6 ms·
Fission 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 s
by jlos 5y ago
Fission 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.