Okloa

Okloa sleeps in the Gabon uranium rock and wakes every few hours. She was a working nuclear reactor two billion years ago.

The Physics

About 2 billion years ago, uranium was richer in U-235 (about 3.7%), because U-235 decays faster than U-238. With groundwater slowing the neutrons, ore at Oklo sustained fission chain reactions, running about 30 minutes on and 2.5 hours off as the water boiled away and returned.

U-235 half-life 0.704 Gyr; U-238 half-life 4.468 Gyr

• U-235, U-238: uranium isotopes
• Gyr: billion years (1 Gyr ≈ 3.16 × 10¹⁶ s)

In 1972 the CEA plant at Pierrelatte found Oklo uranium at 0.7171% U-235 instead of the usual 0.7202%, and as low as 0.296%; Meshik et al. (2004) worked out the on-off cycle.

Honest caveat: It never exploded like a bomb, and Oklo was not the only natural reactor.

On the shirt

Front: Okloa, the pixel-art character, with the title. Back: the isotope numbers, the half-lives, the water-moderated cycle and the 1972 and 2004 sources. Printed on a unisex tee.