Reading aloud About two billion years ago, a natural gas enrichment of uranium-235 produced a chain reaction in a mine in Gabon, setting a nuclear reactor in motion for about 150, 000 years. Scientists at Washington University in St. Louis have now discovered why the reactor remained in operation for such a long time without exploding. According to their study published in the Physical Review Letters (Volume 93 article 182302), the chain reaction of groundwater was moderated so that the reactor was naturally switched off several times a day. The 1972 discovered in Gabon reactor? or rather its remains in the rock strata of a mine? Due to its high concentration of fissile uranium-235, it should have burned out or even exploded in a relatively short time. To find out why this did not happen, Alex Meshik and his St. Louis research colleagues studied the isotopes of the atoms trapped in the rock, created by the uranium chain reaction. These were extracted from the rock by means of a laser-based method and then examined with a mass spectrograph.

The researchers found six different isotopes of the noble gas xenon. The mass ratios of the individual isotopes to one another could not be reconciled with the theory of a continuous chain reaction. Rather, the researchers calculated that the reactor only worked for about 30 minutes and then rest for more than two hours before the chain reaction started again.

Meshik believes that groundwater penetrating the rock causes this moderation by slowing down the neutron needed for the chain reaction. What exactly triggered this periodicity, the researchers can not yet say.

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