Answer to Question #5923 Submitted to "Ask the Experts"

I've read in several sources that around 0.72 percent of any deposit of naturally occurring uranium is ²³⁵U. I'm curious to know whether there have ever been, or potentially could be discovered, deposits of uranium with higher concentrations of the ²³⁵U isotope. I've also read an article that stated a naturally occurring fission reaction made some deposits in Central Africa as low as 0.440 percent. But is it possible that there may be a deposit existing with concentrations as high as a full 1 percent or even more?

Natural uranium is approximately 0.72 percent ²³⁵U by weight and 99.28 percent ²³⁸U by weight, plus a fraction of a percent of ²³⁴U. Higher percentages of ²³⁵U did exist at the time the earth was formed, but slowly decreased to the present level because the decay rate of ²³⁵U is about five times greater than ²³⁸U. Assuming the age of the earth to be about 4.5 billion years, the ²³⁸U has gone through one half-life so there was about twice as much ²³⁸U present when the earth was formed. There would have been about five times as much ²³⁵U at that time than there is today. The naturally occurring fission reaction that you refer to is known as the Oklo Phenomenon and is believed to have taken place a billion years ago when the abundance of the ²³⁵U isotope was about 3 percent. For this to occur, conditions had to be just right—suitable percentage of ²³⁵U in the ores, the proper shape (geometry) and size, and the presence of a neutron moderator such as water.
 
Ron Kathren, CHP