Oops - let me correct a major typo.
The proton-proton chain reaction on the sun is mostly “reversible fusion”. P+P <-> H2 This, of course, should be: P+P < - > 2He (the helium-2 nucleus, which is unstable). It has been posted here many times that the strong force is overwhelming at close range - and will bring too protons in a cavity together, despite Pauli (Pauli exclusion principle). But almost always Pauli prevails and the He2 nucleus which forms, immediately breaks up into the same two protons as if it was an elastic collision. Thus, 99.99+ % of all the fusion reactions, on all the stars in the Universe, can be said to be reversible, and do not produce much energy. The bigger question for NiH on earth is this: does reversible proton fusion produce any net energy at all? The currently favored model based on remote solar fusion from protium says NO, but there is really little way to be sure – except via P+P experiments on earth.
