80km(50マイル)の避難範囲設定はどういった根拠なのだろう。 Warden Message | Embassy of the United States Tokyo, Japan

Warden Message | Embassy of the United States Tokyo, Japan:

Consistent with the NRC guidelines that apply to such a situation in the United States, we are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometers) of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical

80km(50マイル)の避難範囲設定はどういった根拠なのだろう。

単にNRCの規定なのか、それとも、今回の事情を鑑みて？