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In fact, scientists have been doing studies like this since the 1960s, releasing tracers such as zinc sulfide powder or sulfur hexafluoride gas into the stratosphere to study air currents.

It said the worker died from a "mechanical injury" caused by a breach in a container of uranium hexafluoride, a chemical compound used in uranium enrichment.

It is called sulfur hexafluoride, or SF6.

The report said Iran's stock of uranium enriched to 60% and in the form of uranium hexafluoride, the gas that centrifuges enrich, was estimated to be 55.6 kg, an increase of 12.5 kg from the previous quarterly report.

AP also tracked millions of dollars worth of shipments of radioactive uranium hexafluoride from Russian state-owned Tenex JSC, the world’s largest exporter of initial nuclear fuel cycle products, to Westinghouse Electric Co. in South Carolina.