Alexander Nazaryan, born in Russia and raised mainly in the United States, reports for Newsweek on a visit to what was the scene, in 1986, of the world’s worst nuclear accident: “The toxic cloud that enveloped much of Europe that spring has intrigued me ever since. I can name all of the radionuclides it contained: cesium-137, iodine-131, zirconium-95,  strontium-90, ruthenium-103… But I longed to know its origins, the way a naturalist might yearn to see the source of a river somewhere high in the mountains, simply to fulfil the human need to discover beginnings and pay homage to them.”