A model created by the Comprehensive Nuclear Test Ban Treaty Organization and obtained by the New York Times.

The chairman of the Nuclear Regulatory Commission, Gregory B. Jaczko, said Monday that the plume posed no danger to the United States. “You just aren’t going to have any radiological material that, by the time it traveled those large distances, could present any risk to the American public,” he said in a White House briefing.

Mr. Jaczko was asked if the meltdown of a core of one of the reactors would increase the chance of harmful radiation reaching Hawaii or the West Coast.

“I don’t want to speculate on various scenarios,” he replied. “But based on the design and the distances involved, it is very unlikely that there would be any harmful impacts.”