How do you even begin to decontaminate the area like that, not to mention returning people even on a temporary basis?
The Ministry of Education and Science announced the annual cumulative radiation levels within the no-entry evacuation zone within the 20-kilometer radius from the Fukushima I Nuclear Power Plant, and at the highest place - 3 kilometers west by southwest of the plant - it would be 508 millisieverts in one year from March 11, 2011.
From Asahi Shinbun (8/20/2011):
On August 19 the Ministry of Education and Science announced the amount of cumulative radiation within the no-entry evacuation zone within the 20-kilometer radius from Fukushima I Nuclear Power Plant. The highest estimated cumulative radiation in one year from the day of the accident would be 508.1 millisieverts in Koirino in Okuma-machi in Fukushima Prefecture, located at 3 kilometers west by southwest of the plant. It reinforces the fear that decontamination effort would be difficult. The lowest was 3-plus millisieverts in Kodaka-ku in Minami Soma City. The numbers differ significantly within the 20-kilometer radius.
Of 9 municipalities within the no-entry zone, 50 locations in 8 municipalities were surveyed. The cumulative radiation levels were estimated by assuming people would spend 8 hours outdoors, and stay 16 hours indoors in wooden houses everyday for 1 year until March 11 of 2012.
35 locations out of 50 would exceed 20 millisieverts per year, which was used to determine the planned evacuation zone. In Okuma-machi where Fukushima I Nuke Plant is located, all 12 locations surveyed would exceed 20 millisieverts, with 7 exceeding 100 millisieverts. The highest number in Koirino in Okuma-machi is 508.1 millisieverts, which is equal to 500 years worth of the annual radiation exposure limit for non-nuclear workers (1 millisievert).
In Namie-machi, the highest would be 223.7 millisieverts per year at Kawafusa, 20 kilometers northwest of the plant. The lowest would be 4.1 millisieverts at a location 8 kilometers north of the plant.
The temporary return of residents within the 3-kilometer radius from the plant is being discussed, and this result will be consulted, according to the Ministry of Education and Science.
The national government is planning to allow the residents temporarily returning to their homes within the 20-kilometer radius to bring in their cars and load up whatever they can from their homes.