Leo Seren, 83; Physicist on the Atomic Bomb Who Turned Pacifist

Leo Seren, 83, a University of Chicago physicist who called himself a war criminal for the role he played in the development of the atomic bomb, died of amyloidosis Jan. 3 at a hospital in Evanston, Ill.

Seren had just earned his doctorate from the University of Chicago when he went to work with Enrico Fermi on the Manhattan Project in 1942. Seren was one of 51 people present in an abandoned squash court at the university’s Stagg Field on Dec. 2, 1942, when the first nuclear reactor achieved critical mass. Seren’s job was to measure the density of neutrons in piles of graphite, uranium and cadmium control rods used to build the reactor.

He worked on nuclear power until 1960, stopping when he reached the conclusion that there was no way to safely dispose of radioactive waste. He began to focus on renewable energy sources, such as solar, wind and water power. He also joined the peace movement and belonged to a Chicago-based group, now defunct, called NOMOR, an acronym for Nuclear Overkill Moratorium.

In a 1982 speech before anti-nuclear demonstrators at the University of Chicago, Seren spoke of his regret over his role in the Manhattan Project, which led to the devastation of Hiroshima and Nagasaki, Japan, in 1945 and the loss of tens of thousands of lives. He said that if he were tried for crimes against humanity, “I’d plead guilty. And I’d say for mitigating circumstances that at least I decided that I’d never work on nuclear weapons again.”