Fissioning one gram matches tonnes of coal

Complete fission of one gram of uranium-235 releases about as much energy as burning around three tonnes of coal.

Each fission yields about 200 million electron-volts, which is a ridiculous energy for one nucleus and a tiny energy for a hand. There are so many nuclei in a gram that the tiny energies add to around 80 billion joules. Coal, burned, yields on the order of 24 million joules a kilogram. Divide, and a gram of fully fissioned uranium-235 stands in for a few tonnes of coal. That ratio is why a reactor core is measured in tonnes of fuel and a coal yard in trains.

The catch is in the words ‘complete’ and ‘uranium-235’. Reactor fuel is mostly uranium-238. Only a few percent is the fissile isotope, and not all of that is burned before the fuel is retired. A gram of yellowcake, or a gram of a fresh fuel pellet, is not this gram. The energy is in the nucleus. Getting it out is the engineering, and the engineering does not collect the theoretical total.