Two completely different sources of energy
Burning coal, oil or gas releases energy stored in chemical bonds — the forces holding atoms together in a molecule. Breaking those bonds releases a few electronvolts (eV) of energy per reaction, the same kind of energy involved in every chemical reaction you've ever seen. Nuclear fission works on a completely different scale: it releases energy stored inside the atomic nucleus itself, around 200 million electronvolts (MeV) per fission event.
That's roughly a million times more energy from a single event. It's not a marginal improvement — it's a difference in kind, and it's the entire reason nuclear fuel logistics look nothing like fossil fuel logistics.
See the comparison for your own numbers
Enter any amount of uranium and see exactly how much coal, oil and gas it matches.
The numbers, side by side
| Fuel | Energy density |
|---|---|
| Coal | ~8.1 kWh/kg |
| Oil | ~11.6 kWh/kg |
| Natural gas | ~13.9 kWh/kg |
| Natural uranium, realistic reactor yield | ~400,000 kWh/kg |
| Pure U-235, fully fissioned (theoretical) | ~22,800,000 kWh/kg |
Even the conservative, realistic reactor figure puts uranium tens of thousands of times ahead of any fossil fuel per kilogram. The theoretical maximum — fully fissioning pure U-235 — is millions of times ahead, though real reactors only extract a fraction of that theoretical limit.
What this means in practice
A typical reactor's entire annual fuel supply weighs a small fraction of what a similarly sized coal plant burns through in days. That's why nuclear plants need far less fuel transport, why fuel storage on-site is compact, and why nuclear waste — while it requires careful long-term handling — is a tiny volume compared to fossil fuel ash and emissions from the equivalent electricity output.
It also explains why nuclear fission produces no direct CO₂ from the reaction itself, only small lifecycle emissions from mining, enrichment and construction — see the uranium enrichment guide for how that raw uranium becomes reactor fuel, and the reactor output guide for how that fuel becomes electricity on the grid.
Frequently asked questions
Why is nuclear fuel more energy-dense than fossil fuel?
Fossil fuels release energy from chemical bonds, a few electronvolts per reaction. Nuclear fission releases energy from the atomic nucleus, about 200 million electronvolts per event — roughly a million times more concentrated.
How much coal equals 1 kg of uranium?
In a typical reactor, roughly 14,000 to 20,000 kg of coal. Fully fissioning pure U-235 would equal about 2.7 million kg of coal.
Does higher energy density mean less fuel is needed?
Yes — a reactor's annual fuel supply weighs a tiny fraction of the coal a similarly sized coal plant burns in the same period, which is why nuclear's mining and transport footprint per unit of electricity is so much smaller.