That seem like a lot, but come to think about it, 250 divided by 30 equals 8.33. At this point, average 8.33 millisieverts per year exposure in parts of Fukushima, particularly near Fukushima I Nuclear Power Plant, seems almost reasonable.
Anyway, here's Yomiuri reporting on one of several report issued on April 9, 2012 by the Science Council of Japan:
Risk of dying from cancer will increase unless decontamination continues after residents return
The Science Council of Japan's estimate shows that unless decontamination is carried out for a certain period of time after the residents return to the areas surrounding Fukushima I Nuclear Power Plant, the cumulative radiation exposure in 30 years may exceed 100 millisieverts. The estimate was published on April 9. The area surrounding the plant is highly contaminated with radioactive materials. When the cumulative radiation exposure reaches 100 millisieverts, the risk of dying from cancer goes up by 0.5%.
The Council calculated different scenarios for cumulative radiation exposure levels for 30 years, taking into account the amount of radiation exposure before the evacuation, annual radiation exposure levels on return, and decontamination after the return of the residents.
The result shows that if the residents return when the annual radiation exposure levels drop to 20 millisieverts and if no decontamination is done afterwards, the residents may be exposed to nearly 250 millisieverts in 30 years, even with the attenuation of radioactivity.
Oops. The Japanese national government's guideline to return the residents to the areas inside the no-entry zone and the planned evacuation zone is that the annual radiation exposure levels drop down to 20 millisieverts. Judging from the decontamination work that has been done inside the zones and outside, it doesn't look like there will be on-going decontamination.
The original 53-page report by the Science Council of Japan is titled (my translation) "To take a first new step toward dealing with radioactivity - take action based on the scientific probe of facts". The radiation exposure for the residents who return to their homes in the evacuation zones is discussed from page 15 to page 20 in the report.
The average background (natural) external radiation exposure in Japan before the Fukushima nuclear accident was about 0.6 millisievert per year. In 30 years, the cumulative external radiation would have been 18 millisieverts.