Japan's chief cabinet secretary says a hydrogen explosion occurred Monday at the facility's Unit 3. The blast was similar to an earlier one at a different unit at the facility.
Yukio Edano says people within a 12-mile (20-kilometer) radius were ordered inside following the blast. AP journalists felt the explosion 25 miles (40 kilometers) away.
Edano says the reactor's inner containment vessel holding nuclear rods is intact, allaying some fears of the risk to the environment and public.
More than 180,000 people have evacuated the area in recent days.