Japanese reactor operators now have little choice but to periodically release radioactive steam as part of an emergency cooling process for the fuel of the stricken reactors that may continue for a year or more even after fission has stopped. The plant's operator must constantly try to flood the reactors with seawater, then release the resulting radioactive steam into the atmosphere, several experts familiar with the design of the Daiichi facility said.

That suggests that the tens of thousands of people who have been evacuated may not be able to return to their homes for a considerable period, and that shifts in the wind could blow radioactive materials toward Japanese cities rather than out to sea.

Re-establishing normal cooling of the reactors would require restoring electric power -- which was cut in the earthquake and tsunami -- and now may require plant technicians working in areas that have become highly contaminated with radioactivity.