When Mr. Kan had heard the reactor designers warning about the need of a 300 km radius evacuation zone if all the reactors and spent fuel pools are melting down and burning he had been angry why he had not been told by the nuclear regulatory body about the huge amount of spent fuel stored on site. He had been counting the maximum evacuation zone to be 100 km while he wasn't informed about the amount of spent fuel at the area. So after the reactor-3 blew up he had been willing to evacuate all people inside 80 km radius from the plant. But that decision would have caused many people to die because of lack of food and heating at the tsunami disaster area in northern Japan if the no-go zone would have blocked the main roads and no goods could have been delivered to the people suffering of cold and hunger in northern Japan.
So the government decided to order only 30 km stay inside -zone, though a larger zone could have reduced the risks of radiation exposure for the people living there. A quite hard decision to be made.