Also note that due to isotope decay in the ore, a natural reactor is no longer possible. From the wikipedia article:

"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 235U made up about 3.1% of the natural uranium, which is comparable to the amount used in some of today's reactors. [...] the current abundance of 235U in natural uranium is only 0.72%. A natural nuclear reactor is therefore no longer possible on Earth without heavy water or graphite."

Another fascinating detail from the article, due to our understanding of fission, we can get some incredible results:

"The concentrations of xenon isotopes, found trapped in mineral formations 2 billion years later, make it possible to calculate the specific time intervals of reactor operation: approximately 30 minutes of criticality followed by 2 hours and 30 minutes of cooling down"