Zeldovich is regarded as a secret principal of the Soviet nuclear weapons project ; his travels abroad were highly restricted, to Eastern Europe, under close Soviet security. Soon after the discovery of nuclear fission (by German chemist Otto Hahn in 1939) Russian physicists had begun investigating the scope of nuclear-fission physics, and undertook seminars on that topic; Igor Kurchatov and Yulii Khariton were engaged in 1940.