TOKYO—A Japanese lawmaker last year raised in Parliament the possibility that a natural disaster could wipe out a nuclear reactor's backup systems, leading to melting in the core, but the country's top nuclear regulator responded that such a scenario was "practically impossible."

A landslide or earthquake could knock out a nuclear plant's emergency power as well as cut off power supplies from outside, Communist Party legislator Hidekatsu Yoshii told a parliamentary committee in May 2010. Such a sequence of events could remove...