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Why nuclear fusion yields more energy than fission
- hulitu 5y agoThe title should have been "Why Nuclear Fusion Could Yield More Energy Than Fission". Fusion is still not working as expected.
- frankbreetz 5y agoThere is fusion happening inside stars and it yields a lot of energy
- pfdietz 5y agoThis doesn't have much to do with fusion reactors. It's not even the same nuclear reactions.
- sandworm101 5y agoNot really. Stellar fusion is only a few watts per ton of hydrogen fuel. That would never be a practical energy source if replicated on earth, but it is why stars can burn for billions of years.
- jemaddux 5y agoThis is surprising to me. I would have thought the energy output per ton would be way higher. Do you have a source that explains why it is so low?
- bognition 5y agoIt's pretty easy to forget that the sun is a ball of gas. It not very dense (roughly 1/4 the density of the earth). So it's extremely unlikely that a specific hydrogen atom will undergo fusion during the lifetime of the sun. However the sun is also ENORMOUS. So in the aggregate there is a lot of fusion going on and a lot of energy is being produces. However its not super dense. From wikipedia: https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_density_84-0 https://en.wikipedia.org/wiki/Sun#cite_ref-power_production_... The large power output of the Sun is mainly due to the huge size [...] Theoretical models of the Sun's interior indicate a maximum power density, or energy production, of approximately 276.5 watts per cubic metre at the center of the core,[77] which is about the same power density inside a compost pile
- CRConrad 5y agoSo it's low in terms of "amount of energy in relation to amount of hydrogen in the star"? But wouldn't much of the un-fused hydrogen be eligible to fuse at some later time, if for instance convection streams pull it down into the deeper (=higher-pressure) zones of the star? So it seems to me the relevant measurement ought to be the ratio of energy per hydrogen atoms fused, not hydrogen atoms total. (Certainly in a reactor, where presumably all or most of them should be.)
- ZeroGravitas 5y agoA song about the sun being a ball of gas, for those that find it hard to remember: The Sun Is A Mass Of Incandescent Gas https://g.co/kgs/g8rKyh https://g.co/kgs/g8rKyh A follow up song, correcting the incorrect physics of the first one: Why Does the Sun Really Shine? https://g.co/kgs/2QKBdw https://g.co/kgs/2QKBdw
- mannykannot 5y agoInside the sun's core, where the fusion occurs, the density is considerably higher - 150 g/cm3 vs. 5.5 g/cm3 (average) or 13 g/cm3 (inner core) for the earth.
- spaetzleesser 5y agoThat’s why a fusion reactor would use different fuels than a star.
- adgjlsfhk1 5y agono? hydrogen to helium is by far the best.
- xoa 5y ago>no? hydrogen to helium is by far the best. Not necessarily for humans. Stars get to be gravity-mass containment/fusion pressure automoderated, but there is nothing known even in blue sky to manipulate gravity beyond sheer mass. So humans have to deal with containing via electromagnetism and relatively little isolated surrounding matter, which means we have to deal with both ignition thresholds and the issue of neutrons. In stars neutrons provide heat/energy by sheer virtue of hitting stuff as they pass out from the core, and obviously radioactivity isn't a concern. But it's definitely an issue for a power plant, where neutrons mostly represent wasted energy since we don't have any good way to harness them for useful work, plus they'll activate surrounding materials over time which is also an irritation. So the ideal fusion may be aneutronic, which then faces tradeoffs of fuel availability vs ease of production. For example terms of plain ease of engineering/compactness etc, helium-3 would probably be the best fuel. But helium 3 is also super ultra rare and effectively nonexistent naturally on Earth. We'd have to get it from space, or breed it from neutronic reactors which obviously isn't ideal (might be worth it as a compact fuel for far future spacecraft though). On the opposite side hydrogen-boron fusion requires much much higher ignition energy, greatly complicating actually building a working plant, but the fuels themselves are readily available. There are other potential ones between, or maybe we just suck it up and deal with neutron activation (still a lot less radioactivity than in fission plans), which could be moderated with more careful selection of construction materials. At any rate, we're definitely not yet at the point where we can really say what the "best" fusion cycle will be, and it's perfectly possible we'll even end up with multiple (He3 for vehicle/more portable reactors, H-B or D-D for massive stationary ones say).
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- pixl97 5y agoIf it wasn't so low the universe would be dead. We see this is large blue stars. They burn through their fuel supply in millions of years and years and explode with the force of a billion suns. Meanwhile small stars can burn for billions and billions of years because they are conservative with their hydrogen supply.
- aqme28 5y agoThat's not entirely accurate. It's still a lot of power per unit of fuel burned, but you're right that it burns much much slower than people expect.
- sandworm101 5y agoI did saw watts, not watt-hours. The rate of energy output per unit mass is very small.
- deleted 5y ago[deleted]
- rpmuller 5y agoTsar Bomba [1] is a good example of energy release from fusion. [1]: https://en.wikipedia.org/wiki/Tsar_Bomba https://en.wikipedia.org/wiki/Tsar_Bomba
- Ekaros 5y agoOne could think that all energy generated by nuclear fission is just energy stored from fusion. So it should be pretty clear fusion yields more as it is input for all fission.
- pfdietz 5y agoExcept it's not. The energy in fission comes from gravitational collapse, not from fusion (collapse is what allows free neutrons to be liberated to drive the r-process that makes uranium.)
- JumpCrisscross 5y ago> energy in fission comes from gravitational collapse Do you have a source for gravity being in any way involved with fission’s energy release? I thought, and the article says, it’s from “the energy of the electromagnetic force when positively charged parts of the nucleus fly away from one another.” (EDIT: Ah, nvm.)
- usrusr 5y agoI think this was talking about "energy" in the same very loose sense as the original post, which was using the term energy for talking about the provenance of the fuel. The heavy elements used in fission are not created in the almost eternal smoldering suns but in the relatively short events that follow.
- gpm 5y agoYou're missing the point, the energy released in fission is energy stored in the uranium billions of years ago when it was formed by a star collapsing, that collapse was powered by gravity.
- HPsquared 5y agoIsn't it still nuclear fusion, just endothermic? The nuclei are formed from smaller particles colliding and fusing together. Or is it just lots of neutron capture? Edit: I see, it's neutron capture: https://en.m.wikipedia.org/wiki/R-process https://en.m.wikipedia.org/wiki/R-process
- beervirus 5y agoGood explanation, but it kind of buried the lede. The answer is way down at the bottom: > Fission releases the energy of the electromagnetic force when positively charged parts of the nucleus fly away from one another. Fusion releases the energy of the strong force (much stronger at short distances than the EM force) when the small pieces are captured and held into one nucleus.
- marcodiego 5y agoFusion of tritium and deuterium Input: - 1 deuterium atom: 1 proton and 1 neutron - 1 tritium atom: 1 proton and 2 neutrons Output: - 1 helium atom: 2 protons and 2 neutrons Overall: 5 particles become 4, 20% become free to hit other atoms or turn into energy. Decay of uranium-235 into barium and krypton Input: - 1 atom of uranium: 235 particles Output: - 1 atom of barium: 144 particles - 1 atom of krypton: 89 particles Overall: 235 particles become 233, less than 1% become free to hit other atoms or turn into energy. Fission is much more similar to radioactive decay: big atoms turn into smaller atoms releasing a few free particles. Fusion releases basically the same amount of particles with much smaller atoms. Of course, this is an extreme simplification of the process. Energy required to start each reaction is ignored, but the idea is mostly correct, I think. EDIT: fixed tritium number of protons and neutrons and grammar.
- 4WIW 5y agoNot to nitpick but: trituim is 1 proton and 2 neutrons, otherwise it would be helium
- marcodiego 5y agoThanks! I'll fix it.
- cecilpl2 5y agoThis is not quite right. In no cases are nucleons converted into energy. D+T fusion releases a He4 atom and a free neutron - 5 in, 5 out. U235 absorbs a neutron, then decays into Ba144, Kr89, and 3 neutrons - 236 in, 236 out. In the fission case, the neutrons escape to hit other U235 atoms, which causes the well-known chain reaction. In the fusion case, neutrons are not an input to the process. The released energy comes from the binding energy of the nuclei. The nucleons in a U-235 atom are held together weakly. My lay understanding which is probably wrong, is that the protons are trying to repel each other electromagnetically but are held together by the strong force, so splitting the atom means less strong force is "required" to hold the two new nuclei together. I don't have a solid understanding of exactly how the binding energy is lower in a He4 nuclei, and would love someone to explain!
- dghughes 5y agoI was watching "Undecided with Matt Ferrell" on it Dr. Martin Greenwald said that 0.1g deuterium and 0.3 lithium fused in a fusion power plant would release enough energy to power a typical US home for a year. Actually he said energy needs for an American so he may have meant more than electrical power. From what I can see the typical US home uses 10,715kWh/year which is 38,574,000,000 Joules.
- panick21_ 5y agoA single tennis-ball sized thorium ball is enough for your whole live. I really like that visual.
- bronzeage 5y agoThe bottleneck of nuclear fusion in all stars is the proton-proton chain reaction, turning two protons into deuterium. Only deuterium can further fuse, and usually deuterium doesn't survive in the sun for long. This reaction is caused by the weak force, so it's extremely slow. This is what's slowing the fusion in all the stars. "The average proton in the core of the Sun waits 9 billion years before it successfully fuses with another proton. It has not been possible to measure the cross-section of this reaction experimentally because it is so low" https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain https://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain In nuclear reactors, we can start from deuterium, making the process much faster and power intensive than the sun.
- pfdietz 5y agoI sometimes see the argument "fusion produces more energy per mass than fission, therefore fusion is better than fission." This is a completely dumb argument.
- grayhatter 5y agoheh, you're right, look at all these idiots who don't know it's dumb like we do. he he he... (why do we think it's dumb again?)
- pfdietz 5y agoBecause it's a non sequitur. The metric of "energy/mass of fuel" has nothing to do with why any actual consumer would consider the source to be better. Metrics like cost, safety, pollution, etc. are relevant; that metric is not.
- ncmncm 5y agoAnd, thus far, fusion yields exactly zero joules of useful energy (unless you count solar). It hasn't even vaporized anybody yet, never mind anybody you might have wanted to have vaporized. (Tokamak) fusion will never produce commercially competitive energy. Every plausible system design would be overwhelmingly more expensive to build and operate than a fission plant of similar rating. And, fission is already not competitive, and gets less so every day. (I still hold out hope for FRC De-He3 fusion for space probe propulsion--4 years to Pluto--but Tokamak eats all the research dollars, so who knows if we will ever find out?)
- pfdietz 5y ago> I still hold out hope Well, Helion did just get $500M in funding, and is very close to that design space. I have doubts about Helion being able to do it, but if I had to invest in fusion it would be them (and just possibly Zap). Going beyond DT and recovering energy electromagnetically addresses the engineering/economic showstoppers that make things like tokamaks burning DT seem like a dead end.