In the chaotic, fearful weeks after the Fukushima nuclear crisis began, in March 2011, researchers struggled to measure the radioactive fallout unleashed on the public. Michio Aoyama’s initial findings were more startling than most. As a senior scientist at the Japanese government’s Meteorological Research Institute, he said levels of radioactive cesium 137 in the surface water of the Pacific Ocean could be 10,000 times as high as contamination after Chernobyl, the world’s worst nuclear accident. Two months later, as Mr. Aoyama prepared to publish his findings in a short, nonpeer-reviewed article for Nature, the director general of the institute called with an unusual demand — that Mr. Aoyama remove his own name from the paper. “He said there were points he didn’t understand, or want to understand,” the researcher recalled. […]