So $^3$H turns into $^3$He through $\beta^-$ decay, right? The energy of this reaction is only 18.6 keV.
However, according to the constituent quarks model, the only thing different between $^3$H and $^3$He nuclei is that the first one has $d$ quark where the second one has $u$ quark (so the decay of this $d$ into $u$ is what creates an extra proton instead of the initial neutron).
Now, my question is thus: if the mass of $d$ quark is (by some modern estimates) around 5 MeV (plus-minus one - doesn't really matter), and the mass of $u$ quark is about 2.5 MeV (again, rounding a bit), how is the turning of one into other only releases 18.6 keV of energy? Where does the most part of the energy vanish instead? As far as I know, for strong forces neutrons and protons are pretty much the same - so what is going on?