What is the isotopic composition of natural uranium?

Natural uranium consists primarily of three isotopes: uranium-238, uranium-235, and uranium-234, each present in fixed proportions due to their radioactive decay rates. Because uranium-235 has a shorter half-life than uranium-238, its natural abundance was much higher in the distant geological past. This historical abundance allowed natural nuclear fission reactors to form billions of years ago under specific groundwater and oxygen conditions.

What we know

• Natural uranium is composed mainly of three isotopes: uranium-238, uranium-235, and uranium-234. [3, 4]
• Uranium-238 makes up approximately 99.27% to 99.28% of natural uranium. [3, 4]
• Uranium-235 has a natural mole-fraction abundance ranging between 0.7198% and 0.7210%. [3, 4]
• About 1.7 billion years ago, the natural abundance of uranium-235 was around 3.1%, which enabled self-sustaining natural nuclear fission reactors to operate. [2, 3]

Where to go next

• a case in pointHow did the Oklo natural nuclear reactor operate?
• the bigger pictureWhat is isotopic abundance in physics?
• how it workedHow does uranium enrichment change isotopic ratios?
• compared withWhat are the radioactive decay properties of uranium-238?
• whenWhen was the Oklo natural reactor discovered?
• an unexpected connectionHow does ancient algae relate to natural nuclear reactors?Planetary-scale oxygen production by early photosynthetic life was likely essential for dissolving and concentrating uranium into fissionable ore bodies that formed natural nuclear reactors.

Sources

• [1]natural nuclear fission reactor (Wikidata Q64470499) · CC0 1.0
• [2]Natural nuclear fission reactor · CC BY-SA 4.0
• [3]Isotopic abundance · CC BY-SA 4.0
• [4]Enriched uranium · CC BY-SA 4.0
• [5]Uranium-238 · CC BY-SA 4.0