A key factor that made the reaction possible was that, at the time the reactor went critical 1.7 billion years ago, the fissile isotope U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. (The remaining 96.9% was non-fissile U and roughly 55 ppm U.) Because U has a shorter half-life than U, and thus decays more rapidly, the current abundance of U in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite.