question: How can nuclear fusion produce more energy than nuclear fission?
comment: It depends which side of iron you're on Elements heavier than iron give off energy when they split, the heavier atoms like plutonium tend to give off more energy per splitting than lighter ones like Radon. Elements lighter than iron give off energy when they fuse but need energy to split them apart. Heavier forms of hydrogen give off the most energy. Iron is super stable and every step towards it lets the atom get rid of some binding energy.
passages: It has also been proposed to use the neutrons to breed additional fission fuel in a blanket of nuclear waste, a concept known as a fission-fusion hybrid. In these systems, the power output is enhanced by the fission events, and power is extracted using systems like those in conventional fission reactors.
   This is a key concept in the hybrid concept, known as "fission multiplication". For every fusion event, several fission events may occur, each of which gives off much more energy than the original fusion, about 11 times. This greatly increases the total power output of the reactor. This has been suggested as a way to produce practical fusion reactors in spite of the fact that no fusion reactor has yet reached break-even, by multiplying the power output using cheap fuel or waste. However, a number of studies have repeatedly demonstrated that this only becomes practical when the overall reactor is very large, 2 to 3 GWt, which makes it expensive to build.
   The total energy output, 17.6 MeV, is one tenth of that with fission, but the ingredients are only one-fiftieth as massive, so the energy output per unit mass is approximately five times as great. In this fusion reaction, 14 of the 17.6 MeV (80% of the energy released in the reaction) shows up as the kinetic energy of the neutron, which, having no electric charge and being almost as massive as the hydrogen nuclei that created it, can escape the scene without leaving its energy behind to help sustain the reaction – or to generate x-rays for blast and fire.
   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 only practical way to capture most of the fusion energy is to trap the neutrons inside a massive bottle of heavy material such as lead, uranium, or plutonium. If the 14 MeV neutron is captured by uranium (of either isotope; 14 MeV is high enough to fission both U and U) or plutonium, the result is fission and the release of 180 MeV of fission energy, multiplying the energy output tenfold.
   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".
   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.
   When this type of bomb explodes, the fission of the highly enriched uranium or plutonium core creates neutrons, some of which escape and strike atoms of lithium-6, creating tritium. At the temperature created by fission in the core, tritium and deuterium can undergo thermonuclear fusion without a high level of compression. The fusion of tritium and deuterium produces a neutron with an energy of 14 MeV—a much higher energy than the 1 MeV of the neutron that began the reaction. This creation of high-energy neutrons, rather than energy yield, is the main purpose of fusion in this kind of weapon. This 14 MeV neutron then strikes an atom of uranium-238, causing fission: without this fusion stage, the original 1 MeV neutron hitting an atom of uranium-238 would probably have just been absorbed. This fission then releases energy and also neutrons, which then create more tritium from the remaining lithium-6, and so on, in a continuous cycle. Energy from fission of uranium-238 is useful in weapons: both because depleted uranium is much cheaper than highly enriched uranium and because it cannot go critical and is therefore less likely to be involved in a catastrophic accident.
   All proto- and mid-life stars radiate enormous amounts of energy generated by fusion processes. Mass for mass, the deuterium–tritium fusion process releases roughly three times as much energy as uranium-235 fission, and millions of times more energy than a chemical reaction such as the burning of coal. It is the goal of a fusion power station to harness this energy to produce electricity.
   Section::::Fuels.:Lithium.
  
  Fusion reactions involving lithium are well studied due to the use of lithium for breeding tritium in thermonuclear weapons. They are intermediate in ignition difficulty between the reactions involving lower atomic-number species, H and He, and the B reaction.
  
  The p–Li reaction, although highly energetic, releases neutrons because of the high cross section for the alternate neutron-producing reaction p + Li
  
  → Be + n
  
  Section::::Fuels.:Boron.
   In the case of neutrons carrying most of the practical energy, as is the case in the D-T fuel studied in most designs, this neutron energy is normally captured in a "blanket" of lithium that produces more tritium that is used to fuel the reactor. Due to various exothermic and endothermic reactions, the blanket may have a power gain factor a few percent higher or lower than 100%, but that will be neglected here. The blanket is then cooled and the cooling fluid used in a heat exchanger driving conventional steam turbines. These have an efficiency η which is around 35 to 40%.
   Section::::Fuel cycle.
   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.
   Unfortunately, the optimum temperature for this reaction of 123 keV is nearly ten times higher than that for pure hydrogen reactions, and the energy confinement must be 500 times better than that required for the D-T reaction. In addition the power density is 2500 times lower than for D-T, although per unit mass of fuel, this is still considerably higher than for fission reactors.
   Section::::Important reactions.:Neutronicity, confinement requirement, and power density.
  
  Any of the reactions above can in principle be the basis of fusion power production. In addition to the temperature and cross section discussed above, we must consider the total energy of the fusion products "E", the energy of the charged fusion products "E", and the atomic number "Z" of the non-hydrogenic reactant.
   Section::::Engineering considerations.
   In nuclear fusion, two low mass nuclei come into very close contact with each other, so that the strong force fuses them. It requires a large amount of energy for the strong or nuclear forces to overcome the electrical repulsion between the nuclei in order to fuse them; therefore nuclear fusion can only take place at very high temperatures or high pressures. When nuclei fuse, a very large amount of energy is released and the combined nucleus assumes a lower energy level. The binding energy per nucleon increases with mass number up to nickel-62. Stars like the Sun are powered by the fusion of four protons into a helium nucleus, two positrons, and two neutrinos. The uncontrolled fusion of hydrogen into helium is known as thermonuclear runaway. A frontier in current research at various institutions, for example the Joint European Torus (JET) and ITER, is the development of an economically viable method of using energy from a controlled fusion reaction.
   Timeline of nuclear fusion
  
  This timeline of nuclear fusion is an incomplete chronological summary of significant events in the study and use of nuclear fusion.
  
  Section::::1920s.
  
  BULLET::::- 1920
  
  BULLET::::- Based on F.W. Aston's measurements of the masses of low-mass elements and Einstein's discovery that E=mc, Arthur Eddington proposes that large amounts of energy released by fusing small nuclei together provides the energy source that powers the stars.
   The large flux of high-energy neutrons in a reactor will make the structural materials radioactive. The radioactive inventory at shut-down may be comparable to that of a fission reactor, but there are important differences.
   The fission of one plutonium atom releases ten times more total energy than the fusion of one tritium atom. For this reason, tritium is included in nuclear weapon components only when it causes more fission than its production sacrifices, namely in the case of fusion-boosted fission.
  
  Of the four basic types of nuclear weapon, the first, pure fission, uses the first of the three nuclear reactions above. The second, fusion-boosted fission, uses the first two. The third, two-stage thermonuclear, uses all three.
  
  Section::::Pure fission weapons.
   Before detonation, a few grams of tritium-deuterium gas are injected into the hollow "pit" of fissile plutonium or uranium. The early stages of the fission chain reaction supply enough heat and compression to start deuterium-tritium fusion, then both fission and fusion proceed in parallel, the fission assisting the fusion by continuing heating and compression, and the fusion assisting the fission with highly energetic (14.1 MeV) neutrons. As the fission fuel depletes and also explodes outward, it falls below the density needed to stay critical by itself, but the fusion neutrons make the fission process progress faster and continue longer than it would without boosting. Increased yield comes overwhelmingly from the increase in fission. The energy released by the fusion itself is much smaller because the amount of fusion fuel is so much smaller. The effects of boosting include:
   The ratio of fusion power produced to x-ray radiation lost to walls is an important figure of merit. This ratio is generally maximized at a much higher temperature than that which maximizes the power density (see the previous subsection). The following table shows estimates of the optimum temperature and the power ratio at that temperature for several reactions:
   Some atoms, notably uranium-238, do not usually undergo fission when struck by slow neutrons, but do split when struck with neutrons of high enough energy. The fast neutrons produced in a hydrogen bomb by fusion of deuterium and tritium have even higher energy than the fast neutrons produced in a nuclear reactor. This makes it possible to increase the yield of any given fusion weapon by the simple expedient of adding layers of cheap natural (or even depleted) uranium. Fast fission of uranium-238 provides a large part of the explosive yield, and fallout, in many designs of hydrogen bomb.
   In contrast, inertial confinement systems approach useful triple product values via higher density, and have vanishingly small confinement times. In modern machines like NIF, the initial frozen hydrogen fuel load has a density less than water which is increased to about 100 times the density of lead. In these conditions, the rate of fusion is so high that the entire fuel load undergoes fusion in the microseconds it takes for the heat generated by the reactions to blow the fuel apart. Although modern ICF machines like NIF are also extremely large, this is a function of their "driver" design, not an inherent design criterion of the fusion process itself.
   In some ways, fission and fusion are opposite and complementary reactions, but the particulars are unique for each. To understand how nuclear weapons are designed, it is useful to know the important similarities and differences between fission and fusion. The following explanation uses rounded numbers and approximations.
  
  Section::::Nuclear reactions.:Fission.
  
  When a free neutron hits the nucleus of a fissile atom like uranium-235 (U), the uranium nucleus splits into two smaller nuclei called fission fragments, plus more neutrons. Fission can be self-sustaining because it produces more neutrons of the speed required to cause new fissions.
  
presuppositions: Nuclear fusion produces more energy than nuclear fission. Nuclear fusion always produces more energy than nuclear fission.
corrections: Elements heavier than iron give off more energy with nuclear fission. Nuclear fusion only produces more energy than nuclear fission depending on the side of the iron.
raw_presuppositions: Nuclear fusion produces more energy than nuclear fission. Nuclear fusion always produces more energy than nuclear fission.
raw_corrections: Elements heavier than iron give off more energy with nuclear fission. Nuclear fusion only produces more energy than nuclear fission depending on the side of the iron.