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Depleted uranium, primarily uranium 238, is what’s left over after the lighter isotope uranium-235 has been removed from natural uranium.

Seaborg’s approach had only produced plutonium 239, which decays very slowly back to uranium 238.

Ordinary low-enriched uranium fuel contains primarily uranium 238, the most common natural isotope of the element, along with about 5 percent uranium 235, a rarer isotope that splits, or fissions, more readily.

The uranium 238 then captures the neutron and is transformed into a new element, plutonium 239, which is also fuel.

By using up most of the uranium 238, the new design will get 40 times more energy from each pound of uranium, TerraPower says.