In 1949, Zeldovich led a team of physicists that conducted the first nuclear test, the RDS-1, based roughly on the American design obtained through the atomic spies in the United States, though he continued his fundamental work on explosive theory. Zeldovich then began working on modernizing the successive designs of the nuclear weapon and initially conceived the idea of hydrogen bomb to Andrei Sakharov and others. In the course of his work on nuclear weapons, Zeldovich did ground-breaking work in radiation hydrodynamics, and the physics of matter at high pressure.