Per NHK press conference on nuclear reactors:
They are in still in process of putting sea water to cool the nuclear reactor. They will use that as a seal.
Even if that fails, they do not expect a problem within 20km which is why they increased the evac area.
When explosion happened, 4 people got injured. they we near the turbine of the containment and they are at hospital right now.
I remember yesterday, they were saying 6km was adequate. Some reports say they're not letting anyone closer than 60km.