The existence of uranium 237 in “ashes of death” from the Bikini bomb in 1954 suggested that it was a “dirty bomb” because uranium 237 was a typical isotope produced from uranium 238 by bombing with such fast neutrons as those from fusion reactions. The evidence of uranium 237 was first reported by Japanese scientists including Ken-jiro Kimura at the University of Tokyo. It was Joseph Rotblat that first found the Bikini bomb was so-called “3 F” (fission (primary)-fusion (secondary)-fission) bomb. He received the radiological data of the bomb from Yasushi Nishiwaki (1917-2011), then an associate professor at Osaka City Medical College and later a professor at the Tokyo Institute of Technology. This paper studies how Nishiwaki met Rotblat to convey the data, especially an estimation of fallout on the Japanese fishing vessel, Daigo Fukuryu Maru (Lucky Dragon No. 5).

Yasushi Nishiwaki was one of the first scientists who studied radiation of the Daigo Fukuryu Maru at the port of Yaizu, its mother port in Shizuoka Prefecture. With the radiological data gathered by Japanese scientists, he and his American wife Jane traveled for four months, from July to November 1954, to at least about nine European countries including the UK, France, Belgium, Germany, and Sweden. Nishiwaki gave lectures at various places, universities and public halls. The amount of money they needed was a lot. It was said to be around two million yen which is almost two hundred thousand Euros at current rate. Anti-H bomb groups in the Osaka area financially supported their trip. Nishiwaki met at the end of August Rotblat on the occasion of the (first) international conference on radiobiology at Liege University, Belgium. Nishiwaki gave Rotblat a memorandum about ashes of death on the Daigo Fukuryu Maru, from which Rotblat could estimate the amount of radioactive substances from the Bikini test. He was surprised to see an unexpectedly high amount of fallout. This resulted in his speculation that the bomb was surrounded with ordinary uranium which, with fast neutrons from the secondary fusion bomb, produced much more fission products than the primary fission bomb. Rotblat then told Bertrand Russell his speculation about the Bikini bomb. This made Russell write in 1955 what became the “Russell-Einstein Manifesto.”

This paper will also give an account of reactions to Nishiwaki’s message in Europe. It argues that his message helped make Europe aware of the real suffering in spite of American propaganda that radiological effects from the Bikini bomb were not serious.