TOKYO — 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. “I was later told that he did not want to say that Fukushima radioactivity was worse than Chernobyl.” The head of the institute, who has since retired, declined to comment for this article. Mr. Aoyama asked for his name to be removed, he said, and the article was not published.

The pressure he felt is not unusual — only his decision to speak about it. Off the record, university researchers in Japan say that even now, three years after the triple meltdown at the Fukushima Daiichi plant, they feel under pressure to play down the impact of the disaster. Some say they cannot get funds or university support for their work. In several cases, the professors say, they have been obstructed or told to steer clear of data that might cause public “concern.”