Tanaka Mitsuhiko is an engineer who helped build reactor 4’s containment vessel in 1974.  Tanaka says he helped hide a defect in the steel used to make the containment vessel.

Reactor 4 at the Fukushima Daiichi was shut for maintenance when the March 11 quake triggered a tsunami that lead to explosions and radiation leaks.

Tanaka has called reactor 4 a “time bomb”.