Sometimes it takes me a while to get around to it, but I actually read each issue of Science. Good thing too, because the October 29 issue mentioned a paper about the oxygenation of the earth being the possible trigger of the formation of natural uranium reactors triggered (subscriber-only Science article). Here’s the theory, as simply as I can describe it:
- Uraninite, a uranium-bearing mineral, weathered out of rocks and formed sand.
- Uraninite is much denser than most minerals and uraninite sand naturally separates out into uranium-rich deposits.
- Uraninite is soluble in oxygenated water but not anoxic water.
- When the earth oxygenated ~2.5 billion years ago, the uraninite sands dissolved into water.
- Variations in oxygen levels precipitated the uraninite back out of the water in some places – think like salt crystalizing.
- The re-precipitated uraninite built up in large, fairly pure deposits.
- 2.5 billion years ago, the natural ratio of U235 to U238 was high enough that the critical mass of naturally occurring uranium was about 0.1 cubic meters (or about 25 gallons) – easily attainable by natural causes.
- These deposits formed small natural uranium reactors and spewed out radiation and crazy fission products for hundreds of thousands of years right in some of the most biologically interesting zones of the planet.
COOL, huh? We know that natural uranium reactors did form and operate – the Oklo reactors were discovered in 1972 when routine testing found that some parts of the uranium ore had significantly lower ratios of U235 to U238, among other isotopic imbalances. At this point we are merely arguing about mechanism and frequency.
This article particularly struck me because I’m reading The Making of the Atomic Bomb, which is large part about the vast, incredible, unthinkable effort and expense that it took to enrich enough uranium for a critical mass by the end of World War II. Acres and acres of silver-wound electromagnets and nickel-plated pipes, entire towns built from scratch, hundreds and hundreds of millions of (1940s) dollars representing a significant fraction of total U.S. industrial output, all to create something that just kinda… washed up on shore during the Archean. Time’s a bitch.