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Earth鈥檚 crust used to be full of radioactive uranium. In some spots, conditions were right for that uranium to undergo chain fission reactions 鈥� the same reactions that give us nuclear power today. Now, the remnants of these natural nuclear reactors are helping us figure out how best to store modern radioactive waste.

When the isotope uranium-235 is struck by relatively slow-moving neutrons, it breaks apart into smaller elements in a fission reaction, releasing huge amounts of energy. Uranium-235 has a half-life of 700 million years 鈥� after that amount of time, half of the atoms will have decayed into lighter elements.

This decay means there is not much uranium-235 left in Earth鈥檚 crust, which is 聽why we can鈥檛 just use uranium mined straight from the ground to power nuclear reactors 鈥� we have to enrich it first so it will react. But two billion years ago, before the uranium decayed, there were natural nuclear reactors in Earth鈥檚 crust where uranium underwent fission without enrichment.

Evan Groopman at the US Naval Research Laboratory in Washington DC and his colleagues examined samples from the remnants of one such site in Gabon, called the Oklo reactor, to figure out how the relatively heavy elements left behind from those ancient reactions have moved around since then.

One of the most dangerous byproducts of nuclear fission is radioactive cesium, which can cause major health problems if it gets into the soil or water. 鈥淐esium tends to migrate,鈥� says Groopman. 鈥淚f you were to rupture the casks that we store nuclear waste in, cesium is so volatile that it would escape immediately.鈥�

But the researchers found that all of the cesium hadn鈥檛 escaped the ancient reactor. Instead, it was trapped in the molecular structure of chunks of another element, ruthenium. That means ruthenium might help keep nuclear waste contained, Groopman says.

鈥淲hat you really want is for the fission products to be locked up in some sort of mineral that鈥檚 resistant to heating or being hit with a hammer,鈥� he says. 鈥淟ining casks with ruthenium or something similar might be the way to go.鈥�