Nuclear reactions and Q-values
A nuclear reaction a + X \rightarrow Y + b conserves nucleon number A, charge Z, and energy–momentum. Its energy release is again a Q-value: sum the masses before and after. Q>0 is exothermic (energy out, like fission and fusion); Q<0 is endothermic and needs a threshold kinetic energy to proceed.
The reaction Q-value — the master accountant of nuclear energetics.
How likely a reaction is, is a separate question, answered by its cross-section \sigma — the effective target area a nucleus presents, measured in barns (10^{-28}\,\mathrm{m}^2). Reaction rate is flux times number density times \sigma. A reaction can be hugely exothermic yet almost never happen if its cross-section is tiny — energetics and probability are independent.
Fission: splitting the heavy end
In nuclear fission a heavy nucleus such as uranium-235 absorbs a slow neutron, wobbles like an overstretched liquid drop until the Coulomb repulsion wins over the surface tension, and splits into two mid-mass fragments plus two or three fresh neutrons — releasing about 200 MeV per event. Most of that energy is the kinetic energy of the flying fragments. The fragments are neutron-rich and beta-decay onward, which is the origin of radioactive waste.
Chain reactions and criticality
The neutrons released by one fission can trigger the next — a chain reaction. Its fate is set by the multiplication factor k, the average number of those neutrons that go on to cause a further fission. k<1 dies out (subcritical); k=1 self-sustains steadily (critical, a running reactor); k>1 grows exponentially (supercritical). Reaching k=1 requires enough fissile material in the right geometry — the critical mass.
Fusion: climbing the left slope
Nuclear fusion joins light nuclei — but first they must approach within the 1–2 fm range of the strong force, and to do that they must overcome the Coulomb barrier of their mutual repulsion. That demands enormous relative speeds, i.e. temperatures of tens of millions of kelvin. The workhorse reaction for terrestrial fusion is deuterium + tritium.
The D–T reaction releases 17.6 MeV — over 3 MeV per nucleon, far more per nucleon than fission.
Stars do it differently and more gently. In the Sun's core, the proton–proton chain fuses four protons into a helium-4 nucleus, net 4\,{}^1\mathrm{H} \rightarrow {}^4\mathrm{He} + 2e^+ + 2\nu_e, releasing about 26.7 MeV. The Sun's core is only ~15 million K — below the classical barrier — so fusion there relies on the same quantum tunnelling as alpha decay, biased by the Maxwell–Boltzmann tail into the narrow Gamow peak of energies where reactions actually occur.
Nucleosynthesis, and where this leads
Every atom in your body was assembled by these reactions. The first minutes of the universe made hydrogen, helium and a trace of lithium (Big Bang nucleosynthesis). Stars fused hydrogen upward through helium, carbon, oxygen... all the way to the iron peak — but no further, because past iron fusion is endothermic. The elements heavier than iron were forged in supernova explosions and neutron-star mergers, by rapid neutron capture (stellar nucleosynthesis). The binding-energy curve of guide 1 is literally the recipe for the periodic table.
One last worked comparison — why nuclear fuel is so concentrated. Fission liberates ~200 MeV per uranium-235 nucleus. Per kilogram: one kg of U-235 holds N_A/0.235 nuclei \approx 2.56\times10^{24}, times 200\ \mathrm{MeV} = 3.2\times10^{-11}\,\mathrm{J} each, giving about 8\times10^{13}\ \mathrm{J/kg}. Coal yields ~3\times10^{7}\ \mathrm{J/kg}. The ratio is a few million — precisely the eV-to-MeV factor from guide 1, made concrete.