PeterDonis said:
I have no idea where you are getting this from, but none of it is correct.
Well, technically there is some truth to it, but it's wrong in the generality depending on how the word 'fission' is used.
Deuterium and tritium fusion, for example, consists of fusing those two isotopes to helium 5, which then fissions to a neutron and helium 4.
'Fission' is a plain english word meaning to break into parts. However, in nuclear physics, 'Fission' may be a title reserved for the fission of heavy element actinides.
There are non-fissioning fusions which are mediated by a different force. Where fusions result in a prompt fission of an unstable intermediate, these are
mediated by the strong-nuclear force. On Earth we can only aim to perform fusion for energy via strong-force mediated fusion, because this produces fission fragments that are 'hot' (have kinetic energy).
There are fission reactions
mediated by the electromagnetic force (for example carbon 12 and a proton, a step in the CNO cycle produces only a 1.95MeV gamma photon and the resultant nitrogen 13 does NOT fission). This is useless on Earth because the gammas cannot be captured by any efficient means. The Sun can do it because it's so big, the photons get to heat up the solar mass on their 100,000's mile journey outwards.
There are fission reactions
mediated by the weak force, this is the case for the proton-proton fusion, in which two protons momentarily fuse then (ordinarily) just separate (difficult to know if there is actually a helium 2 formed or not, might just be a 'scattering' event) but every so often very very rarely in statistical terms, one of the protons decays to a neutron just at the moment of the fusion and leaves behind deuterium. The emission of the positron and electron neutrino from this fusion event is not regarded as a 'fission' which is usually confined to where multiple massive nucleonic products emerge form the nucleus, carrying away the excess energy as kinetic energy (which can then 'heat something up'). We can't do this on Earth because the reaction rate for weak mediated reactions is so poor it is literally impossible here, one needs a super dense and massive stellar core to do it, there is no other way, and like electromagnetically mediated fusion it still would not produce any thermal products.
Curiously, the very first ever 'controlled' fusion was the proton-lithium 7 fusion (14 Apr 1932 at Cavendish Laboratory by Rutherford/Cockroft/Walton), which results in the unstable beryllium 8 that fissions into two alpha particles. At the time this p-7Li fusion was, and still is often, reported and referred to as 'splitting the atom' rather than 'fusing atoms'.
So actually 'fission', in the technical sense (it is a proper word in its own right, fission means to break into parts), is a part of strong-force mediated fusion and is what we have to do on Earth for usable power here (if it is possible at all).
However, this is a subject of disambiguation because more often than not, in some circles, 'fission' is used as a
title (rather than the english meaning) which is then reserved
exclusively for the fissioning of heavy elements of the actinide series, in which a larger nucleus will break into two other substantial fractions of the nucleus, plus an assortment of 'debris', including neutrons (which can then cause the fission of more of the heavy nucleii).
Back to the point the contributor made, actually, oddly enough, of all the many nuclear fusion reaction types in the Sun, there are only a very few fusions that results in the fission of an intermediate, and these are all part of the CNO cycles, such as p+15N->12C+4He. One might argue that CNO is not even 'essential', as our Sun is mainly a pp fusion reactor, but it does happen.
The dominant cycle, the pp cycle, contains no strong-force mediated fusions (i.e. no fissioning intermediaries).
One curious feature I am interested in is that no solar fusion processes produce any neutrons, not even as intermediaries. I am unclear why that is, whether it 'just is like that' or if there is some actual reason and rational explanation.
So if we wanted to really do what the Sun does, we'd use electromagnetically mediated fusions and capture gamma rays for heat, but we can't so we use other reactions that the Sun
doesn't do, and produce neutrons instead.
In answer to the original ancient OP question from 2013, there might be a simpler answer to it; maybe controlled fusion power is simply not feasible, hence that is the problem. It's an experiment running for over 70 years now, which must be the longest running science experiment ever (and might always be) that people haven't yet formed a negative conclusion on. Edward Teller famously (or infamously) said at a meeting at the Princeton Gun Club Feb 27, 1958, that magnetic confinement fusion was impossible because it required concave magnetic fields in all directions away from a confined plasma, which is impossible. His hypothesis (on magnetic confinement fusion) has yet to be disproven.
