Second of two parts

Yukiteru Naka, a former employee of General Electric who took part in designing and operating reactor units 1, 2 and 6 at the Fukushima No. 1 nuclear power plant, is just one of a few engineers whose knowledge spans the facility's 40-year history.

Naka recalls that he and many of his colleagues had a lingering question about the plant at least since the mid-1980s: Why were the backup emergency diesel generators and DC batteries still located in the turbine buildings' basements?