One of the best sources to answer this question comes from Lisle Rose. He described early public reaction to nuclear testing as popular, and that people flocked to the desert to watch tests. As for the health ramifications caused by radiation, the U.S. government was well aware of its dangers, even to the extent that it barred newspaper reporters and photographers not approved by the government from visiting Hiroshima and Nagasaki. The public didn't really become aware of the danger from radiation until after Japanese fishermen working in the area of the Bikini Atoll tests became sick and the news went public.
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