Why is nuclear fuel so energy-dense?

Here's the fact that makes nuclear special: a fission reaction releases roughly a million times more energy per atom than burning coal or gas. A single fuel pellet the size of a fingertip holds about as much usable energy as a ton of coal.

Everything good about nuclear flows from this. A tiny footprint, years between refuelings, fuel you can stockpile for decades, and a waste stream measured in tons rather than the billions of tons of CO2 a fossil plant dumps into the air. No other clean, dispatchable source comes close on energy density.