Nuclear research in the Soviet Union during this period was limited to skillful laboratory work. Two associates of Soviet physicist Igor Kurchatov reported to the Physical Review in June 1940 that they had observed rare spontaneous fissioning in uranium. “The complete lack of any American response to the publication of the discovery,” writes the American physicist Herbert F. York, “was one of the factors which convinced the Russians that there must be a big secret project under way in the United States.”
