205
votes
Accepted
Why isn’t CERN afraid of a fusion reaction in the LHC?
Proton-proton fusion happens at energies around 15 keV. The LHC currently operates at an energy of 13 TeV, which is literally one billion times larger. Fusion is one of the lowest-energy processes ...
- 35.7k
183
votes
Would a pin head heated to 15 million degrees Celsius kill everyone in a 1000 mile radius?
In this occasion Vsauce rather dropped the ball, I should think. As the other answers show, the claim as stated doesn't make much sense when you put in the numbers, and if you chase the source to its ...
- 135k
94
votes
Accepted
Why doesn't the nuclear fusion in a star make it explode?
The fusion that occurs in the core of the Sun occurs in nothing like the conditions you might be thinking of in a bomb, or a fusion reactor. In particular, it occurs at much lower temperatures and at ...
- 135k
88
votes
Accepted
Why do fusion and fission both release energy?
In general, both fusion and fission may either require or release energy.
Purely classical model
Nucleons are bound together with the strong (and some weak) nuclear force.
The nuclear binding is ...
- 1,531
83
votes
How do stars produce energy if fusion reactions are not viable for us?
The Sun is fabulously inefficient as a fusor: in a given year it consumes only about $10^{-10}$ of its fuel. For the Sun that’s actually a good thing, because it took us five billion years to notice. ...
rob♦
- 92.1k
77
votes
Accepted
Nuclear Fusion: Why is spherical magnetic confinement not used instead of tokamaks in nuclear fusion?
Not an expert, but I believe the answer lies in the hairy ball theorem.
You see, for a magnetic field to turn charged particles back from a surface, the field must be parallel to the surface, which ...
70
votes
Why do fusion and fission both release energy?
Fission releases energy, because a heavy nucleus (like Uranium-235) is like a cocked mouse trap: it took energy to squeeze all those protons and neutrons hard enough together to make them barely stick ...
- 96.8k
65
votes
What stops us from creating a nuclear fusion reactor as we already have the hydrogen bomb working on the same principle of fusion?
The example of a Molotov bomb, a favorite of anarchists, and a car engine are a good analogy. The technology needed to contain the energies in a fusion reaction is much harder than the one needed for ...
- 235k
44
votes
As neutrons are more massive than protons, does the Sun increase its mass while fusioning elements?
While a free neutron does have more mass than a free proton, a bound helium-4 nucleus has less mass than two free protons and two free neutrons. In fact, the helium-4 nucleus has less mass than four ...
- 1,143
42
votes
Accepted
Why don't deuterons produce a helium-4 nucleus when they fuse?
The deuterium fusion reaction is extremely exothermic. It releases about a million times more energy than a typical chemical reaction, and that energy has to go somewhere. If we had two deuterium ...
- 360k
39
votes
How do stars produce energy if fusion reactions are not viable for us?
None of the other answers (so far) mentions gravity. At least one of them mentions confinement though. It talks about the cost of confinement and, about the consequences of not confining a reaction ...
- 15.7k
38
votes
Would a pin head heated to 15 million degrees Celsius kill everyone in a 1000 mile radius?
A pin head is maybe equivalent to a spherical piece of iron with a diameter of 2 mm. That gives it a volume of about 4 mm$^3$ and a mass of $3.2 \times 10^{-6}~\rm{kg}$; computing the heat capacity of ...
- 119k
38
votes
Is fission/fusion to iron the most efficient way to convert mass to energy?
Matter-antimatter annihilation, such as an electron annihilating with a positron to form two high-energy photons, can convert 100% of the mass into radiation. So fission and fusion are far from the ...
- 51.8k
36
votes
Accepted
Can Jupiter turn into a star if it radiated off enough heat?
The smallest objects (given an elemental abundance mixture appropriate to a giant planet like Jupiter) that can attain hot enough interiors to ignite a sustained thermonuclear reaction are about 13 ...
- 135k
35
votes
Accepted
Why can't hydrogen and helium fuse?
Hydrogen and helium can briefly bind together to make lithium-5, but this is an extremely unstable nuclide which falls apart instantly (with a half-life of ${\sim}4\times 10^{-22}\:\rm s$) and which ...
- 135k
33
votes
Is fission/fusion to iron the most efficient way to convert mass to energy?
The most efficient non-gravitational way of extracting energy from ordinary matter is indeed to convert it into elements in the $^{56}$Fe region. There is a fairly broad plateau of nuclides with ...
Buzz♦
- 16.5k
32
votes
What stops us from creating a nuclear fusion reactor as we already have the hydrogen bomb working on the same principle of fusion?
You have several questions there, let's first focus on the main question: why is there no working fusion reactor on earth as we already have the hydrogen bomb?
This is an interesting question, as a ...
- 1,989
32
votes
Why do fusion and fission both release energy?
Your assumption about the lowest energy state when everything is tightly stuck together is incorrect.
It only goes this way until you get iron nuclei - and this is why iron is the heaviest element ...
- 979
31
votes
Can nuclear fusion alone account for the energy output from type 1a supernova?
A back of the envelope calculation (and that is all this is) would go along the lines of assuming that the white dwarf is made entirely of $^{12}$C (it isn't) and is entirely converted into $^{56}$Ni (...
- 135k
31
votes
Why are nuclear fusion reactors difficult?
The key difficulty in fusion power is sustaining a controlled nuclear fusion reaction.
The conditions needed for nuclear fusion here on Earth involve extremely high temperature -- on the order of $10^...
- 1,857
30
votes
As neutrons are more massive than protons, does the Sun increase its mass while fusioning elements?
In fact, the Sun is losing mass all the time. It radiates large amounts of energy, and through the energy-mass relationship $E = m c^2$, radiating energy means radiating mass.
Since the mass of a ...
- 2,645
29
votes
Accepted
How do stars produce energy if fusion reactions are not viable for us?
It is the fact that fusion reactions are very exothermic that makes them so hard to control.
Coal releases its chemical energy so slowly that a coal fire does not need any confinement - it does not ...
- 57.7k
27
votes
Accepted
Proton - neutron fusion?
Of course the reaction is possible. It doesn't even require special environmental conditions. Having no charge the neutrons don't need to overcome a strong Coulomb barrier to interact with atomic ...
27
votes
How do fusion reactors deal with blackbody radiation?
The plasma in a fusion reactor is typically "optically thin"; the radiation isn't really in equilibrium with itself and the plasma particles.
Generally, instead of just modeling the plasma as a black ...
- 14.6k
26
votes
Accepted
Does 60kg of fusion fuel produce as much energy as 400 kilotonnes of coal?
Yes. See for example this table of energy densities.
Let's take 30 MJ/kg for coal (the middle of the range in the table), then 400,000 tonnes of coal gives 1.2*1016 Joule.
Assuming they're talking ...
- 459
25
votes
Accepted
Why all the orifices, protuberances, hinged/bolted panels, etc. on a stellarator?
The external casing belongs to the cryostat that maintains the superconducting magnets at 4 K. It has 254 ports to heat and diagnose the plasma. For heating with neutral beams, you need relatively ...
- 1,057
25
votes
Why are nuclear fusion reactors difficult?
Fission is Easy
Natural fission reactors exist on earth, at a small scale, and low energy. However, the smallest natural fusion reactor is probably the brown dwarf, which has a mass at least 10x ...
- 2,632
23
votes
How exactly is fusion possible?
I am assuming you have had no rigorous mathematical exposure to quantum mechanics. Let me know if you do, and I can point you towards more specific material.
It is difficult to tell what it was ...
- 558
23
votes
How do stars produce energy if fusion reactions are not viable for us?
However, if these reactions are considered endothermic for us, how are they exothermic in stars?
The reactions are still exothermic for us. In fact, they are very exothermic. The fact that they are ...
- 105k
21
votes
Accepted
When will hydrogen no longer be the most abundant nucleus?
We can only give an answer on the basis of what we currently know about cosmological parameters. If indeed these have been correctly estimated, and that the cosmological constant is constant, then the ...
- 135k
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