for leeper, it was a significant step forward.s been working on nuclear disarmament for more than 30 years. >> i'm very happy about that because this is the success of the rebellion. the non-nuclear world is successfully creating international law. >> reporter: leeper's activism started in the 1980s when he came to japan as a translator. he was working in hiroshima when he read a survivor's account of the bombing by the u.s. military during world war ii. it included the memories of a girl forced to abandon her mother who was trapped under rubble, so she could escape approaching flames. >> that one made me think about how horrible the experience was for those children. so then i realized that the bombings were really a war crime. >> reporter: leeper gained prominence for anti-nuclear activities. he became the first foreign chairman of the foundation that runs the hiroshima peace memorial museum. during his tenure, he helped organize the first atomic bomb exhibition in the u.s. americans were able to see with their own eyes the full