Under the guidelines for public safety that would be used in the United States under similar circumstances, the NRC believes it is appropriate for U.S. residents within 50 miles of the Fukushima reactors to evacuate.
In making protective action recommendations, the NRC takes into account a variety of factors that include weather, wind direction and speed, and the status of the problem at the reactors. Here is a link to results of two sets of computer calculations used to support the NRC recommendations: http://www.nrc.gov/reading-rm/doc-collections/news/2011/11-050_Attchmt.pdf.
In other news, the U.S. Embassy continues to update American citizens as the situation develops. U.S. citizens in need of emergency assistance should send an e-mail to JapanEmergencyUSC@state.gov with detailed information about their location and contact information.
Eliot Brenner Public Affairs Director
U.S. NRC Blog 