Fission — 82 terajoules in a kilogram

Split uranium-235 and each atom yields ~200 MeV. Scaled by Avogadro's number (exact since 2019), sealed: 82 TJ per kilogram — 2.737 million times coal at 30 MJ/kg: https://narrowhighway.com/s/8a5e89ab13bc499f39240c64360d90c34a65a291c91c73fcac4fc976131b17c5 . That ratio is why a reactor refuels in years, not trainloads per day, and why a few kilograms mattered so terribly in 1945. The chain reaction (each fission freeing 2–3 neutrons), criticality, and moderation are engineering on top of measured cross-sections; which fragments appear, and their decay heat, are ENSDF tables — nature measured, never derived in one line. The energy arithmetic above, though, is receipts.