On his walk home from the factory, he was drenched with a black rain. His grandmother immediately had him bathe, most likely saving him from radiation-related illness.

His future wife, Akemi, was not as lucky. She was just over a mile from the blast and, though sheltered by a small hill, suffered for years from the effects of radiation poisoning.

In the aftermath of World War II and the dropping of the atomic bomb on Hiroshima and Nagasaki, Dr. Shimomura, now a Nobel Prize-winning chemist, said he largely put the events out of his mind.

But here he was last month, in the birthplace of the atomic era, to deliver a lecture at the monthly Director’s Symposium. Nearby was a museum with Manhattan Project artifacts, and surrounding him were Los Alamos scientists who were curious about how this man, now 84 and a professor emeritus at the Marine Biological Laboratory in Woods Hole, Mass., felt about the bombings in 1945.

That is not, though, what he chose to address in his talk to about 100 Los Alamos scientists and lab workers.

Instead, he recounted the discovery and development of one of the most significant tools for modern biotechnology: the green fluorescent protein, or G.F.P., used widely in cell and molecular biology as a visual tracer. The discovery, which has deepened the understanding of a wide range of fundamental biological processes, brought him the Nobel Prize in Chemistry in 2008, along with Martin Chalfie and Roger Y. Tsien.