus, U and Pu are superior fission fuels. The isotope U is only 0. This is followed by Kazakhstan and Canada.
Most fission reactors utilize U , which is separated from U at some expense. This is called enrichment. The most common separation method is gaseous diffusion of uranium hexafluoride UF 6 through membranes. Since U has less mass than U , its UF 6 molecules have higher average velocity at the same temperature and diffuse faster. Another interesting characteristic of U is that it preferentially absorbs very slow moving neutrons with energies a fraction of an eV , whereas fission reactions produce fast neutrons with energies in the order of an MeV.
Water is very effective, since neutrons collide with protons in water molecules and lose energy. Figure 4 shows a schematic of a reactor design, called the pressurized water reactor. Figure 4. A pressurized water reactor is cleverly designed to control the fission of large amounts of U , while using the heat produced in the fission reaction to create steam for generating electrical energy.
For elements heavier than iron, fusion consumes energy, i. We can use fission today to release energy due to the fact, that some process in the past e. There's energy involved in keeping atoms apart, but also energy involved in holding an atom together. When you smash an atom, this bonding energy is released. In fact, one of the four fundamental forces is responsible for binding atoms together - the strong nuclear force. Fusion works by banging together the same two elements and sticking them together to form a new heavier element.
When you add the masses of the two original elements it is greater than the new element. It is this mass difference that becomes energy.
Here m is 2 x Mass of original element - Mass of new element and c is the speed of light. When the mass of the two original elements becomes heavier the difference between their masses and the new element gets smaller. Fission works by splitting one element into two new lighter elements. When you add the masses of the two new elements it is less than the original element.
Here m is Mass of original element - Mass of new elements and c is the speed of light. I wanted to mention that is technically much more complicated than what I say here. The short answer is still the same: Mass is converted into Energy. Wanted to provide a quick answer, but apparently now is frowned upon to give quick answers in comments, so here it is:.
Roughly speaking, nuclear fission is endothermic for nuclei where nuclear fusion would be exothermic, and viceversa. For nuclei smaller than Iron, fission is typically endothermic, while fusion is exothermic.
For nuclei heavier than Iron, the situation reverses. Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. Why do fusion and fission both release energy? Ask Question. Asked 2 years, 9 months ago. Active 2 years, 9 months ago.
Viewed 21k times. Improve this question. It can also happen when a single slow neutron merges with the nucleus. How does the entropy of the system change when fission or fusion occur and why does this depend on the size of the nucleus? Please keep in mind that comments are to be used for suggesting improvements and requesting clarification on their parent post i.
Add a comment. Active Oldest Votes. Purely classical model Nucleons are bound together with the strong and some weak nuclear force. Equivalently, the binding energy per nucleon behaves similarly.
This image from Wikipedia illustrates the curve in the typically presented manner: However, I prefer to think of binding energy as negative and therefore better visualize iron as being the lowest energy state: For lighter elements: Fission requires energy Fusion releases energy For heavier elements, the opposite is true. The reason we mainly observe the release energy cases is because: It is easier to do It is more "useful". Improve this answer.
Keith Keith 1, 9 9 silver badges 9 9 bronze badges. What makes it "cold" is that supposedly you could somehow get it to happen without going through an intermediate stage that costs a lot of energy to get to -- namely where two nuclei are close enough together that you've spent a lot of energy overcoming their electrostatic repulsion but not yet so close together that the strong force has begun attracting them.
This energy cost is not lost; it will be paid back either when fusion happens, or if fusion fails to happen, then when the nuclei are forced apart again at high speed by electrostatic forces. But getting that much energy concentrated in a pair of nuclei in the first place is the major technological problem in fusion power, and the only known way to achieve it in quantity is to heat up a plasma to insane temperatures. Cold fusion would -- in some way never quite explained -- mean a way to avoid this barrier.
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