The ban on re-entering the evacuation zone around Japan's damaged nuclear reactor is now in effect.

It extends 12 miles from the plant, affecting some 80,000 people. Officials are worried about the radiation danger, and possible looting, and are urging residents to comply or possibly face fines or detention.

Police say they have no idea how many people have previously returned to the zone, or who still might be living there.

A top Japanese official says authorities will arrange brief visits for residents, allowing one person per household to return for a maximum of two hours to collect necessary belongings. They'll be required to go through radiation screening.

No visits will be allowed in a two mile area closest to the plant.