• $summary_{type}$: concept
• $word_{count}$: 160

Nuclear fusion combines light nuclei to form heavier ones, releasing energy as the product moves toward the peak of the BE/A curve. The proton-proton chain: 4H -> He-4 + 2e+ + 2$nu_e$ + 26.7 MeV. Energy per nucleon (~6.7 MeV/nucleon) far exceeds fission (~0.9 MeV/nucleon), making fusion the more efficient energy source. Fusion requires temperatures ~10^7 K (thermonuclear conditions) to overcome Coulomb repulsion between positively charged nuclei. At such temperatures, matter exists as plasma. The Sun's core sustains fusion at ~1.5 x 10^7 K, converting ~4 million tonnes of mass to energy every second. Controlled fusion (tokamak, ITER) remains an engineering challenge — containment of plasma at fusion temperatures is extremely difficult. Uncontrolled fusion powers hydrogen bombs. The CNO cycle (dominant in stars heavier than the Sun) also fuses hydrogen to helium but uses carbon as a catalyst.