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I read that fusion is safer than fission because it emits less particles and is harder to cause a 10k year meltdown. Is that accurate?
by gentleman11 5y ago
I read that fusion is safer than fission because it emits less particles and is harder to cause a 10k year meltdown. Is that accurate?
- jlos 5y agoFission is a chain reaction, Fusion is not. A fusion reaction is so unstable that if the reactor failed the conditions for sustaining fusion would immediately stop. It's basically the hottest plasma we can make suspended in a magnetic donut surrounding by near absolute zero temperatures. https://en.m.wikipedia.org/wiki/Tokamak https://en.m.wikipedia.org/wiki/Tokamak
- gentleman11 5y agoI had assumed both were chain reactions due to stars. That’s interesting
- choeger 5y agoWell, technically, that's right. Fusion is a chain reaction. (albeit, "chain" might be a stretch here. Let's say it is a continuous process in stars.) But it's driven by gravity, not neutrons. You just need to bring about 70 times the mass of Jupiter into one place and that's it. Child's play, essentially ;).
- guscost 5y agoThe enormous pressure inside stars is what sustains the reaction. Neutrons coming off a fusion reaction, even if moderated, can’t really “squeeze together” other particles in any sense. Also, per unit volume, the sun produces about as much power as a compost pile.
- only_as_i_fall 5y agoDoes the volume of the sun include its Corona in that last statement?
- 317070 5y agoNo, iirc that statement is for the part of the sun where fusion actually happens. Per volume, not a lot fusions happen in the plasma on the sun. The crux is that the volume of the compost heap grows order cubed to the radius, while the area through which the heat escapes grows quadratic. And the sun is like, a _really_ big compost heap. In fusion on earth, we want to be considerably more efficient than the fusion process in the sun, as we don't have as much space to work with. ITER is already a pretty big machine.
- ben_w 5y agoThat’s just the core. Go out to the photosphere, let alone the corona, and it’s about about a thousand times less, 0.27 W/m^3: https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2F+volume+of+sun%29+in+watts+per+cubic+meter https://www.wolframalpha.com/input/?i=%28solar+luminosity+%2...
- innot 5y agoThis lead me to the following - what if you make a compost pile the size (mass) of Sun? Meaning, it won't be made of hydrogen, but rather some carbon-based molecules. I'm not sure about other atoms in these molecules, but I think carbon is stable enough not to initiate nuclear reactions. So probably fusion won't start. What then?
- shadofx 5y agoCarbon is capable of fusion, in a sufficiently compressed stellar mass. Everything up to Iron-56 can theoretically sustain fusion.
- thinkcontext 5y agoGravity would compress it. Depending on the exact composition fusion could start, ie it would become a star. According to Wikipedia carbon fusion requires a mass of 8 suns to start but there's a bunch of hydrogen in compost so maybe that would fuse.
- jabl 5y agoMaybe it could work via the CNO cycle: https://en.wikipedia.org/wiki/CNO_cycle https://en.wikipedia.org/wiki/CNO_cycle ?
- Server6 5y agoFusion can be a chain ration. In stars its caused by intense uninterpreted gravitational pressure. In the H-Bomb its triggered by a smaller fission explosion. Fusion chain reactions just require so much energy its hard (if not impossible) to harness on a commercial level. One small break in the the chain reaction containment and it falls apart.
- throwaway894345 5y agoProbably a dumb question, but what are the potential environmental impacts of manufacturing lots of helium? Is it a greenhouse gas? Even if we pivoted to 100% fusion in the next 100 years, is there a chance of releasing enough helium that it would be a problem in practice? I assume we won't need to worry about running out of hydrogen considering all of the water in the world (provided we can efficiently get hydrogen from the more abundant ocean water rather than the limited fresh water)?
- ngngngng 5y agoUK accents will acquire a slightly higher pitch over time
- mbg721 5y agoYet another way Monty Python was ahead of its time.
- mocko 5y agoNot much - it's lighter than the atmosphere so rises to the top and floats off into space.
- XorNot 5y agoHelium doesn't persist in the atmosphere. The mean free velocity of helium gas is above escape velocity, so all of it leaves Earth for outer space.
- throwaway894345 5y agoWhoa, I didn't realize that was possible. Fascinating.
- ClumsyPilot 5y agoWe have a shortage of helium on Earth, we are running out fast, all hydrogen produced can be put to good use.
- FridayoLeary 5y agoWhich is infinitely better then rapidly overheating and vaporizing and melting the reactor housing and spreading radiation which will stay dangerous for 200 years.
- joshmarinacci 5y agoSort of, but it's a vast oversimplification. Fission is very safe relative to something like coal because all of the pollution is concentrated in barrels instead of pushed into the atmosphere, but has the risk of meltdown. To be fair lots of other non-nuclear plants have had disasters that released toxic gasses around the globe too. Fusion in theory has no pollution at all, but that's theoretical until large scale fusion plants are built.
- jeffbee 5y agoFusion reactors don't create fission products, obviously, but why wouldn't fusion create radioactive waste via neutron activation? Materials exposed to neutron flux, in any kind of device, may be activated into radioactive isotopes.
- willis936 5y agoYes, but first wall materials are chosen to have a high melting point and short half life when activated. Close the building off for 100 years then scrap it. It's a far cry from the myriad of nightmare scenarios fission plants need active control against.
- slipframe 5y agoClosing the building off for 100 years might work fine, but that can't be the solution to every problem, it wouldn't be economically viable. Ostensibly simple matters like routine maintenance are very complicated propositions for fusion reactors; you can turn the reactor off but it will still be too radioactive for anybody to work inside. So you either need some sophisticated robotics to repair anything that might ever need repairing, or you have to consider the entire reactor to be disposable. Of course, repairing things inside a fission reactor is no less nasty, but fission reactors are comparably much simpler and much smaller. Swapping a fission reactor out with a new one is comparably much easier than with a fusion reactor.
- DennisP 5y ago
- BobbyJo 5y agoA good way to think of the difference is: Fission requires effort to control. When things go wrong it isn't controlled. Fusion requires effort to create. When things go wrong it isn't created.
- jeffbee 5y agoThat seems like a faulty explanation. Uncontained fusion is massively energetic and destructive. The device in this article is analogous to an H-bomb weapon with a pneumatic rather than fissile primary.
- cogman10 5y agoFusion bombs are when you put fusible materials next to fission explosions. Yes, they are massively energetic, (we wouldn't be harvesting power from it if they weren't.) However, they require a very high input energy to trigger the release of the output energy. With a fission reactor meltdown, the way you get there is by pulling out dampening rods or boiling off all the water, but otherwise leaving the fission rods in the same place. With a fusion reaction, you have to be constantly providing both the energy to keep the fusion going AND the input material to be fused. Interrupt one or the other and fusion stops. I know it seems weird that the bigger energy release is safer, but that's how it is. It's the difference between requiring constant input into the system to produce power vs an idle system with no input producing power.
- tolbish 5y agoTheir explanation is not really faulty. FYI hydrogen bombs are mainly destructive due to fission, not fusion. The fusion step is primarily for bombarding the dangerous fissile fuel with neutrons.
- jeffbee 5y agoThat a fusion bomb has significant energy from fission doesn't seem germane. It still has a great deal of energy from fusion alone. I'm not trying to scaremonger fusion energy, but I think it's intellectually dishonest to portray is as fundamentally sound, with a binary outcome of either inertia or safe energy. This design relies on spherical compression to both initiate and confine the fusion. We should not discount the possibility that if it instead creates a cylindrical or elliptical confinement due to malfunction, it will just explode, at a minimum destroying the device. We know it is possible to initiate fusion with radial compression in a cylinder, because that's how an H-bomb secondary works. The main safety factor in these things comes from the fact that a fusion weapon needs hundreds of kilos of hydrogen, and they are experimenting with much smaller masses. That limits the destructive potential.
- gambiting 5y agoYes, in the sense that fusion requires incredibly accurate magnetic fields to maintain, and the second there's any issue with the reactor chamber, the reaction will just stop. The reactor itself cannot explode in any way shape or form, because there is nothing in there to explode. It also doesn't produce any radioactive isotopes while running, it just fuses(hence fusion) hydrogen into helium, just like the sun does. You can just capture this helium and sell it to make baloons if you want. The reaction itself kicks off a huge amount of neutron radiation, which eventually makes the reactor chamber radioactive - that is the only radioactive waste that will have to be disposed safely eventually. But neither the fuel nor the resulting product are radioactive.
- mikro2nd 5y agoWhat is the sort of lifetime (ball-park) that one might expect before neutron saturation of the reactor walls becomes a serious concern and the reactor has to be scrapped?
- meowkit 5y agoYou can read about stuff like that in some of the ITER technical reports. They actually want to use that neutron radiation to generate tritium, and feed that back into the reactor. I don’t think the reactor would be scrapped, just shutdown for maintenance.
- eloff 5y agoI imagine you just replace the shielding on the inner walls, not the entire reactor.
- Symmetry 5y agoThe General Fusion design covers the walls with a molten mix of lead and lithium held in place by spinning the chamber. The lithium turns to tritium when you bang on it with a neutron to crease more inputs to the D-T fusion process. And lead doesn't really mind getting hit with neutrons. From the diagrams on Wikipedia[1] it looks like there is some normal metal exposed which might get neutron activated. [1]https://en.wikipedia.org/wiki/General_Fusion https://en.wikipedia.org/wiki/General_Fusion
- Kirby64 5y agoFusion byproducts are by and large significantly safer than fission. Fission byproducts are well known to just be a ton of heavy metal junk that decays over thousands of years. I'm sure there's fusion byproducts that are nasty too, but fundamentally you can't get byproducts as high up on the elemental chart as you can with fission strictly because fusion is merging 2 smaller atoms into 1 larger atom. With fission you start with a larger atom and break it into 2 smaller ones. Oversimplified, but you get the idea.
- StreamBright 5y agoThis is the 1990's view on the subject. There were several projects to address the fission leftover problem. Some of these: - Method to Reduce Long-lived Fission Products by Nuclear Transmutations with Fast Spectrum Reactors https://www.nature.com/articles/s41598-017-14319-7 https://www.nature.com/articles/s41598-017-14319-7 - Fast-neutron reactor https://en.wikipedia.org/wiki/Fast-neutron_reactor https://en.wikipedia.org/wiki/Fast-neutron_reactor - Evolution of transuranium isotopic composition in power reactors and innovative nuclear systems for transmutation https://inspirehep.net/literature/1243003 https://inspirehep.net/literature/1243003 And few other things. It is possible to have nuclear power with shorter byproducts than thousands of years.
- Kirby64 5y agoGiven that the US has only 1 reactor that was commercially put into operation after 1990s [1], it's a pretty realistic way to look at it in the US. Newer reactors and spent fuel reprocessing are definitely ways to solve the issue, but the fact is you still create those nasty byproducts in the traditional reactors today. [1]: https://en.wikipedia.org/wiki/Nuclear_power_in_the_United_States https://en.wikipedia.org/wiki/Nuclear_power_in_the_United_St...
- AtlasBarfed 5y agoWell if you're talking about a in-development fusion reactor, then it's fair to use in-development fission reactors for comparison.
- Symmetry 5y agoBasically yes. Both fission reactors and fusion reactors generate a bunch of neutrons. With a fission reactor you have to expose uranium to those neutrons and when it gets hit it either splits into dangerously radioactive components or absorbs a neutron and becomes dangerously radioactive plutonium. There are pipes and stuff that become a bit radioactive too but that's not the part that people are afraid of when an accident happens. Also, the dangerously radioactive byproducts continue to release a lot of energy after the chain reaction is shut down, about 10% of the power the reactor was run at. So even after a fission reactor is shut down you need to keep cooling it otherwise things melt and people become unhappy. With fusion you do need to keep exposing lithium to the neutrons from the reaction to make more tritium fuel. And tritium is radioactive. But if it escapes it'll go straight up into the upper atmosphere where it won't particularly bother people and then dilute. You've still got the radioactive pipes problem you do with fission but, again, that's not really the part that people are afraid of. And once you stop containing the plasma energy generation stops instantly. I'm generally inclined to say that the risks involved in fission are worth the benefits but those risks are worth taking seriously and require careful government regulation more so than other forms of power. With fusion, on the other hand, I'd a lot less concerned with someone building a power plant up wind of me than a coal plant even with similar levels of regulation. Just as long as they aren't deliberately exposing uranium to the neutrons or something like that.