The radiation dose following the Hiroshima and Nagasaki bombings was cut in half every 200 meters from ground zero, Williams said.
But the fallout from a Fukushima could vary greatly depending on the nature of any leak -- whether it's through an explosion of the core, as in Chernobyl, or a slow, controlled release of pressure-building gases. It also depends on the wind.
"Members of the public are not in imminent danger at a distance of 20 kilometers, so long as they are not downwind," Williams said.
The plume from the Chernobyl explosion drifted over large parts of Europe and the Soviet Union, prompting the evacuation of more than 300,000 people.
The scene dredges up memories of the Three Mile Island accident near Harrisburg, Pa., in 1979.
The partial meltdown at the Three Mile Island Nuclear Generating Station, which ended nuclear power construction in the United States, has never been definitively linked to deaths or cancer. But radiation exposure following the Chernobyl meltdown has been directly implicated in more than 50 deaths and suspected in many more.
The true extent of the disaster's effects, such as the cancer risk posed by widespread low-level radiation, remains unclear.
"No one has yet conclusively assessed this question 25 years after the Chernobyl Accident," said John Williams, professor of nuclear and energy engineering at the University of Arizona.
The acceptable level of radiation exposure, up to 10 millisieverts per year, is based on a "no threshold" model, which assumes a direct relationship between radiation dose and cancer risk and implies anything beyond natural levels is harmful. But some experts argue that low-level radiation may be less harmful on a per unit basis.
Japanese authorities deny that the exposures reported so far pose any health risks.