Estimated cumulative dose over one year (in milliSieverts), starting from March 11, to someone spending 8 hours a day outdoors:
What's the bold line (black) at 20 mSv mean? According to reports (e.g. this NYT article), 20 mSv over one year is the Japanese government's threshold for evacuation. So if I interpret correctly, that enclosed area is likely to be evacuated within a month or so, for a long period of time. (Since much of the dose comes from long-lived 137Cs, this could be a very long time. A permanent-exclusion zone.) That's around 500 km2 in there, which based on district-level population densities probably encloses (very roughly) around 50,000 people.
Some related data sources are