Uranium: How energy resides
Why a tiny pellet can rival tonnes of coal
What this tab covers The physical form in which the energy is held or carried, where it comes from, the chain of transformations that put it there, and how concentrated it is.Uranium and thorium hold energy as nuclear binding energy: splitting a heavy nucleus releases millions of times more energy per kilogram than any chemical fuel. These atoms were made in exploding stars before the Solar System formed and have been slowly decaying in the Earth's rocks ever since.
Form Nuclear binding energy in heavy atomic nuclei Origin Nuclear (stellar nucleosynthesis before the Earth formed)How it got there
Supernovae and neutron-star mergers forge heavy elements Uranium and thorium incorporated into the Earth as it formed Concentrated in the crust by melting, weathering and groundwater Ore deposits form in sandstones, unconformities and granites Mining, milling and (for uranium) enrichment Fission in a reactor releases heatHow concentrated it is
| Measure | Value | Note |
|---|---|---|
| Uranium-235 fully fissioned | about 24,000,000 kWh of heat per kg2 | Versus about 8 kWh/kg for coal and 12 kWh/kg for oil: roughly 3 million times coal |
| Natural uranium in a typical light-water reactor | about 45,000 kWh of electricity per kg of natural uranium2 | After enrichment; the same electricity needs about 10,000 kg of oil or 14,000 kg of coal. Equal to roughly 140,000 kWh of heat at 33% efficiency (calculated), so today's reactors use well under 1% of the U-235 figure |
| Average abundance in continental crust | about 2.8 ppm uranium3 | Thorium is about three times as abundant |
| Uranium dissolved in seawater | about 3 mg per m34 | Equivalent to about 0.003 ppm; too dilute to recover cheaply |
When a uranium-235 nucleus absorbs a neutron it splits into two lighter nuclei, releasing energy and more neutrons that can split further nuclei in a chain reaction. Only uranium-235 among natural isotopes fissions readily; the much more common uranium-238 and thorium-232 are 'fertile', meaning a reactor can convert them into fissile plutonium-239 and uranium-233.
The energy density is so high that fuel volumes are tiny compared with coal, even though a light-water reactor extracts only a small part of the energy in natural uranium. The challenge is not quantity but the industrial chain (mining, conversion, enrichment, fabrication) and the handling of radioactive by-products. Thorium has no fissile isotope, so it always needs a starter of uranium-235 or plutonium.
Sources
- Fuel comparison (glossary) European Nuclear Society, 2024
- Supply of Uranium World Nuclear Association, 2025
- Uranium from seawater (IAEA Bulletin 59-2) International Atomic Energy Agency, 2018