question: How can nuclear fusion and fission both release energy? Aren’t they opposites?
comment: They are opposite sides of the same hill. Imagine fusion is the side of the hill toward you and fission is the side of the hill away from you. Any ball placed on the side of the hill is going to roll "down", releasing energy even though they'll be rolling in opposite directions (toward you or away from you). The "fusion" side contains lighter elements like hydrogen and helium. The "fission" side contains heavier elements, like uranium and plutonium. At the top of the hill? Iron, the most stable element and any ball placed there sits there and doesn't move.
passages: Nuclear fusion produces energy by combining the very lightest elements into more tightly bound elements (such as hydrogen into helium), and nuclear fission produces energy by splitting the heaviest elements (such as uranium and plutonium) into more tightly bound elements (such as barium and krypton). Both processes produce energy, because middle-sized nuclei are the most tightly bound of all.
   The latter scenario is the case with nuclei such as helium: to break them up into protons and neutrons, one must inject energy. On the other hand, if a process existed going in the opposite direction, by which hydrogen atoms could be combined to form helium, then energy would be released. The energy can be computed using "E" = Δ"m" "c" for each nucleus, where Δ"m" is the difference between the mass of the helium nucleus and the mass of four protons (plus two electrons, absorbed to create the neutrons of helium).
   Fusion reactions occur when two or more atomic nuclei come close enough for long enough that the nuclear force pulling them together exceeds the electrostatic force pushing them apart, fusing them into heavier nuclei. For nuclei lighter than iron-56, the reaction is exothermic, releasing energy. For nuclei heavier than iron-56, the reaction is endothermic, requiring an external source of energy. Hence, nuclei smaller than iron-56 are more likely to fuse while those heavier than iron-56 are more likely to break apart.
   For lighter elements, the energy that can be released by assembling them from lighter elements decreases, and energy can be released when they fuse. This is true for nuclei lighter than iron/nickel. For heavier nuclei, more energy is needed to bind them, and that energy may be released by breaking them up into 2 fragments (known as atomic fission). Nuclear power is generated at present by breaking up uranium nuclei in nuclear power reactors, and capturing the released energy as heat, which is converted to electricity. 
   The energy given off during either nuclear fusion or nuclear fission is the difference of the binding energies of the "fuel," i.e. the initial nuclide(s), from that of the fission or fusion products. In practice, this energy may also be calculated from the substantial mass differences between the fuel and products, which uses previous measurements of the atomic masses of known nuclides, which always have the same mass for each species. This mass difference appears once evolved heat and radiation have been removed, which is required for measuring the (rest) masses of the (non-excited) nuclides involved in such calculations.
   In the context of atomic nuclei, the same strong interaction force (that binds quarks within a nucleon) also binds protons and neutrons together to form a nucleus. In this capacity it is called the nuclear force (or "residual strong force"). So the residuum from the strong interaction within protons and neutrons also binds nuclei together. As such, the residual strong interaction obeys a quite different distance-dependent behavior between nucleons, from when it is acting to bind quarks within nucleons. Differences in the binding energy of the nuclear force between different nuclei power nuclear fusion and nuclear fission. Nuclear fusion accounts for most energy production in the Sun and other stars. Nuclear fission allows for decay of radioactive elements and isotopes, although it is often mediated by the weak interaction. Artificially, the energy associated with the nuclear force is partially released in nuclear power and nuclear weapons, both in uranium or plutonium-based fission weapons and in fusion weapons like the hydrogen bomb.
   For reactions with two products, the energy is divided between them in inverse proportion to their masses, as shown. In most reactions with three products, the distribution of energy varies. For reactions that can result in more than one set of products, the branching ratios are given.
   Section::::Neutron economy.
  
  A key issue for the fusion–fission concept is the number and lifetime of the neutrons in the various processes, the so-called "neutron economy".
   The net result of the opposing electrostatic and strong nuclear forces is that the binding energy per nucleon generally increases with increasing size, up to the elements iron and nickel, and then decreases for heavier nuclei. Eventually, the binding energy becomes negative and very heavy nuclei (all with more than 208 nucleons, corresponding to a diameter of about 6 nucleons) are not stable. The four most tightly bound nuclei, in decreasing order of binding energy per nucleon, are , , , and . Even though the nickel isotope, , is more stable, the iron isotope is an order of magnitude more common. This is due to the fact that there is no easy way for stars to create through the alpha process.
   The FRC is also considered for deep space exploration, not only as a possible nuclear energy source, but as means of accelerating a propellant to high levels of specific impulse (I) for electrically powered spaceships and fusion rockets, with interest expressed by NASA.
  
  Section::::Comparisons.
   The best-known classes of exothermic nuclear transmutations are fission and fusion. Nuclear energy may be liberated by atomic fission, when heavy atomic nuclei (like uranium and plutonium) are broken apart into lighter nuclei. The energy from fission is used to generate electric power in hundreds of locations worldwide. Nuclear energy is also released during atomic fusion, when light nuclei like hydrogen are combined to form heavier nuclei such as helium. The Sun and other stars use nuclear fusion to generate thermal energy which is later radiated from the surface, a type of stellar nucleosynthesis. In any exothermic nuclear process, nuclear mass might ultimately be converted to thermal energy, given off as heat.
   Section::::Introduction.:Physics of nuclei.:Combining nuclei.
  
  Small nuclei that are larger than hydrogen can combine into bigger ones and release energy, but in combining such nuclei, the amount of energy released is much smaller compared to hydrogen fusion. The reason is that while the overall process releases energy from letting the nuclear attraction do its work, energy must first be injected to force together positively charged protons, which also repel each other with their electric charge.
   Nuclear fusion is the origin of the energy (including in the form of light and other electromagnetic radiation) produced by the core of all stars including our own Sun.
  
  Section::::Modern nuclear physics.:Nuclear fission.
  
  Nuclear fission is the reverse process to fusion. For nuclei heavier than nickel-62 the binding energy per nucleon decreases with the mass number. It is therefore possible for energy to be released if a heavy nucleus breaks apart into two lighter ones.
   Section::::Fuel cycle.
   Iron nuclei are the most stable nuclei (in particular iron-56), and the best sources of energy are therefore nuclei whose weights are as far removed from iron as possible. One can combine the lightest ones--nuclei of hydrogen (protons)--to form nuclei of helium, and that is how the Sun generates its energy. Or else one can break up the heaviest ones--nuclei of uranium or plutonium--into smaller fragments, and that is what nuclear reactors do.
  
  Section::::Introduction.:Nuclear binding energy.
   Fission occurs naturally because each event gives off more than one neutron capable of producing additional fission events. Fusion, at least in D-T fuel, gives off only a single neutron, and that neutron is not capable of producing more fusion events. When that neutron strikes fissile material in the blanket, one of two reactions may occur. In many cases, the kinetic energy of the neutron will cause one or two neutrons to be struck out of the nucleus without causing fission. These neutrons still have enough energy to cause other fission events. In other cases the neutron will be captured and cause fission, which will release two or three neutrons. This means that every fusion neutron in the fusion–fission design can result in anywhere between two and four neutrons in the fission fuel.
   All of the elements heavier than iron have some potential energy to release, in theory. At the extremely heavy end of element production, these heavier elements can produce energy in the process of being split again back toward the size of iron, in the process of nuclear fission. Nuclear fission thus releases energy which has been stored, sometimes billions of years before, during stellar nucleosynthesis.
  
  Section::::Confinement.:Magnetic confinement.
   Many early fusion devices operated for microseconds, using some sort of pulsed power source to feed their magnetic confinement system and used the confinement as the heating source. Lawson defined breakeven in this context as the total energy released by the entire reaction cycle compared to the total energy supplied to the machine during the same cycle.
   In nuclear fission events the nuclei may break into any combination of lighter nuclei, but the most common event is not fission to equal mass nuclei of about mass 120; the most common event (depending on isotope and process) is a slightly unequal fission in which one daughter nucleus has a mass of about 90 to 100 u and the other the remaining 130 to 140 u. Unequal fissions are energetically more favorable because this allows one product to be closer to the energetic minimum near mass 60 u (only a quarter of the average fissionable mass), while the other nucleus with mass 135 u is still not far out of the range of the most tightly bound nuclei (another statement of this, is that the atomic binding energy curve is slightly steeper to the left of mass 120 u than to the right of it).
   Section::::Fusion basics.
   Section::::Engineering considerations.
   For elements that weigh more than iron (a nucleus with 26 protons), the fusion process no longer releases energy. In even heavier nuclei energy is consumed, not released, by combining similarly sized nuclei. With such large nuclei, overcoming the electric repulsion (which affects all protons in the nucleus) requires more energy than is released by the nuclear attraction (which is effective mainly between close neighbors). Conversely, energy could actually be released by breaking apart nuclei heavier than iron.
   The most common nuclear fission process is "binary fission." It produces two charged asymmetrical fission products with maximally probable charged product at 95±15 and 135±15 u atomic mass. However, in this conventional fission of large nuclei, the binary process happens merely because it is the most energetically probable. 
   Section::::Safety.
   BULLET::::- Fusion, two atomic nuclei fuse together to form a heavier nucleus
  
  BULLET::::- Fission, the breaking of a heavy nucleus into two (or more rarely three) lighter nuclei
  
  Section::::Binding energy for atoms.
  
presuppositions: Fission and fusion are opposite reactions. Nuclear fusion and fission can't both release energy because they are opposites.
corrections: Fission and fusion are opposite directionally, but they both release energy. Nuclear fusion and fission involve different elements but both release energy.
raw_presuppositions: Fission and fusion are opposite reactions. Nuclear fusion and fission can't both release energy because they are opposites.
raw_corrections: Fission and fusion are opposite directionally, but they both release energy. Nuclear fusion and fission involve different elements but both release energy.